NUREG/CR-6543

SAND97-2544

Effects of Smoke on

Functional Circuits

P r e p a iRe b y~ liII • ilI

Preare by

T. J. Tanaka

Sandia National Laboratories

Prepared for

U.S. Nuclear Regulatory Commission

4c;~
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NUREG/CR-6543

SAND97-2544

Effects of Smoke on

Functional Circuits

Manuscrip Comnpleted: April 1997

Date Published. Ocoe 1997

Prepared by

T J Tanaka

Sandia National Laboratories

Albuquerque, NM 87185-0747

C Antonescu. NRC Project Manager

Prepared for

Division of Systems Technology

Office of Nuclear Regulatory Research

U.S. Nuclear Regulatory Commission

Washington, DC 20555-0001

NRC Job Code W6051
Previous reports in series:

NUREGICR-6479, SAND96-2633, T. J. Tanaka, S. P. Nowlen, and D. J. Anderson, "Circuit Bridging of

Components by Smoke," Sandia National Laboratories, Albuquerque, NM, October 1996.

NUREGICR-6543 ii
ABSTRACT

Nuclear power plants are converting to digital instrumentation and control systems; however, the effects of abnormal

environments such as fire and smoke on such systems are not known. There are no standard tests for smoke, but

previous smoke exposure tests at Sandia National Laboratories have shown that digital communications can be

temporarily interrupted during a smoke exposure. Another concern is the long-term corrosion of metals exposed to

the acidic gases produced by a cable fire. This report documents measurements of basic functional circuits during and

up to I day after exposure to smoke created by burning cable insulation. Printed wiring boards were exposed to the

smoke in an enclosed chamber for I hour. For high-resistance circuits, the smoke lowered the resistance of the

surface of the board and caused the circuits to short during the exposure. These circuits recovered after the smoke was

vented. For low-resistance circuits, the smoke caused their resistance to increase slightly. A polyurethane conformal

coating substantially reduced the effects of smoke. A high-speed digital circuit was unaffected. A second experiment

on different logic chip technologies showed that the critical shunt resistance that would cause fMlure was dependent

on the chip technology and that the components used in the smoke exposures were some of the most smoke tolerant.

The smoke densities in these tests were high enough to cause changes in high impedance (resistance) circuits during

exposure, but did not affect most of the other circuits. Conformal coatings and the characteristics of chip

technologies should be considered when designing digital circuitry for nuclear power plant safety systems, which

must be highly reliable under a variety of operating and accident conditions.

ioi NUREG/CR-6543
0r
CONTENTS

ABSTRACT ..................................................................................................................................... iii

EXECUTIVE SUMMARY .................................................................................................................. viii

ACKNOWL ME ..................................................................................................................... ix

ACRONYMS ...................................................................................................................................... x

LIST OF SYMBOLS ........................................................................................................................... xi

1. I T ODUCTION ............................................................................................................................. 1

1.1 Statement of Problem ..................................................................................................................

1.2 Previous Smoke Exposure Tests .................................................................................................... 1

1.3 SNL Tests ................................................................................................................................. 1

2. METHODS AND MATERIALS ................................................................................................... 3...3

2.1 Overview of Tests ....................................................................................................................... 3

2.2 Functional Board Configuration ................................................................................................ . 6

2.3 Smoke Exposure Tests ................................................................................................................. 9

2A Circuit Tests and Measurement Procedures ................................................................................. .15

3. TEST RESULTS ............................................................................................................................ 26

3.1 Smoke Measurements ................................................................................................................. 26

3.2 Circuit Measurements ................................................................................................................. 28

4. CONCLUSIONS ............................................................................................................................ 35

REFERENCES .................................................................................................................................. 36

APPENDIX .................................................................................................................................... A-1

v NUREG/CR-6543
LIST OF FIGURES

Figure 1. Front side-LRSTF board ................................................................................................. 4....... A

Figure 2. Two transmission lines for transmission line coupling measurement ........................................... 4.....

Figure 3. Manual measurement matrix switch box .......................................... 5

Figure 4. Comparison of PTH and SMT packages for the same device .................................... 6

Figure 5. Back side-LRSTF board ...................................................................................... ............. 7

Figure 6. Diagram of LRSTF board showing circuit placement .............................................................. 8

Figure 7. Smoke exposure chamber with boards ................................................................................ . 10

Figure 8. Board arrangement for testing .................................................................................................. 11

Figure 9. Smoke exposure activity timeline ............................................. 13

Figure 10. Quartz crystal microbalances for the measurement of soot deposition ......................................... 13

Figure 11. Fuel in combustion chamber ............................................... 14

Figure 12. Thermocouple locations .................................................................................................. 15

Figure 13. HVLC manual measurement circuit ................................................................................ 17

Figure 14. HVLC automatic measurement circuit. ........................................................................... 17

Figure 15. HCLV manual and automatic measurement circuit ................... ................. 18

Figure 16. HSD PTH measurement circuit .......................................................................... . ......... 19

Figure 17. HSD SMT measurement circuit ............................................. 19

Figure 18. HF LPF typical frequency response of circuits .................................................................. 20

Figure 19. BF LPF manual measurement circuit ................................................................................. 21

Figure 20. HF LPF automatic measurement circuit ........................................................................... 21

Figure 21. HF TL forward coupling measurement ................................................................................ 22

Figure 22. HF TL reverse coupling measurement. ............................................................................... 22

Figure 23. HF TL typical frequency response of circuit .................................................................... 23

Figure 24. Leakage circuit-manual measurement .............................................................................. 24

Figure 25. Leakage circuit-automatic measurement ....................................... 24

Figure 26. IPC-B-24 board for surface insulation resistance ..................................................................... 25

Figure 27. Resistive terminators showing corrosive action of smoke ............................. 27

Figure 28. HVLC PTH vs. time for a high-fuel, high-flux burn ............................................................. 28

Figure 29. HVLC SMT vs. time for a iugh-fuel, high-flux burn .............................................................. 29

Figure 30. HCLV PTH vs. time for a high-fuel, high-flux burn ........................................................... 30

Figure 31. HCLV SMT vs. time for a high-fuel, high-flux burn ........................................................... 30

Figure 32. HF TL (50 MHz) vs. time for a high-fuel, high-flux burn ...................................................... 33

Figure A-I. OR gate wiring diagrams ................................................. A-2

Figure A-2. AND gate wiring diagrams ............................................... A-3

NUREG/CR-6543 vi
LIST OF TABLES

Table 1. Fuel amounts burned ........................................... . ........................................................ 9

Table 2. Cable insulation used as fuel ................................................................................................ 12

Table 3. Circuits on functional board ............................................................................................... 16

Table 4. Leakage measurement parameters for automatic measurement system ........................................... 25

Table 5. Temperature ('C) at end of lamp beating ........................... .................................................. 26

Table 6. Fuel burned and smoke deposition .................................................................................... 26

Table 7. Semiquantitative analysis of ashed sample ....................................................................... 27

Table 8. Chemical analysis of soot (gg/filter) ...................................................................................... 28

Table 9. HVLC change in resistance values ...................................................................................... 29

Table 10. HCLV results (AV in mV) .................................................................................................. 31

Table 11. HSD rise time, averages over time range (ns) ....................................................................... 31

Table 12. HSD fall time, averages over time range (ns) ......................................................................... 32

Table 13. HSD delay time, averages over time range (ns) ........... .......................... 32

Table 14. HF TL at 50 MHz, averages over time range (0B) ................................................................... 33

Table 15. Pads measurements, averages over time range (loge) .............................................................. 34

Table 16. Summary of the surface insulation resistance (logj0•.,d--lgjX1..o) .................................... 34

Table A-I. Components tested and results ........................................................................................ A-4

Table A-2. Minimum high level and maximum low levels for logic families .......................................... A-5

vii. NUREGICR-6543
EXECUTIVE SUMMARY

Nuclear power plants are replacing their analog I&C equipment with digital I&C equipment; however, there is

concern about the effects of abnormal or severe environments on these new control systems. A potentially severe

environment is smoke from an electrical fire. Smoke may impair the operation of electrical circuits by shorting

leads, corroding contacts, and inducing stray capacitance. To investigate some of these effects, functional boards

containing circuits sensitive to these failure modes were exposed to smoke from burning cable insulation. The

components on the boards were those commonly used in modem electrical circuits. Three fuel levels were used in

these experiments and corresponded to burning 3, 25, and 50 g per cubic meter of air. All tests were conducted at

75% relative humidity because earlier tests showed that high humidity increases failures. Circuit performance was

measured during the smoke exposure.

The tests showed that the high-resistance circuit (high voltage, low current, HVLC) was the most susceptible to

smoke. Its hugh impedance (50 Mfl) was shorted by smoke during the exposure, but recovered after the smoke was

vented. This shorting occurred for small concentrations of smoke produced by burning 3 g of fuel per cubic meter of

air At medium and high fuel levels, the resistance of the HVLC circuit typically decreased by 90% during the

smoke exposure. Resistance was lost in these circuits because the current found an alternative path between

conductors. Printed wiring boards generally have very high resistance, > 1012 ohms When smoke occurs in the

vicinity of this circuit, however, the resistance outside of the circuit is lower than that through the circuit and the

current leaks around the circmt. Other circuits that were sensitive to a decrease in surface insulation resistance were

also affected temporarily during the smoke exposure

The resistance of the low-resistance circuit (high current, low voltage, HCLV at 1.4 Q) was permanently increased

by the smoke, implying that the contacts were corroded. This is a different failure mode than the failure of the

HVLC circuit described above as current leakage. However, the change was very small (<2%).

A polyurethane conformal coating brushed on half of the test boards substantially reduced the damaging effects of

smoke.

The high-speed digital circuit was not affected by smoke, even at the highest fuel level. The main component tested

in this circuit was an advanced TTL (FAST) chip. An experiment that compared the failure of chip technologies in

the presence of a shunt resistance showed that a FAST chip is one of the most tolerant to smoke. The results of this

experiment show that technologies with a high output current are more smoke tolerant. Previous tests of

complementary metal-oxide semiconductor (CMOS) chips at fuel levels of 100 g per cubic meter caused failures and

the experiment using shunt resistances showed that CMOS chips are more sensitive to smoke than FAST chips.

The smoke density in the current experiment was not high enough to cause FAST chips to fail.

The test results suggest that conformal coatings and the characteristics of chip technologies should be considered

when designing digital circuitry to be used in nuclear power plant safety systems.

NUREG/CR-6543 viii
ACKNOWLEDGMENTS

I would like to thank all of the people who have contributed to this project and the writing of this report. This work

has been sponsored by the USNRC Office of Research under the project management of Christina Antonescu. John

Garcia helped create the smoke environments and cleaned up the equipment for each test. Andrea Hirst assembled

most of the automated measurement system and wrote programs to control it. Karla Waters analyzed and plotted the

data for this report. Dennis Huffman and Dennis Anderson, of the Low-Residue Soldering Task Force (LRSTF),

patiently answered questions on testing boards, helped design the experiment, and provided the manual matrix switch.

Steve Nowlen provided consultation on fire testing and fixed the environmental chamber when necessary. Kathy

King and Mary Ellen Puckett assisted in the assembly of the LRSTF boards, working with manufacturers when

necessary. This report could not have been written without the electrical engineering advice and support of my

husband, Dr. Stephen J. Martin, who volunteered so many good ideas, such as testing gates with a shunt resistor.

ix NUREG/CR-6543
ACRONYMS

CMOS complementary metal-oxide senuconductor

DIP dual-in-line package

FAST advanced Schottky (TrL family)

HCLV high current, low voltage

HF LPF high-frequency low-pass filter

HF TL high-frequency transmission line

HSD high-speed digital

HVLC high voltage, low current

I&C instrumentation and control

LRSTF low-residue soldering task force

NAND not-AND gate

PGA pin grid array

PTH plated-through hole

QCM quartz crystal microbalance

SMB subminiature coaxial connector, type B (mil-C-39012)

SMT surface mounted technology

SNL Sandia National Laboratories

SOIC small-outline integrated chip

TFL transistor-to-transistor logic

USNRC U. S. Nuclear Regulatory Commission

NUREG/CR-6543x X.
LIST OF SYMBOLS

Ammeter QNo connection

AND gate

Connection

Connector

Operational amplifier

OR gate

Oscilloscope

Current source

DC power supply

-0-.- Resistor -WvSine-wave generator -0-

Ground

Inductor

Square-wave generator

Variable resistor

Voltmeter

-0-

-0- NAND gate 11Ž

xi NUREGICR-6543

1. INTRODUCTION

1.1 Statement of Problem

Existing instrumentation and control (M&C) technology

at nuclear power plants is aging, and analog

replacements tend to be obsolete. The added functions

available in digital I&C are motivating utilities to

adopt this technology in their plants. These

technologies have several advantages and in fact have

been in widespread use in the non-nuclear industry for

several years. However, there is concern about their

use in safety-related systems in nuclear power plants

owing to the lack of experience with such equipment in

severe environments.

Since smoke is a severe environment, the United States

Nuclear Regulatory Commission (USNRC) started a

program to determine the impact of smoke on digital

I&C equipment. This report presents the results of

recent USNRC-sponsored smoke exposure tests on

functional circuits at Sandia National Laboratories

(SNL). An earlier report in this program focused on

circuit bridging of components through reduced

insulation resistance between leads that wos caused by

smoke (Tanaka, Nowlen, and Anderson, 1996).

Oak Ridge National Laboratory is reviewing existing

environmental qualification standards for digital I&C

equipment and apphcable regulatory guides. Under

USNRC sponsorship, it is also conducting research on

the environmental qualification of digital equipment

that includes tests relating to electromagnetic

interference, high temperature, and high humidity

(Korsah and coauthors, 1996; Korsah, Clark and Wood,

1994).

1.2 Previous Smoke Exposure

Tests

There are no standardized tests for electronic equipment

in a smoke environment. However, there are standard

tests for measuring the corrosivity of smoke from

different types of materials. Typically they consist of

measuring the metal loss as a result of corrosion or the

acidity of the smoke (Caudill and coauthors, 1995).

Highly acidic smoke contributes to increased corrosion.

While such measurements indicate the destructive

quality of smoke as it corrodes contacts, they do not

account for smoke-caused failures, such as shorted

circuits, increased leakage currents, and arcing.

Most analyses of smoke damage are performed after a

fire and do not take into account the effects of smoke

during the fire (Reagor, 1992). Post-fire analysis is

important for insurance companies in evaluating the

salvageability of equipment, but it does not address the

need of nuclear power plants, which is to be able to

monitor reactor performance continuously throughout

all unusual events By actively monitoring equipment

during a smoke exposure, tests performed at SNL have

indicated that smoke can cause circuit bridging during

as well as after a fire

The SNL program has concentrated on determining the

impact of smoke on a variety of equipment, ranging

from simple components to networked computer

systems, during and 24 hours after a fire These tests

show that smoke may interrupt communications

between computers on networks or between computers

and other microprocessor boards. Tests of empty

component packages show that smoke lowers the

resistance between leads and may therefore cause

electrical shorting, arcs, and increased leakage currents

(Tanaka, Nowlen, and Anderson, 1996).

1.3 SNL Tests

This report summarizes the results of smoke exposure

tests on circuit boards with simple functional circuits

designed by a government-industry cooperative effort,

" Kofi Korsah, Tina Tanaka, and Mahbubul Hassan, Technical

Basis for Environmental Qualification of Microprocessor-Based

Safety-Related Equipment in Nuclear Power Plants Draft report

NUREG/CR-6479, Oak Ridge National Laboratory. Oak Ridge,

7N

I 1 ~NtREG/CR-6543
Introduction

the Low-Residue Soldering Task Force (LRSTF) (Iman

and coauthors, 1995a). The LRSTF designed two

printed circuit boards to test soldering methods. Their

tests were carried out in two phases and the SNL tests

Section 1

used the boards designed for phase 2 (Iman and

coauthors, 1995b). The functionality of the boards was

tested in the presence of smoke for a range of densities

and burning conditions.

6

NUREGICR-65432 2
2. METHODS AND MATERIALS

2.1 Overview of Tests

Fifty-five Low-Residue Soldering Task Force boards

were manufactured for the smoke exposure tests. The

LRSTF boards contain nine simple circuits, which are

described in Section 2.4. These circuits made it easier

to determine the effects of smoke than if a complex

microprocessor-based board had been used. Figures 1

and 2 show the printed wiring boards used for this

series of tests. The board in Figure I contains most of

the active circuits except for the transmission lines,

which are shown in Figure 2. The phase 2 design of

the boards included ihe circuits shown in Figures I and

2 on the same board, but because the transmission

lines were left off of the board shown in Figure 1, the

printed circuit board shown in Figure 2 had to be

manufactured for these tests.

Of the 55 sets of LRSTF boards (each set consisted of

a main board as shown in Figure I and a board

containing the transmission lines as shown in Figure

2), 26 sets were brushed with a conformal polyurethane

coatingt and the rest were left bare. Four sets of boards

were exposed per smoke test for a total of 12 smoke

exposure tests and one nonsmoke, lugh-hunudity test

(13 tests in all). During each test, an uncoated board

and a polyurethane-coated board were exposed to

smoke, and an uncoated board and a polyurethane-coated

board served as controls and were not exposed to

smoke. These unexposed boards were monitored

simultaneously with the other boards being tested.

The test boards were placed in a smoke exposure

chamber inside an environmental chamber in which

temperature and humidity were controlled before and

after the exposure. The smoke was produced in a

manner similar to the tests documented in Tanaka,

Nowlen, and Anderson (1996).

The boards were subjected to one of six different smoke

conditions, which were varied by the amount of fuel

used to produce the smoke and the heat flux used to

burn the fuel. The order of conditions and the selection

of boards were determined by a random draw. Three

amounts of fuel and two heat flux levels were used for

the six smoke exposure conditions. Each condition

was repeated once.

To determine the condition of the boards before and

after the exposures, the procedures for testing the

circuits with a test matrix switch (Figure 3) developed

by the LRSTF were followed. This allowed

comparisons to be made between the boards

manufactured for the low-residue solder tests and those

used for these tests.

A preliminary test of possible nonsmoke effects was

run prior to the smoke exposures to determine if the

electromagnetic fields generated from currents to the

quartz heating lamps would affect the circuits and if

humidity alone would affect the circuits. The lamps

were turned on when the functional boards were in the

test configuration. The test lasted for 15 minutes, the

length of time of the burn for the smoke tests. A

second preliminary test was performed with the same

test specimens. In this test, the humidity was raised to

75±5% RH This determined the effect of lhgh

humidity on the functional circuits.

All of the low-resistance circuits (coated and uncoated)

increased in resistance when the lamps were on, but 30

minutes later returned to their previous resistances

The lamps probably increased the temperature of the

resistors which increased the resistance of the circuits.

Since these circuits are very low in resistance, they

were the most sensitive to the change in temperature.

None of the other circuits reacted to the lamps The

uncoated high-resistance circuits reacted to the change

in humidity by first decreasing in resistance, and then

increasing back to their original values within I hour.

The coated high-resistance circuits did not react as

much to the change in humidity The high humidity

probably lowered the resistance of the surface of the

board slightly, but the currents on the surfaces can heat

the surface and cause the resistance to rise again.

PIBM by Hysol, a solvent-based, one-component urethane coating

cured at room temperature

3 NUREG/CR-6543
Methods and Materials Section 2

&-ift III 1?J1111 11311111114,111111151 6 1', )iAkP)tj-j, *) 7 1 1111111 111111.

,ttNl

Figure 1. Front side-LRSTF board.

Figure 2. Two transmission lines for transmission line coupling measurement.

NUREGICR-65434 4
Section 2

~J LJ U U

C

Methods and Materials

4Pj

A

I

F - .1 11

"1 97 V'

.5. 5.

4-,.

r

Figure 3. Manual measurement matrix switch box.

NUREG/CR-ý6543 5
Methods and Materials Section 2

2.2 Functional Board

Configuration

The LRSTF board was designed to evaluate the use of

low-residue solder flux (a procedure in which the flux

remains on the printed wiring board after soldering).

Both plated-through hole (PTH) and surface-mounted

components (SMT) were used on this board to

represent the two major types of components that may

be used in a modem circuit board. Modern digital

components use smaller packages whose contacts are

also smaller and spaced closer together. An example of

the differences can be seen in Figure 4, which shows

the same logic chip enclosed in two different standard

packages. The dual-in-line package (DIP) on the right

uses plated-through holes to connect to a printed wiring

board, while the small-outline integrated chip (SOIC)

package on the left, an SMT component, is soldered to

the surface of the board.

A variety of circuits were selected to represent the

circuits that could be used in general applications; they

include high-voltage, high-current, high-frequency, and

high-speed digital circuits. For these four types of

circuits, a separate circuit in the form of PTH and SMT

components was included on the printed circuit board.

The functional boards consisted of four layers; that is,

there were two pieces of FR-4* insulated circuit board

material that were laminated together, each with traces

on both sides (three layers of dielectric in all) The

layers were electncaily connected by drilling holes into

the circuit board and allowing solder to flow through

the holes and form "vias." For many of the circuits,

one of the muddle layers served as a ground plane, while

the other middle layer served as an electrical connection

to various circuits.

Figure 4. Comparison of PTH and SMT

packages for the same device.

Figure 5 shows the back of the board in Figure 1.

Leads for PTH components can be seen penetrating

though the back of the functional board in Figure 5,

whereas these leads are absent for SMT components.

The printed circuit board can be schematically divided

into isolated circuits as shown in Figure 6.

Comparisons with Figure I allow identification of the

components in each of the circuits. More detail on

each circuit is provided in the following sections.

Details on the development of each circuit can be found

in Iman and coauthors (1995a).

The original LRSTF board design included several

"dummy" parts that served no function other than

providing crevices in which residue could deposit.

Since the objective of these tests was to determine the

functionality of circuits, only the functional circuits

were completed; the dummy circuits were left bare of

components.

FR-4 is a National Electrical Manufacturer's Association

(NEMA) grade for an epoxy-glass larminated printed wiring

board with a self-extinguishmg reizn system

NUREG/CR-65436 6
Section 2 Methods and Materials

4.

& 3 ~LUI .~ .. ~.

t

.19

~~I 1 I ii 111 i 11 1111 1

Figure 5. Back side-LRSTF board.

7 7 NUJREGICR-6543
Methods and Materials Section 2

SMB,

Figure 6. Diagram of LRSTF board showing circuit placement. (SMB, subminiature

type B connector; HCLV, high current low voltage; PGA, pin grid array; HSD, high.

speed digital; HVLC, high voltage low current; HF LPF, high-frequency low-pass

filter)

NUREGICR-65438 8
Section 2

2.3 Smoke Exposure Tests

2.3.1 Procedure

In order to produce smoke in a standard and reproducible

way, a draft corrosivity test standard produced by the

E5.21.70 subtask group of the American Society for

Testing and Materials was followed. The fuel was

burned using radiant heat from a tungsten-quartz lamp

inside a cylindrical quartz combustion chamber. The

smoke production and exposure equipment is shown in

Figure 7. The radiant beat lamps were adjusted so that

a fixed heat flux was produced at the fuel surface. The

heat flux was measured with a Schmidt-Boelter

(thermopile) heat flux meter before each test to

determine the amount of heat incident on the fuel at the

beginning of the test. Small variations in the

positions of the lamps can affect the heat flux that is

incident on the sample. As smoke is produced, the

quartz chamber becomes coated with soot, which

reduces the heat flux. No attempts were made to

compensate for this effect. Smoke rises up a stainless

steel chimney into the smoke exposure chamber.

Although the combustion cell has a small opening, no

substantial amount of fresh air is allowed into the

chamber during the exposure.

The smoke exposure chamber is contained within an

environmental chamber that allows control of

temperature and humidity before and after the exposure.

The temperature and humidity levels of the

environmental chamber were 24" C (75" F) and 75%

RH for all of these tests. These levels were selected

because they were judged to be realistic for a nuclear

power plant after a fire. Because the smoke exposure

chamber is sealed off from the environmental chamber

during the smoke exposure, there is no effective control

over temperature and humidity during the exposure.

The smoke exposure tests consisted of monitoring four

functional board sets at a time, two sets in the smoke

chamber (Figure 8) and two sets outside the smoke

chamber (Figure 7) while smoke was added to the

exposure chamber and for 24 hours thereafter. In

addition, two IPC-B-24 boards were included in the

tests, one inside the smoke chamber and one outside

the smoke chamber with the control functional boards.

Half of the functional boards were coated with

polyurethane and half were uncoated

Methods and Materials

Three levels of fuel were tested-high (50 g/mO),

medium (25 g/m3), and low (3 g/m)--defined in terms

of fuel available to burn per volume of air available

(Table 1). Each density level simulated the amount of

smoke per volume of air that is likely to occur for

different fire scenarios (Tanaka, Nowlen, and Anderson,

1996). As reported in Tanaka, Nowlen, and Anderson

(1996), typical electrical cabinets in nuclear power

plants range in size from 0.85 to 85 m3. The amount

of fuel in these cabinets averages 25 kg/i 3 . The high

fuel level in Table I corresponds to burning 2% of the

fuel m an average cabinet and then determining the

effect on equipment located within the cabinet. This

scenario of equipment located in the same cabinet as

the fire produces the highest likely smoke density in

which the exposed electrical equipment is not burned

but can be damaged by smoke. The medium fuel level

corresponds to a significant fire in a general room

(smaller than the control room). Other scenarios could

also yield this same smoke density, for example,

electrical equipment located in the same cabinet as a

fire, but with only 1% of the fuel burning. The low

level of smoke density corresponds to a cabinet fire in

the control room. Since the control room is very large

compared with most other rooms in a nuclear power

plant and it contains much of the control equipment, a

special effort was made to include the low smoke

densities that may affect equipment located within a

control room while another electrical equipment cabinet

is burning. For most scenarios, no dilution of the

smoke by ventilation was assumed because fire

dampeis should close the vents to general areas.

However, for the large cabinet fire in the control room,

the smoke was assumed to be diluted by an equivalent

of 10 air changes per hour based on activation of the

control room emergency vent mode (Jacobus, 1984).

Table 1. Fuel amounts burned

Fuellair Amount

Smoke ratio of fuel

Fire scenario level (g/mi) (g)

Large cabinet fire Low 3 0.6

in control room

General area fire Medium 25 5

Small cabinet High 50 10

fire (equipment

in cabinet) -

9 NUREG/CR-6543
0~

Ut

h

IV *

w ~ ",ft. *-.* t

C.

C',

- -wwr

~

Figure 7. Smoke exposure chamber with boards. The control boards are In the lower right-hand corner outside of the

exposure chamber.

Ci2

0

w

I..)
0

Figure 8. Board arrangement for testing.
Methods and Materials

The volume of the smoke exposure chamber used for

this test was 0.2 in3, one-third the size of the smallest

cabinets used in plants It is easier to perform repeated

tests on a small scale and in a small volume than in

full-scale tests. However, some assumptions must

then be made on the amount of fuel that should be

burned to replicate a full-scale fire on the small scale of

these tests. Since we did not know whether the

amount of fuel burned should be scaled by volume or

by surface area of the test chamber, volume was

arbitrarily selected. Earlier, during the circuit bridging

tests, shorting occurred midway through burning the

fuel (Tanaka, Nowlen, and Anderson, 1996). Thus,

for the high level in the current tests we used only 50

g/m3 Since a threshold value for smoke density was

desired, a middle value of 25 g/m3 was also included

The low level was the same as the low level for the

circuit bridging tests on components. All tests were

run at >70% RH, which corresponds to a controller

setting of 75% RH. The tests were run at high and

low heat flux levels: 50 kW/m2 for high (flaming) and

25 kW/m2 for low (smoldering).

Since the smoke exposure chamber volume of 0.2 m3

was smaller than that used for the circuit bridging tests

reported in Tanaka, Nowlen, and Anderson (1996), the

amount of fuel needed was very small. To make the

test valid for a general cable fire rather than a specific

cable material, a mixture of cables was burned. Rather

than cutting small lengths of cable, the cable jacket and

insulation materials were stripped from the conductors

and ground into small particles.

The fuel mixture is summarized in Table 2. The fuel

includes insulation and jacket material from different

environmentally qualified cables currently in use at

plants (Bustard and Holzman, 1994). The mixture was

Section 2

based on a rough estimate of the relative popularity of

each material using the ratio for the number of plants

using that material to the total number in the Bustard

and Holzman report It is intended to be representative

of cables found in all plants rather than a single

material or plant

As indicated previously, the fuel was burned at two

heat flux levels-25 kW/n 2 and 50 kW/m2 These

levels were provided by the quartz lamps used to heat

the fuel The flux level was measured without any fuel

in the combustion chamber. A control program that

compared the heat flux level with the output of the

thermopile determined the voltage level at which the

lamps were to be powered for the smoke exposure.

The 25-kWim2 level corresponds to a smoldering fire,

tests at this level were performed without igniting the

fuel. The 50-kW/m2 level corresponds to a flaming

fire; after 2 minutes of heating, the fuel was ignited

with a butane pilot light and the fire was allowed to

burn until the fuel was exhausted.

The smoke exposure procedure is shown schematically

in Figure 9. At the beginning of each test cycle, the

circuit board functions were measured using the manual

procedure developed by the LRSTF. This was done in

the laboratory area with no humidity or temperature

controls. The boards were then placed in the smoke

exposure chamber and automatic measurements began

as the humichty was increased to 75% RH.

When the humidity reached 75% RH, the smoke

exposure chamber was closed and the lamps turned on

for 15 minutes. While the lamps were on, the fuel was

either ignited (high flux) or allowed to smolder (low

flux). After the lamps were turned off, a small fan was

turned on to mix the smoky atmosphere. The smoke

exposure chamber remained closed for I hour. After

Table 2. Cable insulation used as fuel

Fraction

Cable name Insulation Jacket Plants of plants

Rockbestos Firewall II FRXLPB CSPE 61 0.222

Anaconda Flameguard lkv EPR CSPE 35 0.127

Brand Rex XLPE XLPE CSPE 30 0.109

Okonite Okolon EPR CSPE 26 0.095

Kente HTK 25 0.091

Rockbestos Coax (leI) 24 0.087

Raychem XLPE XLPE 23 0.084

Dekoran Dekorad EPDM CSPE 19 0.069

BIW EPR CSPE 19 0.069

Kente FR 13 0.047

NUREGICR-6543 12
Section 2 Methods and Materials

Measurement

Humidity

Smoke Chamber

Lamps

Time (hrs)

PretestI Autoetah JPost test

Low High Low

Open V Open

Off Off Tom J

I, .. , ,! ,

G

0

I

I 2

24

Figure 9. Smoke exposure activity timeline.

the smoke exposure chamber was vented through a

hood, the chamber doors were opened to allow the

humidity-controlled air into the combustion chamber.

The circuits were continually monitored for 24 hours

starting at the time the lamps were turned on

The control boards were treated in a manner similar to

the exposed boards; their functions were checked before

the test using a manual measurement system. The

control boards were then placed outside the smoke

exposure chamber but inside the environmental

chamber while the humidity was increased. When the

smoke exposure chamber was closed to start the smoke

exposure, the control boards were enclosed in a plastic

bag to ensure that any smoke leaking from the chamber

did not drift over to the control boards The control

boards remained in the bag throughout the exposure

The bag was then opened while the smoke exposure

chamber was vented and remained opened throughout

the remainder of the automatic measunng period.

After the automatic measurement penod, both the

exposed and control boards were measured again using

the manual procedure These measurements were made

within I hour of the end of the automatic procedure,

but were in the general laboratory, which had no

humidity controls and a temperature of approximately

20"C

2.3.2 Measurement Techniques and

Equipment

2.3.2.1 Soot Deposition

Two quartz crystal microbalances (QCM) oriented in

horizontal and vertical directions were used to determine

how much smoke was deposited on surfaces (Figure

10) The QCM oscillates at its natural frequency

(approximately 5 MHz), which is determined by the

oscillation mode and the mass of the crystal As

smoke is deposited, the crystal oscillates at a slower

frequency, much as a loaded spring with a changing

mass The change in frequency is related to the change

in mass by the following equation

AM 4.419E+11

whlere fo is the starting frequency of the QCM, Af is

the change in frequency (Hz) fromfo, and the resulting

change in mass (AM) is in Jig/cm2 . The constant

multiplier (4 419E+1 1) is dependent on the dimensions

of the original QCM crystal.

Figure 10. Quartz crystal microbalances for

the measurement of soot deposition.

13 13 NUREG/CR-6543
Methods and Matenals

2.3.2.2 Fuel Weight Loss

Before the test, the fuel was placed in aluminum trays

on top of a load cell within the quartz combustion

chamber (Figure 11). The trays were weighed before

and after each test to determine how much fuel was

burned. The load cell also provided active monitoring

of the weight loss as the fuel was burned

2.3.2.3 Temperature and Humidity

Temperature was measured using five thermocouples in

the smoke exposure chamber (Figure 12) and two

thermocouples with the control boards.

Thermocouples 1 to 3 were located in the ceiling of the

smoke exposure chamber. Thermocouple 4 was located

along the floor of the smoke exposure chamber.

Thermocouple 5 was located in the same area as the

functional boards and could be used to determine the

temperature near the bottom of the boards.

Thermocouples 6 and 7 were located in the bag with

the control boards during the smoke exposure. They

were exposed to the environmental chamber when the

bag was opened, after the exposure period

Section 2

Humidity inside the environmental chamber (but

outside the smoke exposure chamber) was monitored

and controlled for a 24-hour period. Since the smoke

exposure chamber was sealed during the exposure

period (1 hour), the humidity that was measured during

this time did not indicate the humidity at the board

locations. The plastic bag around the control boards

was sealed just before the start of the smoke exposure

and opened after the exposure; thus the humidity

surrounding the control boards was treated in the same

manner as the smoke-exposed boards, i.e., humidity

was controlled before and after but not during exposure.

2.3.2.4 Chemical Analysis

The soot was analyzed using ion chromatography,

whuch detects specific acidic compounds containing Cl,

Br, F, and S04. The soot was collected on 4.25-cm

diameter ashless filter papers placed on the bottom of

the smoke exposure chamber, away from other

interfering materials.

The fuel was also analyzed for percent bromine,

chlorine, fluorine, and ash after burning. The gross

heat of combustion was measured in this process.

'C -

.5

Figure 11. Fuel in combustion chamber.

NUREG/CR-6543 14
Section 2 Methods and Materials

Located near test boards 0* Located wah control boards

Figure 12. Thermocouple locations.

A seniquantitative spectrographic analysis of the ash

sample was also performed to determine the presence of

metals.

2.4 Circuit Tests and

Measurement Procedures

2.4.1 Overview

As indicated earlier, the manual testing procedures

developed by the LRSTF were followed. All of the

functional boards were tested before and after each

exposure using a method developed by the LRSTF that

employs a manually operated switch box These tests

were performed outside of the environmental chamber,

thus, the humidity and temperature were not controlled.

The conditions were those of the standard laboratory

environment, which was generally 68"F and 10 to 20%

RH. The post-test measurements were performed

within 1 hour of the end of the monitoring period in

the smoke exposure chamber.

Automatic measurement of in situ equipment is

important in order to capture malfunctions that may

occur during a fire. Therefore, an automatic

monitoring system was assembled to test the

functional boards in a manner similar to the manual

measurements, but that could complete the task in 3

minutes for all four sets of boards. This assembly

required more switches and longer cables than the

manual measurement system because the measurement

instruments were located in thegeneral laboratory

outside the testing area Since aditional cable length

and switches tend to decrease high-frequency signals,

direct comparisons between manual and automatic

measurements may not be consistent.

Automatic measurements of the functionality and

leakage currents were taken at approximately 3-minute

intervals. This is the amount of time that it takes to

cycle through all of the measurements. Automatic

measurements of the environment within the smoke

chamber were made at intervals of 5 seconds whule the

lamps were on and at intervals of 1 minute thereafter

A set of specific measurements was made on each of 9

active circuits to characterize the operation of the

circuits during smoke exposures. Since both highfrequency and direct current measurements were made

on these boards, two types of connectors were used:

coaxial subminiature connectors of type B (SMB) and a

29-pin connector. The 29-pin connector was also used

to connect to ground, +5 Vdc, and to the input pulse

for the high-speed digital circuit Table 3 presents a

list of circuts and connectors through which these

circuits were accessed These circuits were designed to

test the limits of the soldering process and are very

sensitive to poor manufacturing practice. Because the

functional board was designed by the LRSTF to study

low-residue solder processes rather than digital I&C

circuits, the board included analog as well as digital

circuits. For example, the high-voltage, high-current,

and high-frequency circuits test for extremes in analog

circuits. Although these analog circuits are not

commonly used in digital I&C circuits, they may be a

part of a sensor package, and their evaluation to

15 15 NUREG/CR-6543
Methods and Materials Section 2

Table 3. Circuits on functional board

Circuit SMT PTH Connector

High voltage low current (HVLC) x x 29 pin

High current low voltage (HCLV) x x 29 pin

High-speed digital (HSD) x x 29 pin and SMB

High-frequency low-pass filter (HF LPF) x x SMB

High-frequency transmission line (HF T) N/A" N/A" SMB

Leakage measurement:

Pads x 29 pn

Gull wing x 29 pin

Pin grid array x 29 p

a The transmission lines are traces on the pnnted circuit boards and have no components to be classified as either SMT or PTH

determine what is most easily affected by smoke is

important. The high-speed digital circuit is a simple

logic gate that is similar to digital I&C design. Many

digital designs also include transmission lines in the

forms of traces on printed circuit boards, but they are

usually covered with a solder mask and do not normally

run parallel to other traces for as long as they do on

this board (which is avoided because it encourages

cross-talk). Leakage measurements serve as a

diagnostic and do not resemble any actual circuits.

In addition to the automatic measurements conducted

on the functional boards, surface insulation resistance

was also measured on the uncoated IPC-B-24 boards.

2.4.2 Individual Circuits

2.4.2.1 High Voltage Low Current

The high-voltage low-current circuit (HVLC) is a series

of 10-MQ resistors and 0.1-mF capacitors designed to

determine if flux residue would lower the resistance

between contacts enough to change the circuit's

performance. A high voltage (300 Vdc) was applied to

this circuit during both the manual and automated tests,

although originally the LRSTF planned.to apply 500

Vdc. However, 500 Vdc was found to cause arcing, so

a lower voltage was suggested by the LRSTF. As

discussed in Iman and coauthors (1995a), the voltage

between traces is expected to be approximately 60 V.

Smoke is expected to lower the resistance between

contacts and thus the HVLC circuit should be highly

affected by the smoke.

The HVLC circuit was measured manually with the

matrix switch box shown in Figure 3 using the circuit

in Figure 13. A total of 300 Vdc was applied across

the circuit and the current was measured using an

ammeter. The 10-kWl resistor in the measurement

circuit would limit the current if the HVLC circuit

completely shorted. Since this was a direct current (dc)

measurement, only the steady-state operation of the

circuit needed to be considered; the capacitors do not

affect the steady-state current. The capacitors, however,

provided additional sites for possible leakage currents.

The expected current can be calculated as follows:

Ri =1OM.l+1OMf2+l10 MK+OOM_

+10 M.Q+ 10 Wd)

= 50.01 Mf

V 300 = V _= =6.0 #A

Rota 50.01 M 6.

The automated HVLC circuit measurement was

performed by measuring voltage instead of current.

These voltages were measured across an additional pair

of in-series resistors that were located outside of the

environmental chamber. The circuit is shown in

Figure 14 The current through the circuit can be

calculated using Ohm's law:

R.W =5x1O Mf+10MQ+1 Mil

=61MfN

V = = 300 V

V. =IxlMil

1V M.0

r 61MQ1

=4.9 V

NUREG/CR-6543 16
Section 2 Methods and Materials

1o0Ma IoMO iOM lOMa 1OMa

-- U - U ,~ U ~. I ~-

10 3gOV KO

- I

Manual Measurement I

Ckcuft

Figure 13. IM

"I

I - :o°iI

w

I

i

-4f 41 6

0.1 pF 0.1 pF OlpF 0. pF 0.1 pF

Prnted Circuit

Board

/LC manual measurement circuit.

IOMO IOMO IOMO IOMO 10 Ma

AM-i ýV- 0 A - Yr

L+ aA

0 IpF 0.1 p: 0.1 pF 0.1 pF 0.1 pF

Automatic Measurement Printed Circuit

-E -

I

I

Automatic Measurement Printed Ckcuit

Circuit Board

Figure 14. HVLC automatic measurement circuit.

These in-series resistors limit the current from the

power supply to 27" pA and divide the supply voltage

so that Vo < 27 Vdc if the HVLC circuit shorts.

Since the measurements from the two systems are

different (one is in terms of current and the other

voltage), comparisons between them must be made by

converting the results-to similar units. The current

through the automated measurement system is not the

same as the current through the manual system because

of the additional in-series resistors The resistance of

the HVLC circuit, however, is independent of the

measurement system and can be calculated for both

cases as follows:

300 V

RHVLJC = 30V-10 kQ

I.a

and

R= 300VxlM•lM~

RHvLC = oM -11M2

Vatomatd

17 NUREG/CR-6543
Methods and Materials Section 2

1np UT V T T WT R

IPnfted Chcuit Board

Manual and Automatic Measurement Circuit

Figure 15. HCLV manual and automatic measurement circuit.

2.4.2.2 High Current Low Voltage where

The high-current low-voltage circuit (HCLV) consists

of parallel resistors and capacitors driven by a current

source as shown in Figure 15. The HCLV circuit was

included on the functional board to test solder joints,

because cracks in these joints increase the resistance of

a circuit. Changes in resistance are measured by the

changes in voltage needed to maintain the high current

level of I A.

Both the manual and automatic measurements of the

boards were conducted in the same manner. A current

source was applied across the HCLV circuit and the

voltage necessary to maintain 1 A across the circuit

was determined. Because the manual and automatic

measurements used different cables to supply this

current (the current supply cable for the manual system

was about I m long; the automatic system supply

included relays and an additional 3 in), the voltages

from the two measurement systems differed by a

constant amount, as shown in the following equations

V... = I + R..)

V.,. = 1A(R&cLv+ R...C)

1 1 1 1 1 1

1 1

7

10o

and R.-•a and R,..,, are the resistances in the cables

of the respective measurement systems. Because the

test conditions maintain a constant current of I A, the

differences between the manual and automatic

measurements should be a constant [1 A(R...meRm..))]. Since this is a steady-state measurement, the

capacitors are not included in the equation for voltage.

Smoke may attack solderjoints because of its corrosive

nature; however, this may take longer than the

exposure and measurement period of the tests. Very

little change in these circuits is expected because the

resistance of the circuit is already very low (1.4 £1) and

it was found in previous experiments that the

immediate effect of smoke is to lower resistance rather

than raise it (Tanaka, Nowlen, and Anderson, 1996)

2.4.2.3 High-speed Digital

The high-speed digital (HSD) circuit consisted of

logical NAND gates (not-AND) that were connected

NUREG/CR-6543 18
Section 2 Methods and Materials

Manual and

Automatic

Measurement

Circull

Figure 16. HSD PTH measurement circuit.

mI

Manuial and

Automatic Measurement Clrcrt

Figure 17. HSD SMT measurement circuit.

into a circuit so that an input pulse would generate a

similar output pulse later in time. Two dual-m-line

chips (SN54FOO) were used for the PTH circuit (Figure

16) and one leadless chip carrier (54FOOLMQB) was

used for the SMT circuit (Figure 17) Each of these

chips consisted of quad-dual-input NAND gates (quad

refers to the four gates in each chip package and dual

input refers to the number of input connections into

each gate). These two circuit are identical except that

the PTH circuit passes through eight NAND gates and

the SMT circuit through only four NAND gates. The

LRSTF decided to include four more gates on the PTH

circuit so that they could investigate the quality of

hand-soldered PTH components. The input signal,

supply voltage (Vcr), and ground were supplied

through the 29-pin connector while the output was

connected through a coaxial SMB connector. One

capacitor was included for each chip (i.e., two for the

PTH circuit and one for the SMT circuit) to prevent

introducing spurious noise signals in the supply

power.

There are several choices in the logic family and

technology of the NAND gate chip, as shown in the

appendix. The technology chosen by the LRSTF was

the advanced Schottky (FAST) because it has one of

19 19 NUREG/CR-6543
Methods and Materials

the fastest switching times of all of the gates. Solder

flux residues were expected to lengthen the switching

time of the gates.

A 20-ns wide +5 V pulse was used as an input signal

and the output of the HSD was measured on a

digitizing oscilloscope. The data collected on the HSD

signal included rise time of the output pulse, fall time

of the output pulse, and delay between the input and

output pulses. The automated and manual

measurements of the HSD circut were performed in a

similar manner, but the cable lengths used for the

manual measurements were much shorter than for the

automatic measurements. The manual measurement

used cables less than I mn while the automatic

measurement used cables that were more than 4 m.

The cable length affected the measurements in two

ways: the longer cable increased the delay time

between pulses and degenerated the signal. Because the

high-frequency components of a pulse are degraded in

longer cables, rise times and fall times of signals

become longer. Thus, for the automatic measuring

systems, all three measurements--rise time, fall time,

and delay time-should be longer than for the manual

measurement system.

2.4.2.4 High-frequency Low-pass Filter

The high-frequency low-pass filter (H11F LPF) was

designed to determine if the flux residues affect the

circuits by adding capacitance or resistance to the

printed circuit traces. The HF LPF consists of a series

Section 2

of inductors with capacitors between them connected to

ground (see Figure 18). The inductor on the circuits

consisted of spiral traces on the printed circuit board

called spiral inductors. If sine waves of different

frequencies are input into this circuit, the output is

attenuated by different ratios; high frequencies are

attenuated more than low frequencies. These circuits

are designed to maintain a 50-0 impedance and connect

easily to hugh-frequency measurement equipment

The HF LPF circuits were measured manually using a

network analyzer that measured the throughput of the

circuit for frequencies between 50 MHz and 1 GHz.

The network analyzer operates by generating a sine

wave electrical signal for a range of specified

frequencies and measuring either the reflected or

transmitted signals from the test circuit. The results

are reported in (nondimensional) units called decibels

(dB), defined as follows:

dB lO log,o(P• 0

=20 log10 (I!VI

where P. and P,, are output and input power, and

V., and V. are output and input voltages,

respectively All of the measurements using the

network analyzer were done in transmission mode

(throughput).

0

-10

-20

-30

c -40 U

-50

-60

-70 I0 OE+00 2 OE+08 4 OE÷08 6 OE+08 8 OE+08 1 OE+09

Frequency (Hz)

Figure 18. HF LPF typical frequency response of circuits.

NUREGiCR-6543 20
Section 2

Measurement of the uncoated circuits yields results

such as those shown in Figure 19. To limit the

information to the minimum required, three data points

were recorded for each HF LPF circuit: the amount of

throughput at 50 MHz, the frequency at which the

signal falls to -3 dB, and the frequency at which the

signal falls to -40 dB. Lines on Figure 19 indicate

these attenuation levels. Note that -3 dB corresponds

to a V,. I Vw ratio of 0.7. Although the inductance

and capacitance values in the two circuits were

equivalent, characteristics of the HF LPF PTH and

SMT circuits differ significantly over 300 MHz. As

Methods and Materials

explained in Iman and coauthors (1995a), additional

capacitance due to the leads on the PTH components

changes the LPF circuit so that the PTH circuit cuts

off at lower frequencies.

The automated test for the HF LPF circuit (Figure 20)

consisted of inserting a 250-MHZ sine wave into the

filter and measuring the output. This differs

significantly from the manual measurement because the

throughput is measured at only one frequency rather

than a range of frequencies. This measurement was

performed using a sine wave generator to produce the

Network

Analyzer

OUT

IN

ý GROLIND ýK

Printed Circuit Board

Manual

Measurement

Circuit

Figure 19. HF LPF manual measurement circuit.

II

Auaomatic

Measurement

Circuit

16 nH 32 ri65

N.T

IP

F

H

-J

Printed Circuit Board

Figure 20. HF LPF automatic measurement circuit.

21 21 NUREG/CR-6543
Methods and Materials

input signal and a digitizing oscilloscope to analyze the

output.

Automated and manual measurements were not

compared because a comparison would require

assumptions on the shape of the LPF curve. Future

measurements on the manual system will include

recording the throughput at 250 MHz.

2.4.2.5 High-frequency Transmission Line

Transmission lines are a means to transfer highfrequency signals from one part of a printed circuit

board to another. They are designed to have constant

characteristic impedances and to match the impedance

of whatever they are connecting. The impedance of a

transmission line is determined by the width of the line

compared with the distance to the ground plane

(thickness of the printed wiring board material) and

dielectric constant of the circut board material. The

Section 2

transmission lines in the LRSTF board were designed

to have a 50-0l impedance.

The high-frequency transmission line (HF TL) was

measured to determine coupling between two

transmission lines located 0.25 mm (0.01 inch) apart

and 12.7 cm long (5 inches), as shown in Figure 2.

The coupling is expected to increase if conductive

materials accumulate between the lines since the ideal

condition is for the circuit board to act as a good

insulator. The coupling is also expected to be

frequency dependent. Two measurement configurations

are possible in determining the transfer of signal from

one transmission line to another, as shown in Figures

21 and 22. In the forward coupling configuration,

connectors J8 and J9 are terminated with 50-11 resistors

(a 50-4 resistor is placed between the signal line and

ground), and in the reverse coupling configuration,

connectors J9 and J10 are terminated. The throughput

between unterminated connectors is then measured.

1500

Friward Coupling

Printed Circuit Board

Figure 21. HF TL forward coupling measurement.

IN

I

I

*1

I

Network

Ana zer

Al

J8

J9

GROUNDi

50Q

.J.- /I

I

Reverse Coupling

Printed Ckcuit Board

Figure 22. HF TL reverse coupling measurement.

NIJREG/CR-6543 22
Section 2 Methods and Materials

0-

-5

-10

~.-15

%-20

C

0

•-25--

0 -30

4C -35

-40

-45

-50 /

O.OE+0O 2.OE+08 4.OE+08 6.OE+08 8.OE+08

Frequency (Hz)

Figure 23. HF TL typical frequency response of circuit.

1.OE+09

Figure 23 shows a typical output of the network

analyzer for forward and reverse coupling.

The manual measurements of the boards consisted of

testing both the forward coupling and reverse coupling

configurations with a network analyzer A range of

frequencies between 50 MHz and 1 GHz was scanned in

both cases. Data were recorded at specified values of

firequency or attenuation:

Forward coupling. 50 MHz, 500 MHz, and

1 GHz.

Reverse coupling- frequency of null point

(Figure 23)

These measurements were made with short, 30-cm

coaxial cables that were compensated for frequency

loss.

For the automated tests, only the forward coupling

configuration was measured because changing the

positions of the 50-fL terminating resistors requires

physically handling the functional boards. The same

forward coupling measurements were performed as

previously described, but because four boards were

exposed simultaneously, an additional 1.3-GHz coaxial

switch was included in the circuit as well as longer

coaxial cables. As in the manual measurement case,

the cables and switches were calibrated prior to the tests

to allow compensation for frequency loss. Since the

cables were longer and the circuit included the switches,

more compensation was required for the automated than

for the manual measurements. This frequency

compensation is performed by the network analyzer.

2.4.3 Leakage Measurements

Current leakage is a primary method of failure that is

expected for electronics that are manufactured with high

levels of residue or are exposed to smoke. Although

leakage measurements are not measurements of the

actual function of a circuit, high levels of current

leakage will lead to short circuits and faulty data

transnmssion. Leakage measurements were performed

in four areas on the circuit board:

(1) pads-plated-through holes that were spaced 0.25

nun (10-mul) apart,

(2) gull wing dummy component (GW),

(3) inner two rows ofpin grid array (PGA/A), and

(4) outer two rows of pin grid array (PGA/B).

The gull wing dummy component was a plastic molded

package that contained some circuitry. The outer two

23 23 NUREG/CR-6543
Methods and Materials

rows of the pin grid array included ad additional solder

mask that the inner two rows did not.

Manual measurements of the board consisted of biasing

the circuit with 5 Vdc and measuring the leakage

Section 2

current as shown in Figure 24. For the manual

measurements, 5 Vdc (Vý

) was applied only during the

measurement and the ammeter was allowed to stabilize

for I minute after the bias was switched on before the

reading was recorded from the ammeter (1Q).

I

UNKNOWN

R

Manuci Measurement

Circuit

Figure 24.

300

Leakage circuit-mannl measurement.

I

UN*JOVM

+

Autocmatc

Measurement

Circuit

Figure 25. Leakage circuit-automatic measurement.

NUREG/CR-6543 24
Section 2 Methods and Materials

The automated measurements were performed using the

circuit shown in Figure 25. The 5-Vdc bias voltage

(V,) was applied constantly during the exposure and

monitoring period. The circuit converts the leakage

current to voltage by the following equation:

where the feedback resistance is 5.1 MCI for all of the

leakage measurements from the circuit board. The

operational amplifier only allows output in the range

of the supply voltage, +15 V. This limits the range of

observations to resistance values that are close in value

to the feedback resistor. When the resistance is very

high or very low compared with the feedback resistor,

the voltage reading will be either zero or limited to +15

V. V,. was measured with the voltmeter function of a

data acquisition system.

The two measurements were converted to resistance

through the component on the boards because of the

differences between the measurements. The equations

for conversion are, per Ohm's law:

V and

where Rh,, was a 30-41 limiting resistor included in

the automatic measurement circuit to prevent shorting

the power supply when the leakage current was too

high.

Figure 26. IPC-B-24 board for surface

insulation resistance.

One board was placed outside the smoke m a bag with

the control LRSTF boards, and one board was placed

inside the smoke exposure chamber in a vertical

orientation. Each IPC-B-24 board contained four

identical interdigitated comb patterns with 0.5-mm

spaces between 0.4-mm wide traces. These boards were

also used in the circuit bridging tests performed at

SNL.

For the four traces, two were biased with 5 Vdc

throughout the test and two were biased with 30 Vdc.

The feedback resistors and current limiting resistors

were also different, as shown in Table 4.

Table 4. Leakage measurement parameters

for automatic measurement system

Bias voltage Rr

Circuit (Vdc) (fQ)

IPC-B-24 combs A and D 30 5 1 k

IPC-B-24 combs B and C 5 51M

LRSTF leakage circuits 5 5 1 M

2.4.4 IPC-B-24 Board

The IWC-B-24 test board (Figure 26) was developed by

the Institute for Interconnecting and Packaging

Electronics Circuits (IPC) to measure surface

insulation resistance on printed wiring boards (Pauls,

1992). High surface insulation resistance indicates a

clean printed circuit board with little solder residue or

ionic contamination. Smoke causes the resistance of

the surface to decrease. Two IPC-B-24 boards were

included in these tests and measured only with the

automated system and the circuit shown in Figure 25

25. NUREGICR-6543
3. TEST RESULTS

3.1 Smoke Measurements

3.1.1 Temperature and Humidity

The chamber temperature was affected by both the flux

level and the amount of fuel because the fuel also

provided additional heat (Table 5). As expected, the

temperatures at the top of the chamber (thermocouples

1, 2, and 3) were higher than those at the bottom of the

chamber (thermocouples 4 and 5). Thermocouple 5

was probably somewhat shielded from high

temperatures by the boards in addition to being near the

bottom of the chamber. Thus, it had the lowest

temperature of all of the thermocouples located in the

smoke exposure chamber. The temperature for the

control boards increased when the lamps were on

because of radiant heating. The highest temperature in

the chamber for the control boards (T6 or 17) was

32.1" C.

The humidity also was affected by the radiant heat from

the lamps, but since the sensor was not inside the

smoke exposure chamber, the actual humidity that the

boards were exposed to was not measured.

3.1.2 Fuel Burned and Soot

Deposition

Soot deposition was higher on the horizontal than the

vertical surfaces. More fuel was burned and more

vertical deposition occurred in the flaming, high-flux

burns than with the low-flux burns; however, the

horizontal deposition was less for some of the highflux burns. Table 6 shows the results of measurements

that were averaged for the tests with the same

conditions.

Overall, as can be seen in Figure 27, the smoke was

very corrosive to reactive metals. The figure shows an

Table S. Temperature ('C) at end of lamp heating

Flux Fuel

level level TI T2 T3 T4 TS T6 T7

(kW/m2 ) (g/m3 )

25 3 44.4 34.7 33.3 28.5 25.9 23.9 25.2

25 25 46.3 35.6 35.0 28.8 25.2 24.7 24.7

25 50 49.3 35.0 35.1 29.6 27.0 24.3 27.9

50 0 47.8 41.0 36.0 31.5 29.9 22.6 22.1

50 3 63.0 39.1 41.4 33.6 29.2 25.0 32.1

50 25 73.5 55.4 48.2 38.2 32.8 24.7 30.7

50 50 65.0 51.6 48.0 36.0 29.7 26.7 28.5

Table 6. Fuel burned and smoke deposition

IIIII III II

Fuel

level

(g/m 3)

3

3

25

25

50

50

Fuel

available

(g)

06

06

5.0

5.0

10.0

10.0

Flux

level

(kW/m2 )

25

50

25

50

25

50

Fuel

burned

(g)

0.27

0.49

3.12

3.95

6.49

7.34

Vertical

deposition

(4tg/cm2)

0.4

0.9

2.1

4.2

3.2

7.3

Horizontal

deposition

(tgncm 2 )

2.8

3.4

45.3

17.6

94.8

58.5 I i I I I I I

NUREG/CR-6543 26
Section 3

exposed and unexposed resistive terminator. The body

of the terminator is brass with a gold-plated finish and

the connector end is brass with a nickel finish. The

gold finish was only slightly affected, but the nickel

finish reacted strongly. The corroded nickel surface

shown in Figure 27 was the bottom surface of the

terminator as it was positioned horizontally during the

exposure. The top surface was sooty, but not nearly as

corroded.

Test Results

3.1.3 Chemical Analysis of Fuel

Chemical analysis of the burned fuel showed these

quantitative results:

% Ash 23.04

% Bromine 0.95

% Chlorine 12.60

" Fluorine 0.49

Gross heat of combustion, 1/kg 2.4x107

The semiquantitative analysis results for metals from

the ashed sample are presented in Table 7.

Filter papers placed on the floor of the smoke exposure

chamber were analyzed for F, Cl, and Br. The results

of the analyses of the soot on the 4.25-cm diameter

filters are shown in Table 8. The amount of chloride

deposited on the filter is higher for the low flux level

than for the high flux level exposures.

Figure 27. Resistive terminators showing

corrosive action of smoke.

Table 7. Semiquantitative analysis of ashed sample.

Aluminum m

Arsenic ND

Antimony t/m

Barium ND

Beryllium ND

Boron ft

Bismuth ND

Cadmium ND

Calcium m

Chromium ND

Cobalt ND

Copper t

Iron ML

Lead M

Lithium ND

Magnesium ML

Manganese ND

Mercuy ND

Notes. P. lOto 100%. M. I to 10%, m,01 to i0%,t,0Of toO1%,

ft, less than 001%, L. lower half of range shown. ND. not detected

Molybdenum

Niobium

Nickel

Phosphorus

Potassium

Sodium

Silicon

Silver

Strontium

Tantalum

Tellurium

Tin

Titanium

Tungsten

Vanadium

Zinc

Zirconium

ND

ND

t

ND

ND

ND

P

t

ND

ND

ND

ND

PL

ND

ND

t/m

ND

27 27 NUREG/CR-654W
Test Results

Table 8. Chemical analysis of soot

(jig/filter)

Fuel Flux

level level Chloride Bromide Fluoride

(g/m 3) (kW/m2)

3 25 95.5 2.1 0

3 50 24 5.5 0.25

25 25 92.6 96.6 0

25 50 73.0 66.9 1.45

50 25 147 181 1.25

50 50 95.4 165 0

Unexposed filter 3.9 0 0.7

3.2 Circuit Measurements

3.2.1 High Voltage Low Current

The measurement for the HVLC circuit determined the

resistance of the cirut. The high-voltage low-current

circuit reacted to both smoke and humidity. The most

common reaction to smoke was to lower the resistance

of the circuit as shown in Figure 28 for the PTH

circuit and Figure 29 for the SMT circuit for high-fuel,

Section 3

high-flux tests. For the PTM circuit, the coated boards

showed a reduction in resistance due to the smoke, but

soon recovered. The bare board circuit began to short

and continued to short throughout the high-humidity

exposure, but recovered as shown by the post-test

measurements. The SMT circuit also shorted during

the smoke exposure, but recovered in the latter stages

of the automated measurement.

The loss of resistance of these circuits is a result of the

current finding an alternative path between input and

output conductors. Printed wiring boards generally

have high resistance between trace lines, above 102 fQ

(Pauls, 1992). The HVLC circuit has 50 M11 across

the circuit. When there is no smoke, the current flows

through the path of least resistance, the 50-MO circuit.

When smoke is introduced, the resistance of the air

surrounding the circuit and the surface of the printed

wiring board is lowered. The current then leaks

through these alternative routes and the measured

resistance of the circuit falls.

The phenomenon of shorting when first exposed to

smoke and later recovery was observed in the circuit

bridging tests at the lower smoke exposures (Tanaka,

Nowlen and Anderson, 1996). Indeed, the "high"

60 -

55-

Coated Control -~~~ ---------

45 -

Coated

I r.-rww

U

C,)

404 Bare Control

354

30 -f

254

204

15-

104

5-

Bare

I-.......--40..-. I I I I I

-2 -1 0 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 18 17 18 19 20 2122 23 24 25

TIME (HR)

Figure 28. HVLC PTH vs. time for a high-fuel, high-flux burn.

NUREG/CR-6643 28
Section 3 Test Results

w

C., 2

ICo uJ

-2-1 0 1 2 3 4 5 6 7 8 9 1011 1213141516171819202122232425

TIME (HR)

Figure 29. HVLC SMT vs. time for a high-fuel, high-flux burn.

Table 9. HVLC change in resistance values

Fuel Flux

level level Bare PTH Bare SMT Coated PTH Coated SMT

(glmn3) (kW/m2 ) 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr

3 25 0.8 0.6 -21 5 -13.3 0.6 0.6 -2.2 03

3 50 -23.3 -100 -3.5 1.0 0.0 0.1 1.0 0.9

25 25 -32.7 1.4 -38.9 -27.2 -09 0.3 07 0.8

25 50 -46.0 -31.6 -46.0 -38.0 -3.7 0.0 0.0 0.8

50 25 -46.6 -38.5 6865 15.7 -1.2 0.0 0.8 0.9

50 50 -47.1 -49.9 -443 -45.5 23.2 24.6 1.9 1.3

exposure in these tests (50 g/m3) is approximately

midway between the "low" and "high" levels of smoke

observed in the earlier tests (3 and 100 g/m3,

respectively) where the "low" levels recovered and the

"high" levels did not. A summary of the changes in

resistance for different smoke exposures is shown in

Table 9 in which changes in resistance from pretest

values are listed. Each value is the average result from

two tests. Most of the bare boards decreased in

resistance (-92% change in resistance) with higher

amounts of smoke except for one test for the high-fuel,

low-bum case where the circuit reacted as if it were

open (-686.5 MO), a 1370% increase. The coated

boards did not react as much as the bare boards.

Analysis of the circuit traces that connect the high

voltage input and ground to the HVLC circuits showed

that the greatest drops in potential per distance between

conductors occurred on the surface of the printed wiring

board. On these surfaces the traces are separated by

0 76 mm (nominal 30 mil by design) and carry a

potential difference of 300 Vdc. In comparison, the

voltage drop across any of the components in the

circuit is at most 60 Vdc and the component leads are

at least 2.mm apart Thus most of the shorting

through smoke should occur between the traces rather

than between soldering surfaces of the components.

Since the trace leads for both the PTH and SMT

circuits are similar in length and separation, both

circuits reacted similarly.

The addition of humidity to the HVLC circuit tended to

cause its resistance to change slightly, sometimes by

increasing and sometimes by decreasing in value by

29 9NURE/CR-6543
Results

approximatelY I MN . This change only temporary in resistiI was and the circuit returned value after 15 minutes. to the original

3.2.2 High Current Low Voltage

The measurement for the HCLV circuit yielded voltage needed the to produce a t-Adc curret tougthe ft circuit. The 

1.47average value was 1.460 V for the PTH

0

> 1.45

Section 3

and 1.472 V for the SMT circuits. The HCLV was not affected by circuit smoke or humidity HVLC circuit. The as much as the addition of smoke tended increase to the voltage, a change that corresponds increase in the to an resistance 

circuits of the circuit. Th PT reacted slightly to the smoke high-flux for a high-fuel, level exposure SMTf circuits (Figure 30), but the uncoated tended to increase in resistance remain and higher (Figure 31). This change was also

I

10 11 12 13 14 15 1 17 18 19 20 21 22 23 24 25

4 5 6• 7 8 ""I

T4ME (HR)

-1 0 1 2 3

s. t. e for a high-ruelt high.flux burn.

Figure 30. ICLV PTH

1.5 

1

1.49

1.48

1.41

Bare

Bare Control

b'~ ...... 

. 'H"202 2 3 24 25

56789 10 11 12 13 14 15 1 17 181 1

TINME (HR)

-1

I I - - o 1 2 3 4

Figure 31. RCJLV SMT vs. time for a high-fuel, high-lux burn. 30

NUREG/CR-6 543
Section 3 Test Results

Table 10. HCLV results (AV in mV)

Fuel Flux

level level Bare PTH Bare SMT Coated PTH Coated SMT

(g/m 3) (kW/m2) 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr

3 25 0.4 -1 5 0.6 -1.6 0.3 -0.6 0.7 -1.5

3 50 0.9 -2.1 1.2 -1.3 1.3 -1.6 1.4 - 1.3

25 25 0.1 01 131 16.9 01 -0.4 1.2 -0.4

25 50 2.2 -1.6 12.1 13.5 2.0 -1.8 2.3 -1.5

50 25 0.3 -2.1 18.5 24.2 0.9 -1.4 1 6 0.0

50 50 -0.5 -4.4 18.6 20.6 07 -4.0 1.1 -3.4

evident in the differences between pre- and post-test

manual measurements. Table 10 shows the average

change in voltage based on two tests. As the data

show, the bare circuits in general reacted more than the

coated circuits.

This HCLV circuit had a very low initial resistance,

1.4 il The resistance of the surrounding printed

wiring board and air probably decreased when the

smoke was added, but since the circuit resistance was

so low, the current continued to flow through the

circuit. Thus the measured resistance did not change

much in most cases. However, for the SMT circuit,

the smoke probably corroded the contacts or solder

joints because the resistance increased (as did the

voltage since the current was held constant) This

failure mechanism is very different from the case of the

HVLC circuit. Although there was a measurable

change in the voltage, the overall effect on these

circuits was small The original measured voltages

were approximately 1.45 V, thus the maximum circuit

change was 1 7%.

3.2.3 High-speed Digital

The measurements performed on the HSD circuit

included rise time, fall time, and delay between the

input and output pulses. The manual pre- and post-test

measurements and the automatic measurements could

not be compared because the cable lengths were a

significant factor for both the delay (increased time was

needed for longer automatic measurement cables) and

rise and fall time (high frequency loss) Nevertheless,

comparisons of the HSD circuit measurements from

early in the test, before the lamps were energized, and

during the smoke exposure show that the HSD circuit

was not significantly affected by smoke at these

exposure levels. The rise times (Table 11), fall times

(Table 12), and delay times (Table 13) show a fairly

constant behavior regardless of flux level or amount of

fuel.

The highert fuel level for these tests was 50 gim3 4-. . In

previous tc t ts such fuel levels resulted in resistances

that were so low that they were at the threshold of the

measurement ability (1000 Q)) for the circuit bridging

tests (Tanaka, Nowlen and Anderson, 1996). This

limitation was partially a result of the wide range of

measurement planned for the circuit bridging tests (10i0

to I0C P). Since these tests were performed, however,

measurements of critical resistance (see the appendix)

Table 11. HSD rise time, averages over time range (us)

Fuel Flux

level level Bare PTH Bare SMT Coated PTH Coated SMT

(g/m 3 ) (kW/mr) 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr

0 50 2.56 2.54 2.61 262 2.58 2.56 2.61 2.62

3 25 2.47 2.50 2.64 2.65 2.52 2.52 2.69 2.66

3 50 252 2.53 2.67 2.69 2.49 2.53 2.77 2.73

25 25 2.52 2.52 2.79 2.71 2.55 2.55 2.73 2.73

25 50 2.52 250 2.72 2.69 2.52 2.52 2.73 2.70

50 25 2.56 2.55 2.81 2.70 2.57 2.54 2.72 2.76

50 50 262 2.53 2.85 2.76 2.54 2.52 2.77 2.73

31 NUREG/CR-6543
Test Results Section 3

Table 12. HSD fall time, averages over time range (ns)

Fuel Flux

level level Bare PTH Bare SMT Coated PTH Coated SMT

(g/m3 ) (kWIm2 ) 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr

0 50 2.96 3.03 3.70 3.83 3.01 3 09 3.91 3.83

3 25 3.01 3.02 3.67 3.72 3.12 3.10 3.75 3.86

3 50 3.13 304 4.03 3.91 2.96 2.88 3.99 3.94

25 25 3 14 3.07 3.92 3.95 3.13 2.98 4.06 4.06

25 50 3.00 3.02 3.93 3 81 3.11 3.07 4.05 3.88

50 25 3.02 2.95 3.55 3.79 2.96 2.91 3.99 392

50 50 3.24 3.11 4.30 4.13 2.98 2.91 391 3.89

Table 13. HSD delay time, averages over time range (ns)

Fuel Flux

level level Bare PTH Bare SMT Coated PTH Coated SMT

(g/m3 ) (kW/m2 ) 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr 1st hr 2-24 hr

0 50 50.8 50.7 45 8 45.9 50.6 50.7 46.1 46.1

3 25 50.5 50.6 45.9 46.0 50.7 50.7 46.1 46.1

3 50 50.7 507 46.0 46.1 50.6 50.6 46.1 46.3

25 25 50.7 507 46.2 46.2 508 50.8 461 462

25 50 50.6 50.7 46.1 46.2 50.6 50.6 46.2 46.2

50 25 50.9 50.7 46.3 46.1 50.7 50.7 463 46.3

50 50 51.1 51.0 46.4 46.5 50.6 50.7 464 46.4

have shown that this range of resistance would cause

failure only in CMOS chips and not in FAST TTL

chips. Since the chips used in this test were FAST

TTL chips, they were not expected to fail as a result of

tests with a fuel load of 50 g/m3.

3.2.4 High-frequency Low-pass Filter

The HF LPF measurement was made to determine the

throughput of a filter at various frequencies. Because

of limitations of the automatic testing equipment, only

one frequency (250 MHz) was used; however, data at 50

MHz, -3 dB, and -40 dB were recorded during the

manual measurements This circuit was expected to

react to stray capacitance that could be caused by any

debris on the circuit. No changes were measured in any

of the tests. The automatic measurement frequency,

250 MHz, occurs at a high section of the throughput

curve shown in Figure 19. At this frequency, changes

in capacitance should not be very obvious. The

capacitors in the circuit act as open elements, and it is

not until the frequency is higher (>500 MHz) that the

capacitors begin to conduct to ground. Tests at these

frequencies would be needed to determine if stray

capacitance is a problem. The pre- and post-test

measurements at the higher frequencies did not change

significantly; however, in many of these measurements

the effect of smoke is most obvious during the

exposure and almost disappears after the smoke is

vented.

3.2.5 High-frequency Transmission

Line

The transmission line measurement determines the

coupling between one transmission line and another

that is very close. As the transmission lines

accumulate debris, the coupling between the two lines

will change because the insulation resistance is reduced.

For low frequencies, there should be more coupling,

but at higher frequencies, the expected result is not

easily m6deled. The manual measurements were made

with both the forward coupling and reverse coupling

configurations, but the automatic measurements were

made only with the forward coupling configuration.

The automatic measurements were made at three

different values: 50 MHz, 500 MHz, and I GHz.

Figure 32 shows the response to smoke for 50 MHz in

a lhgh-fuel, high-flux exposure. The bare circuit (solid

line) shows the most response by increasing in

NUREG/CR-6543 32
Section 3

coupling from -52 dB to -17 dB during the 1-hour

exposure. The coupling at 50 MHz then decreases as

the smoke is vented until the final value is very close

to the pretest value.

Table 14 presents the change in coupling from pretest

measurements (dB) with fuel level and flux level. The

values in Table 14 are averages for two tests and over

the time periods listed in the table (either the fist hour

or from 2 to 24 hours). The bare transmission lines

generally couple more than the coated transmission

lines in response to smoke. The values of the

transmission lines are very close to the original pretest

values by the end of the exposure for both the bare and

coated cases. This behavior is similar to earlier circuit

Test Results

bridging tests in which conductance was higher during

the smoke exposure, but fell as the smoke was vented.

One odd value should be noted in the second row of

Table 14 for the bare BF TL in the 2-24 hour period.

This is a result of one transmission line, which

exhibited an anomalous increase 3 hours after exposure.

This behavior was not repeated in any of the other

experiments.

The results for the 500-MHz and I-GHz frequencies are

similar to those at 50-MHz except that the coupling

changes in the opposite direction for the first hour.

The averages for hours 2 to 24 are similar to the 50-

MHz case; i.e., they are close to the original values.

Table 14. HF TL at 50 MHz, averages over time range (dB)

Fuel Flux

level level Bare Coated

(gim') (kW./m) 1 hr 2-24 hr I hr 2-24 hr

0 50 -0.3 -0.6 -0.2 0.6

3 25 -1.8 7.1 -0.5 0.1

3 50 -0.8 -0.9 1.2 1.6

25 25 45 -1.3 -0.3 -0.2

25 50 7.9 -1.3 1.0 1.1

50 25 23.2 0.0 -1.1 -08

50 50 22.3 -0.6 0.2 0.3

16-

-25-

mv-30-

O-35-

ocA-45-

S S S I I I S I ~T S S S I 2 3 4 I I 7 I I 1 I 2 I I 1 I I 

2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19

2 I I I 2 I 

20 21 22 23 24 25

Bare

L Coated

L P.- Bare Control

-55. V,Coated Control

-60 £

TIME (HR)

Figure 32. HF TL (50 MHz) vs. time for a high-fuel, high-flux burn.

33 NUREGICR-6543
Test Results

3.2.6 Leakage Measurements

Section 3

The pads circuit measures leakage current from two

rows of through-holes that are biased with 5 Vdc.

Leakage current was converted to resistance across the

pads. The logl0 of the resistance values is presented in

Table 15. The starting resistance was 13.4 (in log, 00

units). As the functional boards are exposed to smoke,

the resistance between the pads decreases. In Table 14

the averages over the first hour of the smoke exposure

and over the period between 2 and 24 hours after the

start of the exposure can be compared. The resistance

falls the most for high-fuel, low-burn, bare pads. The

coating significantly reduced the change. As with

many of these circuits in which the shunt resistance is

affected by smoke, the resistance is lowest during the

smoke exposure and increases as the smoke is vented.

The leakage measurements from the IPC-B-24 boards

determined the surface insulation resistance of a 20-

square (a dimensionless measurement of surface

insulation resistance) area. The same type of circuit

was used to determine leakage currents as was used in

the pad measurements. The results are shown in Table

16 in which the values listed are averages of the

differences between the boards that were in the smoke

exposure chamber and the control boards. This

comparison was made because there was no pretest

measurement. In general, the higher the heat flux, the

more loss in resistance during the smoke exposure.

After the smoke was vented, the resistance recovered

somewhat.

Table 15. Pads measurements, averages over time range (log,'f)

Fuel Flux

level level Bare Coated

(g/m3) (kW/m2 ) 1st hr 2-24 hr 1st hr 2-24 hr

0 50 13.4 134 12.7 13.4

3 25 13.4 13.4 12.9 13.4

3 50 11.9 121 13.4 13.4

25 25 7.3 12.3 12.8 13.3

25 50 7.1 9.1 13.4 13.4

50 25 6.9 7.0 13.4 13.4

50 50 6.7 8.1 13.1 13.

Table 16. Summary of the surface Insulation resistance (log10.l.X1osd-log10o4Lo.,,)

Fuel level Flux level 30 V Bias 5 V Bias

(gWm 3) (kW/m2) 1st hr 2-24 hr 1st hr 2-24 hr

0 50 1.2 1.5 0.6 1.4

3 25 0.1 0.7 07 08

3 50 -1.9 -15 0.3 0.2

25 25 -0.2 1.3 -06 1.1

25 50 -3.1 -2.4 -1.5 0.1

50 25 -1.1 0.6 -26 -0.3

50 50 -2.8 -2.4 -1.9 -1 6

NUREG/CR-6543 34
4. CONCLUSIONS

These smoke exposures provided data on the effect of

smoke on simplified functional circuits. The results

showed (1) the predominant effects of smoke, (2) how

circuits are affected by smoke, (3) which types of

circuits are most vulnerable, and (4) under what

conditions circuits are most affected. The advantage of

exposing simplified circuits is that the effect on each

circuit is easier to determine. However, the results of

exposing more complicated circuits would probably be

similar to those found here.

Smoke may cause three different, immediate effects on

electrical circuits: (1) it may lower resistance by acting

like a shunt; (2) it may increase resistance by attacking

solder joints or adding debris to connectors; and (3) it

may increase stray capacitance. All of the circuits in

these tests could be used to determine one of these three

effects. The HVLC circuit, leakage measurements, HF

T1, and HSD determined effects of an added shunt. The

HCLV circuit indicated the effect of increased

resistance. The HF LPF determined the effect of stray

capacitance.

The predominant effect was lowered resistance

(increased conductance) and the most vulnerable circuits

were those that had high input impedance. Clean

printed wiring.boards have resistances higher than 102

Q) between traces. When smoke lowers electrical

resistance in the vicinity of a circuit, a high impedance

circuit will be bridged by the relatively low impedance

of the smoke. Thus the circuits that were most affected

were those that reacted to added shunt resistance, such

as the HVLC. For the HVLC circuit, virtually all of

the current was transmitted through the smoke and soot

deposits instead of through the circuit whien the fuel

level was higher than 25 g/m3

. However, smoke did

not affect the HSD circuit, the shunt resistance was not

low enough to affect the FAST TIL logic chips in the

HSD circuits. FAST TTL chips have a high tolerance

to smoke because they have low impedance and high

output current. If the HSD circuit contained a CMOS

chip instead, it might have been damaged just as the

memory chips in the earlier component tests were

because CMOS chips have lower output current drive.

The resistance of the HCLV SMT circuit measurably

increased when it was exposed to fuel levels of 25 g/m3

or greater, but not in any of the PTH circuits or coated

circuits, which remained stable. The lowered resistance

in the vicinity of the circuit was not low enough to

cause circuit bridging; however, this implies that the

smoke increased the resistance in the solder joints of

this low-impedance circuit. These effects were not

decreased by venting the smoke; however, they made

relatively small changes in circut performance, less

than 2% in the worst cases. Smoke did not cause any

obvious change in the HF LPF circuit, indicating that

there was little change in stray capacitance.

The increased conductance induced by smoke was

highest during the smoke exposure and was reduced by

venting. Therefore, smoke and not just surface

deposition causes increased conductivity by orders of

magnitude. Results from tests earlier in this program

showed that fuel levels greater than 100 g/m3 caused

increased conductivity that remained after the smoke

was vented. This implies that for higher smoke

densities, the increased surface deposition will cause

lingering effects. The polyurethane coating helped

considerably to prevent smoke from increasing

conductance or damaging solder joints.

This test series is the fourth in this program to

determine the impact of smoke on digital I&C

equipment. The tests included exposures of

microprocessor-based A/D boards, an experimental

digital safety channel (computers linked by a network),

and component packages to determine leakage currents.

The fist two tests in this series indicated that

communications could be interrupted internuttantly by

smoke. The exposures of component packages

indicated how components may be affected by smoke,

and the tests reported here show the effect of smoke on

simple circuits. The next test will determine the effect

of different conformal coatings on the same functional

circuit board A future report will summarize the

results of the coating tests and overall insights from

this program. It will also evaluate the effectiveness of

current fire protection regulations intended to shield

electronics from smoke and current recommended

practices in smoke damage recovery.

35 NUREG/CR-6543
REFERENCES

Bustard, Larry D., and P. Holzman. 1994. '"owVoltage Environmentally-Qualified Cable License

Renewal Industry Report; Revision 1," TR-103841,

Electric Power Research Institute, Palo Alto, CA.

Caudill, Loren M., L. Thomas Chapin, R. B.

Commizzoli, Pravin Gandhi, G. A. Peins, and J.

Doug Sinclair. 1995. "Current State of Fire

Corrosivity Testing Preliminary Electrical Leak

Current Measurements," in Proc. International Wire

& Cable Symposium, pp. 432-437, Army

Communications Electronics Command, Fort

Monmouth, NJ.

Iman, Ronald L., Robert V. Burress, Dennis J.

Anderson, Dennis L Huffman, Jeffry F. Koon,

Barbara M. Waller, Mahendra S. Gandhi, Thomas A.

Carroll, Mark J. Shireman, Carol M. Krska, Gary A.

Becka, Robin L. Sellers, David P. Carlton, Roger D.

Nickell, Mark I. Siewers, Gary S. Falconbury, and

Terry L. Munson. 1995a. "Evaluation of LowResidue Soldering for Military and Commercial

Applications: A Report from the Low-Residue

Soldering Task Force," SAND 95-1060, Sandia

National Laboratories, Albuquerque, NM.

Iman, Ronald L., Robert V. Burress, Dennis J.

Anderson, Dennis J. Huffman, Jeffry F Koon,

Barbara M. Waller, Mahendra S. Gandhi, Thomas A.

Carroll, Mark J. Shireman, Carol M. Krska., Gary A.

Becka, Robin L. Sellers, David P. Carlton, Roger D.

Nickell, Mark I. Siewers, Gary S. Falconbury, and

Terry L. Munson. 1995b. "Appendix to the Report

from the Low-Residue Soldering Task Force: Phase HI

Results," SAND 95-1060, Sandia National

Laboratories, Albuquerque, NM

Jacobus, Mark J. 1984. "A Review of Regulatory

Requirements Governing Control Room Habitability

Systems," NUREG/CR-3786, SAND84-0978, Sandia

National Laboratories, Albuquerque, NM.

Korsah, Kofi, R. L. Clark, and Richard T. Wood.

1994. "Functional Issues and Environmental

Qualification of Digital Protection Systems of

Advanced light-Water Nuclear Reactors,"

NUREG/CR-5904, ORNUITM-12164, Oak Ridge

National Laboratory, Oak Ridge, TN.

Korsah, Kofi, Tina J. Tanaka, T. L. Wilson, Jr., and

Richard T. Wood. 1996. "Environmental Testing of

an Experimental Digital Safety Channel,"

NUREG/CR-6406, ORNLITM-13122, Oak Ridge

National Laboratory, Oak Ridge, TN.

Pauls, Doug. 1992. "IPC Cleaning and Cleanliness

Test Program Phase 3 Water Soluble Fluxes Part 1,"

Paper presented at the IPC 1992 Fall Meeting,

October 12-15, 1992, Minneapolis, MN, Institute for

Interconnecting and Packaging Electronics Circuits

(IPC), Lincolnwood, IL, IPC TP-1043

Reagor, Barbara. 1992. "Smoke Corrosivity:

Generation, Impact, Detection, and Protection," Fire

Sciences, v. 10, pp. 169-179.

Tanaka, Tina L, Steven P. Nowlen and Dennis J.

Anderson. 1996. "Circuit Bridging of Components

by Smoke, NUREG/CR-6476, SAND96-2633,

Sandia National Laboratories, Albuquerque, NM.

NUREG/CR-6543 336
Appendix

APPENDIX

DETERMINATION OF CRITICAL SHUNT RESISTANCE FOR

CIRCUIT BRIDGING FAILURES IN LOGIC CIRCUITS

Tina J. Tanaka, Stephen J. Martin, and Kelly M. Hays

Sandia National Laboratories

Summary

Smoke creates leakage paths between component leads

and can therefore cause logic errors in digital systems

For example, if the high output from a lead is shorted

to a low output nearby, a low signal may be

transmitted instead of a high. The loss of resistance

due to smoke was simulated with a variable shunt

resistor to determine the tolerance to resistance loss for

different logic chips. The tests were performed on quad

2-input AND and OR gates from three logic families

(Table A-I): transistor-to-transistor logic (MIL),

complementary metal oxide-semiconductor (CMOS),

and emitter-coupled logic (ECL). The components that

tolerated the lowest resistances had the highest output

drive current. Components with a high tolerance to

resistance loss are more tolerant of smoke. The

components with high tolerance included advanced

Schottky, TrL (FAST), and advanced CMOS (FACT)

Standard CMOS components were the least tolerant to

*a decrease in resistance, especially at low power supply

voltages.

The circuit bridging tests that were performed in 1995-

96' only measured down to resistances of 1000 Ql; this

level of resistance would cause metal gate CMOS

failure, but other components would not be likely to

fail. The circuit bridging tests can be viewed as

providing conservative estimates of smoke levels that

would cause errors in logic circuits.

Evaluation Criteria and Results

Evaluation Criteria

Motorola AND and OR gates were tested to determine

the level of shunt resistance needed to cause erroneous

results in circuits la and lb, 2, and 3 (Figures A-i and

A-2). The simple circuits shown in Figures A-I and

A-2 were assembled xising a single chip. The inputs to

gates A and B were connected to a power supply while

the input to gate C was connected to the output of A

and B. The shunt resistance was always placed between

high and low voltages. The voltage from the output at

gate C was measured to determine if an error was

caused when the shunt resistance was lowered to a

minimum of 10 12. The results are presented in Table

A-i. A more detailed discussion of the results follows.

For the OR gates, the expected result was a high state,

and for the AND gate tests, a low state; failure

resistances were recorded when the output of the OR

circuit was low or the AND gate was iugh. Both OR

and AND gates were tested because sometimes the

addition of a shunt resistance can either pull the "low"

output to "high" or the "high" output to "low,"

depending on the component. Circuits 2 and 3 were

included in the tests because a shunt to the power

supply should be more severe than a shunt to the

output of a gate because of the limited output of a gate.

All of the components tested were selected from the

Motorola Semiconductor Master Selection Guide!

Other families of integrated circuits are also in

production, but this selection is representative of a

range in power consumption, speed, and technology.

In general, the fastest logic circuits have the highest

I Tna I Tanaka, Dennis J Anderson, and Steven P Nowlen.

Circuit Bridging of Components by Smoke, NUREG/CR-6476,

SAND96-2633, Sandia National Laboratories, Albuquerque, NM

(1996)

2 Motorola Semiconductor, Master Selection Guide, Rev 7,

Motorola Literature Distribution, Phoenix Arizona (1994)

.A-1 AL-REG/CR-6543
Appendix

tolerance to smoke because gates have higher drive

currents and can more easily supply enough current to

drive the voltage of the next gate to the correct level.

TTL and CMOS chips operate in saturation; the output

of gate C changes abruptly when the critical resistance

is reached. For these chips, the value of resistance at

which the output of gate C changed state was recorded

as the failure resistance. ECL chips operate in a

nonsaturation region; the output from gate C changes

linearly with input voltage. Since the input voltage

varies with shunt resistance, the output of gate C

varies slowly rather than changing abruptly with

changes in the resistance. A range of resistance was

included in the results for the ECL chips that

corresponds to the voltage limits of the high and low

states (nunimum high-level voltage and maximum

low-level voltage).

These tests provide only an approximate estimate of

what will cause an error in a logic gate because only

one pair of leads is shorted at a time and all gate

outputs are connected to only one input gate (no

fanout). These estimates are conservative because in a

smoke exposure more than one lead is expected to be

shorted at once, causing current to flow through more

than one faulty path. The other condition- allowing

each output to be tied to only one input-is not

conservative, but since the output currents are generally

20 to 10,000 times larger than the input currents,

depending on the technology, this assumption may not

affect the results as strongly.

Circuit la

Circuit 2

Figure A-1. OR gate wiring diagrams

NUREG/CR-6543A- A-2
Appendix

High

Low

Circuit lb

Low

Circuit 3

Figure A-2. AND gate wiring diagrams

A-3 A-3 NUREGtCR-6543
I

Table A-1. Components tested and results

Supply Output Drive

Motorola Gate Device No. Voltage tow* (mA) Critical Resistance (f)

Logic Family AND Chip OR Chip (Vd) (ns) (hl/Io) Circuit la Circuit lb Circuit 2 Circuit 3

TIL

LS SN74LS08N SN74LS32N +5 9 -0.4/8 N/F N/F 29 100

FAST MC74F08N MC74F32N +5 3 -1.0/20 N/F 24 45 31

CMOS

MG MC14081BCP MC14071BCP +5 160 ±0.88 690 N/F 837 1220

+10 65 ±2.25 230 N/F 391 605

+15 50 ±8 8 170 N/F 288 490

HC MC74HC08AN MC74HC32AN +2 75 ±0.02 N/F 50 130 120

+4.5 15 ±4.0 N/F 25 68 57

+6 13 ±5.2 N/F 19 56 49

FACT MC74AC08N MC74AC32N +3 5 ±12 N/F 10 30 15

+4.5 4 ±24 N/F 10 20 30

+5.5 3 ±24 N/F N/F 17 25

ECL

100K MC10104P MC10103P -5.2 0.75 50-0l load* N/F 15-33 47-56 77-105

10H MC1OHI04P MC10HIO3P -5.2 1 50-4l load N/F 18-33 93-66 79-107

t Gale propagaton delay ume N/F means no f dilue

* EC gates give output drive an terms of inpedance instead of mA
Appendix

Analysis and Discussion

General Conclusions

In Table A-I, a comparison of the results for circuits

la and lb shows that only one of these circuits fails for

each logic family. This is expected because these

circuits are designed to either raise the low-level output

to a high level, or the high level to a low level by

partially shorting the output leads together. For each

logic family, one case or the other will occur: the

output source of the high level will overcome the sink

power of the low level, or vice versa. If the low logic

level is raised to a high level, then the AND circuit

output will change from low to high, but the OR gate

will not change. If the high level is pulled down, then

the OR gate will change from high to low, but the

AND gate will not change. Only one of these

outcomes should occur in each logic family.

The failure resistances for circuits 2 and 3 are higher

than for circuits la and lb. This can be expected

because in circuits 2 and 3 the input signals to gate C

are shunted to the power supply instead of one of the

gate outputs. The power supply has more current

available to overcome either the sink or source current

of a gate and therefore these shunts can cause failures at

higher resistance values than shunts between gate

outputs.

The failure resistance was highly correlated with the

output current drive. This is especially evident for the

standard CMOS, where the drive current changes with

supply voltage. Gates with higher current drives can

better supply the current necessary to maintain a

voltage level despite a shunt. For the CMOS family,

higher supply voltages correspond to higher drive

current and to lower failure resistance. CMOS gates

with higher supply voltage will be more impervious to

smoke.

Failure resistances for circuits 2 and 3 are not

necessarily equvalent. For example, the LS AND gate

falls in circuit 3 at 100 Ql and the LS OR gate in

circuit 2 fails at 29 Q. The magnitudes of sink and

source currents for LS gates are not equivalent and

failure shunt resistances for circuits 2 and 3 are

dependent on these currents.

Component Families

Components are categorized into logic famfilies and

further into technologies. Logic families are defined by

the voltage levels required to define high (1) and low

(0) states. Table A-2 lists the minimum voltage level

for a high state and maximUm voltage level for a low

state for the different components tested; they were

obtained from data sheets for each chip. Since CMOS

chips can have a range of supply voltages, the

minimum voltage level for a high state and maximum

level for a low state are dependent on the supply

voltage. These values are typical for most of these

chips when they are operating around 25 *C. Values

are a function of operating temperatures.

The TTL family is very commonly used; it is

inexpensive and has high output drive current. The

two technologies tested offer different advantages; the

1S version uses less power, but the FAST is faster.

The TEL chips are all operated between 0 and +5 V and

have output levels of near 0 and 44.5 to +5 V

The advantages of CMOS chips are lower supply

voltage, less power consumption, and smaller size (for

densely packed logic chips). The standard CMOS can

operate with supply voltages ranging from +3 to +18

V. Since standard CMOS gates were introduced, newer

CMOS technologies with enhanced speed have been

Table A-2. Minimum high level and maximum low levels for logic families

Supply voltage Minimum" high level Maximum low level

Family (V.) (V) (V)

TTL (LS and FAST) +5.0 2.0 0.08

CMOS (HC) +20 15 0.05

CMOS (standard) +5.0 3.5 1.5

CMOS (standard) +10.0 7.0 3.0

CMOS (standard). +15.0 11.0 4.0

ECL (10K and 100H) -5.2 -0.96 -1.65

A-5 INUREG/CR-6543
Appendix

introduced. These newer technologies do not operate

over the same range of supply voltage; the HC and

FACT operate between +2 and +6 V. Although they

are faster, these newer technologies consume more

power.

The ECL family is the fastest of all those tested, but

these gates cost more, are more complicated to use,

require the use of two supply voltages, and consume

more power. To measure the output voltage, a 50-11

resistor must be placed between the -2.0-V supply and

the output of the gate, with the voltage measured at the

gate side of the resistor. Since the high and low states

are separated by less than 1 V, and the power

consumption is high (25 to 50 mW/gate compared to

0.04 mW/gate for the standard CMOS at 1 MHz), such

gates should be much faster than gates of other

families.

Digital circuit designers have many choices of chip

technology and usually base their decisions on

requirements of speed, power consumption, and size.

For nuclear power plants, one consideration may be

smoke tolerance. The experiment described here was a

simple test to determine tolerance to shunt resistance.

These results could also be obtained by modeling the

operation of logic chips (i.e., SPICE3). Based on these

tests and the results of smoke exposure tests, it is clear

that the chip technology used can make a great

difference in tolerance to smoke.

3Ornseppc Massobno and Paolo Antogneti. eds ,Semuconductor

Device Modeling with SPICE, 2nd ed . 479 p, McOraw-Hi-l. New

York (1993)

NUREG/CR-6543 A-6
NRC FORV 335 US NUCLEAR REGULATORY COMMISSION 1 REPORT NUMBER

r2 89 (Aulrod by NRC Add Vol. Suep Rev NRCM 1102 and Adfetndum Numners H an¥ I

no'"o0 0 BIBUOGRAPHIC DATA SHEET

Mef ImuctsOIs on the ,tVtfdj NUREG/.R-6543

2 TITLE AND SUBTITLE SAND97-2544

Effects of Smoke on Func-tional Circuits 3 DATE REPORT PUBLISHED

MONTH i Y,,P

O1997

4 FIN OR GRANT NUMBER

W6051

5 AUTHOR(SI 6 TYPE OF REPORT

T. J. Tanaka Technical

7 PERIOD COVERED hovc,. DV'e.,

&. PERFORMING ORGANIZATION - NAME AND ADDRESS 0 AIr.. pfo,,. Da,,•n Ofkwe'i*q.evonu. U4 Aicka- lUr*aw, . andmwade adaf o altrtaeto, ,r

Sandia National Laboratories

P.O. Box 5800, MS047

Albuquerque, NM 87185-0747

9. SPONSORING ORGANIZATION - NAMIE AND ADDRESS INAC. rv *~ move'o,,e xeW,.eV0NeaC 0w Offce RfsAon. US dr.Jdewtmruar Cofmaso- efd oma", )~s

Division of Systems Technology

Office of Nuclear Regulatory Research

U.S. Nuclear ftgulatory Commission

Washington, DC 20555-0001

10 SUPPLEMENTARY NOTES

C. Antonescu, NRC Project Manager

11 ABSTRACT 12woo worrao l

Nuclear power plants are converting to digital instrumentation and control systems; however, the effects of abnormal

environments such as fire and smoke on such systems are not known. Ther are no standard tests for smole, but previous

smoke exposure tests at Sandia National Laboratories have shown that digital communications can be temporarily interrupted

during a smoke exposure. Another concern is the long-term corrosion of metals exposed to the acidic gases produced by a

cable fire. This report documents measurements of basic functional circuits during and up to I day after exposure to smoke

created by burning cable insulation. Printed wiring boards were exposed to the smoke in an enclosed chamber for I hour. For

high-resistance circuits, the smoke lowered the resistance of the surface of the board and caused the circuits to short during the

exposure. These circuits recovered after the smoke was vented. For low-resistance circuits, the smoke caused their resistance to

increase slightly. A polyurethane conformal coating substantially reduced the effects of smoke. A high-speed digital circuit was

unaffected. A second experiment on different logic chip technologies showed that the critical shunt resistance that would cause

failure was dependent on the chip technology and that the components used in the smoke exposures were some of the most

smoke toleranL The smoke densities in these tests were high enough to cause changes in high impedance (resistance) circuits

during exposure, but did not affect most of the other circuits. Conformal coatings and the characteristics of chip technologies

should be considered when designing digital circuitry for nuclear power plant safety systems, which must be highly reliable

under a variety of operating and accident conditions.

12 KEY WORDS,#DESCR!PTORS (Lots wrm s that w• wistw mieWa•h• ano, aine m Mport 13 AVAILABILITY STATEMEN.T

Fire, Smoke, Corrosion, Electronics 14 SECURITY CLASSIICAIoAt.

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