NAT L INST. OF STAND & TECH NISI

PUBLICATIONS

Test Procedures

for Developing

Solder Data

T.A. Siewert

C.A. Handwerker

QC

100 nist

National Institute of Standards and Technology

Technology Administration

U.S. Department of Commerce

NIST Recommended Practice Guide

Special Publication 960-8

Test Procedures

for Developing

Solder Data

NEMI Task Group on

Lead-Free Alloys and

Reliability

Edited by T.A. Siewert

and C.A. Handwerker

Materials Science and

Engineering Laboratory

August 2002

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Under Secretary for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director
Certain commercial entities, equipment, or materials may be identified in

this document in order to describe an experimental procedure or concept

adequately. Such identification is not intended to imply recommendation or endorsement by the National Institute of Standards and Technology, nor is it intended to imply that the entities, materials, or equipment are necessarily the

best available for the purpose.

National Institute of Standards and Technology

Special Publication 960-8

Natl. Inst. Stand. Technol.

Spec. Publ. 960-8

40 pages (August 2002)

CODEN: NSPUE2

U.S. GOVERNMENT PRINTING OFFICE

WASHINGTON: 2003

For sale by the Superintendent of Documents

U.S. Government Printing Office

Internet: bookstore.gpo.gov Phone: (202)512-1800 Fax: (202)512-2250

Mail: Stop S SOP, Washington, DC 20402-0001
FOREWORD

This publication documents standardized test procedures that can produce

valid and reproducible mechanical-property data for lead-free solders. Such data speeds the application of lead-free solders in high-volume,

automated production of electronic assemblies, especially when current production expectations combine high levels of quality with the lowest cost. Use of standardized procedures facilitates the comparison of data between

laboratories and permits the combination of data from different sources into a single, comprehensive database.

Most dimensions and temperatures are listed without tolerances. Unless

otherwise specified, use ±5% on dimensions, times, and pressures, and

± 3 °C for temperatures.

Many of the procedures assume some skill in the arts of specimen production

and testing. Various textbooks and industry brochures (some listed in the

Bibliography) can provide background information on these skills and any

hazards associated with these procedures.

Solder Database

Various researchers have already generated useful data; often with procedures

similar to those listed above. These data have been collected and are available on the web at www.boulder.nist.gov/div853/. If you have data to offer,

please send it to Tom Siewert at siewert@boulder.nist.gov.

iii

Acknowledgments

ACKNOWLEDGMENTS

A primary source of test methods in this document is the 1998 NCMS

Lead-Free Solder Project CDROM (available from NCMS). Members of

the NEMI Task Force on Lead-Free Assembly participated in the review and refinement of these procedures.

v

Table of Contents

TABLE OF CONTENTS

Solder Database /'// List of Figures viii Introduction 1 0.0 Materials and Processing Details 3

I. Thermal Analysis 4

2.0 Wetting Balance Test 5

3.0 Tensile Specimen Fabrication 6

4.0 Tensile Test Procedure 8

5.0 Creep Test Procedure 9

6.0 Metallography 10

7.0 Measurement of Solid State Intermetallic

Compound Growth Rate (by dip method) 11

8.0 Measurement of Liquid-State Intermetallic

Compound Growth Rate (by the hot plate method) 14

9.0 Fracture Toughness 17

10.0 Thermomechanical Fatigue Testing —

Simulated Solder Joints 19

II. Thermomechanical Fatigue Test Using PWB Test

Vehicles 21

12.0 Thermomechanical Fatigue Test Using Strips 22

13.0 Liquid Solder Dissolution 23

14.0 Acoustic Measurements of Elastic Constants 26

15.0 Ring-and-Plug Test 27

16.0 Stress-Rupture Test 29

17.0 Constitutive Behavior 30

Bibliography 31

vii
T*, c

""'<; oping Solder Data

List of Figures

Figure 1. A typical lead-free solder joint structure after reflow.

This SEM micrograph (1800x) shows the (Ni,Cu)-Sn

intermetallic reaction layer between Ni and solder and

Ag-Sn intermetallic rods crystallized from Sn-Ag-Cu

solder. Include a magnification marker on the image,

so the magnification calibration does not change with

enlargement in printing 13

viii
Introduction

INTRODUCTION

Around the world, electronics manufacturers are considering using alternatives

to the traditional lead-containing solder alloys. Unfortunately, there are much

less data available for most of these lead-free alloys, and what data exist

have often been generated with a variety of procedures. Thus, laboratories

continue to develop the mechanical-property data needed by their customers; sometimes having to replicate the experiments of others because of differences

in procedures. The continual development of new solder fluxes and processing

technology also drives the production of additional data. Broad adoption of

standard procedures could greatly reduce the number of experiments that would otherwise have to be performed.

The procedures described in this publication have been shown to produce

data that are applicable to the design and control of high-volume, automated

production of electronic assemblies. Use of standardized procedures facilitates the comparison of data among laboratories and permits the compilation of data

from different sources into a single, comprehensive database.

The following pages describe the wide variety of mechanical property tests that were selected for this collection, then refined, by the NEMI Task Groups

on Lead-free Alloys and Reliability. Use as many of the tests as needed for

your application. When you perform any of these tests, record as much

information as possible. In the Foreword, we include a link to some data that have already been generated and entered into a prototype public database.

Additional data are welcome.

1

Material and Processing Details

0.0 Materials and Processing Details

Unique identifiers to be reported for the following tests.

0.1 Source of Materials

Batch or lot identification (substrate, solder and flux, as appropriate).

0.2 Specimen Identification

Type, orientation, dimensions.

0.3 Composition

Major elements, any information on trace elements; analysis method.

0.4 Flux (if used)

Standard type (e.g., standard activated rosin flux described in

ANSI/IPC/EIA Standard J-STD-002B), composition or product ID.

0.5 Substrate (if used)

Part number, structure, thickness of layers, purity, plate or wrought, etc.

3
es for Developing Solder Data

1.0 Thermal Analysis

Evaluate melting and solidification temperatures.

1.1 Equipment

Differential Scanning Calorimeter (DSC) or Differential Thermal

Analyzer (DTA).

1.2 Specimens

About 10 mg.

1.3 Parameters

Ramp rate: 5.0°C/min:

Scan: (Scan 3 times, but discard the first scan):

Nitrogen purge.

1.4 Evaluation

Measure peak and onset temperatures of each peak through both heating

and cooling.

Note: The 5.0°C/min cooling rate allows accurate measurement of the melting and solidification temperatures of these alloys, but this cooling rate

is too slow to produce a representative microstructure. DSC or DTA can

also be used for study of undercooling at the higher cooling rates (50

C C/

min to 120°C/min) that more accurately represent the behavior of actual

small-scale solder joints. Collect data in a manner similar to that for melting points.

4
Wetting Balance Test

2.0 Wetting Balance Test

Evaluate wetting behavior of solders and component leads.

2.1 Equipment

Commercial wetting balance tester or equivalent.

2.2 Specimens

The type of specimens depends on whether the solder and flux

combination is to be evaluated and compared with other combinations,

or whether component leads with different surface finishes are to be

evaluated for a fixed solder/flux combination. In the NCMS project,

various lead-free solders were compared using easily- solderable copper

wires. (Oxygen-free high-conductivity (OFHC) bare copper wire was

used, 25 mm long (1 in.), and 0.5 mm in diameter (0.02 in., 24 gauge).)

For testing of various component leads with different surface finishes,

test the specimens with a fixed solder/flux combination as specified

by J-STD-002 with the solder temperature as specified in Section 2.4.1.

2.3 Clean

Clean the OFHC copper wires per Section 3.2.4 of J-STD-002. Store the

specimen in anhydrous alcohol to prevent further oxidation prior to use.

2.4 Wetting Balance Test

(Per Sections 4.3.1 or 4.3.2 of J-STD-002.)

2.4.1 Test Temperatures

Two temperatures are suggested: (1) Liquidus +50°C (test temperature in J-STD-002 for tin-lead eutectic is -50 °C higher

than its eutectic temperature); (2) Liquidus + 30 °C (to be closer

to actual practice for lead-free alloys with most processes).

(Both temperatures allow reasonable evaluation of the

different assembly processes.)

Note: This procedure can also be used to study the wetting

characteristics of surfaces other than OFHC copper.

5
Test Procedures for Developing Solder Data

3.0 Tensile Specimen Fabrication

The specimen preparation procedures are intended to duplicate

the structure (grain size and orientation) in the actual solder joints.

Thus, specimens that are larger than joints should be cooled more

rapidly to obtain equivalent properties. Measurements of

microstructure, microhardness, and crystallographic texture measurements help to establish this equivalence.

3.1 Uniaxial (Bulk Solder) Tensile Specimens

3.1.1 Dimensions

Gauge Length = 31.8 mm (1.250 ± 0.005 in.); Diameter = 3.81 mm (0.150 ± 0.005 in.).

3.1.2 Procedure

3.1 .2.1 Preheat molds (aluminum molds for Sn-Bi alloys to enhance casting

properties and no mold release needed; titanium molds for high tin alloys with graphite powder as a lubricant) to 150°C above liquidus

temperature of alloy.

3.1 .2.2 Maintain solder melt at 100°C above liquidus temperature with argon

shield over liquid solder to reduce dross buildup. The argon flow

should be very slow to avoid agitation.

3.1 .2.3 Allow 20 s between removing molds from oven and pouring the cast (bottom fill). Stir the melt immediately prior to pouring. Pour slowly

and continuously until solder reaches top of mold, 5-7 s pour time.

Do not allow bridging between bars at top of mold.

3.1 .2.4 Immediately after pouring, quench molds in tap water by controlled

filling of quench tank. Water should fill tank in 60 s (using aluminum

molds) for complete solder solidification. Consider using an

intermediate tier in the quench tank to allow for a 10 s water level dwell at the mold midpoint during filling. Run water until molds are completely cooled, and remove castings from mold.

6
Tensile Specimen Fabrication

3.1 .2.5 Typical post-solidification aging: Anneal castings for 16 h at a temperature two-thirds of the alloy's solidus temperature. Report

hardness data or other unusual characteristics of the specimens

(e.g., microstructure). Allow 24 h at room temperature for stress

relief before performing tests.

3.2 Lap Shear Specimens

3.2.1 Materials and Dimensions

Lap shear bar material: AISI type 1045 carbon steel with 0.0127 mm

Cu or Ni plating. Solder joint volume: 6.4 x 3.2 x 0.13 mm (width x

length x thickness).

3.2.2 Procedure

3.2.2.1 Pre-tin joint ends of lap shear bars with solder alloy. Clean with

alcohol.

3.2.2.2 Flux the pre-tinned ends with a standard activated rosin flux (such as

that listed in J-STD-002B), and align lap shear bars in a spring-loaded

fixture to maintain about a 0.013 mm (0.0005 in.) joint gap during

soldering.

3.2.2.3 Use electric hot tweezers to heat joint area to reflow temperature

(operation time 5-7 s).

3.2.2.4 Feed new solder into joint from one side to minimize voids in joint.

Allow to cool, and remove from fixture.

3.2.2.5 Remove solder joint fillets using a fine jeweler's file.

3.2.2.6 Typical post-solidification aging: Anneal for 16 h at a temperature

near two-thirds of the alloy's solidus temperature, and temper 24 h

at room temperature for stress relief before performing tests.

7
IV. ig Solder Data

4.0 Tensile Test Procedure

4.1 Procedure

Follow ASTM procedure E8.

4.2 Specimens

Tensile bar: per Section 3.1; Lap shear: per Section 3.2.

4.3 Test Parameters

Crosshead speed: To achieve strain rates between 10 _3

/s and 10-4

/s, although the full range of lO^/s (creep) to 10_2

/s is of interest. Temperatures: -50 to 150°C, in 25°C steps (9 tests).

4.4 Evaluation

Measure strain with an extensometer; continue up to 20% before

stopping. Discontinue test if the load drops below ~ 2 kg. Measure

load as a function of displacement. Report elastic modulus, 0.2% offset yield stress, ultimate tensile stress, uniform elongation, total elongation,

strength coefficient, and work hardening exponent.

8
Creep Test Procedure

5.0 Creep Test Procedure

5.1 Procedure

Follow ASTM procedure El 39.

5.2 Specimens

Butt joint.

5.3 Test Parameters

The strain rate can be 10-7/s. Also, the loading can be stopped

occasionally to monitor the strain decrease as the load drops to zero, developing on a single specimen, data for a range of creep rates. Temperatures: -50 to 150°C, in 25 °C steps (9 tests).

5.4 Evaluation and Reporting

Measure strain as a function of time. Calculate the strain rate.

9
res for Developing Solder Data

6.0 Metallography

Examination of microstructures including different phases in solder and

along interfaces.

6.1 Mount

Mount the specimen on edge in epoxy. Use a slow-hardening mounting

material (such as Buehler Epoxide Resin with separate hardener) to prevent the mount from pulling away from the specimen. Pull a vacuum

for the first five minutes that the mounting material is hardening. Release

vacuum, and allow 6-8 hours for the mount to cure fully. Use disposable

mounts which are glued together.

6.2 Grinding

Follow the sequence of 240/400/ 600 grit paper (600 grit is soft on top. not

like the coarser grits). Do not use grit coarser than 240. Spend no more

than 2 minutes on each step. Run the wheel at 200-250 rpm.

6.3 Polishing

Follow the media size sequence, 15um/6um/ 1 um/0.25um. starting with

diamond suspension (such as Buehler Metadi Supreme), which is water

based and comes in spray bottles. Use with appropriate polishing wheel

preparation, such as Buehler Met Grip liners (thicker w/ sticky backing).

With 15 urn to 6 um grit, use Texmet 1000 hand cloth. With 1 um to 0.25 urn, use Microcloth (thicker nap). Then, use colloidal silica on microcloth.

Spend no more than 2 minutes on each step. Run the wheel at 300 rpm.

6.4 Etching

Appropriate for alloy. Etching of the specimen in a colloidal suspension

of silica is often used. Also, consider a mixture of 10% HC1 in methanol

for a few seconds to reveal the microstructure of most lead-free alloys,

and a mixture of 4 parts glycerol, 1 part acetic acid, and 1 part nitric acid

at 80°C to reveal the intermetallic surface.

6.5 Photomicrographs

Magnification of approximately 320 x for microstructure and 1000 to 1600x (air or oil immersion with optical microscope, or higher with a SEM) for intermetallic compounds. Use scanning electron microscopy/

energy dispersive microscopy, as required, to identify intermetallic

compounds or phases.

10
Measurement of Solid State Growth Rate

7.0 Measurement of Solid State Intermetallic

Compound Growth Rate (by dip method)

7.1 Equipment

One or more air furnaces having a maximum temperature of at least 250°C, controllable to ±1°C

7.2 Substrates

Metal substrates, such as copper, nickel, or palladium.

7.2.1 Copper Substrate

Try polished, OFHC copper sheets with solder coating (6.4 x 6.4 x

1.6 mm).

7.2.2 Coated Substrates

Coated substrates such as Au + Ni over Fe-Ni-Co alloy (KovarTM) as well as Pd, Ni + Pd, or Ni + Pd + Au over Fe-Ni-Co or Cu substrates

could be used. Starting thicknesses of the substrate and various

coatings must be established prior to testing.

7.3 Solders

Solder should be kept in a pot with a minimum volume of 60 cc. Maintain

the solder pot temperature at 40 °C greater than the liquidus temperature

of the alloy.

7.4 Specimen Fabrication Procedure

7.4.1 Coat the optically-polished side of the specimen with a flux that provides suitable wetting.

7.4.2 Immerse the tab into the solder edge-on with the polished surface on

the bottom.

7.4.3 Immerse the specimen in the bath for 5 s.

7.4.4 Remove the specimen from the solder bath by maintaining the bottom,

polished surface parallel to the surface of the bath.

11
res for Developing Solder Data

7.4.5 Allow the specimen to cool. 7.4.6 Remove the flux residues by a suitable cleaning process.

7.5 Thermal Aging Procedures

7.5.1 Thermal aging is used to accelerate the solid-state growth of the reaction (intermetallic compound) layer between the solder and the substrate or solderable finish. Measurements are reported in terms

of thickness and intermetallic growth rate as functions of time,

temperature, and composition.

7.5.2 Aging temperature to be used per each of the solder compositions, per

aging times of 0. 10, 40, and 100 days, are listed below. Care must be

taken not to exceed the solidus temperature of the solder composition.

Alloy Composition Aging Temperatures (°C)

lOOSn 135.170. 205

96Sn-4Cu 135. 170. 205

95Sn-5In 135. 170. 205

95Sn-5Bi 135. 170. 205

91Sn-4Cu-5In 135. 170. 205

90Sn-5In-5Bi 100. 135. 170

91Sn-4Cu-5Bi 135. 170. 205

93Sn-2Cu-2.5Bi-2.5In 135.170. 205

86Sn-4Cu-5In-5Bi 100. 135. 170

76Sn-4Cu-10In-10Bi 100. 135. 170

7.6 Data Analysis

7.6.1 An optical or SEM micrograph should be taken of the intermetallic

compound layer at each of four different locations along the

solder/ substrate interface, using a minimum magnification of lOOOx

(1600 to 2000x is optimal). Because thickness measurements will

be determined from these micrographs, a magnification calibration

should be provided with each set of data. Figure 1 shows a representative micrograph.

12
Figure 1. A typical lead-free solder joint structure after reflow. This SEM micrograph

(1800x) shows the (Ni,Cu)-Sn intermetallic reaction layer between Ni and solder and

Ag-Sn intermetallic rods crystallized from Sn-Ag-Cu solder. Include a magnification

marker on the image, so the magnification calibration does not change with

enlargement in printing.

7.6.2 Measure and report the thickness of the intermetallic compound layer

at about ten equally-spaced positions along the interface (less for a very

uniform layer; as many as 40 for a layer with very uneven thickness).

Should more than one layer be present, measure the total layer

thickness as well as the thickness of each layer.

L3
Test Procedures for Developing Solder Data

8.0 Measurement of Liquid-State Intermetallic

Compound Growth Rate (by the hot plate method)

8.1 Equipment

Programmable hot plate with a maximum temperature setting of at least 300 °C.

8.2 Substrates

Metal substrates, such as copper, nickel, palladium. Dimensions need to be established prior to reflow (i.e., 10x5x 1 mm).

8.2.1 Copper Substrate

Try polished OFHC sheets.

8.2.2 Coated Substrates

Coated substrates such as Au + Ni over Fe-Ni-Co alloy (KovarTM) as well as Pd, Ni + Pd, or Ni + Pd + Au over Fe-Ni-Co or Cu substrates

could be used. Starting thicknesses of the substrate and various

coatings must be established prior to testing.

8.3 Solders

Solder to be provided in paste form, with 50% of volume being flux.

8.4 Flux

Activated rosin flux per J-STD-002.

8.5 Specimen Fabrication Procedure

8.5.1 Preheat the hot plate to the desired reflow temperature; reflow

temperatures of 10°C, 20 °C, 30 °C, and 40 °C above melting point

or liquidus of solders are preferred.

8.5.2 Monitor temperature with a dummy specimen equipped with a fine-gage Type K thermocouple.

14
Measurement of Liquid-State Growth Rate

8.5.3 Place the substrate on the hot plate once the hot plate reaches the

reflow temperature.

8.5.4 Apply a drop of flux to break-up the oxide layer, and wait 30 s for the substrate to equilibrate to the hot plate temperature. Record

temperature of the dummy specimen using the thermocouple.

8.5.5 Apply known amount of solder paste (same ratio of substrate surface

to solder volume for constant thickness).

8.5.6 Start is the time at which melting occurs. (Monitoring the thermocouple

in the dummy specimen will also provide the start time).

8.5.7 Specimens are removed from the hot plate at specified times (i.e.. 20 s. 40 s. 60 s. 2 min. 4 min, 8 min) and quenched on an iron block or similar

media.

8.5.8 Mount and polish the sample to reveal the intermetallic layer.

8.6 Data Analysis

8.6.1 An optical or SEM micrograph is taken of the intermetallic compound

layer at each of four different locations along the solder/ substrate

interface, using a nominal magnification of 1600 to 2000 x. Because

thickness measurements will be determined from these micrographs,

a magnification calibration should be provided with each set of data.

8.6.2 Measure the intermetallic compound layer thickness at ten equally-spaced

positions along the interface. Should more than one layer be present, measure the total layer thickness as well as the thickness of each

layer. A total of 40 data points shall be obtained for the intermetallic

compound layer thickness for each aging condition.

8.6.3 Plot time versus total intermetallic layer thickness for each reflow

temperature.

15
Test Procedures for Developing Solder Data

8.6.4 Fit the data to the equation x(t,T) = k tn

, where k = kQexp(-Q/RT),

using regression.

8.6.5 Refit the data to the average value of n and obtain k for each

temperature.

8.6.6 Arrehenius plot of k values (In k vs. 1/T) provide the pre-exponential

constant kQ and apparent activation energy Q.

16
Fracture Toughness

9.0 Fracture Toughness

9.1 Equipment

A materials testing machine equipped with a load cell range near 200 kg.

9.2 Specimens

A modified compact-tension specimen with a solder joint between two

machined brass halves plated with sufficient Cu (10.2 um) to withstand

reflow and aging conditions. Specimen dimensions are: approximately

32 mm long, 13 mm high, and 10 mm thick with the holes 25 mm from

the back of the specimen and 5.7 mm from the bottom.

9.2.1 Mask each half of the specimen except the surface to be soldered.

9.2.2 Coat the surface with a standard activated flux, and immerse it into a pot of the appropriate solder at a temperature 40 °C above the liquidus

point to allow the formation of a solder coating of approximately

0.02 mm thick on each half.

9.2.3 Clamp the two halves together with an aluminum shim at the mouth of

the specimen to control the joint separation to 0.25 mm, and then recoat

the joint surfaces with flux.

9.2.4 Slowly immerse the clamped assembly into the molten solder.

9.2.5 Remove the specimen from the solder after capillary action fills the joint

(confirmed by flux being forced out the top of the specimen, but no more than 30 s), and then allow to cool to room temperature.

9.2.6 Remove the clamp and spacer. Polish one side of the specimen to a

1 um finish using standard metallographic techniques; then lightly etch

and polish in a colloidal suspension of silica. This polishing technique

will confirm the thickness of the intermetallic layers and allow post-test

observation of crack location.

17
f Procedures for Developing Solder Data

9.3 Test Procedures and Parameters

9.3.1 Place the specimen into the load frame by inserting pins through the

specimen and corresponding holes in the grips.

9.3.2 Perform the tests at a displacement rate of 2.1 x 10~3 mm/s. Use a high-resolution load cell (~ 200 kg capacity).

9.3.3 Record data through analog plots of load versus displacement (from

stroke motion of the load frame).

9.4 Evaluation

9.4.1 Quantify and analyze the fracture toughness as the value Kq (the

energy required to initiate a crack) per ASTM E399. (This assumes

that the results can be characterized by linear elastic fracture mechanics

and recognizes that E399 is intended for only monolithic materials.

Still, the results are useful for comparison of joints containing different

solder alloys.) The apparent fracture toughness is related to the fracture load and the geometry of the specimen:

Where: - ^

Kq ~ BWl/2 ' f

P = load at fracture (kN)

B = specimen thickness (0.95 cm)

W = specimen width (2.54 cm)

a = crack length (1.74 cm)

f(a/w) = geometric factor (5.84).

aW

9.4.2 Perform optical and SEM analyses on the cross section of the joint and

the fracture surfaces, respectively. Report the elemental composition of

the fracture surfaces in the SEM using energy dispersive x-ray analysis

and the void fraction in the solder joint (from image analysis of macrophotos of the fracture surface).

18
Thermomechanical Fatigue Testing —Solder Joints

10.0 Thermomechanical Fatigue Testing —

Simulated Solder Joints

10.1 Equipment

A servohydraulic 50 kN (10 kip) loadframe equipped with a 250 kg

(500 lb) load cell and a thermal cycling device capable of cycling

temperatures from -80 to 260 °C are required. Displacements are measured with an extensometer. A computer controls all components

of the test and digital data collection including test-frame control electronics. Compressed air is sent through the refrigeration unit, cooled to -80 °C,

and then heated to the required temperature by resistive heating coils

in the arm of the equipment. (Cooling the air first reduces the relative

humidity.) The cooled and reheated air is then directed to the test specimen in the region of the solder joint. Temperature is monitored by

a thermocouple immediately adjacent to the specimen. The equipment

is controlled by an IEEE interface between the computer and the

equipment. A knife-edge extensometer is attached to the specimen

across the region containing the joints. The load frame is run in strain control and the extensometer can measure, and control, the imposed

strain. The expansion and/or contraction of the specimen can be

accounted for by thermally cycling the specimen with only the

extensometer attached over the test temperature range. The 10%

strain can then be added to the expansion /contraction of the specimen

to ensure that 10% strain is imposed on the specimen. The extent of

TMF is monitored by the load drop measured across the joints.

10.2 Specimens

The specimens are made from three Al boards having 9 solder

joints per side defined by machining and Cu electroplating techniques.

When assembled, the ends of the specimen are gripped and strain is imposed along the axis of the specimen resulting in shear strain in each

of the joints.

10.2.1 Pre-tin the copper lands with the appropriate solder alloy.

10.2.2 Weigh out enough solder alloy to make joints that would have a

cylindrical shape 0.25 mm (0.010 in.) thick over the joint footprint.

10.2.3 Reflow the specimen in air, with an RJVIA flux, at temperatures 40 °C

above the melting point or liquidus of the solder alloy.

19
lures for Developing Solder Data

10.3 Test Parameters

10.3.1 Temperature ranges for high-Sn lead-free solders, depending on

application: -55° to 150°C, -55° to 125°C, -40° to 125°C, or 0° to 100°C; (-55° to 90°C, -40° to 100°C, or 0° to 100°C for lower temperature alloys such as Sn-58Bi).

10.3.2 Pre-strain: 10%.

10.3.3 The temperature /strain profile should follow a ramped wave with hold

times at the temperature /strain extremes. The hold times should be

3 minutes at each extreme with a deformation rate of 2.9 x 10_4

/s

from one temperature extreme to another.

10.3.4 Data collection: It is useful to collect data on the resultant load,

temperature, strain applied, and the voltage across the joints.

The data should be collected and stored every 5 s during the cycle.

10.4 Evaluation

10.4.1 Plot hysteresis loops using the strain (or temperature) and the

resultant load for each cycle. The mechanical data collected during

the test can be described as a hysteresis loop during each cycle as a

plot of strain (or temperature) versus the resultant load. The data on

this plot can be described and summarized as: the maximum load on

the high-temperature portion of the cycle (high load), the minimum

load on the low-temperature portion (low load), and the load to which

the specimen relaxes during the high-temperature hold portion of

the cycle (relax load). (No load relaxation was observed during the

low-temperature portion of the cycle.)

10.4.2 Metallography

1 0.4.2.1 Section the specimen and mount in epoxy.

1 0.4.2.2 Grind and polish down to a 1 um diamond finish.

1 0.4.2.3 Etch the specimen in a colloidal suspension of silica.

20
Thermomechanical Fatigue Test Using Vehicles

11.0 Thermomechanical Fatigue Test Using

PWB Test Vehicles

Follow the procedures in the latest version of IPC-9701.

21
Test Procedures for Developing Solder Data

12.0 Thermomechanical Fatigue Test Using Strips

This section is based on a paper by Pao, et al. See the reference in the bibliography for more details.

12.1 Test Specimen

2 long strips (beams of the material couple of interest), joined by

solder at the ends. Aluminum and aluminum oxide (a CTE mismatch

of 14 ppm/°C) were used in the paper to introduce large strains,

but common materials used in electronics manufacturing might also be evaluated.

12.2 Equipment

12.2.1 Strain Gages

Four high-temperature strain gages are mounted on the specimen

faces to measure the strains that occur during thermal cycling.

12.2.2 Wheatstone Bridge

A high-resolution bridge is used to improve the sensitivity to 20 fje I MPa. The beam geometry, together with the high resolution bridge,

permits detection of stresses as low as 0.5 MPa in the specimens.

12.2.3 Thermal Cycling Chamber

Use a chamber capable of spanning the range of interest with

appropriate ramp rates. During thermal cycling, the CTE mismatch

between the two beams will introduce bending stresses.

12.3 Analysis

Calibrate the strain gages per the recommended procedure. Use the

equations in the paper by Pao to convert from strain in the beams to

stress in the solder. After the test, the specimen can be sectioned to confirm the dimensions and to check for cracks. The data can then

be plotted and reported in conventional creep formats.

22
Liquid Solder Dissolution

13.0 Liquid Solder Dissolution

Experiments to determine the extent of dissolution of a substrate by

molten solder.

13.1 Equipment

Two solder baths with a capacity of 1 kg (2 lb) each. Digital imageanalysis system to measure changes in the substrate thickness.

13.2 Specimens

13.2.1 Specimen Materials

Metal substrates, such as copper, nickel, palladium.

13.2.1.1 Copper Substrate

Polished, oxygen-free, high-conductivity copper (OFHC) sheets.

13.2.1.2 Coated Substrates

Coated substrates such as Au + Ni over Fe-Ni-Co alloy (KovarTM)

as well as Ag, Pd, Ni + Pd, or Ni + Pd + Au over Fe-Ni-Co or Cu

substrates could be used. Starting thicknesses of the substrate and

various coatings must be established prior to testing.

13.2.2 Specimen Shape

13.2.2.1 Planar coupons measuring approximately 6.4 x 6.4 x 1.6 mm are used for the tests.

13.2.2.2 Cylindrical specimens of wire, 0.68 mm (0.031 in.) in diameter.

13.2.3 Specimen Preparation

13.2.3.1 Degrease the substrates in trichloroethane (or equivalent), etch in a 1: 1 solution of HC1 and water for 30 s, rinse to remove any acid

residues, and dry.

23
;edures for Developing Solder Data

1 3.2.3.2 Wrap the plate and wire specimens around their respective

perimeters with KaptonTM tape, leaving a 12 mm (0.5 in.) wide

surface "band" around the specimen, extending from the bottom

edge.

1 3.2.3.3 Coat the exposed regions of the plate and wire specimens with a rosin-based, mildly-activated flux.

13.2.4 Solders

Sn-3.9Ag-0.6Cu (Tmelt = 217-220°C);

Sn-3.5Ag (Tonset = 221°C);

Sn-0.7Cu (Teut = 227°C);

Sn-37Pb (Teut = 183°C).

13.3 Test Procedure/Parameters

The immersion-time parameters are 5, 10, 20, 50, 75, and 100 s; three

solder pot temperatures are used per alloy; selected values are shown in the table below.

Pot Temperatures (°C)

Solders Pot #1 Pot #2 Pot #3

Sn-3.9Ag-0.6Cu 240 255 270

Sn-3.5Ag 240 255 270

Sn-0.7Cu 250 265 280

63Sn-37Pb 205 220 235

13.4 Evaluation

13.4.1 Thickness Measurement of Plated Specimens

13.4.1.1 Measure 20 thicknesses on each of the non-immersed and immersed

segments of the coupons, and describe the results by a mean (x)

and ± one standard deviation (s) as the error term.

24
Liquid Solder Dissolution

Determine the dissolution distance by subtracting the thickness in the immersed area from the original thickness of the plate; then,

dividing that value by two (2) so as to represent a single surface.

Determine the error term for the difference as follows:

(1) A fractional error is defined as s/x for each of the data sets from the immersed and non-immersed regions. (2) The fractional errors are added together, the value of which is designated as the

"fractional error for the difference." (3) This "fractional error

for the difference" is multiplied by the dissolution distance to arrive at the ± error term. (4) The two data sets per each of

the time/temperature conditions are combined by combining the

immersed plate thickness data from Trial #1 with the same data

from Trial #2.

13.4.1.2 The same procedure is performed on the plate thickness of the

non-immersed regions. The values of x and s are computed; the

overall dissolution being the difference between the two means

of the combined data (divided by 2 to represent one surface).

The error term is computed in the same manner as described above.

13.4.2 Thickness Measurement of Wire Specimens

13.4.2.1 Measure the thickness reduction of the wire specimens at eight

diametrical positions about the specimen footprint.

13.4.2.2 Use the same procedures for data analysis as outlined above for

the plate specimens.

13.4.1.1.1

13.4.1.1.2

25
Test Procedures for Developing Solder Data

14.0 Acoustic Measurements of Elastic Constants

14.1 Equipment

An acoustic microscope.

14.2 Specimens

Cast the specimens into a cylindrical shape and machine to final dimensions of 20 mm (0.8 in. long by 10 mm (0.4 in.)) in diameter.

14.3 Test Procedure/Parameters

The testing temperatures 23 °C to 125°C with a minimum of 12 testing

temperatures between these extremes.

14.4 Evaluation

Determine the elastic constants as a function of temperature.

26
Ring-and-Plug Test

15.0 Ring-and-Plug Test

15.1 Equipment

A standard test load frame.

15.1.1 Test Fixture

Ring: Inner diameter = 3.175 mm (0.125 in.); Outer diameter = 9.525 mm (0.375 in.); Thickness = 3.175 mm (0.125 in.); Plug: a solid cylinder having a diameter of 2.794 mm (0. 1 10 in.) and

length of 9.525 mm (0.375 in.), and effective joint gap (the difference

between outer diameter of the plug and inner diameter of the ring) is 0.19 mm (0.0075 in.).

15.2 Joint Fabrication

1 5.2.1 Degrease the Cu ring and plug by immersing in isopropyl alcohol for 2min.

15.2.2 Clean in fluoroboric acid (10% HBF in water) for 5 minutes at room temperature with agitation (use caution in handling; also see IPC-CS-70), rinse twice in distilled or deionized water, insert twice

in isopropyl alcohol, and air dry.

15.2.3 Coat with flux, J-STD-004, Type ROL1 (formerly RJVIA).

1 5.2.4 Insert the plug into the ring hole accompanied by four Cu wires of

0.18 mm (0.007 in.) diameter in order to maintain concentricity

between the plug and the ring.

1 5.2.5 Prepare a preform of the particular solder for testing from a short

length of wire by twisting the wire around the plug-ring intersection.

1 5.2.6 Place the assembled unit onto a stainless steel fixture and, in turn, place it upon a hot plate held at 300 °C. Keep the assembly on the

hot plate for a duration of 15 s past the time at which the solder

preform is observed to have melted.

27
ledures for Developing Solder Data

15.2.7 Remove the assembly from the hot plate and allow to cool.

15.2.8 Cut off the protruding ends of the plug with a slow-speed diamond

saw, and polish the two faces flat.

15.3 Test Procedure/Parameters

15.3.1 Displacement rate =10 mm/min.

1 5.3.2 Run four tests per solder alloy composition.

15.4 Evaluation

Report the maximum load achieved. The results are represented as the mean and plus-or-minus one standard deviation of the four tests. Reference: ITRI Report 656.

28
Stress-Rupture ' Test .

16.0 Stress-Rupture Test

16.1 Test Specimens

Tensile bar: 31.7 mm (1.25 in.) gauge length, 3.8 mm (0.15 in.)

diameter; or lap shear: 6.4 mm x 3.175 mm x 0.127 mm (0.25 long x

0.125 wide x 0.005 in. thick), with AISI type 1045 carbon steel substrate

plated with 0.0125 mm (0.0005 in.) Cu or Ni.

16.2 Temperatures

-25°C, 25°C, 75°C, and 125°C.

16.3 Initial Stress

Low-melt alloys: 9 MPa (1250 psi) and 18 MPa (2500 psi);

High-melt alloys: 18 MPa (2500 psi) and 35 MPa (5000 psi).

16.4 Measure

Measure the time to failure and percent elongation at break. With the

assumption that there are no structural changes occurring in the material,

the slope of the stress-rupture line will remain constant. So, instead of

reporting the data as time to failure at a certain stress (e.g., 18 MPa,

2500 psi), report the stress to failure at certain times (10, 1,000, and

10,000 min.) for the comparison of the different solder materials.

29
edures for Developing Solder Data

17.0 Constitutive Behavior

17.1 Bulk Solder Specimens

Fabricate by casting the alloys in a split steel mold. Prior to casting,

heat the alloys 40 °C above their melting point or liquidus, then pour

directly into the chilled mold. This will promote the formation of the

fine microstructure that is found in the small solder joints used in electronic packages. Cut specimens from the cast ingots, and mount

in epoxy for metallographic observation. Hand polish on sequentially

finer nylon wheels using diamond paste; follow by polishing by a

colloidal suspension of silica on a vibratory polisher.

The mechanical response of the various solder alloys can be

determined through a series of isothermal uniaxial compression creep

tests. (The specimens can also be tested after isothermal aging to coarsen the microstructures.) For test specimens, use right circular

cylinders 10 mm (0.4 in.) diameter by 20 mm (0.8 in.) in length. Use

a test machine fitted with an environmental chamber and a relatively

small (such as 55 kN) load cell. Test the alloys at -25°C, 25°C, 75°C,

and 125°C. Each specimen should be allowed to equilibrate in the

environmental chamber for 30 min. prior to testing. A compressive

prestrain of approximately one percent should be applied to the

specimens to ensure contact between the specimen and the platens. Test in load control with stresses ranging from 10 to 80 MPa, applied

using a 1 s linear ramp. Platen displacement should be monitored as a function of time with a digital oscilloscope with data stored on disk.

The ASCII-formatted strain-time data can be plotted to determine the

minimum strain rate for each stress and temperature. A plotting program

for engineering analysis should be used to fit the data according to the

Sherby-Dorn power-law creep equation:

e = A(cr

n /T)exp(-Q/RT)

Attempts might be made to fit the Pb-free solder alloy data to a sink

power-law equation. In past tests, the near-eutectic 60Sn-40Pb alloy

has shown power-law breakdown at temperatures lower than 20 °C,

and these data could be fit with the Garofalo sinh equation.

30
Bibliography

Joint Industry Standard: Guidelines for Chemical Handling Safety

in Printed Circuit Board Manufacturing. ANSI/IPC-CS-70. Rev. ed. Northbrook: IPC.

Joint Industry Standard: Solderability Tests for Component Leads,

Terminations, Lugs, Terminals and Wires. ANSI/IPC/EIS J-STD-002A.

Rev. ed. Arlington: Electronic Industry Alliance or Northbrook: IPC.

Lead Free Solder Project. (A CDROM report.) Ann Arbor: National

Center for Manufacturing Sciences, 1999.

"Mechanical Testing and Evaluation." ASM Handbook, Vol. 8, Rev. ed. Materials Park: ASM International.

Pao, Y.-H., et al. "Measurement of Mechanical Behavior of High Lead

Lead-Tin Solder Joints Subjected to Thermal Cycling." Journal of Electronic

Packaging, 114, June 1992, 135.

Standard Test Method for Conducting Creep, Creep Rupture, and

Stress Rupture Tests of Metallic Materials. ASTM E 139. Rev. ed. West Conshohocken: ASTM.

Standard Test Method for Tension Testing of Metallic Materials [Metric].

ASTM E 8M. Rev. ed. West Conshohocken: ASTM.

31