National Aeronautics and

Space Administration

IS20

George C. Marshall Space Flight Center

Marshall Space Flight Center, Alabama

35812
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i

NASA/TM—2011–216463

Microcoil Spring Interconnects for Ceramic

Grid Array Integrated Circuits

S.M. Strickland, J.D. Hester, A.K. Gowan, R.K. Montgomery, and D.L. Geist

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

J.F. Blanche and G.D. McGuire

Jacobs Engineering, Science and Technical Services Group

Marshall Space Flight Center, Marshall Space Flight Center, Alabama

T.S. Nash

Jacobs Engineering, Science and Technical Services Group/Integrated Concepts 

and Research, Marshall Space Flight Center, Marshall Space Flight Center, Alabama

March 2011

National Aeronautics and

Space Administration

Marshall Space Flight Center • MSFC, Alabama 35812
ii

Available from:

NASA Center for AeroSpace Information

7115 Standard Drive

Hanover, MD 21076 –1320

443–757–5802

This report is also available in electronic form at

<https://www2.sti.nasa.gov/login/wt/>

Acknowledgments

The authors would like to extend thanks to Mike Sampson, NASA Goddard Space Flight Center, Manager 

of the NASA Electronic Parts and Packaging Program, for his support of this effort. The authors would 

also like to recognize the following individuals for their technical support: Dr. Dhananjay Panchagade, 

Auburn University, for his support of the vibration testing, Ron Johnson, NASA Marshall 

Space Flight Center/ES43, for his support of fixture design/fabrication.
iii

TABLE OF CONTENTS

1. INTRODUCTION ............................................................................................................. 1

2. MICROCOIL DESIGN ..................................................................................................... 3

3. TEST VEHICLE DESIGN AND EVALUATION ............................................................. 5

3.1 Interconnect First Attach to Ceramic Substrate ........................................................... 6

3.2 Board-Level Fabrication ............................................................................................... 7

3.3 Thermal Cycling ........................................................................................................... 9

3.4 Vibration ...................................................................................................................... 11

3.5 Radio Frequency Simulation ........................................................................................ 15

3.6 Inductance Measurement .............................................................................................. 16

4. STRESS ANALYSIS .......................................................................................................... 17

5. SUMMARY ....................................................................................................................... 19

6. FUTURE WORK ............................................................................................................... 20

REFERENCES ....................................................................................................................... 21
iv

LIST OF FIGURES

 1. MCS design ............................................................................................................... 3

 2. Ceramic channel configuration .................................................................................. 5

 3. Test vehicle ................................................................................................................ 6

 4. Interconnect attachment fixture ................................................................................ 7

 5. MCS 4-mil stencil ...................................................................................................... 8

 6. Typical wire column .................................................................................................. 8

 7. Polymer cored solder ball .......................................................................................... 8

 8. Thermal cycling results .............................................................................................. 9

 9. Failed MCS (MCS-2-U1-3) thermal cycling .............................................................. 10

10. Failed MCS (MCS-1-U1-4) thermal cycling .............................................................. 10

11. Vibration test vehicle ................................................................................................. 11

12. Wire staking .............................................................................................................. 11

13. Fixture and shaker .................................................................................................... 12

14. Vibration failure locations ......................................................................................... 14

15. Column cracks .......................................................................................................... 14

16. Column displacement ................................................................................................ 14

17. Simplified FEA model—(a) MCS and (b) column ..................................................... 17

18. Solder stress—MCS versus column ........................................................................... 18

19. MCS fillet with 6-mil stencil ...................................................................................... 20
v

LIST OF TABLES

1. Test vehicle configuration ............................................................................................ 5

2. Vibration test levels....................................................................................................... 13

3. Vibration failures ......................................................................................................... 13

4. Power transferred—HFSS simulation—MCSs versus columns .................................... 15

5. Crosstalk (coupling)—HFSS simulation—MCSs versus columns ................................ 15

6. Maximum stress levels ................................................................................................. 18
vi

LIST OF ACRONYMS AND SYMBOLS

Ag silver

AU Auburn University

Au gold

CBGA ceramic ball grid array

CCGA ceramic column grid array

CTE coefficient of thermal expansion

Cu copper

DC direct current

EEE electrical, electronic, and electromechanical

HFSS high-frequency structure simulator 

I/O input/output

MCS microcoil spring

MSFC Marshall Space Flight Center

Ni nickel

PBGA plastic ball grid array

Pb lead

PCB printed circuit board

Pd palladium

RF radio frequency

Sn tin

TC thermal cycling

W tungsten
1

TECHNICAL MEMORANDUM

MICROCOIL SPRING INTERCONNECTS FOR CERAMIC GRID ARRAY 

INTEGRATED CIRCUITS

1. INTRODUCTION

Current aerospace avionic designs routinely use integrated circuits packaged in the area 

array format. Unfortunately, the area array package trades the compliance of ribbon leads for the 

quantity of noncompliant solder balls, which under thermal cyclic loading, severely limits the life 

of the solder joints. 

One approach to overcome this limitation was to replace the noncompliant balls with 

slightly more compliant solder columns. Another approach was to package the silicon microcircuit 

on an organic substrate and plastic encapsulate it rather than put it on a ceramic substrate and 

hermetically seal it. The coefficient of thermal expansion (CTE) of a plastic ball grid array (PBGA) 

more closely matched the CTE of the printed circuit board (PCB) than the CTE of a ceramic ball 

grid array (CBGA). There are always tradeoffs to be made and the price initially paid for using a 

PBGA was that the plastic absorbed moisture and moisture could initiate corrosion and result in 

a failure rate that was unacceptable for high reliability space flight electronics. With time, the processes and materials have improved such that the moisture absorption is much lower but a plastic 

encapsulated microcircuit is still not a hermetically sealed part. Electrical, electronic, and electromechanical (EEE) parts personnel prefer ceramic column grid array (CCGA) packages for highest 

reliability critical applications. 

Reliability of various configurations of flexible interconnects for ceramic area array packages are reported in the literature.1–5 Variation in test methods and test vehicle configurations make 

it difficult to perform direct comparisons with a high confidence level. NASA Marshall Space 

Flight Center (MSFC) initiated a research project to test a variety of flexible interconnects while 

holding constant (as nearly as possible) the ceramic substrate design, PCB design, and test methods 

so that reliability could be fairly compared.

A long-term human presence in space without the benefit of resupply will require confidence levels not previously required. Due to this and existing reliability data on columns, MSFC 

started investigating other potential flexible interconnects to improve the thermal reliability of 

ceramic area array packages. Preliminary assembly experiments with springs of various configurations were conducted. As a result of these preliminary experiments, a microcoil spring (MCS) was 

chosen and introduced into the test program. 

The MCSs provide ample flexibility to withstand the shear forces resulting from the CTE 

mismatch and are much more robust from a handling perspective than high lead columns. However, the MCSs do not have planarized end faces which makes attachment challenging. 
2
3

2. MICROCOIL DESIGN

The MCS (fig. 1) is fabricated from a beryllium copper (Cu) wire 0.0034 in thick and is post 

plated with 100 μin of tin lead (Sn63Pb37). The springs are 0.020 in diameter by 0.050 in long with 

a 0.010 in pitch and two to three closed coils on each end. 

 

D= 0.020 in

0.0034 in

0.050 in

Figure 1. MCS design.
4
5

3. TEST VEHICLE DESIGN AND EVALUATION

The primary ceramic substrate and PCB were designed by MSFC’s Parts, Packaging, and 

Fabrication Branch. The ceramic substrates are 21.5 mm × 21.5 mm × 1 mm. The substrates are 

daisy-chained, two layers and 1 mm pitch. Each substrate consists of four channels (fig. 2) which 

are made up of 100 daisy-chained interconnects. The current ceramic substrates were fabricated 

using a thick film technique with a minimum of three layers of silver (Ag)/palladium (Pd) paste 

to form metallization ≈0.001 in thick. The pad diameter has ranged from 0.025 to 0.030 in. Early 

in the study, 0.025 in was used due to the ball size chosen. Subsequently, the pad diameter was 

changed to 0.030 in to accommodate columns and MCSs. The same pad diameter is used on both 

the substrate and PCB. Early in the study a four-layer ceramic with tungsten (W)-nickel (Ni)-gold 

(Au) metallization was used, but it was abandoned due to cost. The interconnects tested with this 

metallization are noted in table 1. 

1

4

3 2

Figure 2. Ceramic channel configuration.

Table 1. Test vehicle configuration.

Test Vehicle*

No. 

of TV’s

Ceramic 

Configuration

Solder Used 

for First Attach

Stencil Thickness/Aperture 

(PCB Attach) Use

Initial MCS* 2 25 mil/Ag-Pd Tin/lead 4 mil/100% TC @ MSFC

95/5 CCGA 3 30 mil/Ag-Pd Tin/lead 8 mil/100% Vib @ AU then TC @ MSFC

85/15 CCGA* 1 25 mil/W-Ni-Au Tin/lead 8 mil/100% TC @ AU

90/10 CBGA* 2 25 mil/Ag-Pd Tin/lead 4 mil/100% TC @ MSFC

Polymer CBGA* 2 25 mil/Ag-Pd Tin/lead 6 mil/100% TC @ MSFC

Reworked MCS 3 30 mil/Ag-Pd Tin/lead 6 mil/100% (1) and 4 mil/75% (2) TC @ MSFC 

MCS Vibration 1 30 mil/Ag-Pd Tin/lead 4 mil/100% Vib @ AU then TC @ MSFC

*All test vehicles with asterisks were fabricated using a 0.062-in, double-sided PCB with 0.025-in-diameter pads. The remainder were fabricated using 

a 0.093-in, double-sided PCB with 0.030-in pads.
6

Unless otherwise noted, the PCB (fig. 3) is four layers, two of which are ground planes for 

stiffness and is designed to accommodate eight parts. The overall thickness is 0.093 in. 

Figure 3. Test vehicle.

The following interconnects were included in the thermal cycling phase of the study:

• Sn5/Pb95 wire column (0.018×0.087 in).

• Sn15/Pb85 column (0.015×0.087 in).

• Polymer cored ball (650 μm).

• Sn10/Pb90 solder balls (0.025 in).

• MCS (0.020×0.050 in).

Vibration testing was conducted on test vehicles with Sn5/Pb95 wire columns and MCSs.

3.1 Interconnect First Attach to Ceramic Substrate

Except for the Sn15/Pb85 columns, all interconnects were attached by MSFC using a twopiece fixture (fig. 4) that holds a single ceramic substrate. Solder paste is deposited onto the ceramic 

substrate using an automatic stencil printer. After inspection, the substrate is loaded into a graphite fixture stage (bottom) and a graphite interconnect positioning fixture (top) is lowered onto the 

stage. The positioning fixture has apertures that match the pattern on the ceramic with the apertures sized for the interconnect diameter being attached. After the apertures are filled with interconnects, the whole fixture is loaded onto a pallet and reflowed in a vapor phase reflow oven. For 

ball interconnects, the interconnect positioning fixture (top) is removed prior to reflow which aids 

in ball to ceramic pad alignment. 
7

Figure 4. Interconnect attachment fixture.

For the MCS, the stencil thickness, stencil aperture size, and solder alloy were varied. The 

first thermal cycling test vehicles were fabricated using SnPb solder paste for attachment to both 

the ceramic substrate and the PCB. The stencil thickness was 0.004 in, resulting in the best thermal 

reliability to date. To improve yield, the solder paste for first attach (to ceramic) was changed to 

Sn-Ag-Cu (SAC305) to prevent its reflow and MCS movement during second attach. The stencil 

thickness has been optimized at 0.006 in for the MCS design currently being used. This results in 

the best fillets. Recently, a process step was added to remove MCS oxidation during first attach by 

using flux and a double reflow process which has allowed a return to SnPb paste without a decrease 

in yield. Thermal cycling for test vehicles fabricated using the double-pass reflow process and a 

0.006-in-thick stencil is ongoing and results are not presented herein. 

3.2 Board-Level Fabrication

Solder paste was deposited using an automatic stencil printer. Part placement was achieved 

using a semi-automatic, split-vision placement machine. Reflow except for rework was conducted 

in a vapor phase reflow oven. Endoscopic and x-ray inspections were performed prior to initiation 

of environmental testing. A typical MCS from the first thermal cycling test vehicle is presented in 

figure 5. Notice the lack of side fillets. 

A typical wire column (Sn5/Pb95) is shown in figure 6. An 8-mil-thick stencil was used 

to attach the Sn5/Pb95 columns to both the ceramic and the PCB.

A typical polymer cored solder ball is shown in figure 7. The ball is 650 μm and has a Sn-Ag 

over Cu finish. A 6-mil-thick stencil was used to attach the polymer cored solder balls to both the 

ceramic and the PCB. 
8

Figure 5. MCS 4-mil stencil.

 

Figure 6. Typical wire column. Figure 7. Polymer cored solder ball.

The overall test vehicle configuration is presented in table 1 along with the environmental 

tests performed on each unit.
9

3.3 Thermal Cycling

Thermal cycling was conducted at MSFC and at Auburn University (AU). The range was 

–55 to 125 °C with 15-min dwells at the extremes. Auburn University conducted the thermal cycling 

of the 85/15 CCGA test vehicles. MSFC performed the thermal cycling of the remainder of the test 

vehicles.

The AU tests were monitored using a data logger. The failure criteria used was a 50% 

increase in resistance for 10 consecutive thermal cycles. The AU testing consisted of 400 interconnects per channel and one 85/15 CCGA test board with five parts. Testing was conducted in a thermal cycling chamber.

The MSFC tests were monitored using an event detector. The failure criteria were per IPC9701A, 4.3.3.3, with a 1,000 Ω threshold. Testing was conducted in a thermal forcing unit with a 

custom enclosure; ≈19 cycles per day were obtained.

A composite Weibull is shown in figure 8. The initial MCS (MCS on 25-mil pad) significantly outperformed 85/15 CCGAs and 95/5 CCGAs. The initial MCS test vehicles had many 

individual springs that were misaligned to the PCB pads by as much as 50%. The first recordable 

failure (other than one infant mortality at 400 cycles) occurred at 4,760 cycles. For space flight 

systems, first failure is considered a more important factor than characteristic life. 

90

70

50

30

10

5

2

1

.1

Occurrence CDF (%)

10 20 30 70 100 300 500 1,000 2,000 5,000 10,000

Datum (Thermal Cycles –55 to 125C 15 min dwell)

YR2010

W/rr M10D20

MSFC 95/5 CCGA

Reworked MCS

MCS Vibe

Aub 85/15 CCGA

MCS on 25-mil Pad

MSFC HIPB CBGA

Polymer CBGA

ETA Beta pve% n/s

3178 5.155 0.99 64/17

5668 2.524 0.188 126/64

4739 3.09 63.98 32/8

3233 63.13 3.67 57/34

5120 63.13 3.67 57/34

429.9 7.008 1.24 64/6

252.8 6.215 99.9 64/21

Figure 8. Thermal cycling results.
10

Metallographic analysis and endoscopy of failed MCSs (figs. 9 and 10) and 95/5 columns 

were performed to verify failure mode. All MCS failures observed were in the solder joint interface 

with the ceramic pad. The failures appear to have begun parallel to the interface which is expected, 

but as the failure propagated, a perpendicular crack developed in several of the microsections 

observed. Metallographic analysis was also used to verify that the Ag-Pd pad on the ceramic 

was not being leached into the solder joint during multiple reflows. No evidence of pad failure 

or significant leaching was found. 

Figure 9. Failed MCS (MCS-2-U1-3) thermal cycling.

Figure 10. Failed MCS (MCS-1-U1-4) thermal cycling.
11

3.4 Vibration

Vibration testing was conducted at AU. The test vehicles used the same PCB and ceramic 

parts. The only difference from the thermal cycling test vehicles is that an aluminum mass was 

attached to each ceramic to simulate the mass of a comparably sized active ceramic package 

(fig. 11). One 95/5 CCGA and one MCS test vehicle were vibration tested. 

Each test vehicle was hard wired to a shielded cable that attached to an event detector 

system. An event was recorded when the channel resistance exceeded 1,000 Ω, which in each case, 

constituted a failure since these resistance changes were hard and did not show intermittency. The 

wires were staked to the back side of the board (fig. 12).

Figure 11. Vibration test vehicle.

Figure 12. Wire staking.
12

The test vehicles mounted to the vibration fixture and the shaker table are shown in figure 13.

Figure 13. Fixture and shaker.

Prior to performing the random vibration test a resonance search sine sweep was performed 

with the boards mounted in the x axis at a sweep range 5–2,000–5 Hz, sweep rate: two octaves per 

minute, amplitude 0.25 g. 

The z axis is perpendicular to the surface of the unit under test. The x axis is defined as the 

shortest board dimension (4 in). Random vibration testing was performed at three different levels 

for 10 min at each level in the x axis and the z axis, for a total test time of 1 hr. The random vibration levels are presented in table 2.

The testing sequence was 0.25 g sine sweep, 5gx, 5gz, 8gz, 8gx, 10gx, and 10gz. There was 

a system natural frequency at ≈420 Hz and another at 900 Hz. The MCSs were recorded as channels 1–32 and the 95/5 columns as channels 33–64. Throughout the test there were no failures of 

the MCSs. There were 10 failures of the 95/5 columns. The failure data are shown in table 3. The 

location of the failed circuits on the column grid array test board is shown in figure 14. The failures 

were resistance failures only. No parts were shaken off of the boards.
13

Table 2. Vibration test levels.

Level I

20 Hz @

20–50 Hz @

50–1,000 Hz @

1,000–2000 Hz @

2,000 Hz @

0.0025 g2/Hz

6 dB/oct

0.01675 g2/Hz

–6 dB/oct

0.005 g2/Hz

Composite = 5 gm

Level II

20 Hz @

20–50 Hz @

50–1,000 Hz @

1,000–2,000 Hz @

2,000 Hz @

0.00698 g2/Hz

6 dB/oct

0.0438 g2/Hz

–6 dB/oct

0.0109 g2/Hz

Composite = 8 gm

Level III

20 Hz @

20–50 Hz @

50–1000 Hz @

1,000–2,000 Hz @

2,000 Hz @

0.01 g2/Hz

6 dB/oct

0.067 g2/Hz

–6 dB/oct

0.02 g2/Hz

Composite = 10 gm

Table 3. Vibration failures.

g Level Axis

Failed 

Channel

Time Into Run 

(min)

Total Vibration Time

to Failure (min)

5

5

8

8

8

10

10

10

10

10

10

x

z

z

z

x

x

z

z

z

z

z

–

59

61

62

58

57

44

33

41

42

36

–

10

4

7

6

6

3

5

7

7

8

–

20

24

27

36

46

53

55

57

57

58
14

U7

57

59 58

A1 A1 A1 A1

A1 GND1

GND2

GND3

GND4

GND5

A1 A1 A1

U5 U3 U1

U8 U6 U4 U2

61

41

44

42

33

36

62

CH_1

CH_2

CH_3

CH_4

CH_5

CH_6

CH_7

CH_8

CH_9

CH_10

CH_11

CH_12

CH_13

CH_14

CH_15

CH_16

CH_17

CH_18

CH_19

CH_20

CH_21

CH_22

CH_23

CH_24

CH_25

CH_26

CH_27

CH_28

CH_29

CH_30

CH_31

CH_32

CH_1–32

Figure 14. Vibration failure locations.

The failures were investigated by endoscopy, x-ray, and destructive physical analysis. The 

outer rows of columns of failed channels were subjected to endoscopic visual inspection. Cracks 

were observed in the columns generally at the point where the 63/37 solder fillet joined the column. 

Figure 15 shows examples of cracks in both the upper and lower fillets. Figure 16 shows the most 

extreme failure observed. 

 

Figure 15. Column cracks. Figure 16. Column displacement.
15

3.5 Radio Frequency Simulation

Radio frequency simulations were run on the MCS and the column configurations. The 

simulation was performed from the kilohertz range up to 10 GHz. The insertion loss, the coupling 

(crosstalk), and the effective inductance of each configuration was simulated and compared. The 

simulations were done with a high-frequency structure simulator (HFSS) at MSFC in the Data and 

Radio Frequency Systems Branch of the Space Systems Department. 

Table 4 compares the insertion loss for MCSs and columns over a frequency range of 

500 MHz to 5 GHz. The insertion loss shows how much power is transferred from one end of the 

column or MCS to the other. The insertion loss of the MCS configuration is a little worse than the 

column configuration. The insertion loss of each configuration is similar at the lower frequencies. 

As the frequency increases, the insertion loss of the MCS increases slightly faster than the column. 

The difference in performance decreases as the configurations are used at lower frequencies. 

Table 4. Power transferred—HFSS simulation—MCSs versus columns.

Frequency MCS (%) Column (%)

500 MHz

1 GHz

2 GHz

3 GHz

4 GHz

5 GHz

98.3

96.4

89

79.1

69.1

59.8

98.9

97.9

92.5

85.0

77.0

69.3

Table 5 compares coupling (crosstalk) of MCSs and columns. The coupling shows how 

much power is transferred from one column to an adjacent column. The coupling of the MCS 

versus the column is slightly better at the lower frequencies but then becomes much better at the 

higher frequencies. The worst coupling of the MCS configuration was about –22.9 dB (0.52% 

power transfer), which occurred at 4 GHz. At this frequency, the coupling of the column configuration was –13 dB (5% power transfer). However, the coupling of the MCS improves slightly as 

the frequency increases whereas the coupling of the column degrades. The worst coupling of the 

column configuration was about –10 dB (10% power transfer), which occurred at 9.9 GHz.

Table 5. Crosstalk (coupling)—HFSS simulation—MCSs versus columns.

Frequency MCS (%) Column (%)

500 MHz

1 GHz

2 GHz

3 GHz

4 GHz

5 GHz

0.086

0.17

0.59

1.06

1.4

1.7

1.19

0.4

1.5

2.9

4.4

5.8
16

The inductance of the MCS and the column configurations was calculated over the frequency range of the simulation (500 MHz to 10 GHz). The inductance of the MCS was 2.7 nH, 

and the inductance of the column was 2.2 nH. 

3.6 Inductance Measurement

The inductance of one test vehicle channel which is comprised of 100 interconnects was 

determined using a frequency analyzer capable of measurements up to 2.2 MHz. This frequency 

range was sufficient to show the inductive component of the interconnects but it was not sufficient 

to show parasitic capacitances which often show up in higher frequency ranges. The data for each 

test vehicle includes the inductive contribution due to the test leads connecting the test units and 

the instrument. The contribution of the test leads can be subtracted from the data. What cannot be 

removed is the resistance and inductance associated with the interconnections between 100 individual interconnects. It may be assumed that they are small but a more precise evaluation of the interconnects needs to be performed using an inductance/capacitance/resistance bridge or frequency 

response analyzer and a test fixture capable of holding individual interconnects. 

The MCS DC resistance component was measured at ≈3 Ω and the inductive component 

was measured at ≈484 nH (after the inductance of the test leads is subtracted). Since there are 

100 interconnects in series, the per unit resistance and inductance is calculated as follows: 

• Per unit DC resistance = 3 Ω/100 = 0.03 Ω. 

• Per unit inductance = 484 nH/100 = 4.84 nH.

The column DC resistance measured at ≈0.9 Ω and an inductive component was measured 

at ≈591 nH (after the inductance of the test leads is subtracted). Since there are 100 interconnects 

in series, the per unit resistance and inductance is calculated as follows:

• Per unit DC resistance = 0.9 Ω/100 = 0.009 Ω

• Per unit inductance = 591 nH/100 = 5.91 nH.
17

4. STRESS ANALYSIS

A simplified finite element model of an MCS and a column was created (see fig. 17) and 

a stress analysis was performed to illustrate the stress on the two interconnects for given deflections 

(see fig. 18).

(a)

(b)

Figure 17. Simplified FEA model—(a) MCS and (b) column.
18

0 

20,000 

40,000 

60,000 

80,000 

100,000 

120,000 

140,000 

160,000 

180,000 

200,000 

Solder Stress (psi)

 0.0025 0.005 0.0075 0.010 0.0125 0.015

Deflection (in)

Column (psi)

Spring (psi)

Figure 18. Solder stress—MCS versus column.

A preliminary assessment was conducted to determine the Von Mises stress in the solder 

joints associated with the MCS versus column configuration. This was a static analysis conducted 

using the finite element analysis software ANSYS Workbench. For the analysis, a deflection was 

assigned beginning with 0.0025 in and progressing up to 0.015 in in 0.0025-in increments. The 

maximum stress levels determined from this deflection are shown in table 6 regarding the solder 

joints for both the MCS and column configurations. These results are theoretical and have not 

been verified by testing.

Table 6. Maximum stress levels.

Deflection 

(in)

MCS 

(psi)

Column 

(psi)

0.0025

0.0050

0.0075

0.0100

0.0125

0.0150

5,455

10,915

16,282

21,499

26,571

31,525

22,248

44,656

67,477

90,962

115,360

140,915
19

5. SUMMARY

Thermal cycling and vibration testing have shown MCS interconnects to be significantly 

more reliable than solder columns. Also, MCS interconnects are less prone to handling damage 

than solder columns. While attachment of MCS parts to PCBs is more challenging than columns, 

they do not require shearing to obtain acceptable coplanarity.

Initial thermal cycling yielded better results than subsequent testing. The samples tested 

since the initial MCS reported herein were fabricated using a process that resulted in higher yields 

but lower reliability. The current process is very similar to the initial process except for addition of 

a deoxidizing step. 

MCSs are expected to outperform columns in a mechanical shock environment based on 

deflection analysis and preliminary shock testing. 

With further refinement of the attach process, it is anticipated that CGAs populated with 

MCSs can achieve >5,000 thermal cycles prior to first failure without the need for mechanical 

corner support. 

MCSs can be manufactured with overall diameter as small as 0.006 in using conventional 

microcoiling technology. It is possible to use MCSs on column grid arrays down to 0.65-mm pitch.

An important item to note is that the MCS design has not been optimized for reliablility; 

only the part/PCB fabrication parameters have been varied to improve second attach. With a change 

in MCS parameters (i.e., number of active/closed coils, overall length), reliability could be further 

improved. Of course, physical changes to the MCS must be traded with changes in inductance, 

capacitance, crosstalk, and insertion loss. 
20

6. FUTURE WORK

Testing of process improvements to remove oxidation and to improve the MCS to ceramic/

PCB fillets is ongoing. Fillets that result from depositing solder with a 0.006-in-thick stencil are 

shown in figure 19. The fillets are much improved, however, the open coils of the MCS are more 

restrained. 

Figure 19. MCS fillet with 6-mil stencil.

An extensive mechanical shock test program is underway and is currently in the fabrication 

phase. Testing will be conducted by AU and will include MCSs and 95/5 columns. Completion 

of this study is needed to provide confidence for MCSs use in applications with high g liftoff 

environments.

Implementation of MCSs in a digital signal processor board has begun and is currently in 

the procurement phase. After fabrication completion, functional testing of the design will be conducted to judge the effects on circuit function and rise time in particular. 

Testing of the current MCS design on a higher input/output (I/O) daisy-chained ceramic 

substrate is planned within the next year. The substrate will consist of 1,152 I/O. Also, design of a 

smaller diameter MCS is in process for testing on a daisy-chained ceramic substrate with 0.80-mm 

pitch.

A cast Sn10/Pb90 column (0.020 in diameter) is being added to the test program this year. 
21

REFERENCES

1. Kuang, R.; and Zhao, L.: “Thermal Cycling Test Report for Ceramic Column Grid Array 

Packages—CCGA,” Actel White Paper, 26 pp., Publication Year Unknown.

2. “Solder Column Qualification for Ceramic Column Grid Array (CCGA) White Paper,” 

Aeroflex, 2008.

3. Lau, J.: “Reliability Testing and Data Analysis of an 1657CCGA (Ceramic Column Grid Array) 

Package with Lead-Free Solder Paste on Lead-Free PCBs (Printed Circuit Boards),” Proceedings, 54th IEEE Electronic Components and Technology Conference, Vol. 1, pp. 718–725, 

10.1109/ECTC.2004.1319417, 2004.

4. Perkins, A.; and Sitaraman, S.: “Vibration-Induced Solder Joint Failure of a Ceramic Column 

Grid Array (CCGA) Package,” Proceedings, 54th IEEE Electronic Components and Technology 

Conference, Vol. 2, pp. 1271–1278, 10.1109/ECTC.2004.1320277, 2004.

5. Ghaffarian, R.: “Thermal Cycle Reliability and Failure Mechanisms of CCGA and PBGA 

Assemblies With and Without Corner Staking,” IEEE Transactions on Components and Packaging Technologies, Vol. 32, Issue 2, pp. 285–296, 10.1109/TCAPT.2008.921626, 2008. 
22

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Standard Form 298 (Rev. 8-98)

Prescribed by ANSI Std. Z39-18

Microcoil Spring Interconnects for Ceramic Grid Array 

Integrated Circuits

S.M. Strickland, J.D. Hester, A.K. Gowan, R.K. Montgomery, 

D.L. Geist, J.F. Blanche,* G.D. McGuire,* and T.S. Nash**

George C. Marshall Space Flight Center

Marshall Space Flight Center, AL 35812

National Aeronautics and Space Administration

Washington, DC 20546–0001

Unclassified-Unlimited

Subject Category 19

Availability: NASA CASI (443–757–5802)

Prepared by the Space Systems Department, Engineering Directorate

 *Jacobs Engineering, Science and Technical Services Group

**Jacobs Engineering, Science and Technical Services Group/Integrated Concepts and Research Corp.

M–1312

Technical Memorandum

NASA/TM—2011–216463

electrical interconnects, ceramic grid array packages, microcoil springs, solder columns, electronic packaging, 

electronic fabrication

01–03–2011

UU 28

NASA

U U U

As integrated circuit miniaturization trends continue, they drive the need for smaller higher input/output (I/O) packages. Hermetically sealed 

ceramic area array parts are the package of choice by the space community for high reliability space flight electronic hardware. Unfortunately, the 

coefficient of thermal expansion mismatch between the ceramic area array package and the epoxy glass printed wiring board limits the life of the 

interconnecting solder joint. This work presents the results of an investigation by Marshall Space Flight Center into a method to increase the life 

of this second level interconnection by the use of compliant microcoil springs. The design of the spring and its attachment process are presented 

along with thermal cycling results of microcoil springs (MCS) compared with state-of-the-art ball and column interconnections. Vibration testing 

has been conducted on MCS and high lead column parts. Radio frequency simulation and measurements have been made and the MCS has been 

modeled and a stress analysis performed. Thermal cycling and vibration testing have shown MCS interconnects to be significantly more reliable 

than solder columns. Also, MCS interconnects are less prone to handling damage than solder columns. Future work that includes shock testing, 

incorporation into a digital signal processor board, and process evaluation of expansion from a 400 I/O device to a device with over 1,100 I/O is 

identified.

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