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SOLDER PRODUCTS VALUE COUNCIL
ASSOCIATION CONNECTINGELECTRONICS INDUSTRIES ®
The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with
Alloys of Tin, Silver and Copper
A Research Report by the Lead Free Technical Subcommittee
IPC SOLDER PRODUCTS VALUE COUNCIL
The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
TABLE OF CONTENTS
Mission Statement. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .i
Solder Products Value Council Members. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .i
Introduction and Statement of Problem . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1
Review of Test Program. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2
Executive Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5
Void Data Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6
Statistical Analysis of Void and Failure Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Metallographic Cross Section Results . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10
Summary and Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11
Appendix A: Table of Voids and Failure Data. . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12
Appendix B: Statistical Analysis of Voids and Failure Data . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13
i The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
IPC Solder Products Value Council
Mission StatementIn support of IPC’s Mission Statement, IPC solder manufacturers recognize that the PCB and electronics assembly indus-
tries, comprised of the entire supply chain, must grow profi tably. The IPC Solder Products Value Council (SPVC) Steering
Committee’s objective is to identify and execute programs designed to enhance the competitive position of solder manu-
facturers and their customers.
Acknowledgement
It is estimated that nearly $1 million was spent to conduct the round robin lead free testing program from which the data
discussed in this paper was obtained. Each and all members of the IPC Solder Products Value Council contributed not only
funds but also a signifi cant amount of staff time in support of this program. However, like any program of this magnitude,
the following companies and individuals have contributed to the program’s success. The Council wishes to thank George
Wenger and Pat Solan, Andrew Corporation; Engent AAT; Jasbir Bath, Solectron Corporation; Dongkai Shangguan,
Flextronics International; Hallmark Circuits; Jean-Paul Clech; and Dean May, Crane Division-Naval Surface Warfare
Center.
IPC Solder Products Value Council Members
IPC Solder Product Value Council
Lead Free Subcommittee Members
AIM Inc. Henkel Technologies Nihon Superior Company Ltd.
Amtech, Inc. Heraeus, Inc. P. Kay Metal Supply Inc.
Avantec Indium Corporation Qualitek International Inc.
Cookson Electronics Kester Senju Metal Industry
Assembly Material Division Koki Company Ltd. Shenmao Technology Inc.
EFD Inc. Metallic Resources Inc. Thai Solder Industry Corp
Harimatec
Karl Seelig, AIM, Subcommittee Chairman Brian Deram, Kester
Greg Munie, Kester, White Paper Editor Masayuki Nakajima, Koki Company, Ltd.
William Gesick, Advanced Metals Nimal Liyanage, Metallic Resources Inc.
Technology Keith Sweatman, Nihon Superior Co. Ltd.
Patrice Rollet, Avantec Larry Kay, P. Kay Metal Supply Inc.
Paul Lotosky, Cookson Electronics Tippy Wicker, Qualitek
John A. Vivari, EFD, Inc. Hiro Suzuki, Senju/Mitsui Comtek Corp.
Katsuji Takasu, Harimatec Mark Young, Shenmao Technology Inc.
Douglass Dixon, Henkel Loctite Somchai Vorasurayakamt, Thai Solder Industry Corporation Ltd.
Brian Bauer, Heraeus Inc. James Slattery, Indium Corporation
1 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
Introduction and Statement of Problem
Due to marketing and legislative pressures in Asia and
Europe, the electronics industry is moving to the adoption
of lead free solders. These lead free materials are con-
sidered by some to be environmentally preferable to the
current lead containing solders that dominate elec tronics
manufacturing.
Although the issue as to whether lead free solders are
indeed environmentally preferred compared to lead con-
taining solders is still under debate, market and legislative
actions are forcing a change in materials used in electron-
ics assembly.
Accordingly, solder material suppliers are being asked to
provide the electronics industry with solders that are lead
free (per the accepted technical defi nition of that term) and
yet still provide all the needed properties – including ease
of assembly and reliability – the electron ics industry has
come to expect from lead containing solders.
At present, there are a large number of materials that have
been proposed as replacements for Tin/Lead (SnPb) solder.
Primary among these are the Tin/Silver/Copper (SAC)
alloys.
There are several variations of the SAC alloys that have
been suggested as the preferred replacements for SnPb
solders. Two are of special interest: the Japan (JEITA)
adopted alloy of 96.5% Tin, 3.0% Silver and 0.5% Copper
and the North American Electronics Manufacturing
Initiative (NEMI) alloy of 95.5% Tin (Sn), 3.9% Silver
(Ag), and 0.6% Copper (Cu).
Both of these alloys have undergone signifi cant testing.
And both sponsors believe that their particular choice is
the best candidate for replacement of SnPb solders.
The SPVC (Solder Products Value Council) is an industry
council comprised of 23 solder manufactures from around
the world that is addressing issues related to solder
assembly. The IPC Solder Products Value Council (SPVC)
members are technically capable of providing any alloy
requested by their customers.
However, the IPC SPVC, as producers of solder alloys,
believes it is in the best interests of the industry, from the
standpoint of product consistency, quality and the conser-
vation of natural resources to achieve a consensus on a
standard lead free alloy for the electronics industry. To that
end, the SPVC recently fi nished a 36 month, million dollar
study of SAC alloys that included contributions of several
organizations, including Engent Labs, NSWC Crane,
Andrew Corporation, Flextronics, and Solectron. This
study, which was completed in June, 2005, was designed
to fi nd a globally available, default lead free alloy. The
fi ndings of this study determined that SAC305 is the
default alloy.
Along with the performance of the SAC alloys, the study
collected a signifi cant amount of data on solder joint voids
for the alloys. This data shows that process voids found
and thermal fatigue failures seen in testing do not show a
statistically signifi cant dependence for the test vehicles and
alloys examined.
The electronics assembly industry generally considers
voiding in BGAs as a potential defect in manufacturing.
In doing so, the industry has adopted a maximum voiding
specifi cation of 25% of the ball X-ray image area. This is a
debatable point since examination of the void is subject to
energy levels of the X-ray, as well as beam angle.
Extremes in energy levels can result in either a false pass
or a false fail. Additionally, each manufacturer of X-ray in-
spection equipment gives a variety of ranges for the energy
levels used during inspection. Unlike other test criteria
where the pass/fail limits are specifi ed by the particular
piece of equipment, X-ray inspection is not specifi ed. The
type of voids is not specifi ed, e.g. interfacial voids or voids
in the bulk of the solder. However, in spite of this, most
companies engaged in electronics assembly have adopted a
void specifi cation.
In general, voiding seems to have more of an impact
on handheld devices where high G-forces resistance is
required. As a matter of fact, several papers have been
written over the past few years that support the theory that
voiding does not impact reliability.
With advent of lead free solder, the voiding specifi ca-
tion of 25% has been carried over to be used for lead free
assemblies, as well. Lead free solder joints are known
to void more than tin-lead solders, and SAC alloys void
higher than other lead free alloys.
2 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
During the SPVC’s analysis of thermal cycling and
thermal shock failures with SAC alloys, void-to-failure
ratio was studied. The results are in agreement with
previous studies on tin-lead assemblies that demonstrated
that there is no relationship between voids occuring in the
bulk of the solder and thermal stress failures.
The following paper presents data collected from the IPC
SPVC study that supports the claim that voids in the bulk
of the solder do not signifi cantly impact BGA failure. It
also demonstrates that failures occur at the package side of
the ball bond pad, away from the solder joint. This is due
to the die placement and CTE changes that occur across
the package due to die layout. As no interfacial, “cham-
pagne” or Kirkendall voids were observed, this paper
makes no inferences about the effects of those types of
voids on solder joint reliability.
Review of Test Program
Methodology
In order to determine what material is best suited to be the
standard alloy, the IPC SPVC members reviewed the most
likely candidates in the current list of contenders and care-
fully considered:
• What alloys are presently, through general ac-ceptance, most likely to be used as SnPb solder replacements?
• What tests are applicable to make an accurate deter-mination of the differences (if any) in the properties of the most likely candidates?
Alloy Choice
As was previously stated, the majority of potential
“standard” replacement alloys are composed of Tin, Silver,
and Copper with Silver varying between 3 and 4% and Tin
varying between 95.5 and 96.5%. Prior to SPVC testing,
the “front runners” were (in % of Tin/Silver/Copper) the
96.5/3.0/0.5 (JEITA) and 95.5/3.9/0.6 (NEMI) alloys. To
cover that composition range represented by these alloys,
the alloys chosen for testing by the IPC SPVC were:
• 96.5/3.0/0.5 Tin/Silver/Copper (Referred to as Alloy C in this report)
• 95.5/3.8/0.7 Tin/Silver/Copper (Referred to as Alloy B in this report)
• 95.5/4.0/0.5 Tin/Silver/Copper (Referred to as Alloy A in this report.)
Elements of Testing Program
To answer these questions, the IPC SPVC completed a
three year round robin testing program. The elements of
the program were:
1. Assembly Performance (initial screening) by council members of SAC alloys to compare basic alloy properties.
2. Dow n-select testing by Engent of SAC alloys provided by six solder manufacturers before assembly.
3. Assembly of Flextronics and Solectron test PCBs using down-selected SAC alloys. An industry standard eutectic SnPb solder was used as a control.
4. Base Line Metallographic Analysis of completed assemblies by Andrew Corporation
5. Thermal Shock and Thermal cycling conducted by NSWC Crane
As noted above, this Research Paper is not intended to
summarize all phases of the test program. The summaries
of the fi rst and second phases of the work, comparison of
alloy properties and the comparison of assembly results,
as well as the complete overview with all data on thermal
testing and metallographic analysis has already been
presented else where and is now available from IPC. A
complete summary, including all data collected, was pre-
sented in a comprehensive third white paper also available
form IPC. The intent of this work is to discuss observa-
tions made during the testing and metallographic analyses
on the impact of voiding on solder joint integrity. As such
a description of the phases of the testing program and an
executive summary of the conclusions of this part of the
study on voiding in solder interconnections are presented
below.
Board Assembly
In support of the SPVC study, both Solectron and
Flextronics populated 40 boards with each of the three
SAC alloys and their incumbent Sn63/Pb37. Both com-
panies assembled their own test vehicles using process
parameters established by the SPVC Technical Lead Free
Subcommittee. These two test vehicles are shown in
Figures 1 and 2.
3 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
Of primary concern was the refl ow profi le. A single
common time-temperature profi le was used for the three
chosen SAC alloys. Target time-temperature values where
chosen to ensure best possible wetting given the alloys
under testing. It is important to note, that to preserve ano-
nymity of the solder pastes used, there was no optimiza-
tion of the profi le. As a result, it is likely that an optimized
profi le could reduce the level of voiding.
• O2 ppm level of 1000 or less
• Ramp Rate - 0.5˚ to 1.5˚C per second is optimal. The assembly compa nies agree the target would be 1˚ - 2˚C.
• Peak Temperature – 235˚ to 245˚C is recommended. Peak tem peratures may range from 230˚ to 265˚C.
• Time Above Liquidus – 45 to 75 seconds is recom-mended. Time above liquidus may range from 30 to 90 seconds.
• Total Profi le Length – Time from ambient to peak temperature should be 3 to 4 minutes.
At Solectron, the production refl ow oven used had 10
heating zones, with forced convec tion and a Nitrogen
(<100 ppm O2) rich atmosphere. The lead free refl ow
profi le was in the ranges specifi ed. For the lead free
SnAgCu solder pastes at the largest QFP 256 component
on the board, solder joint peak temperature was 241°C
and for one of the smallest components on the board (lead
free 0.5mm CSP) solder joint peak temperature was 247°C
with time over 217°C of 75 to 82 seconds.
A 2D-Xray system was used for initial X-ray inspection
after assembly. Voiding greater than 25% of the area was
observed with all three lead free solder paste assembled
alloy boards specifi cally for the 0.5mm CSP lead free
components. For the tin-lead assembled boards, there was
evidence of some voiding but much less than 25% void
area on the 0.5mm tin-lead assembled CSP components.
It should be noted that the SnPb solder paste used by both
companies was their standard production solder paste
and the time-temperature profi le used for SnPb assembly
was their standard refl ow profi le optimized for their SnPb
solder paste. No rework was performed on the devices
with voids, per agreement with the IPC SPVC technical
committee members.
At Flextronics, the refl ow oven used had 9 heating zones
with a nitrogen (<1000 ppm O2) rich atmosphere. The
refl ow peak temperature was 240˚C-248˚C for the lead
free solder and 217˚C-222˚C for the eutectic Sn-Pb solder.
It also should be noted that the SnPb solder paste used
by Flextronics was their standard production solder paste
and the time-temperature profi le used for SnPb assembly
was their standard refl ow profi le optimized for their SnPb
solder paste.
An X-ray system was used for micro-focus, real-time, non-
destructive inspection of the sol der joints. Voiding greater
than 25% was observed with all three lead free alloys.
Devices PBGA196, C-CSP224, and CSP8 all showed
voids greater than 25% on almost all packages, but no
voids were discovered on the LCC24 and BCC24. X-ray
inspection on the eutectic Sn/Pb solder boards showed
Figure 1: Solectron Test Vehicle
Figure 2: Flextronics Test Vehicle
4 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
no voids. No rework was performed on the devices with
voids, per agreement with the IPC SPVC technical com-
mittee members.
Test Methodology
The test regime consisted of conventional industry
accepted thermal cycle and thermal shock exposures.
Environmental exposures were conducted on both sets
of test boards with functional monitoring during the
exposure. The test vehicle sets included assemblies from
both Solectron and Flextronics. Each company provided
four (4) groups of forty (40) test panels representing the
three SAC solder alloy compositions as well as a baseline
eutectic (tin/lead) solder composition. One board from
each set was used for destructive metallographic analysis
and not included in the thermal cycling study.
The specifi cs relating to the thermal test events are
outlined in the following paragraphs along with specifi cs
on the test equipment, test profi les, test confi gurations,
functional monitoring, and test schedule.
Test Equipment
The thermal cycling equipment incorporates the use of
a BEMCO FW100 thermal chamber. This chamber is
capable of cycling, when empty, from 0°C to 100°C in ap-
proximately 10 minutes. Similarly, this chamber can cycle,
when empty, from -55°C to 125°C in 20 minutes.
The Thermotron, model ATS–320–H–15–15, is the thermal
shock chamber. This chamber provides for temperature
cycling from -55°C to 125°C through physical movement
of the test article within the chambers in less than one
minute. The temperature stabilization would take longer
and would be a function of the thermal mass of the test
article. Typical stabilization times for these temperature
ranges were 5 minutes.
Test Approach
Due to the sizing of the equipment, the payload of the
thermal shock equipment was maximized with a combi-
nation of Flextronics and Solectron test vehicles. The re-
maining balance of test vehicles was allocated for thermal
cycling.
The initial estimate for this distribution consisted of ap-
proximately 24 of each vendors test vehicle subjected to
thermal shock with the remaining balance of 132 of each
vendor test vehicle subjected to thermal cycling. This
approach allowed for 6 of each solder composition to be
exposed to thermal shock with the balance subjected to
thermal cycling. The size of the BEMCO chamber accom-
modated this large number of assemblies.
The thermal cycle profi le refl ects the IPC test regimen
and consists of a low temperature soak (0°C) for ten (10)
minutes with a temperature increase ramp up to 100°C
with a high temperature soak of ten (10) minutes prior to
a ramp down to the low temperature. The total cycle is
typically takes around sixty (60) minutes. The cycle time
is a function of the chamber time to temperature and the
related temperature stabilization of the test article.
The thermal shock test profi le is very similar to the JEDEC
prescribed exposure. It consisted of a low temperature
(-55°C) soak for fi ve (5) minutes, followed by a transition
to the high temp (125°C) with a high temperature soak
for fi ve (5) minutes, with a fi nal transition back to the
low temperature. This cycle would was repeated continu-
ously. The total cycle time was approximately twenty (20)
minutes.
Functional monitoring was provided using Fluke NetDaq
Model 2640A data acquisition units. The test provided 2-
wire resistance monitoring for 700 signals based on thirty-
fi ve (35) NetDaq units with twenty (20) channels each.
In addition to the functional monitoring, metallographic
analysis at every 500 thermal cycles was done at Andrew
Corporation on representative samples from the two sets
of tests vehicles. When comparing the results of X-ray
analysis for voids, failures in thermal cycling and thermal
shock and the metallographic examination of both failed
and functional solder joints after thermal exposure, it is
obvious that voids had little or no infl uence on solder joint
integrity. Follow up statistical analysis, presented here,
confi rms that belief.
5 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
As was previously mentioned, the IPC Solder Products
Value Council Lead Free Technical Subcommittee chose,
because of its widespread use, the tin, silver, copper family
of lead free alloys. The council assumed that although the
high content silver alloys (3.8 % silver or greater) were
being promoted as an alloy of choice, it appeared that
the lower silver (96.5/3.0/0.5 SnAgCu commonly called
SAC 305) lead free alloy would perform equally as well at
lower cost.
Standard tin-lead near-eutectic solder (SnPb) solder, as
a part of this study, was used as a control. However, the
members of the technical committee did not intend for
the test program to be a head to head comparison between
lead free and SnPb solder but an analysis of the SAC alloy
family.
The committee then, working with the appropriate
company or organization, chose the testing protocol and
reviewed each step of the testing program. The results of
each phase of the six-phase test program can be summa-
rized as follows:
Assembly performance screening to compare alloys: No
statistically signifi cant difference was found in alloy
performance when data from participating locations was
compared. Experimentally the alloy properties of melt
temperature (DSC), time to reach zero and maximum force
in wetting balance testing and solder spread as determined
by area and diameter were found to not be statistically dif-
ferent. In some cases a specifi c location found differences
but when the data was averaged between locations for the
same alloy no statistical difference could be found.
Down selection of the solder pastes for assembly: No dif-
ference was found between alloys for the pastes tested for
assembly performance.
Assembly of test vehicles using SAC alloys with SnPb
eutectic solder as a control: No difference was found in
process ability or defect rate between the alloys as as-
sembled at two separate test locations using two separate
test vehicles. Although the materials’ performance was
distinguishable from SnPb eutectic solder there was no dif-
ference between the lead free SAC alloys studied.
Baseline metallographic analysis of the assembled test
vehicles: No metallurgical difference was found between
the SAC alloy solder joints after assembly and before
thermal cycling.
Thermal cycling testing: All three SAC alloys showed
similar failure rates for similar packages. These rates
were distinguishable from the behavior of SnPb solder but
were not distinguishable from one another. When the data
collected in this study was compared to data collected in
previous studies using the NEMI 95.5/3.9/0.6 SAC alloy
the results of the different studies were not distinguishable
by alloy.
Metallographic analysis as a function of thermal cycling:
Metallographic analysis was performed at every 500
thermal cycles. Results showed there was no signifi cant
difference between SAC alloy structures with thermal
cycling.
Statistical analysis of the relationship between voids and
solder interconnection reliability: A key by-product of
this testing program was the data gathered on the much
debated issue of solder joint voiding. Based on comparison
of number and size of solder joint voids to thermal cycle
interconnection failure data collected in this study, there
is no evidence that the type of process-related solder joint
voiding that was observed in the SAC alloy solder joints
has any signifi cant impact on solder joint reliability.
The results of the testing done over all phases of this study
indicate that SAC 305 (Sn96.5/Ag3.0/Cu0.5) should be the
default alloy for use in SAC lead free applications involv-
ing refl ow assembly. The presence of process related
voids in the interconnections formed using the SAC
alloys has been found to have no statistically signifi cant
effect on solder interconnection reliability as tested by
accepted thermal cycling methods.
Executive Summary Section
6 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
Void Data Summary
Overview
The data presented here represents the compilation of
electrical failures, transmission x-ray imaging and metal-
lographic analysis of the test vehicles described above.
The test regimen lasted over 6000 thermal cycles (as per
IPC-9701 test conditions.)
Voiding was noted before the start of the thermal testing in
both the Flextronics and Solectron assemblies. The amount
of voiding in the SAC alloys was considerably greater than
in the near-eutectic SnPb solder joints.
In the Solectron assemblies voiding greater than 25% of
the area was observed with all three lead free SAC alloy
solder paste assembled boards and specifi cally for the
0.5mm pitch CSP84 lead free components on the boards.
For the SnPb assembled boards, there was evidence of
voiding but much less than 25% void area on the 0.5mm
pitch SnPb assembled CSP84 components.
On the Flextronics assembly, voiding greater than 25%
was also observed with all three lead free SAC alloys.
Devices PBGA196, C-CSP224, and wafer-level CSP8 on
the assembled boards all showed voids greater than 25%
on almost all packages.
For both sets of assemblies no rework was performed
on the devices with voids, per agreement with the IPC
SPVC technical committee members. All void locations,
along with other defects that were repaired, were noted
with a red inspection arrow at the component location. It
was hoped that thermal shock and thermal cycle testing
would provide data on the correlation (if any) between
the location and magnitude of the voids and attachment
reliability.
Shown in Figures 1-4 some typical examples of voids
detected on assemblies not yet subjected to thermal
cycling, i.e. the baseline metallographic analysis.
Transmission X-ray images and photomicrographs of
solder joints from the Flextronics solder test vehicle are
shown in Figures 1, 2, and 3. These fi gures along with
many others are from the fi nal SPVC white paper (released
separately and available from IPC). There is also less
solder joint voiding in the SnPb solder joints than there
is in the SAC alloy solder joints. An example of voids
detected during the cross sectioning process for SAC405
alloy A is shown in Figures 3. Note that these images were
made prior to any thermal cycling.
Transmission x-ray imaging was performed on each
component on every board that was removed each 500
cycles. Although the number of x-ray images is too large
to incorporate in a report, the images did reveal that solder
joint voiding was more extensive in the SAC alloy solder
joints than in the SnPb solder joints. In particular the
solder joint voiding, both in number as well as size, in
the CSP84 package solder joints was considerably more
extensive than the other area array packages. Comparison
of the voiding with cross sections of temperature-cycled
packages did not show any obvious correlation of voiding
to interconnect failure.
For example, the cross-sectioned SAC305 solder joints of
the Solectron Board C11, U313 CSP84 package presented
in Figure 4 shows very large voids but no indication that
these voids are contributing to interconnection failure even
though this package was subjected to 4500 temperature
cycles.
The fi nal SPVC white paper confi rms that there were
enough temperature cycle induced creep-fatigue solder
joint failures of the 0.8mm pitch 84 I/O CSP packages
on the Solectron Pb-Free to obtain 2-parameter Weibull
slope (Beta) and characteristic life (Eta) values. A Weibull
plot of the failure distributions for the CSP84 packages is
presented in Figure 5.
The Weibull distributions show that the SAC alloy solder
joints had a longer characteristic life than the SnPb solder
joints (4713 to 6810 cycles for SAC alloy compared to
1595 cycles for SnPb). However, the transmission x-ray
images and cross sections that were done on the non-
monitored boards every 500 cycles showed considerably
more and larger voids in the SAC alloy solder joints than
the SnPb solder joints. Because of this it was decided to
x-ray each and every CSP84 package upon completion
of the 6000 temperature cycles and attempt to correlate
voiding with cycles to failure. It needs to be emphasized
that although the voiding in the SAC alloy solder joints
was greater than the SnPb solder joints, the voiding is
believed to be due to the use of a non-optimized assembly
process for the SAC alloy boards. As indicated earlier, the
7 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
IPC SPVC defi ned a “common refl ow profi le” to be used
for the SAC alloy assembly. The SnPb solder paste used
by Flextronics and Solectron was the standard production
solder paste used at each company and the refl ow profi le
used was their standard optimized production profi le and
not the IPC SPVC “common refl ow profi le”. To validate
that the voids in the SAC alloy solder joints were due to
the use of a non-optimized assembly process, Solectron
provided boards that they assembled using SAC396 with
an optimized assembly process. The transmission x-ray
images presented in Figure 6 and 7 compare the solder
joint voiding in CSP84 packages assembled using SAC396
with an optimized assembly process voiding to CSP84
packages assembled using SAC387 with a non-optimized
assembly process.
Figure 1: Transmission X-Ray comparison of U1 PBGA196 showing voids bigger with Pb-free sac soldering
Figure 2: Transmission X-Ray comparison of U43 Wafer-Level CSP8 showing voids bigger with Pb-free sac soldering
Figure 3: X-Ray and cross section of Flextronics Board A9 U2 SAC405 assembled C-CSP224
Figure 4: X-Ray and cross section of Solectron Board C11 SAC305 assembled U313 CSP84 Row 12
Figure 5: Weibull 2-P Distributions for the CSP84 packages on the Solectron Boards
8 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
The x-rays in Figure 6 and 7 clearly show larger
voids in the SAC alloy solder joints that were
made using a non-optimized assembly process.
The other interesting point to note in these fi gures
is the Weibull Beta and Eta values. These sta-
tistics are based on the 24 CSP84 packages that
were part of the IPC SPVC reliability test and
the 60 CSP84 packages that were part of the
Solectron reliability test. Although there are large
voids in the SAC387 solder joints assembled
using the “common refl ow process”, there is no
statistically signifi cant difference in the character-
istic life of the solder joints. In fact, the average
value of the characteristic life of the solder joint
with large voids is greater.
To better quantify the solder joint voiding, each of
the CSP84 package x-ray images was magnifi ed
and the number of solder balls with voids for each
package was counted. The number of solder joints
with voids greater than 25% of the PCB pad area
was also counted. The magnifi ed images present-
ed in Figure 8 shows the voids were counted. The
red numbers indicate the solder joints with voids
greater than 25%. It is interesting to note that
although there are approximately similar numbers
of solder joints with voids in both fi gures, only
the solder joints made using the non-optimized
assembly process have voids greater than 25%.
Although the CSP84 packages were monitored
during temperature cycling and the number of
cycles to failure for each package is known, it was
impossible to remove each package at the moment
it failed and cross section the solder joints to
determine which solder joints failed fi rst.
However, if there is an infl uence of voids on
failure mode this effect should be detectable
graphically. For example, plots of the distribu-
tion of voids > 25% for failing and non-failing
packages at 6000 cycles should be distinctly
different.
Figure 6: X-Ray comparison showing process effect on solder joint voids
Figure 7: Another X-Ray comparison showing process effect on solder joint voids
Figure 8: Magnifi ed X-Ray comparison showing process effect on solder joint voids
9 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
A comparison of the distributions for failed and non-failed
packages is presented in Figure 9.
If the infl uence of voids was a negative one on intercon-
nect reliability the two sets of distributions would diverge,
i.e. fewer and smaller voids would be tracked by the red
triangle distribution (across the bottom of the graph) while
the blue circle distribution of interconnections with more
and larger voids would climb steeply from left to right.
However, both sets of distributions track each other within
the expected scatter of such a plot. This would tend to
imply that in this test there is no obvious effect of voids on
interconnection reliability.
To present the voiding data in a slightly
different manner Figure 10 is a summary
of the voiding data showing the relation
between size and number of voids in the
interconnection and interconnection cycles
to failure
Two data sets are shown: interconnec-
tions with voids greater than 25% area and
interconnections with total voids regardless
of size. As above, if voids had a signifi cant
effect the >25% voids would be expected
to cluster in failures early on. However,
both sets of data maintain a common
scatter, typical of scatter in thermal
cycling failures, across the entire span
of the testing. Note the early failures at
the bottom left hand area of the plot. The
black circles represent the SnPb assembled
CSP84 package failures. Although these
failed earliest in the testing they had essen-
tially no large voids! The total number of
voids in the SAC alloy assembled CSP84
packages denoted by the blue triangles in
Figure 10 are fewer than the total number
of voids in the SnPb assembled packages.
The SAC alloy packages’ cycles to failure,
however, were considerably greater.
Note that if voids impacted the attachment
reliability of the packages then the occur-
rence of large voids/many voids would
result in a high failure rate at a low number
of cycles. However, the plot of size and oc-
currence versus cycles to failure is essen-
tially fl at within the scatter of the data for the three SAC
alloys and SnPb. This implies that voids have, for this set
of test conditions, no impact on attachment reliability.
Neither the size nor the number of voids in a solder joint
as observed in this study appears to have any effect on at-
tachment reliability. As noted previously in this paper, the
study does not discount the possibility that other types of
voids not observed in the study—Kirkendall voids— may
have an impact on solder joint reliability.
Figure 10: CSP84 voiding verses cycle to failure
Figure 9: Distributions for comparing failed and non-failed CSP84 packages show no void effect
10 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
Many of the monitored packages on the Pb-Free test
vehicles were cross sections after completing 6000 tem-
perature cycling. The cross section confi rmed the large
voids observed in the transmission x-rays made prior to
sectioning. The photomicrographs presented in Figure 20
shows a transmission x-ray and cross sectional comparison
of CSP84 SnPb and SAC396 solder joints. These packages
had been assembled using a refl ow process optimized for
their respective solder pastes. The photomicrographs are of
the solder joints at Row 10 that is immediately under the
edge of the die where the local CTE mismatch would be
greatest. Also presented in Figure 20 is the transmission
x-ray and cross section of SAC305 solder joints that were
made using the non-optimized IPC SPVC “common refl ow
profi le”. As can be seen there are large voids in many of
the SAC305 solder joints.
Magnifi ed image comparisons of the individual solder
joints for the three packages are presented in Figures 21a,
21b, and 21c. All of the Row 10 solder joints on the SnPb
package assembled with an optimized process have com-
pletely cracked at the interface to the CSP84 package after
the 6000 temperature cycles.
The original failure of this package occurred during
temperature cycling at 1413 cycles. Eight of the 10 (solder
joint M10 is not shown for any of the three packages) Row
10 solder joints on the SAC396 package assembled with
and optimized process have completely cracked at the
interface to the CSP84 package after the 6000 temperature
cycles. The original failure of this package occurred during
temperature cycling at 3075 cycles.
Although there are large voids in fi ve of the 10 Row 10
solder joints of the SAC305 package that was assembled
using the non-optimized IPC SPVC “common profi le”,
none of the Row 10 solder joints are completely cracked at
the interface to the CSP84 package after the 6000 tempera-
ture cycles. The original failure on this package occurred
during temperature cycling at 4196 cycles.
Figure 20: Typical Cross sections of CSP84 Packagea after 6000 Temperature Cycles
Figure 21a: CSP84 Solder Joints After 600 Cycles
Figure 21b: CSP84 Solder Joints After 600 Cycles
Figure 21c: CSP84 Solder Joints After 600 Cycles
Metallographic Cross Section Results
11 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
Statistical Analysis of Void and Failure DataListed in Appendix A is a table of voids and failure data.
This data was used in the statistical analysis of voids and
failures shown in Appendix B. In the table, packages
outlined in green did not fail during the 6000 temperature
cycle testing. Those packages that failed are highlighted in
pink.
Appendix B, Figures 11 through 19 features a graphi-
cal analysis of the void and failure data using the Minitab
Software package available from SBTI Inc. Nine different
methods of statistical analysis were used comparing failure
rates of packages with and without voids.
Data from the IPC Solder Products Value Council reli-
ability study on SAC alloys has been used in a comparison
of voids in SAC interconnections and thermal cycles to
failure. Nine separate methods of statistical analysis com-
paring cycles to failure looking at both voids greater than
25% of the interconnection area and total voids have been
done. Absolutely no correlation between voids and failures
under thermal cycling has been demonstrated.
Based on comparison of the number and size of solder
joint voids to interconnection failure in our thermal cycling
data there is no evidence that the type of solder joint
voiding observed in the SAC alloy solder joints has any
signifi cant impact on solder joint reliability.
Summary and Conclusions
12 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
Appendix A: Table of Voids and Failure Data
At 6,000 Cycles: Green—Did not fail; Pink—Failure
13 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
Figure 11 shows a boxplot of the cycles to failure for
all CSP84 package solder joints versus number of voids
greater than 25% of the area of interconnection.
Note that cycles to failure do not track the number of
voids. In addition it should be noted that assemblies with
Sn/Pb solder paste (red boxplot) failed at approximately
1200 cycles even though they had zero voids above 25%.
Figure 12 shows an analysis of mean cycles to failure for
all alloys.
Assemblies with Sn/Pb solder paste (red square outlier)
are statistically different than all the SAC alloys. However,
the SAC alloys show no correlation between mean cycles
to failure and number of voids greater than 25% of the
interconnection area.
Figure 13 is a main effects plot of cycles to failure. No
correlation is shown between the number of large voids
or the total number of total voids and the cycles to failure
except for alloy. While SAC alloys all perform well, SnPb
alloys fail earlier in cycling. (The points in the upper left
hand plot at the end of the line of cycles to failure versus
board ID.) However, there is no correlation between total
number of voids and failure – boards with less than 10
total voids failed before components with more then 39
voids.
Figure 14 is a boxplot of cycles to failure for all alloys.
Note that only SnPb stands out. No correlation between
voids and cycling is apparent for SAC alloys.
Figure 11: Voids Greater than 25% Area and Cycles to Failure
Figure 12: Analysis of Means of Cycles to Failure
Figure 13: Main Effects Plot
Figure 14: Cycles to Failure All Alloys
Appendix B: Statistical Analysis of Void and Failure Data
14 The Effect of Voiding in Solder Interconnections Formed from Lead Free Solder Pastes with Alloys of Tin, Silver and Copper
Figure 15 is a matrix plot of cycles to failure and number
of voids greater than 25% of the interconnection area.
There is no correlation between void size and cycle
failures by SAC alloy.
All the Sn/Pb assemblies did however fail with very few
cycles and with very few voids.
Figure 16 is a one-way analysis of mean cycles to failure
for all alloys.
There is a statistically signifi cant difference between alloys
and cycles to failure but only for the comparison of SnPb
to all SAC alloys, i.e. no void effect is noted for SAC
while SnPb shows a greater failure rate in the absence of
voids.
Figure 17 is a one-way analysis of means for all alloys
comparing voids greater than 25% area versus cycles to
failure.
There is no statically difference between SAC alloys the
number of voids greater than 25% and failures. The fi rst
data point, which shows a statistical difference, is Sn/Pb
solder paste.
Figure 18 is a main effects plot of cycles to failure for all
alloys. Note that SnPb is the only statistically signifi cant
stand out in effects. Voids have no effect on cycles to
failure.
SAC Alloy composition is not a main effect with regard to
cycles to failures.
Figure 19 shows a similar analysis to Figure 18. However,
here all voids, including those less than 25% are examined.
No statistical difference between SnPb and SAC alloys
concerning cycles to failure and total void count is found.
Figure 16: Mean Cycles to Failure All Alloys
Figure 17: One Way Analysis of Mean Cycles to Failure
Figure 18: One Way Analysis of Mean Cycles to Failure for All Alloys
Figure 19: Main Effects Plot of Cycles to Failure for All Alloys
Figure 15: Matrix Plot of Cycles to Failure for Voids Greater than 25% Area
Recommended