INTERCONNECTION RELIABILITY OF INTERPOSER AND
REBALLING OPTIONS FOR BALL GRID ARRAY
BACKWARD COMPATIBILITY
Richard Coyle1, Michael Meilunas 2, Richard Popowich1, Martin Anselm2,
Peter Read1, Mike Oswald3, and Debra Fleming1
1
Alcatel-Lucent
Murray Hill, NJ, USA
2
Universal Instruments, Inc.
Conklin, NY, USA
3
Interconnect Systems, Inc.
Camarillo, CA, USA
richard.coyle@alcatel-lucent.com; anselm@uic.com; mike.oswald@isipkg.com
ABSTRACT
High reliability electronic equipment producers that
continue to manufacture and support tin-lead (SnPb)
electronic products are faced with an increasing number of
availability issues with SnPb ball grid array (BGA)
components. These supply chain constraints are driving
development of backward compatible assembly solutions,
which requires integrating a Pb-free BGA component into a
printed circuit board assembly (PCBA) being manufactured
with SnPb solder processing. This paper addresses two
backward compatible assembly solutions: 1) SnPb assembly
using an interposer card to mount the Pb-free BGA and 2)
SnPb assembly using a SnPb component created by
reballing the original Pb-free component. The primary
objective of the study was to determine if the interposer or
reballing assembly processes degraded the board level
attachment reliability of a 676 I/O plastic ball grid array.
The intermetallic solder attachments and microstructures
were characterized before and after board level assembly
using optical metallography and scanning electron
microscopy. The reliability was characterized using a 0 to
100 C accelerated temperature cycle with 10 minute ramp
and 10 minute dwell times. The reliability of the backward
compatible assembly solutions is discussed relative to that
of the basic SnPb and Pb-free assembly processes.
supply chain that is rapidly converting to Pb-free offerings
and thus has a decreasing motivation to continue producing
SnPb product for these low-volume end users [2]. Until
recently these supply chain constraints were limited to high
volume, small ball grid array (BGA) packages components
such as memory devices. Now there is a growing concern
surrounding availability of larger SnPb BGA components.
There are product and performance constraints that can
preclude complete Pb-free conversion and consequently,
these component availability issues can force companies to
use Pb-free BGAs with the SnPb solder assembly process.
The requirement to integrate a Pb-free BGA into product
that is assembled with a typical SnPb surface mount process
is referred to as backward compatibility or back ward
compatible manufacturing. The ability to address backward
compatibility effectively enables a viable alternative
manufacturing path when complete product conversion to
Pb-free manufacturing is not possible.
There are three common options for addressing Pb-free
BGA backward compatibility: 1) assembly with mixed alloy
processing which requires soldering the Pb-free BGA
directly with a SnPb soldering process, 2) assembly using an
interposer card, and 3) basic SnPb assembly using a SnPb
component created by reballing the original Pb-free
component. Mixed alloy processing is the most prevalent
BGA backward compatibility production option despite the
fact that it is discouraged almost universally by equipment
producers, contract manufacturers, and component
suppliers. The technical issues associated with mixed alloy
processing have been addressed in a significant number of
studies [3-52] and those results have been reviewed and
discussed in detail in previous publications [3, 48, 49, 50,
51]. Mixed alloy assembly is not drop-in replacement
process and requires custom process development. Further,
there has been on ongoing debate over the solder joint
thermal fatigue reliability of BGA packages assembled with
mixed processing. As the need for backward compatible
assembly has grown, the interposer and reballing options
Key words: Pb-free solder, backward compatibility,
interposer assembly, reballing, thermal fatigue, accelerated
temperature cycling
INTRODUCTION
Most high reliability electronic equipment producers
continue to manufacture and support tin-lead (SnPb)
electronic products despite the increasing trend for design
and conversion to Pb-free manufacturing. These equipment
suppliers can continue to use SnPb solder assembly and
remain in compliance with the RoHS Directive (restriction
on certain hazardous substances) by invoking the European
Union Pb-in-solder exemption [1]. At the same time,
companies are facing increased pressure from a component
894
have emerged as potential alternatives for BGA backward
compatible assembly.
finish is high temperature organic solderability preservative
(OSP).
Interposer assembly allows the manufacturer to maintain the
continuity of SnPb assembly without altering the assembly
profile established for the product board. From the assembly
perspective, the BGA interposer subassembly is treated as a
distinct SnPb component. Although there are logistical
issues with interposer implementation, they can be
overcome with adequate planning. However, the interposer
subassembly introduces mechanical complexity and an
additional level of solder attachments, which creates a
concern for overall solder joint reliability. There appears to
be no solder joint reliability data in the literature for
interposers but there is at least one BGA connector study
that showed that increased mechanical complexity could
reduce the inherent fatigue life of a BGA [53].
There is one daisy chain net for each land pattern. Daisy
chain nets from each of the components patterns are brought
out to a card edge connector and soldered connections are
used to monitor the resistances of the daisy chain nets
during temperature cycling.
BGA reballing processes have been used by the defense and
aerospace industries in order to avoid Pb-free conversion
[54]. However, commercial equipment suppliers generally
have shunned BGA reballing due to the process
uncertainties related to the overall reliability of the device,
package, or solder interfaces and attachments. As a whole,
component suppliers are concerned about the impact on IC
device and package reliability caused by the additional
number of thermal excursions required to remove the Pbfree balls and attach new SnPb balls. There is a minimal
amount of data in the literature evaluating reballing [55-58]
and the most complete study did not include completed
results for attachment reliability of reballed BGA
components [55]. The shortage of characterization and test
data is indicative of the limited exposure of this process in
practice.
Figure 1: Top view of a populated printed circuit board.
The primary test vehicle contains SnPb, reballed SnPb, and
interposer components soldered with a SnPb reflow profile.
The components for the SAC305 test cell were assembled
on dedicated test boards using a Pb-free reflow profile.
Table 1: The package attributes for the 676 I/O PBGA.
Package Attribute
Value
Package Type
PBGA
I/O
676
Body Size
27 x 27 mm
(1.06 x 1.06 in)
Die Size
16.5 x 16.5 mm
(0.650 x 0.65 in)
Die Thickness
0.34 (13 mils)
Ball Pitch
1.0 mm (40 mils)
Ball Diameter
0.5 mm (20 mils)
Solder Ball Alloy
63Sn37Pb
Sn3.0Ag0.5Cu
Substrate Pad Design
Circular – Solder Mask
Defined (SMD)
Substrate Pad Solder Mask
0.45 mm (18 mils)
Opening Diameter
Substrate Pad Surface Finish Electrolytic Ni/Au
Substrate Type
BT, 2 metal layer
Substrate Thickness
0.61 mm (24 mils)
Package Thickness
1.73 mm (69 mils)
This paper presents results from attachment quality and
reliability evaluations of a 676 I/O PBGA assembled with
interposers and reballed components. Test boards were
assembled containing Pb-free components mounted on SnPb
interposer cards, Pb-free components reballed with SnPb
attachments, and virgin SnPb and Pb-free components as the
baseline control samples. The basic objectives were to
determine if the SnPb board level attachments were
degraded by the interposer or by the reballing assembly
processes or if Pb-free solder attachments were degraded by
using an interposer card assembly.
EXPERIMENTAL
BGA Test Vehicle
The top view of the populated printed circuit board test
vehicle is shown in Figure 1 and the attributes of the ball
grid array package are listed in Table 1.
Interposer and Reballed BGA Test Vehicles
A schematic representation of the interposer sub-assembly is
shown in Figure 2 and the interposer attributes are provided
in Table 2. The 2nd level attachments of the interposer card
are eutectic SnPb balls with 0.4 mm diameter (16 mils) nonsolder mask defined (NSMD) pads on the underside of the
card to enhance board level reliability. The Pb-free
(SAC305) BGA was assembled onto the top side of the
interposer card using a Pb-free solder paste and a Pb-free
The test board has 8 layers with dimensions 222 mm x 140
mm x 2.36 mm (8.75 in. x 5.5 in. x 0.093 in.) and contains
six identical BGA component footprints. The component
sites have non-solder mask defined (NSMD) land patterns
that are 0.35 mm (14 mils) in diameter and the board surface
895
reflow profile. In this study, the interposers were provided
in JEDEC trays to enable surface mount assembly.
The components and the test circuit boards were daisy
chained to allow electrical continuity testing after surface
mount assembly and in situ, continuous monitoring during
thermal cycling. The resistance of each loop was
independently monitored during the temperature cycle test.
All assembled circuit boards were thermally cycled from 0
C to 100 C with a 10 minute ramp between temperature
extremes. The dwell times were 10 minutes in accordance
with the IPC-9701A industry test guidance [59]. The solder
joints were monitored continuously during thermal cycling
using an event detector set at a resistance limit of 1000
ohms. A spike of 1000 ohms for 0.2 microseconds followed
by 9 additional events within 10% of the cycles to the initial
event was flagged as a failure. The failure data are reported
as characteristic life  (typically the number of cycles to
achieve 63.2% failure) and slope  from a two-parameter
Weibull analysis.
The Pb-free BGA ball removal (“deballing”) was
accomplished using a proprietary wave solder-type process.
The process was engineered specifically for the 676 I/O
BGA used in this study to minimize thermal exposure and
avoid damage to the solder ball pads on the package. The
SnPb balls were attached with a flux-only process in a
nitrogen environment using a SnPb soldering profile in
compliance with typical component manufacturer
specifications. Following the reballing, all packages were
subjected to 100% automated optical inspection (AOI) to
document and confirm coplanarity and flatness.
Microstructural Characterization and Failure Analysis
The intermetallic solder attachments and bulk solder
microstructures were characterized before and after board
level assembly. Failure analysis was performed after
temperature cycling to document the location of the failures
and the failure mode. Microstructural characterization and
failure analysis was done with optical microscopy and
scanning electron microscopy (SEM). Phase identification
and elemental analysis was accomplished using energy
dispersive spectroscopy (EDS) and backscattered electron
imaging (BEI). Optical metallography was the primary tool
used to verify the failure mode. The SEM operating in the
backscattered electron imaging (BEI) mode was used to
differentiate phases.
Figure 2: A schematic representation of the interposer subassembly. The interposer solution addresses backward
compatibility by incorporating the Pb-free BAG into a subassembly that subsequently can be attached to main board
with a SnPb soldering process.
Table 2: 676 I/O PBGA interposer attributes.
Interposer Attribute
Value
I/O
676
Ball Pitch
1.0 mm (40 mils)
Ball Diameter
0.63 mm (25 mils)
Solder Ball Alloy
63Sn37Pb
Substrate Pad Design
Circular – Non-Solder
Mask Defined (NSMD)
Substrate Pad Diameter
0.4 mm (16 mils)
Substrate Pad Surface Finish Electrolytic Ni/Au
Substrate Type
High temperature FR-4
Substrate Thickness
0.4 mm (16 mils)
RESULTS AND DISCUSSION
Baseline Characterization – Interposer Card Assembly
Low magnification metallographic cross sections of an
interposer subassembly are shown in Figure 3 (compare to
the drawing in Figure 2). Higher magnification images of
the soldered interfaces and bulk solder of the PBGA and
interposer are shown in Figures 4 and 5. These
microstructures are typical of Pb-free and SnPb solder joints
respectively.
Surface Mount Assembly and Test Matrix
The build matrix for the accelerated temperature cycling
tests is shown in Table 3. An initial sample size of 18
component sites was used for all test cells except for the
SAC305 baseline, which had a sample size of 24.
Additional boards were populated to provide samples for
baseline microstructural analysis.
Table 3: The build matrix for the temperature cycling tests.
676 I/O PBGA Build Matrix
Component Variation
Sample Size
SnPb as-received
18
SnPb reballed from SAC305
18
SAC305 as-received
24
SAC305 on SnPb interposer
18
Accelerated Temperature Cycling
896
Figure 3: An interposer subassembly in cross section with
SAC305 BGA balls on the package and SnPb balls on the
interposer. The interposer uses non-solder mask defined
pads to enhance solder joint reliability.
are attached. In both cases, the BGA pads are exposed
directly to molten solder, which has the potential to create
more severe damage than a simple thermal excursion.
The Figure 6 shows a series of high magnification
backscattered electron micrographs of the intermetallic
compound (IMC) layers and the bulk solder microstructures
for as-received and reballed SnPb (reballed from SAC305)
ball grid arrays. The intermetallic layer is similar in both
samples and nominally is 1µm thick, which is typical for the
Ni3Sn4 intermetallic layer that forms on an electrolytic
Ni/Au BGA pad. The highest magnification images (lower)
reveal a slightly more uniform IMC layer in the reballed
BGA, which is consistent with the additional reflow cycles
during reballing. Note that this is evident only when viewed
at the highest magnification, which originally was 10,000X.
The bulk solder microstructure of the reballed BGA is
slightly coarser (larger Sn-rich and Pb-rich regions) than the
as-received BGA but this is a minor difference and is not
considered a detriment to reliability. The reballing process
does not appear to induce any substantial changes or
anomalies in the soldered structure.
Figure 4: Backscattered electron images of the solder
interfaces and bulk solder microstructure of the SAC305
BGA assembled onto the interposer card.
Figure 5: Backscattered electron images of the solder
interfaces and bulk solder microstructure of the SnPb
interposer card.
Figure 6: A comparison of IMC layers and bulk solder
microstructures for as-received and reballed SnPb BGA
packages before surface mount assembly.
Baseline Characterization – BGA and BGA Reballing
A significant concern regarding reballed BGA package
reliability is the effect of thermal cycles on the integrity of
the solder ball attachment interface and BGA pad resulting
from final ball attachment. Reballing involves two major
additional thermal cycles above the melting point of the
solder. The first cycle occurs above the melting point of the
Pb-free solder to enable solder ball removal. The second
occurs above the SnPb melting point as the new solder balls
Accelerated Temperature Cycling Test Results
The thermal cycling test results are summarized in Table 4
in the Weibull plots in Figure 7. The results show that the
difference in characteristic lifetime for the as-received SnPb
and the reballed SnPb is less than 5%, which is considered
to be within experimental error. There also is no statistical
897
difference between two data sets at the 90% confidence
level. Based on these results, it is reasonable to conclude
that reballing causes no significant degradation of the
thermal fatigue performance of the 676 I/O PBGA package.
A comparison of the data for the as-received SAC305 and
SAC305 assembled onto the interposer card shows that the
SAC305 BGA fatigue life is degraded by approximately
10% with use of the interposer. A slight degradation
associated with additional mechanical complexity is
consistent qualitatively with the data published previously
for BGA socket attachments [53]. Regardless, it should be
noted that even with interposer assembly, the SAC305
reliability remains significantly better than the SnPb
reliability.
edge of the BGA package. Thermal fatigue failures were
detected in the first full row of solder balls under the die of
the SAC305 BGA, which is a common location for fatigue
failures in full array BGA packages. There are two
significant findings here: 1) the fatigue failures occur
exclusively in the SAC305 BGA, and 2) there is no
detectable fatigue damage in the SnPb interposer balls. The
former result might seem surprising considering that
SAC305 typically outperforms SnPb in short-dwell time
BGA tests [60]. However, the SnPb interposer is designed
with NSMD pads on both sides of the solder joint
specifically to improve attachment reliability. The better
thermal fatigue performance of the SnPb interposer joints
demonstrates the ability of the NSMD pad features to delay
fatigue crack initiation. Additionally, the coefficient of
thermal expansion (CTE) mismatch is expected to be
minimal between the interposer and the PCB. Therefore, the
SnPb solder joint stress is dominated by this minimal CTE
mismatch plus some effect due to the SAC305 component
soldered onto the interposer.
Table 4: Summary of 0 to 100 °C thermal cycling statistics
for the 676 I/O PBGA.
676 I/O PBGA Thermal Cycling Data
Characteristic
Component Variation
Lifetime  (cycles)
SnPb as-received
2049
SnPb reballed from
1969
SAC305
SAC305 as-received
4863
SAC305 on SnPb
4402
interposer
Slope
β
9.36
8.99
7.87
7.69
Figure 7: Weibull plot comparing the performance of the
SnPb and SAC305 packages to the interposer and reballed
backward compatible assembly options.
Figure 8: Failure map and optical photomicrographs for an
interposer subassembly that failed by thermal fatigue in the
SAC305 BGA solder balls (upper images). There is no
evidence of fatigue crack initiation in the eutectic SnPb
interposer balls (lower images).
FAILURE ANALYSIS
Failure analysis was performed on temperature cycled
samples to document the failure mode and specific location
of the failures in the components and interposer
subassemblies. Figure 8 shows the failure map and solder
joint failures for an interposer subassembly.
The photomicrographs in Figure 9 compare the failure mode
of the as-received SnPb BGA to the reballed SnPb BGA.
The critical characteristics of the two failures are identical,
which indicates that the reballing process has no appreciable
Cross sectioning was initiated in the outer row of the
subassembly and proceeded incrementally towards the die
898
CONCLUSIONS
The accelerated temperature cycling test results demonstrate
acceptable thermal fatigue reliability for the interposer
subassembly and the reballed BGA. Based on thermal
fatigue requirements, interposer assembly and reballing are
alternative solutions for backward compatible assembly of
the 676 I/O PBGA.
affect on the failure. The crack paths are at the package side
of the solder ball near the IMC layer but contained in the
bulk solder. The higher magnification SEM images of
additional failed joints in Figure 10 show that the crack path
clearly is between the Sn-rich and Pb-rich phases in the bulk
solder. In the case of SnPb solder, failure is preceded by
grain coarsening in the microstructure [61].
For the case of the Pb-free (SAC305) BGA assembled onto
a SnPb interposer card, the solder joint failures occur in the
SAC305 BGA solder joints. The test data indicate that using
the interposer results in a slight degradation in attachment
reliability (measured by characteristic lifetime) compared to
the case of the same SAC305 BGA soldered directly onto a
circuit board. Despite this minor degradation, the resultant
solder joint attachment reliability of the interposer
subassembly is substantially better than the reliability of the
SnPb version of the identical BGA. The failure analysis also
indicates that the SnPb interposer solder interconnections
are inherently robust from a thermal fatigue perspective,
presumably a result of using non-solder mask defined
(NSMD) pads in the substrate design and the minimal CTE
mismatch between the interposer and the PCB.
Figure 9: Comparison of the fatigue fractures in as-received
SnPb and SnPb reballed from SAC305.
For the case of the reballed SnPb BGA, there is no
statistically significant difference in thermal fatigue
reliability of the solder joints between an as-received
version of the 676 I/O SnPb BGA and the SnPb version
created by reballing a SAC305 BGA. Although there is a
slight difference in time-zero microstructure between the asreceived and reballed packages, it would be highly
speculative to attribute the small differences in thermal
fatigue reliability to the microstructural differences.
Figure 10: Secondary electron images (SEI) showing solder
fatigue failures in the SnPb and reballed SnPb samples.
ACKNOWLEDGEMENTS
The authors thank Sherwin Kahn and Marc Benowitz from
Alcatel-Lucent, and Prashant Joshi, Karen Hille, and Glen
Griswold from Interconnect Systems for supporting this
work.
SUGGESTIONS FOR ADDITIONAL WORK
This work focused specifically on assessing thermal fatigue
performance of the solder attachments of a 676 I/O BGA
interposer subassembly and a reballed 676 I/O BGA. While
thermal fatigue resistance is considered critical for
telecommunication applications, resistance to drop, shock,
and vibration are important in other applications. It would
be beneficial to expand the interposer and reballing
evaluation to include mechanical testing to address other
application requirements. Mechanical integrity might be
particularly critical for the interposer subassembly. There
also would be value in performing similar studies on larger
BGA packages.
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