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Large Low-CTE Glass Package-to-PCB Interconnections with Solder Strain-Relief Using Polymer Collars
Gary Menezes, Vanessa Smet, Makoto Kobayashi
+, Venky Sundaram, Pulugurtha Markondeya Raj, and Rao Tummala
3D Systems Packaging Research Center, Georgia Institute of Technology, Atlanta, USA
+ Namics Corporation, Niigata, Japan
Abstract This paper reports the use of circumferential polymer
collars as a strain-relief mechanism to improve the fatigue life
of low-CTE package-to-PCB solder interconnections, while
preserving SMT-compatibility and reworkability. Acting as a
partial underfill, the polymer- collar serves to block shear
deformation at the solder-package interface, and redistributes
the load to reduce the overall plastic strain concentration in
the solders. It also suppresses failure initiation from defective
surface sites and,thus further enhances reliability.
Ultra-thin glass 100µm interposers were fabricated in
18.4 mm x 18.4 mm size to model, design and demonstrate
the reliability enhancement with the polymer-collar approach.
The detailed interposer design and fabrication process with
laminated dielectric and metallization layers on both sides is
presented. A new class of epoxies with low modulus, without
the incorporation of silica fillers, was used to act as the
polymer collars. The polymer collars are formed by spin-
coating with an optimized thickness to provide the best
compromise between the effective strain relief and
reworkability. Board-level assembly was performed using
standard SMT processes for glass interposers with and without
polymer collars. Thermal cycling reliability testing (-40°C to
125°C) of interposers, assembled on PCBs with and without
polymer collars for various thicknesses of the collar was
performed.
Introduction Large, low-CTE packages are emerging as major
need to interconnect multiple chips with fine-pitch chip-level
interconnections with advanced RDL ground rules, both for
high-performance 2.5D packages and ultrathin consumer
products. These low-CTE packages are also needed to
minimize stress on the ultra-low k on-chip dielectrics. This
low-CTE package approach, however, creates CTE-mismatch
related stress issues for package-to-board-level
interconnections, resulting in degradation of board-level
reliability. Innovative, low-cost, interconnection technologies
at finer pitches are therefore highly sought after for direct
assembly to the board, thus eliminating the need for additional
interposer packaging level. For wider acceptance of low-CTE
packages in high-volume production, it is also critical to
assemble them using standard SMT with high degree of
automation, reduced labor costs and higher production rates.
A number of unique stress-relief interconnection
approaches have been reported in the literature, originally
developed for WLP, but then extending them to solve the
board-level interconnection reliability challenges. The G-helix
[1] and stretched-solder interconnections [2] are based on
compliant metal interconnection structures to provide stress-
relief and improve the reliability. The double solder ball
wafer-level technique by IZM [3] is another such approach to
improve the interconnection compliance. Ball-on-polymer [4]
and ELAStec WLP [5] place the joints on a layer of compliant
polymer to reduce the strain in the joint itself. Wide Area
Vertical Expansion (WAVE) [6] technology introduces a
compliant low-modulus layer between the die and the
interconnection layer. These approaches compromise the
electrical performance, increase process complexity, or create
additional challenges by increasing the package height, or use
non-standard assembly processes.
GT-PRC is pioneering novel low-cost stress-relief
approaches with minimal additional process steps using
standard package- to board SMT processes by extending them
to large package sizes at fine pitch. Past work on such glass
package-to-PCB interconnections at GT-PRC used a dielectric
strain buffer on either side of the glass to enhance reliability of
7.2 mm x 7.2 mm packages. Two variations of glass CTE
were studied, 3.8 ppm/K and 9.8 ppm/K. Both variations of
glass were shown to give better reliability than silicon
interposer at board level, undergoing up to 1500 thermal
cycles before the first occurrence of joint failure [8].
This paper reports the use of circumferential polymer
collar as a strain-relief mechanism to improve the fatigue life
of low CTE package-to-PCB solder interconnections, while
preserving SMT-compatibility and reworkability. Acting as a
partial underfill, the polymer collar serves to block shear
deformation at the solder-package interface, and redistributes
the load to reduce the overall plastic strain concentration in
the solders. It starts with thermo-mechanical modeling to
predict the solder joint strains ,leading to design of the
polymer-collar structure for meeting the reliability
requirements, followed by process development to achieve the
polymer-collar structure design, large glass package test-
vehicle fabrication and surface mount assembly process for
high-volume manufacturing, and finally ends with reliability
characterization and model-to-hardware correlation.
Finite Element Modeling Two-dimensional half-symmetry models of glass
interposers were built along the diagonal of the package to
simulate the plastic strain in the furthest solder joint. The left
boundary of the package was given symmetry boundary
conditions with the bottom corner pinned.
The modeled assembly was first subjected to a drop in
temperature from 260˚C to 25˚C to simulate the cool-down
phase of the SMT reflow process. Five thermal cycles between
-40˚C and 125˚C (following the JESD22-A106B thermal
978-1-4799-2407-3/14/$31.00 ©2014 IEEE 1959 2014 Electronic Components & Technology Conference
shock standard) were applied. The ramp-up and ramp-down
times were 1 minute and the dwell time at the temperature
extremes was 5 minutes. Equivalent plastic strain range was
extracted after cycling.
Lead-free SAC105 solder material was used during
modeling and test-vehicle fabrication due to better
performance under drop test [9]. Solder was modeled as a
visco-plastic material using parameters derived from Anand’s model. Copper was modeled as a bilinear elastic-plastic
material while all other materials were considered elastic
(Table 1). The viscosity of polymer materials is neglected for
the temperature range in consideration.
Table 1: Material properties used for FEA
Material
E (GPa)
CTE (ppm/˚C)
Poisson’s ratio
Material model
Glass (High CTE)
74 9.8 0.23 Elastic
Glass (Low CTE)
77 3.8 0.22 Elastic
Silicon 130 2.7 0.28 Elastic
FR-4 24 16 0.3 Elastic Solder Temp.
dep. 22 0.34 Visco-plastic
Copper 121 17.3 0.3 Elastic-plastic
Finite element modeling results and analysis FEA was used to analyze the strain relief mechanism
behind the polymer collar approach. Five variations in
polymer height on the solder ball were simulated so as to
provide guidelines for experimental design (Figure 1, Table
2).
Figure 1: Variations in polymer collar height
Table 2: Variations in polymer collar height
Collar type
Height between solder balls
UF Collar material as underfill
T0 ~170 µm
T1 ~110 µm
T2 ~80 µm
T3 ~55 µm
Strain relief mechanism
A high-CTE glass package assembly at 18.4 mm x 18.4
mm size was simulated with each of the collar types. Plastic
strain was extracted at the four critical locations on the outer
most solder ball (Figure 2).
Figure 2: Critical locations for maximum solder plastic
strain with (left) and without (right) polymer collar
The underfilled structure shows the same plastic strain at
all four points on the joint (Figure 3). With the addition of the
polymer collar, the region of high plastic strain was
transferred from the package interface to that between the
joint and collar, decreasing the package side strain as
compared to the UF case. Presence of the collar reduces
plastic strain range at all four critical points, though maximum
strain range is still seen on the board side. The collar cannot
be applied on the board side as this would hinder
reworkability.
A trend of decreasing strain range with increasing collar
thickness is seen from the results of these simulations. The
thickest collar variation (T0) provides the largest buffer to
shear deformation resulting in the lowest strain range at the
bottom left of the joint. Reduction in strain range for this case
is 16.7%. The polymer collar therefore acts as a partial under-
fill.
Figure 3: Plastic strain range at the 4 critical locations on the
joint
Glass panel fabrication Glass-interposer test vehicles were designed at a
body size of 18.4 mm x 18.4 mm. High CTE, 100 µm thick
interposers were fabricated with two dielectric build-up layers
RXP-4M (20 µm) and ZS-100 (22.5 µm), laminated on either
side of the 6” x 6” bare CF-XX glass panels provided by
Asahi Glass Company. The dielectric materials were procured
from Rogers Corporation and Zeon Corporation respectively.
The panels were then metalized to provide patterning for test
daisy-chain structures. Two variations in package pads were
introduced with non-solder mask defined (NSMD) pads
created with dry-film photosensitive solder resist. The second
variation with resin mask defined (RMD) pads were created
by laminating an additional layer of ZS-100 over the
patterning and drilling holes in the polymer to create openings.
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ENEPIG was used as the surface finish on the copper
pads (carried out at Atotech, Germany). The interposer
fabrication flow is depicted in Figure 4. The panels were then
sent to Nanium for attachment of 250 µm solder BGAs by ball
drop and interposer dicing.
Figure 4: Panel-level fabrication process flow for glass interposers. (a)-(g): Substrate cleaning, dielectric lamination, seed copper, lithography, electroplating, seed etching, surface finish and solder resist processing.
Polymer collar process Epoxy materials without fillers were spin-coated onto
the BGA side of the diced-glass interposers to form the
circumferential collars. The interposers were then oven dried
at 70 ˚C for 1 hour to allow the solvent content to evaporate
(Figure 5).
Figure 5: Polymer collar application and drying process
When a square substrate is coated with polymer
collar, its corners experience high air friction resulting in a
higher rate of evaporation at these locations. The material at
the corners dries up and impedes flow of material, being
driven radially outward by the centrifugal forces. This
behavior leads to material build-up at the corners of the
substrate.
Figure 6: Material build-up at substrate corners during
spin coating
The shape of the larger 18.4 mm interposers deviates
even more from an ideal circular substrate. Trials using a
simple ramp-up and dwell process resulted in highly non-
uniform deposition (Figure 7).
Figure 7: Non-uniform coating on 18.4mm interposer
Two enhancements were therefore used to improve
spin-coating uniformity. Firstly, dummy interposers were
placed around the test interposer to simulate a larger substrate
size and eliminate corner effects (Figure 8).
Figure 8: Enhancement to make region around test
substrate radially uniform
In addition, the coating profile was changed to
include spreading and uniformity phases to allow the material
to deposit uniformly over the surface of the test interposer
(Figure 9, 10). The final height of coated material depends on
the spin coating rate applied in the uniformity phase. It is
therefore possible to control the height of the polymer collar
using this parameter. The maximum variation in coated height
after these enhancements was about 20% (Figures 11,12).
Figure 9: Coating profile for 18.4 mm interposers
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Figure 10: Uniform coating on 18.4 mm interposers
Figure 11: Measured thickness of deposited collar material
on 18.4 mm interposers
Figure 12: Cross sections of uniformly deposited 18.4 mm
interposers
Polymer collar residue removal
SEM imaging was used to confirm the presence of a
polymer residue on the ball surface after the application
process (Figure 13). Although the material is designed to flow
from the bonding area, this residue could be perceived as
contamination of the solder balls by automated defects
detection systems used in industry. Samples were assembled in
order to check for contamination, however, no difference in
the surface topography and chemical composition was seen at
the solder-pad interface after careful analysis of the
intermetallic compounds by SEM/EDX. The next generation
of collar material may contain fillers, making residue removal
a possible challenge.
Figure 13: Coating with polymer collar leaves a residue on
the ball surface
With the objective of manufacturability, three options for
residue removal were explored.
1. Plasma etching The parameters used in the plasma etching approach are listed
in Table 4. It was observed that while removing residue from
the surface of the ball, etching also attacks the circumferential
collar (Figure 14). In addition, longer etching times can
change the microstructure of the ball surface, which is not
acceptable by industry standards.
Table 3: Plasma etching conditions Parameter Condition
Composition CHF3 (10 sccm) / O2 (50 sccm) Pressure 10 mtorr Power 300 W Time 15 and 30 minutes
Figure 14: Side profile of solder ball before (left) and after
(right) 15 minutes of plasma etching 2. Sandblasting
Sand blasting of the ball surface was explored as a possible
approach (Table 5). With 250 µm diameter pumicite, the
solder surface and, in some samples, the interposer suffered
serious damage. Trials done with 35 µm diameter pumicite
suffered no visible damage up to 10 seconds of processing.
Pumicite was found embedded in the collar material for all
variations in process time.
Table 4: Sand blasting conditions Medium / grain size: pumicite / 250 µm
Pressure 30 psi Time (sec) 10, 20, 30
Medium / grain size: pumicite / 35-75µm Pressure 25 psi
Time (sec) 5, 10, 30
While this process removes the residue from the ball surface,
it is very hard to optimize. Damage to the ball and embedded
particles that compromise the integrity of the collar prevent it
from being used as a manufacturable solution.
Solvent cleaning Acetone was used to wipe the ball surface right after the
polymer drying stage. As the residue is not cured at this stage,
it is easily removed (Figure 15). The material forming the
collar remains intact as the solvent comes in contact with only
the top surface of the ball (Figure 16).
Figure 15: Close-up of ball surface before (left) and after
(right) solvent wipe cleaning
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Figure 16: The polymer collar remains intact after
cleaning
A comparison of the results from the three
approaches shows that the solvent wipe method is most
effective in removing the polymer-collar residue with
negligible damage to the collar. The process can possibly be
implemented by dipping the balls in acetone and then IPA
using infrastructure similar to that used for flux dipping prior
to assembly.
Assembly for reliability
Tacky flux was first applied to the PCB, followed by a
pick-and-place step using a Finetech Matrix flip-chip bonder
with a 20 mm x 20 mm pre-leveled vacuum-locked spring
gimbal tool. Reflow conditions were optimized to match the
recommended profile by the flux supplier. Less than 25%
voiding was achieved on assembled samples as required by
industry standards.
Assembly process evaluation
In samples assembled with collar, leakage of polymer
material onto the PCB was observed. This occurs because the
melting point of the collar material is lower than the reflow
temperature. The most severe occurrence of board-side
leakage is seen in the case of thick collar. A compromise has
therefore to be made between buffering capability of the
thicker collar and board-side leakage which hinders
reworkability.
Figure 17: Leakage of material onto ball and PCB after
assembly with 90µm thick collar
Self-alignment of solder joints
An important consideration for assembly with
circumferential collars is the ability of the solder joints to self-
align during reflow. Samples with thickest collar, serving as
the worst possible scenario, were used to evaluate self-
alignment capability. Increasing degrees of misalignment were
introduced during the pick-and-place process. Samples were
x-ray imaged before and after reflow to document the
introduced misalignment and evaluate alignment after reflow.
The experiment showed that introducing a thick
collar around the solder balls does not prevent self-alignment
even with the interposer is off by 3/4 of the PCB pad ( ~170
µm) unless the ball is placed in contact with multiple pads.
This verifies that the polymer collar is compatible with the
SMT process.
Preliminary Reliability assessment Reliability testing was performed using JESD22-A104D
standards for thermal cycling with assembly, following a pre-
conditioning step comprising of 2 additional reflows at 260
°C. Thermal cycling was then performed between -40 °C and
125 °C with a 15 minute dwell time and a rate of 1 cycle/hour.
A dummy Si die (12mm x 12mm and 100µm thickness) was
underfilled on the interposer to see the impact of die-attach on
board-level reliability. The daisy-chain resistances were
measured after every 50 cycles. No failures were detected
during the testing. The reliability characterization data
indicates that polymer collar was successful in preventing
solder-joint reliability failures.
Conclusion
Solder-pad interfaces on the package side play a critical
role in determining the board-level interconnection reliability,
particularly with low CTE packages. This paper develops and
demonstrates a polymer-collar approach to alleviate the
failures at such interfaces. The polymer collar, which forms a
circumferential layer around the solder joint on the package
side, blocks the shear deformation of the joint at this interface,
transferring the high plastic strain to the lower side of the ball
which has lower propensity for failure. The collar therefore
acts as a partial under-fill and maintains the reworkability of
the assembly. This circumferential polymer collar is deposited
around the solder ball on the package side by spin-coating.
SMT-compatible assembly processes for large 18.4 mm x 18.4
mm glass interposers with polymer collars were demonstrated.
Results demonstrate that the reliability of low-CTE glass
interposers on organic packages or baords is significantly
enhanced with this polymer collar approach.
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Acknowledgments This study was supported by the “Interconnection and
Assembly” focused program at the Georgia Tech Packaging
Research Center. The authors would like to thank industry
mentors, especially Dr. Jaesik Lee from Qualcomm, for their
active guidance, suggestions and support. The authors would
also like to thank Asahi Glass Company for providing glass
substrates, Namics Corporation for their polymer collar
material, Atotech for surface finish processing, Nanium for
BGA attachment and interposer singulation, and Zeon
Corporation and Rogers Corporation for providing dielectric
materials. The authors also thank the Toshitake Seki from
NTK/NGK, Yoichiro Sato from Asahi Glass Company and
Akira Mieno from Atotech for their support and advice on
fabrication, and Anna Stumpf and Anne Matting for their
experimental contribution.
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