2015 Reports

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Effect of TIM Compression Loads on BGA Reliability
Authors: Nicholas Graziano and Harry Schoeller
Abstract: Compressive loads are often applied to Thermal Interface Materials (TIMs) to improve their thermal performance. This stress must be accommodated by the underlying solder joints. The effect of compressive loads on BGA reliability was analyzed for SAC305 solder in thermal shock (-40/125°C with 15 minute dwells).

Assemblies were temperature cycled using two different simulated heat sinks with multiple load magnitudes. One delivered the load directly to the die while the other loaded the outer BGA rows through contact on the substrate. Die loaded samples were tested with and without a backplate. Additionally a set of samples were pre-aged for up to 1000 hours at 125C under compression prior to thermal shock.

BGA thermomechanical reliability under different loading configurations and magnitudes was compared using Weibull failure rate distribution plots. Cross sectioning and dye and pry analyses were performed to identify the location and mode of failure. This work aims to provide guidance for the potential trade-offs between thermal performances and second level interconnect reliability.

Effect of Cu Foil Treatment on Pad Strength
Author: Pericles Kondos
Abstract: Copper foils of different roughness of their resin side surface were used to produce PCB coupons with pads appropriate for bump pull tests, in order to look for a potential effect of copper roughness on pad strength. Two different resins were used to make these coupons. Hot Bump Pull and Cold Bump Pull were used to test pads of at least two different sizes from three nominally identical boards and the failure surfaces were carefully examined to ensure that comparison was made between pads that had the same failure mode. The presence of a primer layer (adhesion promoter) in the two smoothest foils complicated the interpretation of the data, but gave the additional opportunity to study the effect of the primer on pad strength, an effect that proved quite significant.

Effect of Solder Composition Surface Finish and Solder Volume on Drop Shock Reliability
Authors: Shuai Shao, Francis Mutuku, Tom Pittella, and Babak Arfaei
Abstract: In this study, drop reliability test is performed for six lead-free alloys, SAC105, SAC305, Innolot, SAC-M, SN100C, SN99CN. A redesigned, non-JEDEC-standard test board is utilized. All assemblies are built using the same paste as the solder alloy. The dominant failure mode is observed as solder failure, either solder bulk failure or intermetallic compound cracking. Four boards are dropped simultaneously. Acceleration distribution on the drop table is monitored. After test boards are mounted, location sensitivity is investigated by performing strain measurements. Preliminary microstructure analysis is performed for failure modes and Sn grain morphology. The effect of alloy silver content on drop reliability is studied. Drop test data with two surface finishes, Cu-OSP and immersion silver, are compared for their influences on reliability performance. BGA and LGA interconnections are used for studying the effects of solder volume. SAC305 shows better performance than all the others, for both BGAs and LGAs.

Testing Underfilm as a Potential Replacement of Underfill
Author: Pericles Kondos
Abstract: A material called “Underfilm” which, placed under BGAs, melts during reflow and solder collapse and solidifies upon cooling, was evaluated as a lower-cost replacement for conventional underfills. The TB2014U with a 1-mm pitch PABGA and a 0.8-mm pitch memory device were used for the tests, which involved both drop tests and ATC. Cross-sections and flat-sections studied the spreading of the material during reflow.

The performance of the underfilm-reinforced assemblies in both tests was compared with that of conventionally-underfilled and of non-reinforced sets. Finally, some parts were removed in a rework station and the behavior of the underfilm during removal was observed.

Broadband Printing I: _Stencil Technology
Author: Michael Meilunas
Abstract: Broadband printing is the term often applied to an electronic assembly process in which both fine and coarse pitch componentry is assembled through a stencil printing process. Broadband printing applications may require step stencils in order to effectively deposit solder paste over both large and small features. The following report describes an experiment performed in conjunction with an AREA member designed to evaluate the effectiveness of several stencil technologies. Included in the evaluation were laser cut and electroformed foils and the step forming techniques of patching, chemical etching and electroform growing.

Thermo-mechanical Evaluation of Alternate Pb-Free Die Attach Materials
Authors: Harry Schoeller, Sandeep Mallampati and Junghyun Cho
Abstract: High Pb alloys such as 92.5Pb-5Sn-2.5Ag are traditionally used for die attach applications because of their excellent thermo-mechanical performance and desirable thermal and electrical properties. However, with the Restriction of Hazardous Substances (RoHS) exemptions for die attach set to be revisited in 2016, it is now imperative to consider Pb-free alternatives, especially for those applications that have relied on Pb-based technology in the past. One of the steps toward industry adoption of Pb-free alternatives is thermo-mechanical evaluation of suitable candidates.

This work examines the thermo-mechanical performance and fatigue mechanisms of two commercially available die attach materials, and an experimental BiSbCu alloy with 92Pb-5Sn-2.5Ag serving as the control. Assemblies were exposed to liquid to liquid shock -50/150°C for up to 2000 cycles. The test was periodically interrupted to document fatigue damage using both destructive and non-destructive techniques. The accumulation of fatigue damage during thermal shock was documented by x-ray imaging, cross-section, and z-section.

Joints surviving 2000 cycles of thermal shock were compared by measuring the degradation in shear strength before and after thermal shock. The results of this work aid in the development of Pb-free materials to replace high-Pb alloys for die attach applications.

Effect of Conformal Coating on Surface Mount ATC Reliability – SnAgCu
Author: Martin Anselm
Abstract: Conformal coating of printed circuit board assemblies has been used primarily by the military/aerospace electronics industry as a protection against moisture ingress and corrosive gasses. As a consequence, the use of the material has been primarily on eutectic SnPb solder assemblies which, as a general rule, are more tolerant of environmental stresses. Therefore the need for lead-free characterization of the coating in accelerated life testing becomes a necessity. This research compares the accelerated thermal cycling reliability of SAC305 solder joints in uncoated control samples to those coated with urethane and acrylic coating materials with differing thicknesses and processing conditions. Characterization of the boards, components and materials are compared to the failure modes and characteristic life of the components.

The results illustrate the effect of coating on the characteristic thermal cycle life for many component classes, three coating materials (both urethane and acrylic) and two levels of coating thickness.

Statistical Variations of Solder Joint Fatigue Life in Varying Amplitude Cycling
Authors: Sa’d Hamasha and Peter Borgesen
Abstract: Common damage accumulation rules fail to predict the fatigue life of solder joints under realistic service conditions. A modification of Miner’s rule of linear damage accumulation has been proposed that accounts for effects of amplitude variations in, for example, vibration of microelectronic assemblies with lead-free solder joints on the average or characteristic fatigue life. We are however obviously much more concerned with early failure. Prediction of, say, the first failure out of 10,000 or a million requires the extrapolation of experimental failure distributions and the assumption of a shape of this distribution. No current statistical distribution can account for the combination of the strong but bounded effects of Sn grain orientation and other contributions to the statistical variations in fatigue life.

Individual ball grid array scale SAC305 and SAC105 solder joints were cycled in shear at room temperature with combinations of two different stress amplitudes. Relying on our modified Miner’s rule and the associated understanding of the effects of amplitude variations we show that the statistical uncertainty in the fatigue life of solder joints under a specific set of realistic service conditions must be significantly greater than measured in fixed amplitude cycling tests.

The predicted failure distribution was best fit by a Weibull distribution over a limited range, but we argue that the assumption of such a distribution is likely to be seriously misleading when it comes the prediction of earlier failure.

Effect of Multiple Reflows on the Pad Strength of High Tg Boards
Author: Pericles Kondos
Abstract: The project, associated with HDPUG’s “Lead Free PWB Materials Reliability Phase 4”, studied the effect of multiple reflows on the strength of BGA pads. Boards made from nine different high-Tg resins and in high and low resin variants were used for this study. Several boards from each group were reflowed 6 times using a profile with long time above 217°C and relatively high Tmax. Then they were balled, together with non-reflowed boards, and cold bump pull was used to measure pad strength. The strength distributions of reflowed and non-reflowed boards with the same resin material and resin content were compared.

A secondary use for the data was to allow comparison of pad strength for boards made with different materials.

Isothermal Fatigue of High Temperature Solders
Author: Harry Schoelle
Abstract: With electronics being integrated into higher temperature environments such as those associated with deep well drilling and distributed controls for “under the hood” automotive applications, higher melting temperature solders are needed. While limited data on bulk high temperature solders is available, data on joint level behavior is woefully lacking. Further, the data presented is often measured at room temperature and cannot be extended to higher service temperatures where the solders are actually used.

In this work, a test methodology was developed for high temperature isothermal shear fatigue of individual solder joints. A total of six solder alloys, five Pb-based alloys and one Pb-free alloy were tested at 25°C and 200°C. Joints tested at 200°C were first stored at 200°C for 1000hrs to measure performance in a simulated high temperature environment. The microstructure was studied in conjunction with the fatigue results to understand the structure-property-performance relationship. Result show those alloys with a dendritic structure had a greater characteristic life at room temperature.

At 200°C coarsening of the microstructures led to dramatically different results.

Comparison of Drop Performance of Glass and PCB Substrates
Authors: Shuai Shao and Babak Arfaei
Abstract: Organic and silicon interposers have been utilized widely for 2.5D and 3D packaging technology. However organic interposers suffer poor dimensional stability resulting in limited via size and I/O density. The main downsides of silicon interposer are their high cost. Recently glass interposers have attracted great research interest due to significant advantages of glass including rigidity, low cost and high electrical resistivity.  In this study, a specific glass fixture is designed to reduce stress concentration around constrained corners. Drop testing is performed on glass substrates with the maximum acceleration (G) levels observed before glass failure being measured.

Glass substrates with different sizes and coefficients of thermal expansion (CTE) are compared in drop test. The glass fixture and traditional screw fixture are compared in drop for glass and printed circuit board (PCB) substrates.

Failure Mechanism and Microstructural Evolution of Pb-free Solder Alloys in Thermal Cycling Tests: Effect of Solder Composition and Sn Grain Morphology
Authors:  Babak Arfaei, Francis Mutuku, Richard Coyle, Eric Cotts and Jim Wilcox
Abstract: An improved understanding of how materials and processing affect solder joint microstructure provides insight into how to enhance the fatigue performance of Pb free solder joints. Our previous results showed that differences in solder volume can alter the Sn grain morphology of a near eutectic, SnAgCu Pb-free solder joint and change the lifetime in thermal cycling test. Solder composition and PCB surface finish were also shown to influence Pb free solder joint microstructure and the lifetimes of package interconnects in an ATC test. However, the effects of those parameters on failure mechanisms, particularly recrystallization behavior and crack propagation, are not well understood. Thus, in the current work, Pb-free solder joints were assembled with a range of materials, and the microstructure and reliability of those solder joints were characterized and correlated with the different fabrication parameters. In each case, the failure mechanism was examined.

Solder joints of various sizes were assembled onto test boards with Cu-OSP, ENIG or ENEPIG PCB surface finishes using commercial solders alloys with different microalloying elements, such as Ni and Mn. For comparison, Sn3Ag0.5Cu (SAC 305) and eutectic SnPb alloys were included in the sample set. Accelerated thermal cycling testing was performed on the assembled vehicles. Microstructural analysis was performed on as-reflowed, on partially cycled samples, and on samples after electrical failure, to understand the initial microstructure and its evolution during the test. Thus, selected samples were taken out of the thermal cycling chamber at specific intervals of characteristic life (i.e., 20% and 50% of their characteristic life) for microstructural analysis. Polarized light microscopy and electron backscatter diffraction (EBSD) techniques were used to assess the Sn grain morphology of solder joints.

The effect of solder composition, Sn grain morphology and PCB surface finish on solder joint microstructure and lifetime was evaluated. The effect of microstructure on the recrystallization behavior and on crack initiation, and thus the failure mechanism of joints in thermal cycling test is reported.

Effect of Reflow Profile and Composition on Isothermal Shear Fatigue of SAC Solder on Cu-OSP
Authors: Francis Mutuku, Babak Arfaei and Eric Cotts
Abstract: Changes in the sample and process parameters significantly affect the microstructure and performance of materials. Thus we have examined the effect of such parameter changes on the reliability of the Sn-Ag-Cu solder alloys. Changes in the concentration of Ag and other alloying elements were correlated with failure rates during isothermal shear fatigue testing. We examined the effect of changes in thermal history such as cooling rate from the melt, or hold time at peak reflow temperature, on Sn-Ag-Cu solder microstructure and performance.

This study focuses on individual solder joints informed by the fact that the failure of a single joint in a device may render the device to malfunction or fail altogether. The examinations of the effect of the reflow profile on the solidification behavior and shear fatigue life of 20mil lead free solder joints were performed using the Differential Scanning Calorimeter and Dage4000Plus bond tester, respectively. Microstructural analysis was performed with Scanning Electron and Optical microscopy.

It is evident from our results that the isothermal room temperature shear fatigue life increases with increase in the amount of Ag, and that the other alloying elements contribute to the improved performance during isothermal shear fatigue tests.

On Vibration Testing Methodology for Electronic Assemblies
Authors: Farhan Batieha, S’ad Hamasha, and Peter Borgesen
Abstract: The most obvious concern of any reliability engineer is to ensure that the dominant damage mechanism is the same in accelerated testing as it is under the service conditions of actual interest. We argue that resonance tracking or adjusting the input to compensate for resonance drift in vibration testing may be very misleading. Instead, we recommend that the acceleration be kept low enough to prevent significant drift and, of course, to ensure the same failure mode as in service.  In the absence of manufacturing defects the ultimate life of a microelectronics assembly in long-term vibration is likely to be limited by solder joint fatigue. So far relatively limited efforts have been dedicated to the quantitative prediction of this life. However, even day to day ‘engineering tests’ may not be very useful if they do not offer at least a ranking of alternative materials, designs or processes in terms of performance in service.

The assessment of solder joint life in long term vibration under realistic service conditions is complicated by effects of cyclic loading on the solder and, in accelerated testing, the PCB deformation properties. Cycling induced damage to the PCB often goes unnoticed but may have profound consequences for the solder fatigue. Also, preliminary isothermal cycling results suggest that variations in cycling amplitude may lead to a change in creep properties. This would invalidate attempts at Finite Element Modeling and is believed to be the reason for the dramatic break-down of Miner’s Rule of linear damage accumulation. A preliminary test was conducted to show that this applies to vibration to millions of cycles, at least.

These issues are of major concern for the ESS testing commonly required in military and aerospace electronics manufacturing, and they may have consequences for the interpretation of random vibration test results.

MAT7E – Influence of Microalloying Elements on Reliability of SnAgCu Solder Joints
Author: Babak Arfaei
Abstract: New solder alloys (SAC with microalloying elements such as Ni, Mn and Ti, Sb or Bi) are under evaluation as potential replacements for SAC305 lead free solder. Researchers seek a lead free solder alloy with greater resistance to drop shock that also demonstrates acceptable thermal fatigue resistance. The effects of various microalloying additions to SAC alloys on the evolution of solder joint microstructure and on failure mechanisms must be understood in order to optimize the implementation of these new materials.Thus various commercial solders alloys with microalloying elements, such as SN100C, SACM and Innolot, were examined and compared to observations for SAC105, SAC 305 and eutectic SnPb alloys. Both mechanical and thermal cycling tests were performed on solder joints assembled on Cu-OSP or ENIG. The thermal history of each individual solder joint was carefully controlled and monitored by differential scanning calorimeter. Thus the effects of varying reflow parameters on the mechanical behavior in shear and shear fatigue test were analyzed. ATC reliability test was conducted on BGA components using a -40/125°C profile.Careful microstructural analysis was performed on as reflowed and failed samples. Polarized light microscopy and electron backscatter diffraction techniques were used to assess the effect of Sn grain morphology on life time of solder joints, particularly on early and late failures. The effect of variation in solder volume, composition, Sn grain morphology and PCB surface finish on solder joint microstructure and lifetime was carefully evaluated.