2013 Reports

Effect of Thermal Cycling on PCB Pad Strength
Author: Pericles A. Kondos
Abstract: Expensive printed circuit boards are often reworked when one of their components fails, typically from thermally-induced stresses causing failure in one or more joints: The failed component is heated until its solder melts, removed from the PCB, and a new one is put in its place. The expectation is that the new component will last a similar length of time and fail, if it does, with the same failure mode. This however assumes that none of the previous steps have caused latent damage to the PCB itself, which will lead to early failure and a change in failure mode from joint failure to pad cratering, especially in cases of lead-free assemblies which generally are more prone to this kind of failures. Thermal cycling is typically used to simulate, in an accelerated way, the loading history of assemblies that in service are subjected to thermal stresses. The purpose of the project described in this report was to examine whether this thermal cycling can cause latent damage to cycled parts that, combined with any possible extra damage due to rework, weakens the pads to a significant extent.

Authors: Babak Arfaei, Sam Mahin-Shirazi, Shantanu Joshi, Martin Anselm, Peter Borgesen, Eric Cotts, James Wilcox and Richard Coyle
Abstract: Previously crack propagation and joint failure in thermal cycling tests were correlated with recrystallization of Sn grains in SnAgCu (SAC) ball grid array (BGA) solder joints. Generally recrystallization of the Sn grains was observed to occur in the high strain region before solder joint failure. In an effort to better understand this failure mechanism in SnAgCu solder joints subjected to mild thermal cycling profiles, and in smaller solder joints that have interlaced Sn grain morphologies, both conventional (-40/125°C, 0/100°C) and mild (20/80°C) accelerated thermal cycling (ATC) tests were performed on various SAC solder joints. Correlations between microstructure and failure mechanism for solder joints on various BGA packages, chip scale packages (CSP), and quad-flat no-lead (QFN) packages were examined. The microstructure of samples was carefully analyzed; selected samples were removed from the chamber after different numbers of cycles in order to investigate the evolution of the SAC solder joint microstructure. 

Author: Pericles A. Kondos
Abstract: The APL’s Dage 4000HS bond tester was used to evaluate the effect that varying (within ranges recommended by industry standards) a single component design or material parameter (e.g. ball size) has on the strength, fracture energy and failure mode of bumps on otherwise identical components. The tests were performed on 1-mm pitch PBGAs with SMD Cu/OSP pads populated with SAC spheres; the pad and ball diameters were among the parameters being varied, but they had a base value of 500 µm and 600 µm respectively. The JEDEC JESD22-B117A standard was generally followed. Shear speeds varying from 50 mm/s to 500 mm/s were used. The results are valuable for the original design of components and for design modification if an improvement in the performance of the component is needed. They can also be used for developing a comprehensive database of strength, energy, and failure mode distributions with respect to material and design features. The procedure was described in detail but due to the large number of results only some cases were presented. The full data and result sets are available to the AREA Consortium members.

Authors: Mohammad Gharaibeh, Quang Su, James Pitarresi and Martin Anselm
Abstract: A proof-of-concept study was carried-out to demonstrate how existing drop testing data in the Consortium database could be used, in conjunction with finite element (FE) models, to estimate the stresses at various points in the assembly. The strain correlation procedure was implemented by matching the measured strain values obtained during drop testing of the samples to that in the FE model at the same location. Once the values matched, the stresses from the FE modeling were extracted. From these stresses and number of cycles to failure from experiment, a crude S-N curve was developed.

Authors: Mohammad Gharaibeh, James Pitarresi and Martin Anselm
Abstract: A methodology to correlate finite element (FE) models of component and circuit board assemblies, developed in ANSYS, with experimentally derived vibration characterization data, is described. The correlated models can be used to predict the response of assemblies under dynamic loading. The correlation procedure is based on modal measurements (natural frequency and mode shape information). This information is used to fine-tune the FE model (e.g., modify material properties and boundary conditions) of the assembly. This technique results in a methodical means for developing accurate models accounting for both actual material properties and boundary conditions. Using this methodology, mode shape and frequency estimates from the model are within a few percent of those measured for the test board used in this study. Moreover, the transmissibility during random vibration testing was also correlated with excellent accuracy. The developed model can be used to study strain, stress, and displacement for the assembly and can also be employed for studies involving stress in the solder joints to be used in conjunction with measured failure data to develop S-N curves for various solder alloys under dynamic loading.

Authors: Q. Su, J. Tam, N. Schmaling, J. Arikunnel, G. Joshi, M. Anselm
Abstract: A harmonic vibration reliability test method that tracks and adjusts for the resonance frequency of the circuit assembly under test has been implemented. The test setup includes an electrodynamic shaker with vibration control, resistance monitoring event detection, and vibration logging. The vibration responses of the test vehicles used in this study were found to be nonlinear, with different transmissibilities being measured at different input acceleration amplitudes between 0.1 to 3 g’s. It was also seen that the resonant frequencies of the assemblies change over the test duration, and the resonance tracking vibration controller successfully adjusts to have the vibration excitation frequency match the resonant frequency. Preliminary failure analysis show the primary failure mode is trace failure, with evidence of solder fatigue.

New Pb-free Solder Alloys: Dependence of Sn-Grain Morphology on Solidification Temperature
Authors: Shantanu Joshi, Babak Arfaei
Abstract: The Sn grain morphologies of commercial lead free samples were studied and found to be correlated with the sample solidification temperature; the lower the solidification temperature, the higher the degree of interlacing observed. This was found to be consistent with the interlacing observed in either Sn-Ag system or with Sn-Ag-Cu system. At intermediate degrees of undercooling, a mixed Sn grain morphology is observed, with the interlaced portion associated with the region closer to Sn nucleation in these samples. It was observed that smaller samples show higher degree of undercooling as reported in the previous results. Samples reflowed on a substrate showed significant drop in the undercooling as compared to free standing samples.

Comparison of the Failure Mechanism of SAC Solder Joints in Mild and Harsh Thermal Cycling Test Conditions
Authors: Sam Mahin-Shirazi, Babak Arfaei
Abstract: In order to have a better understanding of the failure mechanism in SnAgCu (SAC) solder joints subjected to harsh (0/100°C) and mild (20/80°C) accelerated thermal cycling (ATC) tests, extensive microstructural investigation were performed on various SAC solder assemblies  to examined the correlations between microstructure and failure mechanism. Both recrystallization and intergranular crack growth were observed in tested SAC solder joints after thermal cycling. Distinct coarsening of precipitates was observed in the recrystallized areas near the cracks, consistent with strain enhanced coarsening. The recrystallization was observed to be more in 0/100°C as expected. The 20/80°C reliability test results suggested that the failure mechanism of SAC assemblies is similar to that of harsh 0/100°C commonly performed in industry.

Board-Level Drop Test: Comparison of Two ANSYS Modeling Approaches and Correlation with Testing
Authors: Mohammad Gharaibeh, Quang Su, James Pitarresi and Martin Anselm
Abstract: Two ANSYS-based transient analysis approaches for simulating the board-level drop test of an electronic packaging assembly and correlation with measured responses are described. The procedures for the two finite element approaches, the Large Mass Method and the Direct Acceleration Input method, are discussed. Additionally, a method for including damping estimation for numerical transient analysis, based on measured data, is presented. Both modeling analysis techniques provide reasonable correlation to the test data, however, the large mass method more closely mimics the initial acceleration response as observed in the measurements and it is therefore the recommend approach within ANSYS.

Effect of Sn Grain Morphology on Failure Mechanism and Reliability of Lead-Free Solder Joints in Thermal Cycling Tests
Authors: Babak Arfaei, Martin Anselm, Shantanu Joshi, Sam Mahin-Shirazi, Peter Borgesen, Eric Cotts, James Wilcox,Richard Coyle
Abstract: The failure mechanisms in SnAgCu solder joints subjected to either conventional (-40/125°C, 0/100°C) or mild (20/80°C) accelerated thermal cycling (ATC) tests were examined. The precipitate and Sn grain morphologies of LGAs, BGAs, CSPs and QFN were characterized, and correlations with the failure mechanisms in these SnAgCu solder joints were sought. Both beach ball and interlaced Sn grain morphologies were observed in these components, and recrystallization and intergranular crack growth were observed in both cases. For larger as-reflowed SnAgCu (SAC) ball grid array (BGA) solder joints that showed beach ball structures, strain enhanced coarsening close to the crack area was also observed. Smaller samples that exhibited interlaced structures displayed relatively long fatigue lives. While these samples were completely recrystallized by the time they were cycled to 25% of their projected life, crack initiation was delayed for their joints. The recrystallized Sn grains grew, with cracks ultimately propagating intergranularly along those recrystallized Sn grains. Isothermal pre-aging for 10 days at 125°C did not significantly change the reliability behavior of these solder joints (Sn grain growth was characterized for such anneals).

Effect of Variation in the Reflow Profiles of Pb Free Solder on Lifetimes in Room Temperature Fatigue Tests
Authors: Francis Mutuku, Eric Cotts, Babak Arfaei, Martin Anselm
Abstract: In room temperature fatigue tests, the lifetimes of SAC solder joints showed distinct dependences on both solder composition and reflow profile. As observed previously in accelerated thermal cycling tests, distinct increases in room temperature fatigue lifetimes with increases in Ag content were observed. Room temperature fatigue test lifetimes varied significantly with changes in reflow profiles. In particular, increasing the cooling rate from the melt generally caused increases in characteristic lifetimes. Changes in lifetimes were also apparent when the duration of the reflow profile was changed. Thus, it was shown that room temperature fatigue tests display promise as a relatively simple and sensitive means to monitor the effect of different assembly parameters on solder joint reliability. 

The Evaluation of Dip Solder Paste for the Assembly of Through-Molded Via (TMV) Package on Package (PoP)
Authors: Guillaume Jaudeau, Harry Schoeller
Abstract: The solderabilty of two dip solder pastes was evaluated to determine their viability in Through Molded Via (TMV) Package on Package (PoP) assembly. This study complements a previous AREA Consortium study by Roggerman et al. which evaluated assembly yield of dip materials for TMV PoP assembly. Two test methods were used to evaluate the pastes, the Solder Ball Test (IPC-650-2.4.43), and the Wetting Test (IPC-650-2.4.45). The impact of the solder paste milling time on solderability was also evaluated.

Comparing LGA and BGA Assemblies in Terms of Solder Pad Cratering
Authors: Y. Zeng, P. Kondos, J. Jiang, M. Anselm, and P. Borgesen
Abstract: Land Grid Array (LGA) packages are attractive for a number of reasons. Unlike with SnPb solder, recent work showed lead free LGA assemblies to survive at least as long as corresponding Ball Grid Array (BGA) assemblies in thermal cycling, in spite of the much shorter solder joints. The superior thermal fatigue resistance of the smaller solder joints was found to be associated with a change in Sn grain morphology in the SAC305 joints, and work is ongoing to determine materials and design parameters that might ensure the reproducibility of this microstructure. On the other hand, the greater rigidity of the shorter joints raises concerns with respect to solder pad cratering, a failure mode that is increasingly dominant under various isothermal loading conditions and is even becoming more common in thermal cycling. The risk of pad cratering increases further because of the greater hardness due to the aforementioned difference in morphology which adds further to the already high stiffness of the joints. Nevertheless, cyclic bending experiments showed delayed cratering in LGA assemblies compared to BGA assemblies.

A Comparison of Four SnAgCu Alloys in Isothermal Cycling with Varying Amplitude
Authors: Sa’d Hamasha, Mazin Obaidat, and Peter Borgesen
Abstract: The behavior of lead-free solder alloys under complex loading scenarios is still not well understood. Extrapolations of accelerated test results to life under realistic long term service conditions fail to account for important effects of variations in cycling amplitude. Depending on the actual spectrum of amplitudes Miner’s rule of linear damage accumulation has been shown to overestimate life by more than an order of magnitude and greater errors are predicted for other combinations. This is a result of the effects of cycling parameters on subsequent materials properties. These effects are not yet fully understood or quantitatively predictable, preventing straightforward modeling. Damage accumulation has, however, been shown to scale with the inelastic energy deposition (work), even if amplitudes vary. This and the observation of effects of loading history on subsequent work per cycle provide for a modified damage accumulation rule which allows for the prediction of life.

Individual joints of four different Sn-Ag-Cu based solder alloys (SAC305, SAC105, SAC-Ni, and SACX-plus) were cycled in shear with combinations of two different amplitudes, monitoring the evolution of the effective stiffness and work per cycle. Deviations from Miner’s rule varied systematically with the combination of amplitudes, the sequences of cycles, and the strain rates in each. The severity of deviations also varied systematically with Ag-content in the solder, but major effects were observed for all the alloys. A systematic analysis was conducted to assess whether scenarios might exist in which the more fatigue resistant high-Ag alloys would fail sooner than the lower-Ag ones.

Effect of Isothermal Pre-Aging on the Failure Mechanism of SAC Solder Joints in Mild and Harsh Thermal Cycling Test Conditions
Authors: Sam Mahin-Shirazi, Babak Arfaei
Abstract: The present study addresses the effect of isothermal pre-aging on microstructural evolution and the failure mechanism of SnAgCu (SAC) assemblies subjected to mild (20/80 ͦC) and harsh (0/100 ͦC) thermal cycling test conditions. The microstructure of various assemblies such as various BGAs, wafer level chip scale package (WLCSP), quad-flat no-leads (QFN), surface mount resistor (SMR) and ceramic oscillator were studied in as-reflowed and with/without pre-aging prior to thermal cycling test in both mild and harsh conditions. The common failure mechanism for the pre-aged samples was coarsening and recrystallization in high strain region and crack intergranular crack propagation. Observations from the microstructure study suggested that the failure mechanism of pre-aged samples in mild and harsh samples looked similar. No supportive evidences were found that possibly verify that pre-aging could have improved the fatigue life or could cause the second failure mode on the studied samples.

On the Inapplicability of Current Models to Thermal Cycling Induced Fatigue of SnAgCu Solder Joints
Authors: Peter Borgesen, Jianjun Jiang
Abstract: This report is the first in a serious of three that together define our current understanding of the evolution of damage and failure in BGA/CSP scale SnAgCu solder joints in thermal cycling. It documents the inapplicability of current models based on the Engelmaier and Norris-Landzberg expressions, scalings with plastic strain energy deposition or entropy, and recent models based on correlations between microstructure evolution and damage. Taking advantage of a unique set of thermal cycling test results we show that most currently used models do not account for observed dependencies on individual test parameters. The systematic deviations show that we cannot have confidence in acceleration factors predicted by the models. We argue that they can also not be used to help interpret comparisons between accelerated test results for different materials or designs. Finally, none of the models are compatible with our current understanding of the rate controlling mechanisms.

Fatigue Life of SnAgCu Solder Joints under Realistic Service Conditions
Authors: P. Borgesen, S. Hamasha, M. Obaidat, and A. Qasaimeh
Abstract: The life of a solder joint under cyclic loading may be limited by pad cratering, or by crack growth through the intermetallic bond to one of the pads, as well as by fatigue of the solder itself. Interpretation of accelerated test results in terms of life or relative performance in service should account for effects of variations in loading type and/or amplitude on each of the competing mechanisms, but current damage accumulation rules break down for each of them, albeit for different reasons. All the mechanisms are affected by variations in the solder deformation properties with the specific thermomechanical history. Coarsening of secondary precipitates can be accounted for by microstructurally adaptive constitutive relations but isothermal cycling of individual solder joints reveal additional effects ascribed to the evolution of different dislocation cell structures in the large tin grains. In addition testing of individual solder pads, eliminating effects of the solder properties, show variations in cycling amplitude to systematically reduce subsequent acceleration factors for solder pad cratering. Results of varying the temperature range in thermal cycling and the stress amplitude in isothermal cycling are illustrated and explained.

Effects of Thermal Cycling Parameters on the Life of SnAgCu Solder Joints
Authors: Michael Meilunas, Awni Qasaimeh, Sa’D Hamasha, and Peter Borgesen
Abstract: This report summarizes a systematic matrix of thermal cycling experiments on specially designed area array test vehicles with SAC305 solder joints. The different designs employed 12, 16, 20, and 30mil diameter solder balls, respectively, reflowed between Cu pads on the components and substrates scaled to ensure the same solder joint shapes. Pad layouts were the same except that pitches were scaled to ensure similar thermal cycling induced strain ranges for corresponding joints in the different assemblies. All designs were subjected to cycling from -40°C to 125°C, or from 0°C to 100°C, or from 0°C from 80°C, with dwell times of 15 or 60 minutes.
    The duration of the low temperature dwell was found not to have any significant effect on the failure distributions. Both the high temperature dwell and the cycling temperatures did, however, affect the characteristic life in the direction anticipated. Notably, a previously observed effect of solder joint size on the acceleration factors was confirmed and further details on this important trend documented. The observed systematics form a critical part of the basis for a series of three separate reports that together define our current understanding of the evolution of damage and failure in BGA/CSP scale SnAgCu solder joints in thermal cycling.

PCB Laminate Evaluations IT150DA, IT200LK, EM828
Author: Michael Meilunas
Abstract: ITEQ IT150DA, ITEQ IT200LK and EMC EM828 are epoxy-glass laminates for printed circuit board fabrication.  IT150DA is a ”low-loss”, low permittivity material while IT200LK and EM828 are “mid-loss” materials. The AREA consortium designed and acquired a printed circuit board with each laminate system and performed reflow, pad pull, trace peel, spherical bend, cyclic bend, drop test and thermal cycle tests on the substrates in an effort to compare the three materials.