2011 Reports

Bondline Assembly Process Manual

Author: David F. Rae

Abstract: It is generally desirable to reduce the thermal path resistance between components and the ambient heat sink to improve device performance and system reliability. In electronics and solar packages, filled-polymeric thermal interface material (TIM) systems are widely used to minimize the thermal path resistance by filling gaps between components with a material that conducts heat far more efficiently than either air or a vacuum. The thermal resistance contribution of a TIM bondline to the thermal path is strongly influenced by the TIM’s properties and the bondline assembly process, which affects the bondline structure and consequently in situ thermal performance.

This assembly manual provides guidance on how to produce high performance filled-polymeric bondlines in an automated and repeatable fashion. We present our best recommendations based upon years of process development work with these materials by researchers working with the AREA Consortium. The objective of this document is to guide process development engineers and product designers from material selection, through process development, and onto qualification. This document contains sample questionnaires, process development checklists, process development/troubleshooting guides, and various other sample process travelers and documentation that are aimed at minimizing the time and effort required to develop a new qualified process.

Assembly Study of 0.4/0.5mm Pitch Through-Molded-Via (TMV) Package on Package (PoP)

Authors: Brian Roggeman and David Vicari

Abstract: The successful integration of package-on-package (PoP) stacking utilizing through mold via (TMV) technology hinges on a robust assembly process. In this study, seven dip materials were investigated for high quality TMV PoP assembly by optimizing machine settings to achieve proper material transfer. Film thickness was varied for each material to transfer enough material (target of 50% ball coverage) while preventing parts from sticking within the film. Assemblies were reflowed in both air and N2 atmospheres and yields were quantified. It was determined that flux dipping provides for better TMV assemblies in air reflow due to the flux’s ability to wet to and subsequently protect the TMV solder ball during reflow. All paste dipped materials experienced significant fallout in air reflow due to a non-coalescing of the TMV solder joint. All materials provided 100% assembly yields in N2 reflow.

Print Volume Influence on Mechanical Test Performance of 0.4mm Pitch CSPs and LGAs

Authors: Brian Roggeman and Jeff Schake

Abstract: Drop testing represents an amplified simulation of accidental mishandling that routinely occurs with handheld electronic appliances. A carefully structured drop test accelerates the damage induced on populated circuit boards, permitting efficient discovery of failure mechanisms that can be used to improve the mechanical stress tolerance designed into next generation products [1]. The driving purpose of this work was to correlate effects of different stencil printed solder paste volume on drop shock reliability. We found that drop test performance improved by simply increasing the printed solder volume for balled array components (i.e. CSPs). Identical components supplied without pre-attached solder balls (i.e. LGAs) were assembled and drop tested to compare. The LGAs exhibited consistently high drop reliability for both high and low printed solder volumes. Further investigation into solder joint failure mechanisms show that the combination of low solder print volume with air reflow atmosphere can inhibit wetting enough to form stress concentrations which drastically reduce drop reliability for CSPs.

Quick Assessment of the Minimum Peak Reflow Temperature Required in Low-Ag Solder Assembly

Authors: Pericles A. Kondos and Michael Meilunas

Abstract: There is an ongoing interest in minimizing peak process temperatures for some SMT assemblies, notably when using inexpensive printed circuit board materials or complex multi layer substrate constructions. Each combination of materials and designs may, however, call for a different minimum. The thorough qualification of a proposed process is a major undertaking that usually allows for the inclusion of only a very few, if any, alternative parameters. There is therefore often a need for an initial rapid identification of the specific process parameter values to include in the final test matrix, e.g. for an initial prediction of the peak temperature range to consider. This was accomplished for selected low-Ag alloys through a combination of microstructure analysis and simple tests, the criterion being that structure and properties should not differ significantly from those achieved using more conventional, higher temperatures.

Thermal Cycle Reliability Assessment of Agere 1936 I/O BGA Author: Michael Meilunas

Abstract: In 2007 the UIC Advanced Process Lab asked AREA Consortium members to suggest lead-free surface mount devices for the upcoming thermal cycle reliability assessment program. Fifteen package designs were provided and copper OSP and ENIG motherboards were designed to accommodate the samples. The devices were assembled and testing began in March of 2008. The following report documents the basic assembly processes, test procedures and results for the Agere 1936 I/O BGA.

Reliability of Lead-Free LGAs and BGAs : Effects of Solder Joint Size, Cyclic Strain and MicrostructureLIC STRAIN AND MICROSTRUCTURE

Author: Denis Barbini and Michael Meilunas

Abstract: An accelerated thermal cycle experiment comparing similarly constructed area array devices representing Land Grid Array (LGA) and Ball Grid Array (BGA) technology with 0.254, 0.30, and 0.40mm diameter SAC305 solder balls was performed. The devices were subjected to three thermal cycle conditions in order to promote 2nd level solder fatigue. Failure data was compared using Weibull analyses. The results show that time to failure is highly influenced by the package pitch and thermal cycle temperatures in a manner predicted by simple mechanics. However, there were instances in which the effect of solder ball size did not fit the traditional solder joint reliability model in which increasing solder joint standoff height improves reliability (i.e. more cycles to failure). A theory is proposed that substantial differences in SAC305 solder joint Sn grain morphology may explain, at least partially, the discrepancies and evidence to support this theory is presented.

Methodology for Assessing the Reliability Impact of Partial Pad Craters

Author: Brian Roggeman and David Rae

Abstract: PCB pad craters are generally associated with single overstress events that cause immediate failure. However, significant reliability concerns are raised when partial non-catastrophic cracks are present under the pads of an assembly at the beginning of service life. This investigation empirically examines the effects of partial pad craters on board reliability. Partial pad craters were created through bending over-stress events which simulate damage modes experienced during assembly, handling, test, and shipment. Crack frequency and crack area distributions were measured for a variety of board flexure magnitudes. Final long-term reliability was then characterized for specific pre-damage levels in cyclic bending environments. The results revealed a dramatic degradation in reliability which was then correlated back to the initial crack distribution from the pre-reliability characterization damage event. Furthermore, the relative effect of pre-damage on reliability was markedly greater as the fatigue stress levels were reduced.

Effects of 'Latent Damage' on Pad Cratering: Reduction in Life and a Potential Change in Failure Mode

Author: Venkatesh Raghavan, Brian Roggeman, Michael Meilunas and Peter Borgesen

Abstract: The transition of electronics manufacturing to lead-free soldering has led to the emergence of pad cratering as an increasingly common failure mode. This first caused high concern among manufacturers of servers and other high reliability products who started out with a focus on failure due to a single overload in testing, handling or transport. More recently, a growing number of manufacturers report early failures by cratering in thermal cycling, a phenomenon virtually unheard of for SnPb soldered assemblies. We show how such changes of failure mode are likely to be results of latent damage induced by a preceding overload in assembly, in-circuit test or handling. Testing of three different laminates showed how latent damage can significantly reduce the pad cratering fatigue life in subsequent cycling. Notably, the relative reduction in life increases as cycling amplitudes are reduced. For purposes of illustration extrapolations suggested that a 10% reduction in accelerated test life might well reflect a life in service reduced by more than a factor of 4!

Recrystallization Behavior of Lead Free and Lead Containing Solder in Cycling

Author: Awni Qasaimeh

Abstract: The present work addresses the effects of thermomechanical history on the recrystallization behavior of lead free and backward compatible solder joints. 30 mil SAC305 balls were reflowed onto BGA pads using either a SAC305 or a eutectic SnPb paste. Systematic variations of the rate of recrystallization with precipitate coarsening as well as with cycling and annealing parameters were characterized and correlated with observations from thermal cycling experiments on lead free BGA assemblies. The introduction of a few percent of Pb has been seen to not only affect the distribution of secondary precipitates but also add minute inclusions of Pb within the individual Sn dendrites. These also act as barriers to dislocation motion but do not coarsen as rapidly as the precipitates, and we find recrystallization in cycling to be strongly delayed.

On the Evolution of the Properties and Microstructure of Backward Compatible Solder Joints during Cycling and Aging

Author: Younis Jaradat, Awni Qasaimeh, Pericles Kondos, Babak Arfaei and Peter Borgesen

Abstract: A number of groups have published accelerated test results comparing backward compatible (mixed) solder joints to pure SnPb and lead free ones, but the question remains as to the actual reliability under realistic long term service conditions. We are addressing this in a comprehensive fundamental study of the evolution of the microstructure and resulting joint properties and performance for mixed solder joints. The present work reports results of reflowing 30 mil SAC305 balls onto Cu coated BGA pads with different amounts of SnPb paste, aging and/or cycling the joints and inspecting the microstructure by cross polarizer microscopy and SEM. The addition of small amounts of Pb was found to affect the solidification during cool-down from reflow, and thus the initial microstructure, in a profound fashion. The shapes and distributions of secondary precipitates were, as expected, different from those observed in ‘pure’ SAC305 joints and a large number of very small Pb inclusions were distributed within the Sn dendrites. As a result the mixed joints started out harder and did not soften as rapidly during thermal cycling and aging. In fact, an initial break up of elongated Ag3Sn precipitates first led to a slight hardening and strengthening. In addition thermal cycling induced recrystallization of the Sn was delayed and acceleration factors were lower. However, the strongly reduced sensitivity to aging suggests that mixed joints may compare more favorably to their ‘pure’ lead free counterparts in long term service than reflected by accelerated test results.

Crack Evolution and Rapid Life Assessment of Lead Free Solder Joints

Author: Awni Qasaimeh, Susan Lu and Peter Borgesen

Abstract: This paper reports on the crack behavior and microstructure evolution of lead free solder joints during thermal and isothermal fatigue testing. Daisy chained Ball Grid Array (BGA) assemblies were subjected to either accelerated thermal cycling or cyclic bending at room temperature. In the case of thermal cycling effects of strain range were assessed by comparing assemblies with different distances to neutral point. Crack areas were measured by dye and pry and recrystallization detected by optical microscopy with crossed polarizers. The stochastic nature of the fatigue crack growth associated with variations in Sn grain orientation was addressed by using pattern recognition with Artificial Neural Network (ANN). Different statistics were calculated for the different crack curves at specific life percentages to address the change in crack growth distributions at these different live intervals. Results suggested that the number of cycles to failure could be predicted by the measurement of crack lengths after less than 10% of a typical thermal cycling test. Preliminary results suggest that this may also be true for isothermal cycling, but trends appear to be less easily generalized.

Damage Accumulation in Pb-Free Solder Joints for Complex Loading Histories

Author: Peter Borgesen, Linlin Yang, Awni Qasaimeh, Babak Arfaei, Liang Yin, Brian Roggeman and Michael Meilunas

Abstract: The long term life of solder joints in service is commonly assessed by extrapolation of the results of accelerated tests such as thermal cycling or vibration according to some model or analytical expression. However, what is commonly ignored is that realistic service conditions are almost never well represented by cycling with a fixed, even if much lower, amplitude. A limited number of efforts have shown life under alternating or simultaneous thermal cycling and vibration to not obey Miner’s rule of linear damage accumulation, i.e. it could not be easily predicted based on measurements of life in the two types of cycling individually. This was ascribed to differences between crack initiation under the two types of loading, but in the case of lead free solder joints significant effects must also be associated with differences in recrystallization. Strong deviations from linear damage accumulation were also observed for sequences of a single type of loading with different amplitudes. To make matters worse, preconditioning by cycling with a particular amplitude did not only lead to a very different life than predicted by Miner’s rule in subsequent cycling with other amplitudes, it also affected subsequent acceleration factors. We show how predictions may overestimate life in service by orders of magnitude. Based on comprehensive research into the nature and behavior of realistic SnAgCu solder joints we discuss when and how current damage accumulation concepts must fail to account for life under various combinations of loading. This includes predictions of consequences for assessments of life under realistic service conditions, i.e. whether we expect these to be conservative or overly optimistic.

Dependence of SnAgCu Solder Joint Properties on Solder Microstructure

Author: Babak Arfaei, Tariq Tashtoush, Nam Kim, Luke Wentlent, Eric Cotts and Peter Borgesen

Abstract: It is well known that variations in the microstructure of lead free solders greatly affect their thermomechanical properties. Sn grain size, orientation and number, as well as secondary Ag3Sn and Cu6Sn5 precipitate sizes and numbers, are all seen to influence the mechanical response of solder joints during isothermal and thermal cycling. The solidification temperature of a SnAgCu solder joint dramatically affects its microstructure. Generally, smaller solder balls (e.g. CSP) undercool more, and thus their microstructure and properties are very different than larger solder balls (e.g. BGA). We report results of a study of the effects of solder joint volume, and pad sizes, on the microstructure and

thermomechanical properties of solder joints. Solder joint shapes and dimensions spanned the ranges typical of BGA and CSP assemblies. Temperatures of solidification during cool-down were quantified by differential scanning calorimetry. Sn grain structures were characterized by crossed polarizer microscopy and scanning electron microscopy with electron backscattered diffraction. Precipitate sizes and distributions were measured using backscattered scanning electron microscopy. Corresponding properties, including hardness, strength and fatigue

resistance were measured before and after aging for various lengths of times at temperatures up to 125ºC. Smaller solder joints on smaller pads were shown to be harder and stronger than larger ones, but to age faster and eventually end up softer and weaker.

Effects of Variable Amplitude Loading on Lead Free Solder Joint Properties and Damage Accumulation

Author: Younis Jaradat, J. Chen, J. Owens, Awni Qasaimeh, Luke Wentlet, Babak Arfaei and Peter Borgesen

Abstract: Current extrapolations of accelerated test results to life in long term service may be greatly misleading when it comes to lead free solder joints. Notably, realistic service conditions are almost never well approximated by cycling with a fixed amplitude. However, recent results have demonstrated the consistent breakdown of common damage accumulation rules. In isothermal cycling tests the remaining life, after a step-down in amplitude, was invariably shorter than predicted by such a rule, while a step-up tended to have the opposite effect. The present work offers a mechanistic explanation for this and a basis for the development of a practical approach to the assessment of life under realistic service conditions. Realistic BGA joints were cycled individually in a micromechanical tester, monitoring the solder stiffness and the inelastic energy deposition. Cycling was seen to first cause rapid hardening, followed by leveling off in a ‘cyclic saturation’ stage and eventually the initiation and growth of a crack until failure. A temporary increase in amplitude during cycling caused a lasting reduction in hardness, and thus enhanced inelastic energy deposition and damage evolution, afterwards as well. This would dominate during repeated increases and decreases, eventually shortening the remaining life dramatically.

Damage Mechanisms and Acceleration Factors for No-Pb LGA, TSOP and QFN Type Assemblies in Thermal Cycling

Author: Luke Wentlent

Abstract: Despite the prevalence of reliability tests throughout the electronics packaging industry, only a relatively small number of them are used to help predict the actual in-service life for an electronics assembly. Much more often ‗engineering testing‘ is essentially focused on comparisons to previous performance or between different designs, materials, or process parameters. Empirical testing can, however, often be misleading. For example, Ball Grid Array (BGA) assemblies on a specific product are found to have longer lives than Thin Small-Outline Packages (TSOP) assemblies in accelerated thermal cycling. In order for this result to be meaningful a vital question is then: Are the acceleration factors the same and, if not, how do they differ?

Significant knowledge has been gathered about the performance of lead-free solder joints in thermal cycling. A systematic mechanistic understanding has been established in the case of typical BGA joints and we are starting to develop guidelines for comparisons of accelerated test results [1-4]. Nevertheless, a serious concern is that very different solder joint configurations (e.g., those with very different shapes and/or ratios of solder volume-to-pad areas) may lead to different solder microstructures and, therefore, different acceleration factors. We have addressed this concern for Land Grid Array (LGA) , TSOP, and Quad Flat No-Lead (QFN) assemblies with SAC 305 solder joints by a combined study of the initial post-reflow solder microstructure, its subsequent evolution, and the associated type and propagation of damage. Based on a discussion of the results, judgments were made as to whether acceleration factors to long term service conditions are likely to be similar to or different from those for BGA assemblies.

Underfill Selection Methodology for Component Underfilling Process

Author: Varun Kesavakumaran

Abstract: Underfilling may improve the reliability of a BGA or CSP assembly, but the wrong underfill may be detrimental. An approach is presented for the rapid down-selection and identification of promising materials. This is focused on reducing the cost of initial testing through simple and innovative non assembly tests. Results on representative materials are compared to results of drop and thermal cycling of assemblies. Two solder mask surfaces, PR77 and PSR4000, were tested with five different corner and edge bond materials as well as six capillary underfills.