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Solder joint reliability of lead free solders (Sn-Ag-Cu) in drop testing has been an issue in mobile and handheld electronics. Since lead free solders have lower drop performance compared with Pb-Sn solders, many efforts have been reported to improve solder joint reliability with various lead free solders. In this study, standard JEDEC drop reliability tests were performed for a CSP (chip scale package) prepared with two different compositions of lead free solders (SAC405 alloy: Sn-4Ag-0.5Cu and SAC105 alloy: Sn-lAg-0.5Cu). Lead free solders were assembled on substrates with a Au/Ni surface finish. It was seen that SAC 105 alloy solder demonstrated better drop reliability compared with SAC405 alloy solder and failure analysis conducted to understand the differences in failure modes & drop reliability performance.. The fundamental cause of improved drop reliability performance of SAC 105 solders and the advantages of using low Ag content SAC lead free solders for microelectronic devices is discussed. Finally, a finite element model was developed and validated with failure analysis to investigate the high stress concentration distribution and failure mode in solder joints for drop test stress conditions. IntroductionThe improvement of drop/shock reliability of very fine pitch Ball Grid Array (BGA) Chip Scale Package (CSP) with lead free solder has been the key focus for mobile and handheld electronic products as lead free solders typically show increased brittle solder joint failures than that of Pb-Sn solder. [1] Extensive studies on improving drop reliability with lead free solder have been focused on two directions, namely investigations on the effect of surface finish methods of substrate [2,3,4] and different composition of lead free solders, or combination of both factors [1,5]. For the effect of lead free solder composition, improved drop resistance on board level reliability had been reported with low Ag content solder, such as Sn-lAg-0,5Cu (SAC 105) [1,5]. However, there has been limited discussion in the literature on the failure modes of low Ag content solder and the fundamental understanding of its improved drop performance.In this study, drop reliability data with chip scale package of two different compositions of lead free solders (SAC 405 & SAC 105) were collected. In order to conduct an in-depth study on the effect of low Ag content of solders, one substrate surface finish technology (electrolytic Ni/Au) was analyzed. Drop test solder joint failures from both SAC 105 and SAC 405 solder were analyzed with various techniques, such as dye & peel, FIB X-section, optical microscopy, SEM, and TEM to understand the failure mode differences across various scales. The fundamental root cause of improved drop reliability performance of SAC 105 solders and the advantages of using low Ag content SAC lead free solders for microelectronic devices are discussed. Finally, a finite element model was developed and validated with failure analysis to investigate the high stress concentration distribution and failure mo...

indium solder TIM as a function of integrated heat spreader metallization thickness, TIM bond line thickness, and die size. Also studeied were the steps taken to improve its temperature cycle performance. Analyses were performed using thermal resistance measurements, scanning-electron microscopy, scanning-acoustic microscopy, and transmission-electron microscopy to characterize the solder TIM thermal performance, interfacial microstructure, and failure mechanisms.

When electric current flows in a solder bump, electromigration generates stress, but creep relaxes it. After some time, the bump develops a steady-state stress field. We present a theory to show that the two processes -electromigration and creep -set an intrinsic length. When the intrinsic length is large compared to the height of the bump, electromigration is fast relative to creep and the steady-state stress field is linearly distributed in the bump. When the intrinsic length is small compared to the height of the bump, electromigration is slow relative to creep and the steady-state stress field nearly vanishes in the bump, except in a thin layer along the boundary of the bump. We further show that a critical electric current exists, below which the bump can sustain the steady-state stress field without forming voids. Theoretical predictions are compared with existing experimental observations.

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