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...
In this paper a nonlinear, nonuniform cohesive zone is employed to study the detailed features of quasi-static defect evolution in a simple, planar elastic system consisting of a circular inclusion embedded in an unbounded matrix subject to different remote loading configurations. The inclusion-matrix interface is assumed to be described by Needleman-type force-separation relations characterized by an interface strength, a characteristic force length and a shear stiffness parameter. Interface defects are modeled by an interface strength which varies with interface coordinate. Infinitesimal strain equilibrium solutions, which allow for rigid body inclusion displacement, are sought by eigenfunction approximation of the solution of the governing interfacial integral equations. For equibiaxial tension, quasi-static defect initiation and propagation occur under increasing remote load. For decreasing characteristic force length, a transition occurs from more or less uniform decohesion along the bond line to propagation of a crack-like defect. In the later case a critical failure load is well defined and interface failure is shown to be defect dominated (brittle decohesion). For interfaces with large characteristic force length, the matrix "lifts off" the inclusion accompanied by a delay in defect propagation (ductile decohesion). The decohesion modes ultimately give rise to a cavity with the inclusion situated within it on the side opposite to the original defect. Results for small characteristic force length show consistency with England's results for the sharp arc crack on a circular interface (England AH (1966) ASME J Appl Mech 33:637-640) Stress oscillation and contact at the tip of the defect are observed primarily for small characteristic force lengths under extremely small loading. Results for remote tension, compression and pure shear loading are discussed as well. In the final section of the paper the results obtained in the first part are utilized to estimate the plane effective bulk response of a composite containing a dilute distribution of inclusions with randomly oriented interface defects.
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