Improved Drop Reliability Performance with Lead Free Solders of Low Ag Content and Their Failure Modes

Abstract

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...

Mechanical Shock Durability Studies of Sn-Ag-Cu-Ni BGA Solder Joints on Electroless Ni-P/Au Surface Finish

“…Leadfree solders like SnAg 3 Cu 0.5 , which are very brittle and much weaker than tin-lead ones against impact loads, are used for the BGA solder joints of the package. [1][2][3][4][5][6][7][8][9][10] These solder joints tend to come off easily during handling or transportation. To resolve this problem, the improvement of the solder joints against impact forces and boardlevel thermal cyclic properties is required.…”

Drop impact reliability of edge-bonded lead-free chip scale packages

“…The fundamental mechanics of board level drop testing was studied by Wong et al [14]. Several studies have been done to improve the reliability of lead-free solder joints by adding micro-alloying additives [15,16] or lowering Ag content [17].…”

Reliability Analysis of Low-Silver Bga Solder Joints Using Four Failure Criteria

“…Due to the higher strength and lower acoustic impedance, SAC305 and SAC405 transfer stress to the component more readily [3]. Low silver alloys (SAC105 and 205) have shown to have better drop shock performance than SAC305 and SAC405 [2]- [5]. 10 This study includes reliability analysis of a variety of sphere alloys which include low-Ag SAC spheres as well as spheres with dopants, particularly bismuth (Bi) and nickel (Ni).…”

“…For the lead-free SAC (SnAgCu) sphere alloys used in Ball Grid Array (BGA) assemblies, low-silver sphere alloys have become increasingly popular due to their improved drop shock performance over the more traditional SAC305 (3% Ag) and SAC405(4% Ag) spheres [2]- [5].…”