Thermal shock and drop test performance of lead-free assemblies with no-underfill, corner-underfill and full-underfill

摘要

A wide array of lead-free alloys is available in the market and distinguishing one over the other is not an easy task. Several factors have to be considered before making a choice. Reliability under both thermal and mechanical conditions is one such factor. This experimental research aims at making a comparison of the different lead-free solder ball alloys for area array and advanced packages, assembled using lead-free solder paste. Package reliability will be compared for No-underfill, Corner-underfill and Full-underfill. Assembly reliability was evaluated by subjecting the assemblies to 30 mechanical drops in the as-assembled(AS) condition and after 200 and 500 thermal shock cycles (TS). The scope of this paper is limited to the performance evaluation for area array packages (UCSP, PBGA676, PBGA1156, PoP, CVBGA). Solder ball alloy for the area array packages include SAC305, SAC405, SAC105 and SnAg. The solder paste used for the assembly is SAC305 with Type 3 solder particle size. Three different PCB surface finishes, electroless nickel immersion gold (ENIG), SnPb hot air solder level (HASL), and immersion silver (ImAg) are used. Different solder ball alloys and surface finish combinations will provide data to compare intermetallic thickness. Assembly reliability was evaluated by subjecting the assemblies to 30 mechanical drops in the as-assembled condition and after 200 and 500 thermal shock cycles. After each drop the components were checked for the continuity of the total daisy chain. The number of drops for the first failure was used in analyzing the performance of the components for various combinations. Since each component had many independent daisy chains, the failure of the individual daisy chains was later used in determining the location of the failure and how it progressed. Test results gathered for no-underfill, corner-underfill and full-underfill assemblies indicate SnAg alloys for the solder balls to be performing better than the SAC305 and SAC405 al--loys for PBGA676, irrespective of the PCB pad surface finish. The location on the PCB could have had an influence on these packages. An improvement in drops to failure was also observed for some packages with corner-underfill. But with full-underfill the improvement was observed for all packages. It was also observed that UCSP failed to withstand 500 thermal shock cycles for no-underfill and corner-underfill assemblies. But the package was able to withstand 500 cycles of thermal shock for full-underfill assemblies. Solder joint analysis reveals pad cratering and crack formation to be the root cause for failure. These packages were also cross-sectioned in order to record the changes in intermetallic thickness. This paper will provide a detailed analysis of the findings.