Effect of Solder Composition, PCB Surface Finish and Solder Joint Volume on drop Shock Reliability

Abstract

ABSTRACT Drop shock reliability testing was performed on circuit boards assembled with several different lead-free solder alloys including SAC305 (Sn3.0Ag0.5Cu). The solder compositions tested range in Ag content from 0 to 3.0% by weight. Alloys with various secondary alloying elements were also included. All drop test boards were assembled such that the solder paste composition matched that of the BGA solder ball alloy to produce homogeneous solder joints of known compositions. An alternative test board design (not JEDEC standard) was used for this drop test evaluation. The test board contains a centrally located CABGA 256 package (17x17 mm body, 1 mm pitch). The board was designed with soldermask defined pads to minimize the occurrence of pad cratering failure modes in the laminate material. The test package was soldered to the drop board using either BGA or LGA interconnections to explore the effects of solder joint volume. Drop shock events were characterized with acceleration monitoring on the drop table and strain gage measurements on the mounted test boards. All samples were dropped until electrical failure. Solder joint microstructural analysis was performed on failing parts to establish the failure modes. The dominant failure mode was observed to be solder joint failure, either in the bulk solder or cracking along the interfacial intermetallic compound on the board pad. The effect of alloy silver content on drop reliability is noted. SAC305 solder joints were found to produce the best drop performance of all alloys tested for both BGA and LGA joint formats.