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30 SMT Magazine • January 2017 by Jim Wilcox, Ph.D. and Francis Mutuku, UNIVERSAL INSTRUMENTS CORP.; and Shuai Shao and Babak Arfaei, Ph.D., BINGHAMTON UNIVERSITY Abstract Drop shock reliability testing was performed on circuit boards assembled with several dif- ferent 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 as- sembled 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 sol- der-mask defined pads to minimize the occur- rence of pad cratering failure modes in the lam- inate material. The test package was soldered to the drop board using either BGA or LGA in- terconnections to explore the effects of solder joint volume. A direct comparison in drop per- formance was made between test boards having two common PCB surface finishes: OSP and im- mersion Ag. All samples were repetitively dropped until electrical failure. Drop shock events were charac- terized with acceleration monitoring and strain gage measurements on the mounted test boards. Solder joint microstructural analysis was per- formed on failing parts to establish the failure modes. The dominant failure mode was obser ved to be solder joint failure, either in the bulk sol- der or cracking along the interfacial intermetallic compound on the board pad. For the PCB lam- inate material used, SAC305 solder joints were observed to produce the best alloy drop perfor- mance in both BGA and LGA joint formats. Introduction Lead free solder joint reliability in drop shock loading has been a recurring issue in mobile and handheld consumer electronics. Changing solder composition may offer an op- portunity to improve joint drop reliability. Low Ag alloys such as SAC105 have for instance been reported to have better drop performance than high Ag alloys such as SAC305 1-5 . Some investi- gations suggested that this was because the fail- FEATURE

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