Drop Test Assessment of a Medium Complexity Assembly for High Reliability Applications

ABSTRACT The mechanical behavior of printed circuit assemblies (PCA) at high strain rates is very important for the reliability of products used in harsh environments. The transition to Pb-free materials in the general electronics industry significantly impacts the mechanical reliability of solder joint interconnects, as widely recognized by the consumer electronics industry. Numerous mechanical behavior studies using a drop test have been reported on components with different Pb-free solders. This study is focused on leaded and leadless components in comparison with ball grid array components assembled with Pb-free solder on medium complexity boards. This study is part of a large scale NASA DoD project and utilized the same board design, assembly, and rework processes of that larger project. Components were attached to the boards using SnPb and Pb-free solder SAC305. The leaded and leadless components TSOP-50, TQFP-144, QFN-20, and CLCC-20 were then hand reworked using conventional SnPb solder to address the sustainment issue. The ball grid array components BGA and CSP underwent hot air rework also using conventional SnPb solder. In the present work, a board-level drop shock test was performed on two sets of boards; each board had 63 components attached. The first set consisted of 9 boards and the testing was focused on leaded and leadless component behavior. The second set consisted of 20 boards and testing was focused on the BGA components. Each board was monitored for shock response and net electrical resistance for all components. In addition, three cards were monitored for board surface strain. The assemblies were fixtured to a drop table 3-up and subjected to either 340G or 500G shocks. The first set of 9 cards was subjected to 20 drops per board. The shock response, net resistance and strain were recorded in-situ during each drop. The vast majority of the electrical failures occurred on the PBGAs. Only three of the leaded and leadless components experienced electrical failure. For the first set of 9 cards damage from the drop shock test was assessed by examining electrically failed and non-failed non-BGA parts by dye-and-pry and cross-section analyses followed by microstructural examination and defect mapping. It was found that the predominant failure mechanism was board side pad cratering. The cracks propagated through the board material between the laminate and glass fiber under the pad. Electrical failure was only observed when the Cu trace was broken. Of the leaded components that were still electrically functional after drop testing, approximately one third were found to be mechanically damaged with pad cratering after dye and pry inspection. This hidden damage may be a reliability concern depending on the field use conditions. There was no correlation found between the number of reworks and the amount of electrical or mechanical failure since only three non-BGA components failed in the test. Most importantly, this sample set showed no difference in drop test performance between SnPb-reworked and non-reworked Pb-free solder joints for non-BGA components. The second set of 20 boards was tested to evaluate BGA drop performance. The boards were subjected to 500G shocks, for total of 10 drops per board. Although the number of samples evaluated was low, due the large number of variables, drop testing of the PBGA parts showed the following trends: 1) BGAs with mixed SnPb/SAC 305 solder joints failed before pure SnPb BGAs, 2) When joints are mixed, mixed joints with SAC 305 in the ball and SnPb paste were more robust than those mixed with SnPb balls and SAC 305 paste, and 3) for both pure SnPb and pure Pb-free PBGAs, increasing the number of reworks reduced the resilience of the BGAs to drop testing.

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