Electronics Assembly Knowledge, Vision & Wisdom
Drop Test For Lead-Free Assemblies
Drop Test For Lead-Free Assemblies
In this work, a board-level drop shock test was performed on 9 assemblies. Each board was monitored for shock response and net electrical resistance.
Analysis Lab

Authored By:
P. Snugovsky, J. Bragg, E. Kosiba, M. Thomson
B. Lee, R. Brush, S. Subramaniam, M. Romansky
Celestica International Inc.
A. Ganster, Crane Division NSWC
W. Russell, Raytheon
J. P. Tucker, C. A. Handwerker, Purdue University
D. D. Fritz, SAIC
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Summary
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 ball grid array components with different Pbfree 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 Pb-free solder SAC305. The TSOP-50, TQFP-144, QFN-20, and CLCC-20 components were then hand reworked using conventional SnPb solder to address the sustainment issue. Both 1x and 2x reworks were performed on the non-BGA devices. The PDIP components were also reworked; however, their analysis is not covered in this paper.

In the present work, a board-level drop shock test was performed on nine assemblies, each with 63 components attached. Each board was monitored for shock response and net electrical resistance for all components. In addition, three of these 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, for a total of 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, which were not reworked in this study.

Only three of the leaded and leadless components experienced electrical failure. Damage from the drop shock test was assessed by examining electrically failed and non-failed non-BGA parts by dye-andpry 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 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.

Only three leaded components electrically failed, two that were reworked with SnPb solder and one that was not reworked and contained the original SAC 305 solder. Of the two reworked joints that failed electrically, only the TQFP-144, the more compliant leaded component, showed signs of SnPb solder joint fatigue fracture. The failure of the other two components was due to pad cratering and severed traces. 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 Pbfree solder joints for non-BGA components. More data will be available upon completion of the NASA DoD Pb-free project.
Conclusions
It was found that the predominant damage mechanism in drop testing is pad cratering. Cracks propagate through the board material between the laminate and glass fiber under the pads. Electrical failure was only observed when the Cu trace was completely broken. Of the leaded components that were electrically functional after drop testing, approximately one third were found to be mechanically damaged with pad cratering after dye-and-pry inspection. Whereas only three leaded components electrically failed (less than 1%): two were reworked and one was not reworked. Of those two reworked joints that failed, only the TQFP, the compliant leaded component, showed signs of SnPb solder joint fatigue fracture.

The failure of the other two components was due to pad cratering. There was no correlation found between the number of reworks and the amount of electrical or mechanical failure since only three leaded 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.

Since none of the BGAs were reworked in this study and the test resulted in only a small number of non-BGA electrical failures, the authors are unable to determine the comparative strength of Pb-free vs. SnPb-reworked samples other than to state that both survived the current test plan. Another important finding is that electrical testing is not enough to ascertain interconnect robustness during drop testing. Significant post-test destructive analysis is required to determine the level of mechanical damage.
Initially Published in the IPC Proceedings
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