Authored By:
Thomas A. Woodrow, Ph.D.
Boeing Research and Technology
Seattle, WA USA
Summary
Mechanical shock testing was conducted by Boeing Research and Technology (Seattle) for the NASA-DoD Lead-Free Electronics Solder Project. This project is follow-on to the Joint Council on Aging Aircraft/Joint Group on Pollution Prevention (JCAA/JG-PP) Lead-Free Solder Project which was the first group to test the reliability of lead-free solder joints against the requirements of the aerospace/military community.
Twenty one test vehicles were subjected to the shock test conditions (in four batches). The Shock Response Spectrum(SRS) input was increased during the test after every 100 shock pulses in an effort to fail as many components as possible within the time allotted for the test.
The solder joints on the components were electrically monitored using event detectors and any solder joint failures were recorded on a Lab view-based data collection system. The number of shocks required to fail a given component attached with SnPb solder was then compared to the number of shocks required to fail the same component attached with lead-free solder.
A complete modal analysis was conducted on one test vehicle using a laser vibrometer system which measured velocities, accelerations, and displacements at one hundred points. The laser vibrometer data was used to determine the frequencies of the major modes of the test vehicle and the shapes of the modes. In addition, laser vibrometer data collected during the mechanical shock test was used to calculate the strains generated (using custom software).
After completion of the testing, all of the test vehicles were visually inspected and cross sections were made. Broken component leads and other unwanted failure modes were documented.
Conclusions
The results of this study suggest that for most component types, the lead-free solders tested are as reliable as eutectic SnPb solder with respect to mechanical shock. Most of the components tested (including reworked components) successfully passed the tests defined in MIL-STD-810G 33 times each no matter which solder was used. These tests are the Functional Test (Flight Equipment); the Functional Test (Ground Equipment); and the Crash Hazard Test (Ground Equipment).
These results suggest that the Pb-free solders tested can be used on designs that will be exposed to mechanical shock and will perform as well as currently used eutectic SnPb solder under many use conditions.
Initially Published in the IPC Proceedings
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