High-speed mechanical impact reliability of solder interconnections in high-power LEDs

Light Emitting Diodes (LED) are being implemented more and more into demanding applications like automotive and high-brightness general lighting. From the reliability point of view, the automotive environment is extremely harsh and challenging. Automotive electronics have to withstand exposure to high temperature fluctuations, mechanical shock impacts and vibration. The cyclic thermal load of solder interconnections can be up to 150°C for several thousand cycles. For harsh environments, high-Pb and eutectic AuSn solders are currently being used as the interconnection material. Recent developments of SAC-based solders provide alternative lead-free solutions with lower processing temperature. However, little is known about the reliability and failure mechanism of these solder interconnections, especially under mechanical impact loadings. For a successful reliability test of the solder interconnection under the impact, it is important to find a widely accepted mechanical test method of measuring the degradation of the interconnection. In this study a newly designed high-speed impact tester that is based on the use of a pendulum was used to achieve this target. The purpose was to investigate the mechanical behaviour of lead-free solder interconnections under different loading conditions. The studied carrier had multi dies mounted onto a ceramic sub-mount, which was soldered to a Cu substrate. In total 60 samples were tested with five different test velocities (0.7, 1.0, 1.4, 1.8 and 2.2 m/s). The test results showed that the primary failure mode was the ductile failure in the solder bulk and only some local fractures at the solder interfaces were observed. This indicates that there was no dependency of the test velocity, since all the test velocities induced similar failures in the bulk solder of the solder joints.