Solder Joint Reliability on 3-D MID LCP Substrates

Abstract

This paper presents the results from an experimental reliability study. Two different kinds of SMD (surface mount device) components were assembled with Sn/Ag/Cu solder paste onto injection molded, thermoplastic LCP (liquid crystal polymer) substrates patterned with MID/LDS technology. Two different LCP grades were chosen for the experiments. The assembly was also realized on FR-4 PCBs (printed circuit boards), which served as control samples. The assemblies were exposed to a thermal cycling test followed by failure analysis. As predicted, failures in solder joints occurred earlier on the LCP thermoplastic substrates than on the FR-4 PCB laminates. It was noticed that with one of the LCP grades, the CTE was not the major factor determining the lifetime of the solder joints. A more prominent factor was the dimensional changes that had occurred in the substrates. An SEM/EDS (scanning electron microscope/energy dispersive spectrometry) analysis on the microstructure of the LCP substrates was performed to explain the findings. The analysis showed that there were differences in the detailed microstructure between the two LCP grades. These differences seemed to be related to the different filler particles.