Material Characterization and Die Stress Measurement of Low Expansion PCB for Extreme Environments

This study characterizes a low expansion PCB composed of STABLCORreg/FR4 laminate that is suitable for flip-chip applications exposed to extreme operating environments from -55degC to +150degC. We demonstrate that the STABLCORreg/FR4 laminate exhibits approximately 80% reduction in CTE and greater stiffness characteristics as compared to typical FR4 material. Additionally, we demonstrate that the laminate successfully passes flammability, thermal vacuum stability, and toxicity testing as required for pressurized and un-pressurized Space applications. Fabricated (111) silicon test chips incorporating integral piezoresistive sensors were utilized to measure the die stresses during the underfill cure process, and in the final cured assemblies. Additionally, die stress measurements were performed as a function of temperature post cure with each PCB type. During the snap cure cycle of the underfill, changes in the stresses on the die surface contacting the underfill were observed due to encapsulant shrinkage. However, it was also found that the majority of the final assembly die stresses are built up during the cooling of the flip chip assembly after cure. The STABLCOR7FR4 laminate was found to have significantly lower magnitude of final assembly die stresses as compared to typical FR4 materials at most sensor locations