Characterization of Die Stresses in Plastic Packages Subjected to Moisture and Thermal Exposures

Abstract

Stress sensing test chips are a powerful tool for measuring in-situ stresses in electronic packages. In this study, we have applied (111) silicon test chips to perform a variety of measurements of die stresses in plastic packages. In particular, stresses were characterized in 240 pin Quad Flat Packs (QFPs) subjected to various thermal and moisture loadings. The utilized 10 × 10 mm sensor chips incorporated optimized eight-element piezoresistive rosettes that were capable of measuring the complete state of stress at the die surface (including the interfacial shear stresses). The fabricated test chips were initially used to measure die stresses in the QFPs after molding and post mold bake. Measurement results were correlated with finite element simulations of the molding process. Subsequently, the effects of thermal cycling on the measured die stress distributions for selected packages were investigated. After these initial measurements, the samples were stored at room temperature and ambient humidity for 17 years. The samples were then re-measured after this long term storage to evaluate the degree of die stress relaxation that had occurred. Several packages were then exposed to a harsh high temperature and high humidity environment (85 C, 85% RH) for various time durations, and allowed to absorb moisture. The die stresses at several locations were characterized as a function of time during the hygrothermal exposure. The weight variations in each sample were also measured during the 85/85 exposure to gauge the moisture uptake, and reversibility tests were conducted to see whether the effects of moisture uptake were permanent. Using these measurements and numerical simulations, valuable insight has been gained on moisture induced failure phenomena in plastic packages. Good agreement was found between the predicted and measured die normal stress distributions occurring after molding of the QFP. The magnitudes of the in-plane normal and shear stresses were found to have decreased by up to 30% after moderate levels of thermal cycling. After long term storage, the experimental measurements showed that the die normal stresses in the QFPs relaxed significantly (up to 40%), while the die shear stresses exhibited only small variations. In addition, the 85/85 hygrothermal exposures had strong effects, generating tensile die normal stress changes of up to 130 MPa. Upon fully redrying in reversibility tests, it was observed that the moisture-induced normal stress changes were not recovered. Good correlations were observed between the variations of sample weight (increases in moisture content) and the variations of the die normal and shear stress changes.