Thermal-mechanical fatigue reliability of pbsnag solder layer of die attachment for power electronic devices

PbSnAg solder was widely used in die attachment for high power chip packaging, and the thermal-mechanical reliability of PbSnAg solder layer is a key factor to evaluate the quality of high power devices packaging. Viscoplastic finite-element simulation methodologies were utilized to predict Pb92.5Sn5Ag2.5 solder joint reliability for die attachment under accelerated temperature cycling conditions (-55degC to +125degC, 10 min ramps/20 min dwells). The behavior of solder under accelerated temperature cycling was described by Anand´s viscoplastic constitutive equations and the fatigue life prediction was investigated in volume-average-energy method by R. Darveaux. Three finite-element models (2D plane model, 3D slice model and 3D quarter model) were established to validate the results, and the response of different chip size for die attach was discussed to optimize the fatigue life. The results show that, the maximum plastic stress, strain, and plastic energy were found at the corner or on the edge of the solder layer, which is the week point leading to initial crack damage in the leadframe-solder-die interfaces. With increasing in temperature cycles, the stress-strain hysteretic loops of the dangerous position have a steady trend. The plastic energy accumulated within temperature cycle becomes larger while the increment of plastic energy per cycle trends to be stable. Also, chip size has some influences on the thermal reliability. With increasing in chip size, the maximum plastic stress and strain at the dangerous position increase, which could lead to poor reliability of the die attachment.