Methods of analysing thermomechanical stress in plastic packages for integrated circuits

Thermomechanical stresses are built up in the plastic packages during the manufacture of an integrated device and also throughout the working life of the device, arising due to a mismatch between the coefficients of thermal expansion of the package materials. Excessive levels of thermomechanically induced stresses may lead to cracking in the packages, which could in turn lead to failure of the device. It is important, therefore, to be able to predict and measure thermomechanical stresses in order to improve package reliability. This paper describes the use of finite-element techniques and strain gauges to determine package stress levels. The effect of delaminations at the interfaces of the package materials is discussed also, and the scanning acoustic microscope is introduced as a complimentary tool to identify stress-related defects in plastic packages.