Effect of geometry on the strain in electronic packages

Due to mismatch in the coefficients of thermal expansion of the components of an electronic package, mechanical strains are induced which may cause package failure under thermal load. An experimental method of fractional fringe Moire interferometry enhanced by digital image processing was used to investigate those strains. This technique is a full field displacement measurement tool possessing high sensitivity and excellent spatial resolution. Therefore, the relatively small regions, such as chip corners, which are thought to be high strain concentration zones, were easily examined. Several packages which had chips of different sizes were investigated. The resulting Moire patterns were recorded and analyzed. The results revealed that the mechanical strains which are due to mismatch in the coefficients of thermal expansion of the chip, leadframe, and encapsulant are on the order of thousands microstrains and the strains in the long chip package are lower than those in the package with a short chip specimen at 90°C