Evaluation of thermal shear strains in flip-chip package by electronic speckle pattern interferometry (ESPI)

In this study, we measured the thermal deformations of flip-chip solder joints by an electronic speckle pattern interferometry (ESPI) system. For the scale of evaluation required, the measured displacement resolution was modified to allow application to micro materials with long working-distance microscopes, iris and zoom lens. The flip-chip, a micro material constructed with silicon chip, printed circuit board (PCB) and Sn-36Pb-2Ag solder, is shear-deformed by temperature changes from 25°C to 125°C in a thermal vacuum chamber. From our experimental results we were able to deduce that the thermal expansion difference between the chip and PCB leads to shear strain at the solder joint. The local deformations of solder balls were measured at a sub micrometer scale, and the local shear strains of the solder balls were calculated from the measured displacements. In addition, the experimental results from ESPI demonstrated that the thermal deformations are similar to those results predicted from finite element analysis (FEA)