High precision thermal stress study on flip chips by synchrotron polychromatic x-ray microdiffraction

The bending and residual stress of flip chips caused by the mismatch of thermal expansion between the chip and the substrate have been measured by polychromatic microfocused synchrotron x-ray beam. Precise orientation information as a function of position on the chip was obtained from Laue diffraction patterns, so that the bending angle with respect to a reference position at the center of the chip can be calculated at each position. This in turn allows deducing the local curvature of the entire flip chip. Local stress distribution was then mapped by applying a modified Stoney's stress-strain equation to the measured curvature. Our study shows that thermal stress on the circuits and the solder joints in a flip chip strongly depend on temperature and the distance from the center of the chip, indicating that interconnects at the corner and edge of a flip chip are of reliability concerns.