Reliability testing and stress strain estimations of flip-chip joints made by stud-bump-bonding technique

Abstract

The stud bumping with ball bonder is a flexible method for making bumps suitable for adhesive joining of flip-chips. We have made test structures with stud bumped chips onto alumina and FR4-substrates using isotropically conduc Live adhesives and underfill materials selected to be compat ble together and with the type of substrate used. The chips protected by underfill dispensing were tested to remain sti.ble up to 500 cycles in thermal cycling between -40 and 120 'C. Because of the limited mechanical strength of adhesive joints especially when the area of joints is very small compared with the chip area, good reliability in temperature cycling requ res an underfill material to increase the mechanical strength of joining and to reduce the thermally induced deformation caused by different thermal expansion of the chip m d substrate.Calculations have been made to estimate the stress-stmin behavior of adhesive joints in temperature cycling usin:: a developed 1 -dimensional model together with assumption: of the material behavior. The effects of various geometr cal factors on the reliability have been studied. The application of underfill material, temperature dependence of its e h t i c modulus, difference of the thermal expansion coefficients of the chip and substrate as well as the stiffness of the structure adjacent to joints seem to have large effects on the deformation of the joints in temperature cycling.