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

Stud bumping with a ball bonder is a flexible method for fabrication of bumps suitable for adhesive joining of flip-chips. We made test structures with stud bumped chips on alumina and FR4 substrates using isotropically conductive adhesives and underfill materials selected to be compatible both mutually and with the type of substrate used. The chips protected by underfill dispensing were tested to remain stable up to 500 cycles in thermal cycling between -40 and 120°C. Due to the limited mechanical strength of adhesive joints, especially when the joint area is very small compared to the chip area, good temperature cycling reliability requires an underfill material to increase the joint mechanical strength and to reduce thermally induced deformation caused by the differential thermal expansion of the chip and the substrate. Calculations were undertaken to estimate the stress-strain behavior of adhesive joints in temperature cycling using a developed 1D model together with assumptions about the material behavior. The effects of various geometrical factors on reliability were studied. The application of underfill material, temperature dependence of its elastic modulus, the difference in the chip and substrate thermal expansion coefficients, and the stiffness of the structure adjacent to the joints seem to have major effects on joint deformation during temperature cycling