The mechanism of the low-k stress reduction in chip assembly by shorter solder bump height

To solve the low-k delamination in chip assembly for highend servers, our hypothesis is proposed that the low-k stress is determined by the bending moment and the stress relaxation of a joint. In our hypothesis, the low-k stress decreases as the joint height (SnAg bump height) becomes shorter, such as below 80μm, in 150μm-pitch joints. Our hypothesis is supported by simulation, in which the low-k stress is investigated as a function of the joint height, the joint material and also the joint width (the joint pitch). Finally, experiments are performed to evaluate the low-k delamination as a function of the joint height and our hypothesis is also supported by experiments.