Rapid die heating for low-stress die attach

During traditional isothermal die attach assembly, significant thermomechanical stress develops in the solder joints between the die and board. The coefficient of thermal expansion (CTE) of the silicon die and the woven composite circuit board materials are widely different. Under isothermal die attach, there is, hence, a mismatch between the thermal expansion displacements of the die and substrate, thereby leading to stress in the solder joints and die interconnect layers. One avenue to alleviate these stresses is to use alternate die attach processes that rely on localized heating of the die and solder joints so as to minimize the thermal expansion displacement mismatch. Die attach stress can be reduced significantly through rapid die heating (RDH), which results in the die being hotter than the board at the solder solidification point. Analytical modeling shows that RDH can reduce residual stress by up to 80% compared to traditional, isothermal die attach processing. Limited experimental results demonstrate 40% stress reduction to date. This paper will detail these results and physical analysis of the resulting solder joints.