Effects of assembly process variables on voiding at a thermal interface

Too often, the effects of assembly process parameters are not sufficiently accounted for in the optimization of thermal interface performance. This becomes increasingly critical as demands on this performance continue to grow and alternative processes are developed. Notably, stencil printing is proving a competitive alternative to the traditional dispensing of thermal interface materials (TIMs), with potentially significant gains in units processed per hour (UPH) for some applications. The two techniques may, however, pose quite different challenges in terms of material flow, the resulting filler particle distribution and the risk of air bubble entrapment. Another part of the adhesive attachment process certain to affect void formation and growth, as well as possibly filler particle distribution, is the final cure. In addition, voids may severely reduce assembly robustness and reliability. The present work offers a discussion and a first case study to identify and illustrate voiding mechanisms for a particular TIM between a heat spreader and the back of a flip chip. Pronounced differences were observed between stencil printing and dispensing in terms of initial void formation, apparently related to the specific properties of the material. Measurements of the effects of heat ramp rate and peak temperature showed the subsequent evolution and final void size distribution to be determined by the initial part of the cure profile up to the material gelling temperature.