Delamination mechanisms of thermal interface materials in organic packages during reflow and moisture soaking

A thermal interface material (TIM) is typically a compliant material with high thermal conductivity that is applied between a heat-generating chip and a heat spreader in an electronic package. For a high-conductivity polymeric TIM, the adhesion strength between the TIM and its mating interfaces is typically weak, making the TIM susceptible to degradation when subjected to environmental stresses. At typical chip operating temperatures which are below the curing temperature of the TIM, a compressive force acts on the TIM at the chip center due to the CTE mismatch between the die and the organic chip carrier. Conversely at high BGA(Ball Grid Array) or card-attach reflow temperatures, the TIM center is under tension and the TIM tends to either cohesively separate or adhesively separate from the interfaces. Also, during moisture soaking, such as 85C/85%RH, the organic chip carrier absorbs moisture and expands. The hygroscopic expansion of the organic chip carrier is of the same order of magnitude as the thermal expansion. This expansion reduces the compressive force acting on the TIM, and for certain package constructions, this can lead to degradation of thermal performance. In this paper, the delamination mechanism of a polymer-based thermal interface material in an organic package during reflow and moisture soaking is investigated. The in-situ deformation of the TIM bondline was measured by a digital image correlation (DIC) method on a cross-sectioned part. The TIM bondline deformation was also captured by a digital camera. The coefficients of thermal expansion and hygroscopic expansion for different organic materials were measured, and a finite element analysis of the hygroscopic expansion and TIM bondline deformation was conducted. The affect of T&H stress was analyzed using an equivalent CTE concept.