Thermal modeling for warpage effects in organic packages

INTRODUCTION Maintaining the integrity of the heat dissipation path for high end microelectronic devices has become increasingly challenging as the industry migrates from ceramic to organic packaging. Typically, flip chip organic packages undergo significant thermal and mechanical stresses throughout the manufacturing process, including chip join, underfill, encapsulation, BGA attach and card join. As a result of the mismatch of thermal and mechanical properties between the components, significant warpage is generated in the organic substrate, the chip, and the thermal interface material (TIM1) on completion of the assembly. Warpage affects not only the substrate coplanarity but also the thermal performance of the TIM1. At room temperature, the center of an adhesive TIM1 is under compression between the lid and chip while the corners and edges are under tensile stress. The effective thermal conductivity for the portions of the TIM1 under tensile stress can be significantly lower due to the elongation of the gap and the narrowing of the heat flow area. The present paper describes an approach to model the thermal performance of an adhesive thermal interface material taking into account the warpage effects and the inherent out-of-flatness in the heat spreader. Reasonable agreement is obtained between the modeling results and thermal measurements for a representative thermal test vehicle. The present modeling approach can potentially be used to optimize the component design and the bond and assembly process to achieve optimum thermal performance.