TIM degradation in flip chip packages

Abstract

The main purpose of the thermal interface material (TIM) is to conduct the heat from the die surface to the Ni finish Cu heat spreader. The key requirement for a reliable package is TIM thermal conductivity or thermal interfacial resistance and bond line thickness (BLT). Additional important parameters are modulus, ability to either be dispensed, low volume manufacturing (LVM) or screened, HVM (high volume manufacturing). To achieve these requirements, the TIM formulation focuses on filler type, size, loading, etc. Typically low modulus gel and grease type TIM are Ag or Al filled. Fillers are designed for high-performance, and high power processor applications. The cross-linking properties of gel or grease type TIM should have enough strength so it can comfortably overcome delamination or pump-out issue during highly accelerated package reliability testing (highly accelerated stress test (HAST), high temperature storage (HTS), and Temperature Cycle B condition (TCB)). Most of the package designers focus on the thermal issues for TIM performance evaluation, characterization, and formulation, but there is not much literature available on TIM degradation during actual package reliability tests. Costly gel and grease type materials have very good thermal properties, but during actual package reliability test gel or grease type material may perform poorly due to excessive voiding, pump-out, and other degradation issues. In this study, besides high-performance and costly gel and grease type TIM, low cost adhesive based TIMs are also studied. Comprehensive material and package characterization was done to select a low cost, reliable TIM for low and medium power flip chip application.