Development, testing, and application of metallic TIMs for harsh environments and non-flat surfaces

Abstract

Important determinants of semiconductor device reliability are operating temperature and proper temperature control within a given operating environment. Maintaining a semiconductor device at or below the maximum rated junction temperature (Tj) is accomplished through careful thermal design, including selection of well-performing thermal interface materials (TIM) that minimize efficiency losses in packaging and between the semiconductor package and a heat dissipating component. Minimizing thermal resistances through the material stack is an important aspect of maintaining required operating temperatures. Selection of a TIM increasingly requires examining differences in requirements, as improvements in package design have reduced internal resistance and the TIM has become a correspondingly greater percentage of total losses. Heat dissipation has become more critical across an increasingly wide array of different types of semiconductor packages. While many packaging and system engineers look for a single proven TIM that can be applied uniformly across different requirements, simplifying testing requirements and streamlining assembly, the increasing diversity in demanding applications has led recently to development of even more varied and specialized TIM types. An example is TIM selection for application between the semiconductor package and the heat sink for an RF semiconductor, where poor electrical conductivity may impact electrical signal performance. A highly thermally and electrically conductive TIM will provide uniform electrical and heat transfer mechanism between the contacting surfaces. Transition to gallium nitride for RF devices has increased the need for TIMs capable of handling higher heat flux and higher power output in smaller footprints. While a desirable goal is a minimized TIM thickness between two surfaces, this is often not possible when dealing with such application requirements as relatively large contact surface areas; increasingly high power IGBT power semiconductors with significant potential mechanical movement of a baseplate due to rapid temperature change during operation; and highly specialized burn-in and test applications where non-coplanar surfaces are encountered due to use of gimbaled test heads. Each of these is an example of why new forms of metallic TIM preforms have recently been developed and implemented for integrated circuits, power semiconductor, and RF semiconductors. Defining differences in requirements and selection of appropriate TIM types will assist in meeting system design requirements and these differences and a family of new metallic TIMs will be described.