Solving Thermal Grease Issues by Using Graphite TIM

Introduction

TIMs (thermal interface materials) are materials used to fill gaps between heat-generating components and heatsinks to reduce the contact thermal resistance in order to increase heat dissipation effects. Although thermal greases are the most commonly used TIMs, they have several issues.

This article will introduce a problem solving case example for power modules used in power conversion applications, such as inverters, by replacing the TIM from thermal grease to Graphite TIM.

Thermal Management Using Thermal Greases and Their Issues

Power modules, which generate large amounts of heat, are usually connected to heatsinks, etc., to dissipate heat. When mounting a heatsink on a power module, joining them as they are will inevitably create gaps between the base plate and heatsink because the base plate is slightly distorted. As a result, the dissipation effects of the heatsink cannot be fully obtained due to the contact thermal resistance caused by inadequate adhesion.
As a countermeasure, as shown in the diagram below, a thermal grease is applied between the base plate and heatsink as a TIM to fill the gaps to reduce the contact thermal resistance, thereby improving the dissipation effects.

1. Thermal resistance variations due to voids

   Gaps may not be completely filled because of the application amount and uniformity of thermal greases, and the resulting voids may cause the inadequate reduction in contact thermal resistance.

2. Increase in thermal resistance due to dryout phenomenon

   The dryout phenomenon refers to a condition whereby the components of thermal greases evaporate due to heat, separating oil and filler. When greases degrade due to dryout, they become hard and crack, increasing the contact thermal resistance.

3. Increase in thermal resistance due to pump-out phenomenon

   The expansion/contraction of base plates during long-term use causes the pump-out phenomenon in which greases are gradually discharged. The greases become thinner due to this phenomenon, creating gaps that result in an increase in the contact thermal resistance.

   

   

4. Increase in total costs

   It is also necessary to implement periodic maintenance (disassembly → cleaning → application → assembly) to address performance degradation due to dryout and pump-out phenomena.
   Therefore, although thermal greases themselves are cheap, total costs are high.