TIM characterization for high performance microprocessors

TIM characterization is critical for the thermal solutions of higher power IC chips, like microprocessors. This paper firstly reviews the interface resistance between two contacting solids, thermal interface materials and standard test methodology based on ASTM standard. It shows that the ASTM standard testers can only find out the pure material properties, like heat conductivity of the TIMs under ideal conditions. However, the TIM performance are affected by many factors and thus TIM should be evaluated under specified application conditions. Thus, a TIM characterization system for microprocessor applications is proposed and demonstrated in detail in this paper. The set up can cater all the important factors, such as clamping force; roughness and flatness of the contacting surfaces; the heating area and the path of the heat transfer, etc. Several TIMs were characterized for both lidded and lidless packages. It is found that the process of the TIM preparation and attachment can affect the performance significantly. Inappropriate attachment of solder A leads to an overall thermal resistance even higher than that of dry contact. The area of the contact surface also plays an important role. For lidded packages, the larger contacting surface makes the overall thermal resistance less sensitive to the TIM used. The measurement of the TIMs under the actual conditions can thus be used as the thermal criteria for the TIM selection. The impact of the clamping force and the number of insertions has also been investigated and quantified under the actual application conditions. Based on the current studies, the TIM preparation process and the application parameters can be optimized to achieve better thermal performance and longer life span. Although the present set up was for mainly for microprocessor packages, the concept and methodology can be implemented to the applications of other IC chips. Finally, the criteria for TIM selections are discussed and it is recommended that TI- - M be selected on the evaluation of all the important factors