Issue link: https://iconnect007.uberflip.com/i/472660
14 SMT Magazine • March 2015 metallic TIm Testing to address Failure mechanisms The use of a metallic TIM has been shown to provide an improvement in thermal perfor- mance through an interface, with reductions in tested thermal resistance values when compared to organic TIM compounds, such as thermal greases, phase-change compounds, gels, and other formulations. While a metallic TIM may meet certain types of application requirements, such as those where a minimal clamping force is available or where an electrically-insulating TIM is necessary, the use of flat indium foils has been prevalent for certain industry segments for decades. An example is the use of flat foils as shims beneath flange-mount RF power semicon- ductors for wireless and communications appli- cations and military and aerospace systems. A set of comparative thermal resistance test data is shown in Figure 1. This graph indicates how TIMs typically perform as pressure is in- creased in an assembly. The performance of sili- cone-based thermal greases, with relatively low bulk thermal conductivity of the compound, is dependent on a high degree of surface wetting to obtain even the relatively poor thermal re- sistance values at low clamping forces. The low bulk thermal conductivity of these compounds is typically less than 1 W/mK and, at the lowest pressures, the compound is relatively thick. Be- cause metallic TIMs are solids, they do not have the ability to wet at very low pressures. This is also true of graphite sheet TIMs. Referring again to Figure 1, the high bulk thermal conductivity of the metallic TIM, com- bined with the patterning applied in the Indium Corporation Heat-Spring® product, results in higher performance (measured as lower values for thermal resistance). A crossover in relative thermal resistance is achieved as pressures reach values greater than approximately 45 psi (or ap- proximately 3 bar). The demonstrated thermal performance of the patterned metallic TIM shown in Figure 1 has proven valuable for applications that in- clude diode lasers, large enterprise server pro- cessor modules, power semiconductors, and RF power semiconductors. These thermal performance improvements are also combined with other product attributes DEvELOPmEnTS WITh mETaLLIc ThErmaL InTErFacE maTErIaLS continues Feature figure 1: developments with MTIMs.