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March 2015 • SMT Magazine 15 that eliminate traditional failure mechanisms addressed above. Indium metallic TIMs have been imple- mented for decades for spaceborne applications where NASA, ESA, and other space agencies have specified zero outgassing potential for any TIM. Metallic TIMs and associated indium and metal alloy shims, also meet other system-level requirements and address these tra- ditional failure mechanisms: a. A metallic TIM contains no ionic contaminant, silicone oil, or other constituent sub- ject to potential outgassing. b. Metallic TIM products contain no organic com- pounds or carriers, which are subject to concerns related to pump-out, dry-out, or other separation. This is demonstrat- ed in bake, humidity, HAST, and power cycling testing. El- evated temperature testing has shown that thermal resistance over time and temperature cycling improves as the metallic TIM continues to adapt to minute surface imperfections under clamped conditions. This is distinctly contrary to results shown with silicone thermal grease TIMs. c. Metallic TIM preforms do not contain any compound that will run out of the interface in a vertical mounting orientation. additional attributes of metallic TIms While metal shims, principally manufactured as die-cut indium or indium alloy preforms, have been characterized and used in electronics sys- tem manufacturing for decades, the recent intro- duction of a patterned foil product as a TIM has led to increased use of these metallic TIMs for a wider range of electronics applications. Patterning The application of the patterning is not for cosmetic purposes. The texturing creates the equivalent of an array of compressible columns. Each of these miniature columns is available to conform to minute local imperfections on the surfaces of the interface, distributing via an ap- plied clamping force across the mating surfaces. The application of a metallic TIM product in an assembly, when a given clamping force is applied, results in an initial compression of the pattern columns. Each of these columns exhibits plastic flow at the yield strength un- til enough area of the TIM is com- pressed to support the load. The Heat-Spring spreads the load in a defined array. There is a time- dependent flow phenomenon that accounts for the improve- ment in thermal resistance over the life of the product, as the alloy continues to mold to minor surface imperfections. Note that different pat- terns have been developed for applications that require differing loads, surfaces, and TIM thicknesses. These differ- ent patterns are uniquely var- ied and are an additional tool in the designer's set of potential available solutions for specialized interface requirements. Thickness and relative Bulk Thermal conductivity An important attribute of metallic TIMs is that interface thermal resistance is very weak- ly affected by the thickness of a TIM, given the very high bulk thermal conductivity. cTE mismatch Metallic TIMs can adapt to CTE mismatch requirements between two dissimilar materials or metals, including when an indium metal or indium alloy is used. Electrical conductivity and Performance Metallic thermal interface materials may not be selected for any application, of course, which requires electrical isolation, such as a power transistor package with an exposed elec- trically-live metal tab at the mounting surface to the heat sink. This is an inherent drawback of metallic TIMs for these applications. DEvELOPmEnTS WITh mETaLLIc ThErmaL InTErFacE maTErIaLS continues there is a time-dependent flow phenomenon that accounts for the improvement in thermal resistance over the life of the product, as the alloy continues to mold to minor surface imperfections. " " Feature