Comparative Test Data for TIM Materials for LED Applications

Developments in thermal interface materials (TIMs) continue across the industry with a variety of different types of materials. Thermal and mechanical design engineers are often confronted with the need to select which type or category of TIM material is the most appropriate for a specific LED module application, which can be confusing, and how to determine which materials provide the best thermal performance. The next step is understanding which TIM material types meet requirements for ease of shipping, handling, placement, cost, and rework. These are important distinctions, in addition to thermal performance. This presentation will illustrate comparative testing results for a set of thermal interface materials (TIMs) in different categories, using different TIM testing procedures. Test data prepared using three different test methods will be compared:1. ASTM D5470-06 with known temperatures and clamping forces;2. In-situ testing with industry-standard semiconductor modules, at known temperatures and estimated clamping forces;3. In-situ testing utilizing a thermal test vehicle (TTV) for TIM2 performance for a processor module. In-situ testing has been performed at an independent power semiconductor manufacturer, using both industry-standard and commonly-available modules and a custom-designed module with a relatively small footprint, capable of high operating junction temperatures. This testing data can illustrate how different types of TIM materials perform in laboratory testing conditions, for precise comparisons on thermal performance alone; and how different types of materials perform in what are termed as “in-situ” test procedures. This term is used for application-specific conditions, where additional variables are encountered in the testing (such as non-flat surface conditions and unknown clamping force values), which is significantly different from the laboratory conditions used to generate ASTM D-5470 test values. The comparative testing that has been undertaken will be described, showing that images of various power semiconductors with several different materials tend to correlate with the thermal resistance of materials measured with the ASTM D 5470-06 method. These thermal interface materials were also tested on a TTV supplied by a major processor module. The relevance of the thermal imaging, the TTV and the ASTM values will be discussed. This presentation is intended to illustrate the differences in experimental data from one TIM material to another, as well as the differences in testing procedures.