A Methodology for Thermal Evaluation of Strongly Bonded Packaging Materials

A test apparatus and methodology have been developed to provide accurate measurement of the thermal resistance of interface materials for predictive modeling simulations and selection of optimal cooling solution for high-power electronic modules. A guarded heat flow apparatus, based on the ASTM D5470 standard, using calibrated RTDs embedded in precision-machined meter blocks, has been developed to measure the bulk and contact thermal resistance of both single layers and stacked assemblies of thermal materials. This test station has demonstrated a differential temperature accuracy of 0.006 degC and a repeatability of <0.001 degC/W for a single layer of thermal interface material (TIM) for both 2-in. diameter and 1-in. square area test surfaces. A new test strategy has been developed to facilitate measurement of strongly adhesive TIMs in a bonded assembly. A thermal test vehicle composed of two non-instrumented heat spreader (HS) coupons directly bonded by an adhesive TIM is mounted between the GHF test surfaces. For a calibrated instrument TIM and well-controlled test vehicle geometry, the TIM joint resistance in OHFC Cu assemblies has been measured to an uncertainty of ~0.005 degC/W. We have verified that it is possible to extract both bulk and contact resistances within experimental accuracy. The method has been tested on a matrix of high performance TIMs and HS materials, including Cu and diamond composites, filled adhesives and metal (solder) interfaces. In addition, this technique has been used to compare the thermal performance of nominally "good" samples with those having defects, such as voids or delamination, in the thermal interface layer, and also to modeling predictions