Comparative Characterization of Electronic Packaging Materials through Steady State Thermal Conductivity Measurements

Abstract

The decreasing feature size and increasing power generation of modern electronic devices have created a need for increasingly effective and efficient thermal management solutions to remove the high heat fluxes being generated. The complexity of these devices makes it difficult to model the effects of changing a single component on the system as a whole and often necessitates the use of experimental measurements when comparing alternative solutions.

In harsh environments, including high shock and vibration, magnetic devices such as transformer coils are potted to enhance thermal performance and provide mechanical protection. One potting compound frequently used is epoxy containing alumina particles. A nominally isotropic and uniform potting compound consisting of approximately 70 to 80% by volume 14-28 mesh (0.6 to 1.2 mm across) alumina granules in low viscosity epoxy was tested to determine its thermal properties. Examination by optical microscopy revealed significant variation in volume fraction of alumina particles by location. The specific heat and thermal conductivity of the compound were measured using a Differential Scanning Calorimeter and a comparative cut bar apparatus based on ASTM D 5470. The thermal properties were found to vary with time, location, and temperature; with the specific heat ranging from 1.00 J/g K 14% at 298 K to 1.22 J/g K +- 12% at 398 K and an apparent thermal conductivity of 2.56 W/m K +- 23%. Users of such compounds should be aware that the thermal properties are not necessarily constant in time or uniform, and assuming that they are could lead to significant errors when modeling their performance.

Steady state thermal conductivity measurements were used to compare printed circuit boards (PCB) manufactured from the same design by different vendors and the effect of vias filled with an epoxy versus unfilled vias on the thermal resistance of a PCB. It was found that the thermal resistance of the PCBs varied as much as 30% between vendors and that the PCBs with the unfilled vias performed slightly better than those with the filled vias.

A non-destructive method was used to determine the effects of thermal cycling on the thermal performance of a PCB attached to an aluminum substrate with a thermal adhesive. This method allows for a comparison of the thermal performance of various thermal interface materials (TIM) in an industrial application. Testing was done on FR4 and Flex boards, both with and without overmolding, attached using pressure sensitive adhesive (PSA) and an alternative proprietary adhesive. Baseline measurements were taken, then the boards were cycled from 233 to 398 K on a 90-minute cycle with 15-minute dwells at the target temperatures. It was found that both adhesives showed an increase in thermal conductivity, possibly due to curing, and delamination occurred at 17 out of 35 locations with the alternative adhesive within the first 1500 cycles while no delamination occurred with the PSA.