Do you really know the thermal conductivity of your boards?

Thermal engineers, tasked with ensuring reliable operating temperature of devices, routinely have to estimate or more likely guesstimate the bulk thermal conductivity of the board using the limited industry-published data or resort to outdated and unproven closed form correlations for use in simulation tools. This can have an adverse effect on the projected operating temperature of sensitive devices, the accuracy and eventually the reliability. As a first reported attempt at Teradyne and perhaps in industry, a custom measurement instrument has been developed and is used for measuring the effective thermal conductivity of a large variety of test board constructions with various package land patterns, via density, via-fill, and dielectric material which match closely the production boards' construction. The instrument has been calibrated using prepared materials of known or measured conductivity which include Lexan, Pyrex, S. Steel and Brass alloy. To account for the difference and changes in various board designs over time, a commercial thermal software tool, capable of importing the ECAD/Allegro data base electronically, has been acquired for simulation and the projections are compared with the collected measurements. This paper will describe the challenges and the findings as a mean to inform and equip thermal engineers in their quest for a more reliable design and faster to market product introduction.