Heat Conduction in Printed Circuit Boards: Part II — Small PCBs Connected to Large Thermal Mass at Their Edge

Abstract

The PCB in high density packaging environment often provides a critical heat conduction path from the package to the electrical connectors or the system enclosure that serves as a solid heat sink at the edge of the PCB. Where convective heat transfer on the PCB surface is negligible, heat flow is almost unidirectional from the area under the package footprint to the solid heat sink. However, the heat conduction analysis requires a large computational resource, if the full details of PCB’s internal organization are taken into account. To perform the board and system level thermal analysis with available computational resources, it is imperative to replace the actual PCB’s internal structure by a simplified model. In the most popular modeling the PCB is replaced by a medium having orthotropic thermal conductivities. The assumption of equivalent thermal conductivities, however, is not a straightforward practice, as illustrated by the present study. Two samples are subject to heat conduction analysis; they are rectangular bar specimens cut out from the PCB, one measuring 6mm (L) × 0.5mm (W) × 1.27mm (HPCB), and the other 12.35mm (L) × 1.27mm (W) × 1.27mm (HPCB). The specimen contains two layers of continuous copper, and one row of through-vias in the zone under the package footprint. Numerical solutions are obtained taking into account the full details of the via zone structures. Classical solutions are also obtained for a two-zone continuum model having different sets of orthotropic thermal conductivities for the via zone and the remaining zone. The thermal conductivities in the continuum model are varied to bring the key temperatures to those obtained by the numerical analysis. Presented in this paper is a methodology to estimate the equivalent thermal conductivities for a given set of dimensional data and thermal boundary condition.