Embedded mini heat pipes as thermal solution for PCBs

Abstract

Rapidly ongoing miniaturization in combination with increasing electronic functionality — particularly in high end applications — leads to the need of progresses in the PCB cooling technology. Improved thermal performance of PCBs should allow, at lowest possible cost, to remove the power loss from components without exceeding the specified maximum temperature. Heat decentralization, i.e., heat spreading and heat guiding in the PCB is one opportunity to achieve high cooling effectiveness from passive systems. In order to provide an effective heat transport from the heat source into the lateral direction of the PCB, embedded miniaturized heat pipes are a promising solution for the heat spreading problem. Heat pipes have an extremely high effective thermal conductivity compared to solids because they are based on a different heat transfer mechanism. Heat pipes are basically a closed vessel (usually a copper pipe) whereas fluid and gas (often water and air) are trapped inside at low pressures. Inner structures called wick are made in such way to provide means for fluid water flow inside the vessel. When heat is applied on the evaporator section of the heat pipe, the fluid inside is vaporized and flows to the other end of the heat pipe (condenser) due to the difference in pressure. At the condenser section the heat flow is absorbed, so that the vapor condenses into a so called wick structure. Capillary forces acting inside the porous wick structure drives the liquid back to the evaporator. However, since this process is governed by vaporization and condensation of the medium inside the heat pipe along a certain distance, the heat transport capacity is limited related to the geometrical dimensions. Within the scope of this work, the heat transfer performance of miniaturized heat pipes (diameters 1.5 mm and 2.0 mm) is quantified by thermal 3D simulations. The heat spreading capability of miniaturized heat pipes embedded in four different test PCBs are investigated. The effective thermal conductivity and the thermal resistances between the location of the temperature maximum and a defined point of the heat pipe at ambient temperature are determined in dependence on different design parameters, respectively. One important parameter is the thermal interconnection between the copper structure of the PCB and the heat pipe: interconnects by copper filled slots or vias — both standard manufacturing steps in the AT&S embedding process — proved to be superior solutions. The simulation results are compared with experimental ones from a previous study and are in satisfying agreement.