A monolithic aluminum circuit board structure

Abstract

When mounting high power devices on circuit boards, heat spreader or heat sink is often attached on the device top to remove more heat. The commonly used board material is alumina, to which copper (Cu) sheets are bonded by a direct Cu-bonding process. The circuit board is soldered to a thick Cu basis plate. Even though the thermal conductivity of alumina, 30 W/m-K, is reasonable good, the alumina is thicker than 250 μm, resulting in significant thermal resistance. Alumina thinner than 250 μm is not strong enough to survive during manufacturing. The alternative board is metal core printed circuit board (MCPCB), consisting of aluminum (Al) or Cu basis plate as heat spreader laminated with ceramic-epoxy as insulating layer and Cu foil. The ceramic-epoxy layer has low thermal conductivity, 2-3 W/m-K; so that it does not entirely solve the heat conduction problem. In this study, we developed a new Al circuit board technology. Thin alumina layer is monolithically grown on Al basis plate by anodization processes. Alumina has much higher thermal conductivity than ceramic-epoxy. The alumina thickness can be controlled from 25 to 100 μm, depending breakdown voltage needed. Thin chromium (Cr) and Cu are then deposited, followed by Cu electroplating process to build up the Cu layer. The boards produced in this project were evaluated rigorously using scanning electron microscope (SEM). They also went through 500 cycles of thermal cycling test between -40 to +85 °C and 100 hours of high temperature storage test at 250 °C. We believe that this monolithic Al circuit board structure will open up numerous existing and new applications in power electronics, in particular, power modules for automobiles, oil exploration, and space and wireless communications.