Initial Success on Aluminum Circuit Board Technology

In this paper, aluminum circuit boards (ACBs) were designed, fabricated, and tested to demonstrate the possibility and advantages of the ACB technology. Processes were developed to grow high quality alumina $({\rm Al}₂{\rm O}₃)$ on Al boards and coat thick copper (Cu) layer over the alumina to produce an Al/alumina/Cu structure. The measured resistance and breakdown voltage of the as-formed 50 $\mu{\rm m}$ alumina layer is ${>}{\rm 40}~{\rm M}\Omega$ and 600 VDC respectively. In this design, heat generated by a high power circuit component attached to the Cu layer can conduct through the alumina layer and reach the Al base. Alumina has much higher thermal conductivity than epoxy-glass insulating layer of the popular FR-4 printed circuit boards. The quality of the boards produced in this paper was evaluated rigorously using scanning electron microscope. To test the reliability of the boards, they were put through 500 cycles of thermal cycling test between ${-}{40}^\circ{\rm C}$ to ${+}{85}^\circ{\rm C}$ and 100 h of high temperature storage test at 250 $^\circ{\rm C}$ . To ensure its compatibility with soldering operations, 10 mm $\times\,$ 12 mm Cu substrates were bonded to the Al boards using a fluxless tin process. The thickness of the joint is 9.4 $\mu{\rm m}$ including the intermetallic layers. Despite significant coefficient of thermal expansion mismatch of the structure and large Cu size, the bonded samples show no sign of cracks, breakage, or degradation.