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Abstract-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 (Al 2 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 μm alumina layer is > 40 M 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°C to +85°C and 100 h of high temperature storage test at 250°C. To ensure its compatibility with soldering operations, 10 mm × 12 mm Cu substrates were bonded to the Al boards using a fluxless tin process. The thickness of the joint is 9.4 μm 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.

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The high thermal conductivity and light weight properties of aluminum (Al) make it a promising material in high power device packaging and automotive design applications. A primary challenge is its high coefficient of thermal expansion (CTE) of 23ppm/°C. In this research, we investigated the possibility of surmounting this challenge by bonding large Si chips to Al substrates using fluxless tin (Sn). Si versus Al pair probably has the largest CTE mismatch among all bonded structures in electronic packaging. In experiments, 0.1μm Cr layer and 0.2μm Cu layer were deposited on Al substrates, followed by an electroplated thicker 25μm copper (Cu) layer. The Sn solder layer was then electroplated over the Cu followed immediately by thin (0.1μm) silver (Ag) layer. The bonding process is entirely fluxless. The joint thickness was controlled either by bonding pressure or by Cu spacers. Microstructure and composition of the joints were studied under scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDX). Despite the large CTE mismatch, the bonded structures did not break. This preliminary result suggests potential adaption of Al substrates in electronic packaging where Al is avoided because of its high CTE.

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