Formation of through aluminum via for noble metal PCB and packaging substrate

Aluminum oxide provides high impedance insulation layer on the surface of the aluminum (alloy) body and many 1-dimensional PCBs or module arrays based upon aluminum anodization have been proposed. However 3-dimensional packages or modules can hardly be found. This mismatch is because reliable vertical interconnection through aluminum i.e. `through aluminum via' (TVA) has not been established. Due to the volumetric expansion during the oxidation of Al to Al<sub>2</sub>O<sub>3</sub>, it has been believed preventing cracks in the vicinity of sharp edges is hardly possible. In this paper, we will introduce a vertically conductive `through aluminum via' (TAV) that can be made by aluminum anodization and industry standard PCB manufacturing process. With this structure, we succeeded in constructing an `anodized metal substrate' (AMS) that has supremacy in thermal conductivity, long term reliability and cost effectiveness. The manufacturing process of the substrate was almost identical to that of standard PCB except anodizing process; Mechanical drilling or etching to make the vias, anodizing to form an electrical insulation layer on the whole surface of aluminum plate including the inside wall of the vias, metal layer deposition on the anodized surface by sputtering and electro plating, and patterning by chemical etching. The two key processes are the anodization and metallization of the anodized surface. The electrolytes for anodization contained complex acidic solutions. Industry-standard soft anodizing procedure at room temperature was used and the thickness of the oxidation layer was about 30~40 um. Adhesion metal and copper directly on the anodized surface were deposed by inline sputter. Average electrical resistance between the pads that include the vias was over 100 mega ohm at 250 V and the yield rate was over 90 %. AMS is thermally ideal for its high thermal conductivity of aluminum and aluminum oxide and the cost effectiveness is very high for its material cost and process cost.