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September 2014 • The PCB Magazine 21 circuit routing. This will be discussed again later in the paper. Aluminum is an attractive choice as a circuit substrate owing to a combination of different properties, which include a coefficient of ther- mal expansion reasonably proximate to that of copper, dimensional stability which exceeds that of FR-4, relative light weight, good thermal spreading capability, and low cost (aluminum is roughly $2 per kilogram, while FR-4 in quan- tities of 2,000 kilograms coming out of China runs around $6 per kilogram [3] ). It is also worth noting that aluminum comprises 8.3% of the earth's crust and is highly recyclable position- ing it among the most sustainable of all circuit substrate choices. Turning attention back to the steps in the process, a sheet of aluminum is prepared with cavities wherein components will be placed. Be- cause the substrate is solid metal, the cavities can be created by any of a number of steps in- cluding chemical machining, mechanical ma- chining, laser cutting and punching. The sub- strate could also be embossed or cast with the cavities if desired. Aluminum is a unique metal that can be an- odized, converting the surface to aluminum ox- ide, also referred to as alumina. As a conductive material, aluminum can also be coated electro- phoretically with a plateable insulating mate- rial, making the surfaces nonconductive. Such techniques are commonly used in the coating of a wide range of metals used in products of every imaginable type from toys and household appliances to automobiles and space craft. The cavities receiving the components are ideally formed such that the depths will match the components' height so that when components are placed into their assigned cavities with leads facing up, the lead termina- tions will be flush with the surface to facilitate further processing. Figure 2 illustrates the ba- sic steps that follow and will be discussed in more detail. ALuMINuM BASE CIRCuIT TECHNOLOGy continues figure 2: the basic process steps for double-sided aluminum circuit assembled and interconnected without solder are illustrated from the top left; aluminum material is provisioned with cavities by mill- ing (as illustrated),etching or embossing, wherein components are placed and then coated with an insulating material. holes are drilled and then filled with insulating material, then re-drilled. at the same time, vias are formed to access component terminations. the circuit pattern is then plated and circuits sealed after last layer is complete, leaving open feature required for interconnection and power (open features not illustrated). the metal core can serve both as heat spreader and power or ground layer.