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78 SMT007 MAGAZINE I JULY 2019 a building material in the construction of auto- mobiles, aircraft, bicycles, building frames, etc. Other uses range from electrical connec- tors, packaging cans and foils, and household utensils. While it is a material of choice in the above fields, it is second to copper in the field of flexible circuits. This is despite the various advantages that aluminum has over copper. Aluminum is more than three times lighter than copper. The den- sity of aluminum is 2.7 gm/cm 3 while that of copper is 8.92 gm/cm 3 . Its electrical resistivity is 26.5 nΩ·m (at 20°C) while that of copper is 16.78 nΩ·m (at 20°C). Also, its thermal con- ductivity is 237 W/(m·K) while that of copper is 401 W/(m·K) [1] . Although it is not as good an electrical and thermal conductor as copper, it can radiate heat better than copper due to its lower den- sity. Overall, aluminum has 68% of the con- ductivity of copper but has only 30% of the weight of copper. This means that a bare wire of aluminum weighs half as much as a bare wire of copper that has the same electrical resistance [2] . This will be similar for aluminum traces in the case of flexible circuits. Also, aluminum is generally less expen- sive when compared to copper conductors. A recent check indicated the price of alumi- num was 35% less than that of copper [1] . It is three times less expensive than copper on an equal weight basis and six times less expen- sive on an actual usage basis. This is the big- gest advantage that aluminum has over cop- per. Table 1 lists the comparative properties of the two metals relevant to flexible circuits. Flexible Circuits and Al-PET Substrates The majority of flexible circuits are made using copper on polyimide (Cu-PI) substrates. These consist of copper foil laminated onto polyimide film. Varying the thickness of cop- per and polyimide gives rise to various com- binations of thicknesses of Cu-PI to suit the conductivity and dielectric requirements of the end applications. Traces are formed using pho- tolithography followed by a print-and-etch pro- cess. Components are soldered on to make the finished circuits. A reasonable selection of sol- ders is available that can easily bond to copper traces without the need for any special surface treatment. An increasingly popular method to make flexible circuits is by using aluminum on PET (polyethylene terephthalate) or Al-PET sub- strates. These are available in varying thick- ness of aluminum foil laminated onto PET film (Figure 1). While aluminum is less expensive than cop- per, PET is also significantly cheaper than polyimide film. Hence, lower material cost is a major driver for the increasing use of Al- PET substrates, but their use has been limited because of processing challenges. The process to generate the traces on alu- minum substrates is similar to that of copper. A dry-film or liquid resist is used for photoli- thography, which is then followed by chem- ical print-and-etch to form aluminum traces. But attaching the components onto aluminum is a challenge. Unlike copper, it is not easy to solder to aluminum. Soldering to aluminum is difficult because of the presence of a thin layer of aluminum oxide. This layer forms naturally when the bare metal is exposed to air. Since most flexible circuit manufactur- ing is done under atmospheric condi- tions, all aluminum surfaces will have an oxide layer. While the formation of this natural oxide is self-limiting, its presence cannot be overcome by the Table 1: Properties of aluminum and copper [3] . Figure 1: Typical laminated construction of Al-PET substrates.