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76 SMT007 MAGAZINE I JULY 2019 Feature by Divyakant Kadiwala AVERATEK CORPORATION Abstract The majority of flexible circuits are made by patterning copper metal that is laminated to a flexible substrate, which is usually poly- imide film of varying thicknesses. An increas- ingly popular method to meet the need for lower cost circuitry is the use of aluminum on polyester (Al-PET) substrates. This mate- rial is gaining popularity and has found wide use in RFID tags, low-cost LED lighting, and other single-layer circuits. However, both alu- minum and PET have their own constraints and require special processing to make fin- ished circuits. Aluminum is not easy to solder components to at low temperatures, and PET cannot withstand high temperatures. Soldering to these materials requires either an additional surface treatment or the use of conductive epoxy to attach components. Surface treat- ment of aluminum includes the likes of elec- troless nickel immersion gold plating (ENIG), which is extensive wet chemistry and cost-pro- hibitive for mass adoption. Conductive adhesives, including anisotropic conductive paste (ACP), are another alterna- tive to soldering components. These result in component-substrate interfaces that are infe- rior to conventional solders in terms of per- formance and reliability. An advanced surface treatment technology will be presented that addresses all these constraints. Once applied on aluminum surfaces using conventional printing techniques such as screen, stencil, etc., it is thermally cured in a convection oven at low temperatures. This surface treatment is non-conductive. To attach a component, a sol- der bump on the component or solder printed on the treated pad is needed before placing the component. The aluminum circuit will pass through a reflow oven, as is commonly done in PCB assembly. This allows for the formation of a true metal-to-metal bond between the solder and the aluminum on the pads. This process paves the way for large-scale, low-cost manu- facturing of Al-PET circuits. Introduction Aluminum is the most abundant metal in the earth's crust. Its alloys have found wide use as