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JULY 2019 I SMT007 MAGAZINE 79 flux used in existing solder pastes. If harsher fluxes are used within solder pastes to address the aluminum oxide problem, they will cause corrosion of the very thin aluminum layers and thus reliability problems. There are two methods currently used to attach components to Al-PET substrates: one is the zincate and plating process while the sec- ond is using conductive epoxy. Al-PET Circuits Using Zincate and Plating Finish Special processing can be done on the alu- minum pads to remove and prevent the for- mation of aluminum oxide. These include ENIG, nickel-palladium, or nickel-silver plat- ing. These processes require surface prepara- tion called the zincate process [4] . The purpose of zincation is to clean the aluminum surface for plating of the nickel or other metal. Figure 2 shows an example of a zincate process. As can be seen in Figure 2, the zincate pro- cess involves extensive wet chemistry. The dwell time, which can vary per the alloy of aluminum, ranges from 5–17 minutes. In addi- tion, a plating finish must follow zincation before components can be attached. These add extra costs that make it difficult for large scale adoption of Al-PET substrates in the field of flexible circuits. Al-PET circuits Using Conductive Epoxies Al-PET substrates are widely used to man- ufacture radio-frequency identification (RFID) tags, smart tags, and low-end LED lighting. These are patterned using a roll-to-roll process for print and etch. Components are attached roll-to-roll or in panels, using silver-based con- ductive epoxies, including anisotropic conduc- tive paste (ACP). The assembly process begins with the application of conductive epoxy on the pad or chip. The chip is then flipped onto or placed on the pad followed by heat and pressure. This cures the epoxy and the chip is attached. While they are used in very small amounts in regular RFID tags, they are a big part of the total expense in making smart tags. These tags are larger than RFID tags and have a lot more components that demand more con- ductive epoxy per smart tag. Conductive epoxies have their own chal- lenges. They are made of adhesive epoxy filled with conductive metal particles—usually sil- ver. They are typically syringe applied, require longer cure times, have pot-life issues, and are electrically inferior to conventional solders. In addition, they must be stored at low tempera- tures in special freezers to control the polym- erization of the epoxy. Overall, they are very expensive and limit the use of Al-PET sub- strates [5] . Al-PET Circuits Using Advanced Surface Treatment An advanced surface treatment chemistry has been developed as a new alternative. It enables soldering to aluminum without zinc- ate and plating. It can be printed directly on Figure 2: Zincate process used to prepare the aluminum surface for plating.