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A fabrication process for vertical thermopiles embedded in a 75-mum-thick polyimide foil has been developed for flexible printed circuit boards (flex PCBs). The vertical connections consist of electrodeposited antimony-and nickel-plated through-hole vias. The plated through-hole vias consist of multiple wires, with a total metal content that is 1% of the total via volume. The via fabrication technique is similar to standard flex PCB wet etch and metallization processes. The main difference is that the foils are pretreated with ion irradiation to induce highly selective vertical etch rates. The thermopiles were characterized by measuring their voltage response to an applied temperature difference across the foil thickness.