Bendability Enhancement and Miniaturization of Through-X Via (TXV) Based on Flexible FOWLP with Tiny Cu Pillar Assembly

Through-hole vias in microelectronic packaging can be formed by combining laser or dry-etch drilling with electro- or electroless plating techniques. However, the formation of long through-hole vias makes it difficult to fill the holes completely with conductive materials. The former laser drilling process also has limited resolution and aspect ratio, whereas the latter requires additional photolithography processes. Therefore, this study presents a new formation technology for long through-hole vias, called through-X via (TXV), where X is polydimethylsiloxane, epoxy, or hydrogel, without electroless or electroplating processes. Tiny copper pillars with a diameter/height of 300/300 and 60/200 μm are assembled through a pick-and-place method and molded with soft epoxy as a flexible substrate based on die-first fan-out wafer-level packaging. Double-sided metallization was performed on the epoxy to interconnect the embedded copper pillars. The resulting TXV exhibits relatively low resistance and good mechanical stability against repeated bending.