Wafer Level Electroless Nickel/Gold ― Plating on Copper Pad Metallization A Solution for Wire Bonding and Flip Chip Technology

摘要

Electroless Nickel/Gold under bump metallization is becoming a state of the art - accepted technology for under bump metallization for Aluminum pad wafers. On these wafer types, the wire bonding process is performed directly on the Aluminum. For flip chip bumping processes, an additional wettable under bump structure is required, since solder does not wet directly on the Aluminum. Besides the sputtering techniques consisting of Chromium/ Copper or Titanium/Tungsten layers in combination with Copper and other metals, electroless Nickel/Gold is becoming a low cost alternative implemented in several products world wide. The new wafer types coming onto the market with Copper pads are causing basic evolution and revolution in wire bonding and flip chip technology. Wire bonding directly on Copper pads is not possible or very difficult with the existing standard wire bond equipment and processes. So, an additional alternative metallization step has to be done on these wafers in order to make them suitable for wire bonding. Here, electroless Nickel/Gold offers a unique chance to provide a wire bondable surface for Copper pad wafers. Electroless Nickel/Gold is already used in CSP's, BGA's as metallization on the printed circuit boards. The implementation of this process on wafer level offers unique solutions to obtain a homogenous metallization system. This will provide optimal reliabilities on all interfaces of wire bond technology. This means, the bond pad on the IC and the bond pad on the substrate. Additional, the Nickel/Gold UBM can be used as a structure wetable for solder bumps and for flip chip technology. This paper presents the process flow and the bonding results for this unique metallization which offers a roadmap to further cost reduction for both - flip chip and wire bonding technology.