The effect of prebonding heat treatment on the separability of Au wire from Ag-plated Cu alloy substrate

Abstract

The wire bonding technology that relies on subsequent wire separation of a connection has a possibility of becoming a key packaging technology in the environmentally friendly aspects of the recycling of mounting materials, such as the reworking of chip on board (COB) mounting and the application of the interposerless chip size package (CSP) in the manufacturing process. This study investigated how heat treatment before bonding affects separability at the ball bonding area. For the investigation, we used bonding pairs of Ag-plated Cu alloy substrate and Au wire.Heating a board to 150 C before wire bonding degraded wire bondability but improved separability. An analysis of Ag plating by auger electron spectroscopy (AES) and x-ray photoelectron spectroscopy (XPS) revealed that the heat treatment caused the Cu on the board to diffuse into the Ag plating and to deposit concentrations of Cu in the form of Cu(OH) 2 and CuCl 2 on the Ag surface.The analysis also showed that the thin Ag plating was remarkably affected by the concentrated Cu and that bondability deteriorated even when the heat treatment period was short. These results demonstrate that wire bonding that is intended for separation at the connection requires the selection of an adequate plating thickness while, at the same time, considering the previous heat treatment of the substrate.