Post wirebonding coating for prevention of corrosion of wire bonded packages by chlorine containing foreign particles

Reliability failures of wirebonded packages due to corrosion caused by chlorine containing foreign materials (Cl-FM) is recently reported [14], [15]. Eliminating Cl-FM incorporated in packaging bill of materials (BOM) is an obvious solution. However, the assembly process is conducted in a non- clean room environment and it is practically impossible to eliminate all possible sources of Cl-FM. A post wirebonding coating is explored as a possible solution for corrosion prevention as it can physically restrict Cl-FM reaching the corrosion susceptible areas, Al-Cu IMC (intermetallic compounds) and the aluminum bonding pad of the copper wirebonded (CuWB) packages. As Cl-FM can produce chloride ions that can diffuse in presence of moisture, any material with no or minimum moisture absorption and permeability will be the ideal coating material. Different materials are evaluated as potential coating materials. In addition to corrosion resistance, the coating must be compatible with packaging process and BOM. Coating process is expected to cause no wire shorting or out of specification wire sweep. An important parameter is good adhesion of coating material to molding compound and free of interfacial delamination under various packaging reliability stresses. Packaging compatibility of coating materials is evaluated under various stress conditions such as temperature cycling (TC -55-155° C), highly accelerated stress test (HAST)110°C/85% RH and high temperature storage life (HTSL 175° C). Preliminary evaluation of corrosion prevention effectiveness is conducted with samples containing deliberately added Cl-FM. Limitations and benefits of different coating materials for corrosion prevention will be discussed.