Microstructure and mechanical properties of laser ablation cleaned NiP platings for aluminum wire bonding

Aluminum wire bonding to nickel surfaces is often used in automotive applications. For assurance of a high quality contact a clean substrate without any contaminants is required. In this study lead frame structures consisting of matte nickel, bright nickel and electrolytic deposited NiP (up to 0.5 m thickness) were used for Aluminum wire bonding. The lead frame was partially plated with tin at the connector end (opposite from the wire bonding surface). During the tin plating process an unintentional tin layer of about 10-20nm was deposited onto the NiP wire bonding surface. Laser ablation was used to clean the NiP areas before wire bonding. This paper presents microstructural investigations of the NiP/Sn platings with and without laser ablation. Using FIB/SEM/TEM the thickness and crystalline structure of NiP layer was analyzed. The phosphorous concentration gradient across the depth of the NiP layer was investigated by nano-spot EDX. Using nanoindentation measurements, the microhardeness of the laser etched and non-laser etched areas was determined and correlated to the microstructural phenomena. The investigations show that not only does the laser treatment remove the tin contamination but it also removes the P-rich surface film. The lasered samples also showed a very coarse grain structure close to the Ni film which indicates a temperature-induced recrystallization effect. These results correlate with the mechanical investigations: At a depth of 50nm to 200nm significant lower indentation hardness was measured in the lasered specimen compared to the non-lasered specimen as detected by CSM nanoindentation measurements as well as standard nanoindentation measurements