Electroless Au/Ni plating is intensively applied to high-density printed boards. In this process, local corrosion often occurs between the deposited nickel and the deposited gold. Generally, nickel tends to diffuse from the local corroded areas to the deposited gold surface after thermal treatment due to its strong affinity for oxygen. These areas cause surface mounting failures. Recently, electroless Au/Pd/Ni plating has been actively studied as a substitute for electroless Au/Ni plating because it suppresses the nickel corrosion reaction. In this study, we investigate the influence of the nickel microstructure and the thickness of the palladium and gold on wire bondability. The wire bonding strength is increased with increased palladium and gold film thickness. The deposited nickel microstructures also influence the wire bonding properties after thermal treatment. It was confirmed that good wire bonding properties can be achieved using a nickel film with a layered microstructure rather than a columnar microstructure. From the AES analysis, we confirm that preparation of a uniform layered microstructure of the nickel film is a key factor to keep the gold concentration on the gold film surface after thermal treatment.

With increasing thickness of electroless gold films (greater than 0.5 μm) as a top layer, Au/Ni film shows good wire bondability. However, we confirmed that the deposited nickel structure influences wire bondability for thinner (less than 0.5 μm) electroless gold plating after heat treatment because of the different diffusion behavior of gold on the deposited nickel used as an underlayer. In this study, two types of electroless nickel plating structure with columnar and layered structure were observed and bondability was evaluated. Auto-catalytic gold of 0.2 μm thickness on the layered nickel deposits shows good wire bondability after heat treatment because the gold was deposited uniformly. In contrast, for a columnar structure, the gold on the Ni films shows poor wire bondability after heat treatment because of local corrosion occurring at the initial stage of gold immersion and because the subsequent auto-catalytic gold is not evenly distributed.

For the field of nickel electroforming, where thickness of about 100-500 μm is obtained at the current density of 20-50 A/dm 2 , increase in the current density is demanded for higher plating speed. Regarding the environmental impact, a plating bath without boric acid is also in demand.Results obtained from studying various organic acids to replace boric acid show that acetic acid facilitates high-speed plating without burning at a current density of up to 200 A/dm 2 . The effect of acetic acid was attributed to its excellent buffering capability.

No abstract