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.

Electroless nickel/gold plating and electroless nickel/palladium/gold plating processing have been applied widely to copper patterns as a surface finishing treatment for electronic devices. Recently, miniaturization of copper patterns has advanced to increase the functionality of electronic devices. Conventional electroless nickel plating processing applied to fine copper patterns causes nickel growth in the lateral direction. As a result, the copper pattern distance narrows, which might cause short circuits. To resolve this difficulty, electroless palladium plating on copper has been investigated. This study specifically examined pretreatment processes involving cleaning and soft etching. Results show that the wire bonding characteristics on the electroless palladium/gold plated film differed remarkably depending on the selection of the pretreatment solution used for cleaning and soft etching. Particularly, using a neutral cleaner and a thiol type soft etching solution suppressed dissolution of the copper during the palladium catalyst treatment. Results show that the possibility of obtaining a palladium plated film with fewer pinholes and voids. For electroless palladium /gold plating processing, results show that the soft etching step and the selection of the pretreatment solution used in the cleaning step are important.

Fine pattern formation (L/S＝less than 50 μm) using electroless nickel/gold plating, extraneous nickel deposition on the resin area of a fine pattern is often observed because of residual palladium catalyst on the patterns. Thinner nickel plating was applied to solve the problem. However, a tendency for local nickel corrosion caused by the immersion gold plating step increases. Also, the solder connection reliability decreases compared to conventional nickel/gold processing. Accordingly, this study investigated the influence of palladium catalyst deposition conditions on electroless nickel/gold plated film properties. We propose direct electroless nickel strike plating on copper patterns without the palladium catalyst. Generally, solder bonding strength with electroless thin nickel /gold plating decreased concomitantly with increased palladium catalyst treatment time. In contrast, the nickel strike plating time does not influence solder bonding strength. Uniform nickel deposition and good solder bonding strength were achieved by applying the electroless nickel strike plating method.

The local corrosion of nickel

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.