Capabilities: ENIG Plating for Aluminum Wedge Wire Bonding
The thin flash Gold layer acts as a barrier layer protecting the Nickel from being oxidize. The wire bonding process actually connects the Aluminum wire to the Nickel plating. The major advantage of this method is low cost, as ENIG plating is an inexpensive process and it also helps to make wire bonding simpler. However, Aluminum wires require a larger bond pad pitch.
With the ENIG process, manufacturers can achieve a PCB surface that is more even and flat, as compare to what the HASL process offers. For fabricators and assemblers, ENIG is a suitable surface for Aluminum wire bonding. ENIG uses electroplate Nickel and electroplate Gold, and these are the traditional finish manufacturers prefer for Aluminum wire bonding. The Gold plating makes the surface an extension of the Gold thermosonic wire bonding process, which incidentally, requires a minimum of 50 MIL Gold thickness. However, for Aluminum wire bonding, manufacturers recommend a thinner layer of Gold, closer to 25 MIL, over a 50 MIL layer of Nickel.
Wire bonding, similar to soldering, depends on proper process control the manufacturer has over their ENIG process. There are two processes for plating—electrolytic plating or electroplating, and electroless plating. Although electroplating is a less demanding process compare to electroless plating, the former requires bussing prior to Gold plating. Individual fabricators may have different setups and process preferences that give rise to price differences.
Another factor affecting the quality of wire bonding is the purity of materials manufacturers use during the plating process. Purity of Nickel is critical for Aluminum wedge bonding That means Nickel should be soft as possible, preferably without contaminants and voids. Additionally, manufacturers need to exercise good control over their plating chemicals and the pH of plating bath. For good quality Aluminum wedge wire bonding, Gold thickness should minimum of 3 MIL, with purity at least 99.99%, and with no traces of Thallium.
Some manufacturers using better chemicals, mainly from Atotech, report a higher quality of Aluminum wedge wire bonds than traditionally achievable. For instance, in place of usual brushing prior plating, manufacturers use pumice powder spray on Copper surface of PCB.