Capabilities: ENIG Plating for Aluminum Wedge Wire Bonding

Manufacturers typically use Aluminum wire bonding for COB assembly. In this technique they bond an Aluminum wire to a PCB with an ENIG plating on its surface. ENIG or Electroless Nickel with Immersion Gold is a low cost plating method, where the PCB manufacturer plates the Copper layers with a layer of Nickel, and subsequently flashes a thin Gold layer on the Nickel layer.

The thin flash Gold layer acts as a barrier layer protecting the Nickel from being oxidized. 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, as they are less flexible compared to the flexibility of Gold wires, especially for applications that are prone to vibrations.

With the ENIG process, manufacturers can achieve a PCB surface that is more even and flat, as compared to what the HASL process offers. For fabricators and assemblers, ENIG is a suitable surface for Aluminum wire bonding. ENIG uses electroplated Nickel and electroplated 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 compared 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 the Nickel should be as soft as possible, preferably without contaminants and voids. Additionally, manufacturers need to exercise good control over their plating chemicals and the pH of their plating bath. For good quality Aluminum wedge wire bonding, the Gold thickness should be a minimum of 3 MIL, with a purity of 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 the usual brushing prior to plating, manufacturers use pumice powder spray on the Copper surface of the PCB. This reduces the roughness of the Copper surface, allowing better plating, and consequently, offering a better quality, reliability, and durability of the Aluminum wedge wire bonds.


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