PCB Plating for Chip on Board Assembly

COB wire bonding
Chip on Board Wirebonding

Selecting the right PCB plating for Chip on Board assembly is crucial. One of the most frequent questions we are asked at Majelac Technologies is how should my Printed Circuit Board be plated in order to ensure wire bond-ability. PCB Plating for Chip on Board Assembly must be planned in advance to ensure manufacturability of the device. This planning begins with design rules and selecting the wire bond method to be used. Common wire bond materials include aluminum or gold wire.

Aluminum Wedge Wire Bonding and ENIG Plating

One of the most common wire bond methods for COB assembly is aluminum wire bonding. For aluminum wire bonding to a PCB the plating can be a low cost method known as ENIG or Electroless Nickel with Immersion Gold. With ENIG plating a PCB with Copper traces is plated with a Nickel layer and then the nickel layer has a flash gold layer on top. The Flash Gold is a barrier layer to protect the Nickel from oxidation. The wire bonding connection is actually from the aluminum wire to the Nickel plating. The main advantage of this method is inexpensive plating and relatively easy wire bonding. The disadvantage is Aluminum wire bonding is less flexible than gold bonding and generally requires larger bond pad pitch compared to gold bonding.

Gold Ball Wire Bonding and soft gold plating

For Gold Ball Bonding to Printed Circuit Boards a thick layer of soft gold has traditionally been required. In this Case the copper traces on the PCB are plated with a nickel layer and then a soft gold layer measuring between 30 and 50 micro-inches. The gold is electroplated and when plated correctly  provides an excellent surface for wire bonding. This method has the advantage allowing for fine pitch gold ball bonding. The disadvantages are high cost and the thick gold layer can cause reliability problems with surface mounted components due to gold dissolution into the solder.

Gold Ball Wire Bonding with ENEPEG plating

In recent years Electroless Nickel, Electroless Palladium, Immersion Gold plating has been proven to be the best compromise between cost and manufacturability. This method of plating is known and ENEPIG and is common referred to as the universal plating method. It is called universal plating because it offers good wire bond ability and good solder ability. It is suitable for RoHS solders as well as Eutectic SnPb solder. The typical plating thickness is 100 to 150 micro inches of nickel followed by 4 – 10 micro inches of palladium followed by 1-2 micro inches of gold. ENEPIG is suitable for both Gold and Aluminum wire bonding however for Gold Bonding the plating process must be well controlled. At Majelac technologies we receive PCB’s from many vendors and wire bond ability varies greatly. One simple test is to try and attach a ball bump on the PCB trace and then do a shear test. If the ball comes of cleanly there is an issue. If the ball comes off and exposes the palladium layer there is an issue. If the ball leaves some residual gold wire behind we are confident the material will work with automatic ball bonding.

At Majelac for PCB plating with Chip on Board we recommend using ENEPIG as a preferred plating method. For more information about Majelac Technologies Chip On Board wire bonding services please click the link below:

http://www.majelac.com/assembly-services/wire-bonding/cob-wire-bonding/

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