ENEPIG: Is this the universal finish?
We are asked quite often about the availability and benefits of ENEPIG. As I was reading a recent article in PCB007 by Mike Carano that discusses various solderable surface finishes, I thought our customers would appreciate the information presented and wanted to specifically highlight the information presented on ENEPIG. A link the entire article is attached below.
If hyper-corrosion and black pad are of concern, ENEPIG is a solution. Here, the gold deposits onto the palladium, not the nickel. There is no hyper-corrosion effect as there is with gold over nickel. ENEPIG is often referred to as the “universal finish,” capable of good solderability and wire bondability. However, one must look at this more expensive finish in the context of the circuit board and its intended use/environment.
One area in which ENEPIG has found use is the IC substrate market. ENEPIG can function as one finish for both wire bonding and solder attachment. While it is true that ENIG can perform the same functions, ENEPIG is more robust with respect to gold wire bonding. Typical plating thicknesses for this three-metal-stack over copper are as follows:
- Au Layer: 0.03-0.06 micron;
- Pd Layer: 0.10-0.50 micron; and
- Ni Layer: 3.0-6.0 micron.
The nickel present on the surface benefits from a Pd or Au protective layer to improve solderability by reducing brittleness and oxidation of the solder joint. The basic idea is to achieve improved solderability and wire bonding at reduced palladium and gold thicknesses. For the majority of ENEPIG systems, the palladium is deposited as an electroless reaction. Commercial palladium systems are based on one of two reducing agents, hypophosphite or formate. The former will co-deposit 1-6% phosphorous into the deposit, while the latter is nearly 100% pure palladium.
There is no industry specification for ENEPIG, although one is under development. A key component of any specification is the verification of both solderability and wire bondability at varying palladium thicknesses. Again, lower thicknesses of both palladium and gold will enhance the economic viability of this finish as long as the solderability/wire bonding requirements are met.
Mike Carano’s full article discussing factors to be considered when choosing a solderable surface finish, a review of the latest solderable finishes and special considerations when using these finishes can be found at: http://www.pcb007.com/pages/zone.cgi?artcatid=0&a=85243&artid=85243&pg=1.
Mike will be joining us for a PCB Coffee Talk webinar session later this year discussing surface finishes.