The perfect electronic device has to be light and small while offering the capability of maximum electronic functionality. The PCB industry has driven towards advanced packaging methods to meet this pre-requisite.
It involves increasing the integrated circuits’ density on a PCB board as well as combining several functionalities into a single dense package.
Electroless Nickel Immersion Gold (ENIG)
Electroless Nickel Immersion Gold is a twin layered metallic surface finish of 2 to 8 micro inch Au over 120 to 240 micro inches Nickel.
The Nickel here works as a barrier for the copper and provides a surface to which you can solder your components. The gold is responsible for protecting the nickel plating during storage time and offers low contact resistance, which is a requirement for thin gold deposits.
ENIG is undoubtedly one of the most used surface finishes in the PCB industry. It is all due to the implementation and growth of RoHS regulations.
Surface Finish Viable for Wire Bonding Process
While electrolytic nickel gold surface offers immaculate performance for gold wire bonding, it does suffer from three deficiencies. Each of these shortcomings causes a major roadblock for ENIG’s use as the leading surface finish application for circuit boards. Here are the three deficiencies.
- The process is very expensive and usually requires a high gold thickness.
- As you use higher gold thickness, your solder joint reliability may reduce drastically. It is because of the tin-gold inter-metallic formation.
- The requirement of electrical bussing for establishing connections to the feature amidst the plating process will limit the densities of features you wish to achieve.
The limitations mentioned above also provide an opportunity for an Electroless process. These may include ENEG and ENEPIG that involves Electroless gold and Electroless palladium.
This surface finish has its own advantages, such as cost and packaging reliability. The cost especially becomes one of the most worrying aspects of the process. With a recent spike in gold price value, the price of this type of surface finish is becoming more difficult to control.
However, the cost of palladium metal is comparatively far lower than gold. Hence, manufacturers now have the option to replace gold with palladium for a cost-effective yet identical quality process.
ENIG is a commonly used surface finish that uses a nickel layer, which actually is a nickel-phosphorus alloy layer. This phosphorus content has two categories that are phosphorus nickel or high phosphorus nickel. The application of both is not the same.
There are certain advantages associated with Nickel, such as it is suitable or a lead-free soldering process. The surface it produces is flat, fine, and smooth. You can store it for longer periods considering the storage conditions are decent and not too harsh.
Additionally, Nickel is suitable for binding with aluminum and suitable for thick panels that can withstand attacks of several environmental elements.
Advantages and Disadvantages of ENIG
Here are some of the advantages and shortcomings of ENIG surface finishes.
Benefits
- It produces flat surfaces
- It offers completely lead-free surface finish for PCBs
- It is good for Plated Through Holes (PTH)
- It offers longer shelf life for the circuit boards
Drawback
- This process is very costly due to the high gold prices in the market
- The surface finish is non-reworkable
- Black nickel and black-pad
- The surface finish can cause damage to ET
- The circuitry on the PCB may experience signal loss (RF)
- The whole process of the ENIG surface finish is quite complicated
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