PCBs are now a crucial component of different electronic circuits. The circuit boards must withstand the challenges of various operational conditions. As a result, such circuit boards were covered in a variety of surface treatments.
On a Circuit board, a layer of copper that comes into touch with air oxidizes. Thus, the soldering quality is diminished. Surface treatments improve electrical performance, solderability, and oxidation reduction. There are many different surface treatments on the market. One such significant variety, the ENIG surface finish, as well as the ENIG plating thickness will be the focus of this article.
What Does ENIG Plating Mean?
A surface coating known as ENIG referred to as Electroless Nickel Immersion Gold) can be used to cover copper pads present on circuit boards in order to prevent corrosion as well as other irregularities. This copper pad was initially covered with a layer of nickel (Ni), which is followed by one layer of immersion gold (thin Au). The circuit board’s great electrical performance is achieved by ENIG’s superior oxidation resistance, great surface planarity, and ease of soldering.
Even though ENIG is more expensive and complex than other PCB processes for plating like HASL, it complies with the RoHS standard and is one of the most popular circuit board surface finishes as a result.
Nickel serves as both a barrier to copper pad as well as the material on which components are attached in ENIG, which is a metallic coating of two layers.
On the contrary, gold offers reduced contact resistance while also safeguarding the nickel during storage. Gold thickness ranges from 0.05 to 0.023 mm while typical nickel thickness is from 4 to 7 mm. A temperature for processing of about 80 °C is necessary for ENIG.
Getting Started with ENIG Plating Thickness
Commonly termed Electroless Nickel Immersion Gold, the surface finish is made up of an electroless nickel plating that has a thin coating of immersion gold on top of it. With immersion gold, displacement creates a gold layer on top of a nickel layer. It keeps going until the nickel layer covers the created gold layer. This is the reason for the gold layer being very thin. Nickel is shielded from oxidation by this layer.
Gold typically has a thickness of 0.05 to 0.23 μm (2.0 to 9.0 μ in) and electroless nickel typically has a thickness of 2.5 to 5 μm (100 to 200 μ in). The PCB’s thickness is constant throughout.
The thickness specifications for the ENIG deposit thickness are set out in the IPC-4552B standard for use in applications such as wire bonding, soldering, as well as contact finishes. IPC-6010 documents family including IPC-6018, IPC-6013, as well as IPC-6012, have performance standards that are specified by the IPC-4552B standard.
Benefits of the Immersion Gold Finish
Despite being a pricey surface finish process, ENIG is nevertheless widely used for the reasons listed below:
Immersion gold is highly chemically reactive. Impressive wettability, coplanarity, surface planarity, as well as a lengthy shelf life are some of these characteristics. Nickel and the gold layer both contribute to the characteristics.
The nickel layer serves two purposes in ENIG. It serves as a form of barrier and therefore can prevent the fusion of copper and gold. However, when combined with tin, it will generate the intermetallic compound called Ni3Sn4. This substance has great solderability.
Numerous benefits of a gold coating include reduced contact resistance, max strength, a low likelihood of oxidation, as well as antifriction. These characteristics aid in meeting the requirements for circuit conductivity and shield the copper and nickel layers from any oxidation. Additionally, this results in great solderability.
Neither dummy plating nor excellent re-flow cycles are necessary with ENIG. It has strong adhesion and therefore is renowned for being easy to electrically test.
The circuit board’s holes are well-plated with immersion gold. The coating can be bonded with gold wire and soldering can be done easily as well.
The optimum surface finish for the fine pitch items is ENIG because the pads or tracks have flat, square edges. Also, ENIG works in line with all the RoHS requirements. Computers, cell phones, automotive electronics, and consumer electronics all employ ENIG PCBs.
What are the Drawbacks of ENIG Plating?
ENIG plating’s drawbacks include the following:
- ENIG surface finish is an expensive technique
- It has undesired magnetic characteristics
- It is not suitable for rework therefore makes circuit board repair exceedingly challenging
Specifying the Plating Thickness of Circuit Boards
The average plating thickness of circuit boards is in the range of 100 micro-inches. The normal thickness might be as little as 10 micro-inches for OSP and immersion silver. It’s simple to specify thickness and type of PCB plating: just write it down in your notes for fabrication.
You will have the chance to specify your plating type on the manufacturer’s form if you’re generating a prototype plus they offer a standard quote form. If you require a particular thickness, be sure to state it in the forms as they may not request for it. Your fabricator is responsible for guaranteeing that the plating could be reliably applied to the requisite thickness immediately you’ve supplied the necessary plating value.
The impact it will have on losses is another reason to be concerned about PCB plating thickness. Low-speed digital transmissions as well as sub-GHz radios didn’t have to worry as much regarding PCB plating thickness at low frequencies because you probably will not detect any affects on frequency.
At different mmwave frequencies including short range radars of at least 24 GHz, the problem with losses becomes apparent. Particularly on the RF substrates of low loss like Rogers, copper roughness starts to become a very observable contribution to losses at all these frequencies. Signals will be subjected to varying degrees of roughness as they travel through the plating, which will be reflected in the resistance of skin effect.
Conclusion
The ENIG plating thickness should fall between 2.5 and 5 μm thick, while the layer of the immersion gold should fall between 0.05 and 0.23 µm in thickness. The likelihood of the black pad as a result of process difficulties increases with the immersion gold layer’s thickness.
