Complete Guide for PCB Plating
Circuit board fabrication is a fairly expensive process. PCB manufacturers prefer their components to be additionally protected against corrosion, stresses, environmental conditions, and other factors. This way, costly electronics will last for a longer time.
PCB plating is what can ensure the prolonged shelf and operational life of boards. Let’s find out what plating options are there and what plating methods you can employ.
What is PCB Plating?
Electroless plating is a chemical process that is used to coat the outer layers of a PCB with extremely thin copper layers. In this way, plating ensures that holes and vias internal surfaces and well as board surfaces have a coating on them.
The obtained metal base serves as the foundation for the following electroplating. Enginers clean the holes, prep them with micro etching, cover them with a photoresist material, expose them to UV light, and pill off the photoresist covering. After that, engineers may proceed with electroplating.
Basically, the process is the immersion of boards into a plating solution. The latter is a liquid with ions of metal dissolved into it. The application of a coating is done by forcing an electrical charge to go through the solution with boards in it.
After execution of one or another plating type method, manufacturers may proceed with the soldering step or release components.
Types of Materials Used for PCB Plating
Technically, there are two subtypes of plating. Each of them requires the usage of a certain material:
- Via planting. It’s the coating of internal surfaces of vias. Engineers use copper alloys here and the copper plating method.
- Surface finishing. It’s the covering of the entire surface area of a PCB with a metal alloy. Options here are normally tin, gold, silver, nickel, and molten resin.
Finish in PCB manufacturing process matters. Be sure to choose materials applicable to the design of your circuit boards.
Surface PCB Plating Process Options
You may perform PCB surface finish in one of these ways:
Hot Air Solder Leveling (HASL)
This method requires using some synthetic resin — solder epoxy. HASL is simply immersion of a PCB in the pot with molten resin. After that, the excess epoxy is removed by blasts of hot air, as blades would do.
HASL is a low-cost plating. It’s fairly reworkable and widely available. It ensures excellent shelf life. Unfortunately, the method leaves rather uneven surfaces. It’s not perfectly suitable plating for the through-the-holeR method (detailed below). It is also prone to thermal shocks and solder bridging.
Electroless Nickel Immersion Gold (ENIG)
It’s one of the fairly popular finishes for premium quality boards. Its working principle is two-step coating. First, manufacturers chemically plate a layer of nickel, serving as a protection for copper. Then, in the same way, they plate a thin layer of gold.
ENIG is ideal for soldering pitch components. It also complies with the Restriction of Hazardous Substances (RoHS) provisions. It supports the plating through the hole method.
However, besides being expensive, this method is prone to signal losses and black pads. Such plating is not reworkable, and its application process is complex.
Electroless Nickel Electroless Palladium Immersion Gold (ENEPIG)
You may regard ENEPIG as a modification of ENIG. It’s a three-step coating. Nickel, palladium, and gold applied in this order provide excellent quality of finish.
Such boards are ideal for high-frequency applications. They are reliable and withstand handling easily. ENEPIG outperforms all the other finishes in terms of corrosion prevention capability. It’s wire bondable, lead-free, and not vulnerable to black pads.
However, it’s the most expensive plating method. It’s hardly reworkable and has some processing limits.
Immersion Silver
This is a non-electrolytic finish that can be applied over copper layers by immersion in a tank with silver ions liquid. You may regulate the suitable thickness by the exposure time.
The method is comparatively inexpensive. It’s also more environment-friendly than both ENIG and HASL. It leads to more even surfaces and ensures a good shelf-life.
Its limitations are the vulnerability to oxidation. It can tarnish easily with inadequate handling. It’s also not that durable compared to other ways of plating.
Immersion Tin
This metallic plating is grounded on the chemical displacement reaction done over the copper laters. Tin does not oxidize in the way copper does, so it ensures desirable shelf life.
Immersion tin ensures that surfaces are fairly flat at a low cost. Such plating is well-reworkable.
The bad thing about the method is the inevitable diffusion of tin and copper into each other. The process leads to tin whiskers, which deteriorate the quality of boards overall. Immersion tin plating also is prone to handling damage. The coating is hazardous as utilizes Thiourea — carcinogen.
Organic Solderability Preservative (OSP)
The last method is a conveyorized process that applies a thin layer of a water-based compound onto copper surfaces. The protective layer bonds to copper, forming an organometallic layer.
OSP is the most environmental-friendly method. Its application is simple and cost-effective. It ensures flat surfaces and is reworkable.
As for its limitations, there is no way to measure the thickness. Shelf life is mediocre, and handling is rather sensitive.
As you can see, these types belong to several groups like electroless, immersion, electroplating, organic, and HASL is in its own league.
Types of Plating Methods for PCB Manufacturing
The mentioned options were ways in which PCB plating is applicable to PCB finish. And here are the plating methods you may combine with the process options:
- Finger Plating
It’s the use of gold to provide lower contact resistance and higher abrasive resistance. Basically, tin coating, previously applied, is stripped, rinsed, and rubbed. Then, you immerse PCBs in sulfuric acid, plate them with nickel, and perform gold solution treatment and plating.
You receive excellent, most expensive plating of contacts this way. It still is more affordable than ENIG and, obviously, than ENIG and ENEPIG.
See what gold fingers may look like in the image below.
- Through-hole plating
As evident from its name, its the coating of inner surfaces of PCBs. After the drilling process, сopper foil and synthetic resin are damaged and melted. Surely, it leads to deterioration in a board’s performance.
To mitigate the issue, you may coat inner surfaces with a conductive film. After that, your components will require thermal curing to harden fully. Yet, it’s a one-step and rather an inexpensive process.
- Reel Linkage Selective Plating
It’s selective platting of some components such as connectors, integrated circuits, and transistors. The idea is basically the same as with the finger plating.
You may go with a manual or the automatic method. Its working principle is flattening the ends of metal foil to a required thickness. After that, it’s die-cut, cleaned, and plated.
Platting is simply the coating of the area with a resist film and then plating over the copper foil.
- Brush Plating
This selective plating is applied when the limited area is plated, and not all the parts are immersed in an electrolyte. Usually, manufacturers use this technique over board edge connectors. Brush platting finds its application in repair shops as a method of quick reworking of a finish.
The working principle is wrapping an anode in absorbent material and bringing the plating solution to parts of a PCB for plating.
Summary
In essence, you have six materials that you may use in the plating process in one or another way. Besides, you have six methods of surface finish or surface plating. And finally, you have four ways of rather selective plantings that can be combined with ways of surface finish.
Utilize these options to select the complete plating process that would be a perfect fit to your PCB fabrication needs.
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