You may not know who Forrest Mars or Bruce Murrie is, but if you combine their last names’ initials, they may sound familiar. In 1941 Mars and Murrie formed a company that produced small chocolates surrounded by a candy shell, a treat we know today as M&Ms. Their product was initially sold exclusively to the U.S. Army, who saw the value in soldiers being able to carry chocolates in hot climates without melting. Not only was the hardened sugar candy shell tasty in itself, but it also protected the more vulnerable chocolate inside.
Printed circuit boards are also coated or plated for their protection, and often this is done with a surface finish of solder or some other material. Not only does this help to protect the exposed metal on the board from corrosion, but it helps with component soldering. These aren’t the only plating processes used to fabricate a circuit board. Here is a more in-depth look at the different PCB plating processes used during circuit board manufacturing.
Plating Copper During Printed Circuit Board Fabrication
A printed circuit board comprises a combination of conductive and non-conductive materials arranged in multiple layers to provide electrical connectivity throughout the board. The conductive material, which is usually copper, will be etched away from the inner layer pairs leaving only the circuitry’s patterns. Once these inner layers are completed, they will all be laminated together into one composited board. The next step is drilling all of the holes needed for components and vias, and the circuit board will then be ready for copper plating of its exterior layers and thru-holes.
The first PCB plating process will be an electroless procedure that will chemically coat a thin layer of copper to the board’s surface areas, including the drilled holes’ interior. This plating operation aims to create a thin metal base on the board and in the holes that will serve as the foundation for the electroplating process. Next, the board will be prepared for electroplating:
- The holes are cleaned of contaminants and residues from drilling.
- The holes’ interior is prepped with a micro etching to help the copper binding.
- Both the top and the bottom of the board are covered with a photoresist material.
- The photoresist is exposed to ultraviolet light except for where the metal circuitry patterns are to be plated.
- The pliable photoresist covering the circuitry patterns is washed off, leaving the rest of the board surface areas masked by the hardened photoresist.
The board is now ready for the copper’s bulk to be electroplated onto the external layers and into the holes. It will be connected to the negative charge, or cathode, of an electrical current and immersed in a chemical bath for plating.
Dissolved copper will be drawn through the solution to the negative charge and deposited onto the circuit board’s exposed copper circuity. The electroplating process includes dipping the board alternatively into the plating and cleaning baths under controlled conditions to ensure that the copper is evenly applied. Once the copper plating is completed, tin will be plated onto the copper to protect the circuitry while the photoresist is removed and the non-circuitry copper is etched away. Finally, the tin will be removed, leaving only the bare copper circuity.
While this concludes the circuit board’s copper plating, another vital plating process is necessary to complete the circuit board’s fabrication.
The PCB Plating Process for Applying Surface Finishes
As we have seen, printed circuit boards are fabricated with copper features for electrical connectivity. Although the traces and area fills will usually be covered and protected with a solder mask, the pads and holes must be left out in the open for soldering. This exposure will pose a problem because copper left unprotected for any length of time will begin to oxidize and deteriorate, making the circuit board unusable. For this reason, a surface finish is applied or plated onto the exposed copper for its protection.
Different types of surface finishes can be used on a circuit board depending on what kind of protection the board needs:
HASL (Hot Air Solder Leveling)
Up until recently, HASL has been the most widely used surface finish in the industry. The circuit board would be dipped into a molten pool of solder and then run through hot blasts of air to remove the excess. HASL is a low-cost operation available almost anywhere. It has a long shelf life and is easy to use. The downside of HASL is that it leaves uneven surfaces that can cause solder bridging and are not detailed enough for fine pitch components.
ENIG (Electroless Nickel Immersion Gold)
ENIG has become one of the most popular surface finishes currently being applied to circuit boards. It is composed of two layers of metal coatings, with the first being a layer of nickel that is chemically plated to the board. The nickel provides a barrier of protection for the copper and then, in turn, is protected from oxidation by a thin layer of gold. ENIG has excellent surface planarity making it ideal for soldering fine pitch components. It complies with the Restriction of Hazardous Substances (RoHS) requirements and is durable with a long shelf life. On the other hand, it is more expensive than other finishes, such as HASL.
This finish is often used in high-speed circuit boards as the silver has low losses in high-frequency applications. Immersion silver is deposited onto the copper using a chemical process and has excellent surface planarity for fine pitch parts’ solderability. Immersion silver is also RoHS compliant. The downside is that this finish is sensitive to contaminants in the air or on the board and must be kept in protective packaging to prevent it from becoming tarnished. Even with special packaging, its shelf life is roughly 12 months and must be used quickly after air exposure.
There are many other types of surface finishes in the PCB plating process, including; lead-free HASL, immersion tin, organic solderability preservative (OSP), and electroless nickel electroless palladium immersion gold (ENEPIG), and hard gold. While these all have their uses, HASL, ENIG, and immersion silver are the most commonly used surface finishes for circuit boards. The question then is, which of these different plating processes will be best to use on your circuit board?
How Your Local PCB Contract Manufacturer can Help with PCB Plating Questions
Weighing all of the board’s parameters and requirements to come up with the correct copper weight, plating types, and thicknesses are essential for copper plating. Data will include the current calculations, copper distribution (balancing), the board’s size, and its PCB technology type. When it comes to surface finishes, considerations must include how many boards will be built, how long the expected shelf life is, and what components will be soldered to it.
At VSE, we can give you the help you need to make plating and surface finish decisions for your board. Our engineers understand how the board’s power requirements will dictate the copper weight required and the fabrication vendor’s materials to build to those weights. Our years of experience working with different PCB finishes and can help you make the best choices for your board.