In the production of printed circuit boards, the steps of coating and finishing are essential. PCBs have copper on their surfaces. If copper is left unprotected, it will deteriorate over time. A surface finish provides a vital layer between the component and the board. The finish shields the copper circuitry and provides a surface that can be soldered in the production process (when the components need to be soldered to the PCB). The result is a board that will function efficiently and withstand heat, oxygen, and other elements.
There are a number of different PCB finishes, depending on your budget and what the board is intended to do. Here’s a look at some of the most popular finishes:
HASL (hot air solder leveling). This is the most commonly used finish in printed circuit board production. To apply the HASL finish, technicians immerse the board in a molten alloy of tin and lead, then propel hot air across the board to get rid of the superfluous solder. The hot air presents an added benefit in that it ferrets out any problems with separation of lamination layers. HASL is a good option because it is cheap, resistant to high heat levels, easy to source, and lasts a long time. It does contain lead, however, which makes it a no-go for lead-free projects. If you want the benefits of HASL without the lead, there are some lead-free HASL options.
Gold. The gold amalgam used for finishing is known as “hard electrolytic gold,” which is a layer of nickel that is plated with a layer of gold. It is very durable, so it is a popular choice for areas that get a lot of wear and friction. It’s not good for solderability, however. The drawbacks of a gold finish include the high cost of materials and the labor-intensive plating process that it requires.
Tin. This metallic “immersion tin” finish is deposited by a chemical displacement process directly over the copper circuit board. The tin provides a flat surface, is lead free, and keeps the copper from oxidizing. One disadvantage is that it can form what are known as “tin whiskers” over time. It is also hard to gauge the thickness of the tin application.
OPS/Entek. If environmentally-friendly finishes are a top priority for your organization, Organic Solderability Preservative is a good choice. This organic compound is water-based and easily deposited on the exposed copper of a printed circuit board. It protects the copper before the soldering process begins. It is re-workable, lead-free, and low cost. As with immersion tin, it is hard to measure the thickness of the OPS application. Another shortcoming of OPS is that it is not very durable.
At EMS Solutions, we take pride in our printed circuit board assembly processes. Our many years of experience allow us to choose the best finish options based on our clients’ budgets and the PCB’s performance requirements.