PCB Trace plating is also identified as a copper coating or surface finish. Done with the purpose of protecting PCB from oxidation and the possible deterioration of the copper layer, plating provides protection from various environmental factors. Plating protects the exposed copper traces on the PCB and provides a clean surface for soldering components. A variety of plating methods extant with different advantages and disadvantages.
- Hot Air Solder Leveling (HASL)
One of the most economical copper plating methods, HASL involves dipping the board in molten solder which is later leveled-off using a hot air knife. Apart from being cheap, HASL is widely available and leads to longer shelf life. Although it is ideal for most through-hole and SMT packages you might run into some issues with relatively smaller SOIC and LQFP SMT components. Other disadvantages of this method include uneven surfaces and solder bridging. Perhaps most importantly, HASL incorporates lead. So, if you are seeking RoHS compliance, HASL might not be the way forward. Although some manufacturers do offer lead free HASL plating with added cost.
- Immersion Tin
Immersion Tin utilizes a chemical process that leads to the deposition of the protective metal layer directly over the copper traces. The deposited layer is flat and the process doesn’t involve lead providing RoHs compliant PCB surface.
- Electroless Nickel Immersion Gold (ENIG)
Known for the best fine-pitch flat surface and lead-free compliance, ENIG is a two-step process that creates a layer of gold over a thin coating of nickel. While the nickel protects the copper layer from exposure, gold provides cover for nickel during storage leading to longer shelf life. The only minor drawback of this process is that it is complicated and expensive.
- Organic Solderability Preservative (OSP)
OSP process is done to make PCB surface oxidation resistant. This method is popular as a lead-free, low cost, and eco-friendly process. In this process, no metal to metal displacement occurs.
- Hard Gold
One of the most durable methods, Hard gold or hard electrolytic gold consists of a layer of gold plated over nickel. It is mostly used in edge connector fingers that are usually high-wear areas. The gold used is typically 98 percent pure and the coating is around 30 to 50 micro-inches deep However because of its relatively poor solderability it is not applied to solderable areas.
- Carbon Ink
The economical version of the Hard gold plating method is Carbon ink. In place of gold, carbon-ink is used and further the process cost is optimized using a simple screen-printing process for deposition. Since carbon ink is hard and durable, the PCB surface offers high robustness and can withstand >1 million push-button operations without showing wear or increased resistance.
PCB trace should never be left unprotected as it can lead to failures in finished products. PCB copper trace coating methods offer a viable solution to several environmental challenges like oxidation and corrosion. Although there are various methods available, it is mostly dependent on the application circuit and desired results to select the convenient one.
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