Scratch the surface and you’ll find there’s a lot more to PCB finishes than you think. Actually, don’t scratch the surface… that is very much the point. It is the finish will ultimately protect the copper on the board from oxidation.
The possible finishes to your PCB are varied, and can have very different impacts on the actual PCB. The end use for the PCB will very much dictate what finish you are likely to need but it is also worth mentioning that they also come at a variety of price points, which will therefore affect the materials cost of your final electronic product. As with anything, this requires some thought… saving money in the short term might be storing up longer-term pain.
However, in order to introduce you to the four best surface finishes, we have put together this introductory guide, to help you make your choice:
Made in much the same way as immersion tin, immersion silver is better for middle range volume production. The silver doesn’t react with the copper traces and, properly stored, can last more like a year, although you will have to make sure you use the boards within 24 hours, once out of the packaging. Silver is, of course, more expensive, which also needs to be factored in.
Moving up through the price points, we arrive at Immersion Gold or, to give it its technical title, Electroless Nickel/Immersion Gold, or (ENIG). As you might expect at this price point, ENIG finishes offer a better shelf life, with 75 micro inches of nickel and around 5 micro inches of gold, over a two-part process. Often seen as the best flat, lead-free surface, there is, once again, a price factor, as well as possible availability issues. Best to contact Hi5 if you feel immersion gold is the finish for you.
Originally used by IBM in the ‘70s, the first PCBs with an OSP finish could only withstand a couple of heat cycles, and exhibited a shelf life similar to tin. Things have improved vastly since those early days. Modern OSP formulas have a shelf life of more like a year and are much more hardy, accommodating further heat cycles, as well as being designed for lead-free assembly.
This is often seen as the cheapest solution, although it should be noted that Hi5 use lead-free HAL, as opposed to Hot Air Solder Leveling (HASL) that you may have seen mentioned elsewhere. Nevertheless, it remains an economic option, if that is a driving factor in your PCB design. Here, the PCB is dipped in molten solder, which is then “leveled’, leaving solder on the copper traces. HAL can be used for through-hole PCBs and some SMT boards, but you should also bear in mind that the process can increase the stress on high layer circuit boards, which can have knock-on consequences, in terms of reliability.
Incidentally, you may have read about Immersion Tin finishes. We don’t offer this as a finish and would recommend that you avoid anyway. It’s great for soldering, and also very cost effective, but regarding the cost vs quality debate, these PCBs will need to be used within six months.
So, it’s now time for this rather “superficial” look at surface finishes… to finish. We do hope it’s provided some food for thought. The best advice is simply to contact Hi5, and let us talk you through the options, and find the right finish for you.