Plating on Silver: What’s Old is New Again

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About three decades ago, immersion silver, a nitrate-based process, gained a lot of market share in the world of PCB final finishes. More economical than ENIG, flat, solderable, and conductive, it had everything going for it—everything but corrosion resistance in a harsh environment, that is. 

Champagne voids were also an issue, along with line reduction. But the worst drawback, the characteristic that made the part short over time, was creep corrosion. A build-up of copper sulfide salt that grows in contact with a sulfur-rich environment, heat, and moisture resulted in failures in the field. This was enough to scar the process for good. 

The market today for silver as a final finish is pretty meager and therefore does not justify much research. Fortunately, there is an alternative immersion silver, originally developed for other applications, that has proven to be a reliable solution. Immersion silver is nitrate-free, slightly alkaline, and deposits a slow, pore-free pure silver deposit. I had a strong feeling that this process would be a solid alternative to nitrate-based chemistry, yielding the benefits without the drawbacks.  

Work started about 10 years ago with this product, on a very limited basis, because as mentioned before, the interest in silver is just not there. One of our customers raised the fact that their silver was acting somehow galvanic, and some isolated lines were reduced. The customer works in the RF field, and this was a major issue for them. I offered to test this bath, feeling that it could be a solution to the problem.   

We had nitrate-free silver, which builds up about 10–15 microinches of silver in five minutes, so we tried simple tests with it. To our surprise, it solved all their issues, and they became an instant customer for this silver. They had had ongoing champagne voids and line reduction issues, and this silver solved that for them.  

Strong on that success, we made a few installations where those issues were identified. As I mentioned, the silver market was slow, and we did not push because we did not know of any corrosion benefit yet. The nitrate-based process can deposit up to 20–30 microinches in one minute, and for us, that is the main issue. We felt that thinner, pore-free silver had to be better than thicker, fast deposited silver.  

We had in mind to do a side-by-side bench comparison of these two silvers. The nitrate base is still dominating the market and is also directly related to the minuscule market share of silver as a final finish. Then the pandemic hit, and I had a lot of free time to involve myself in that long-desired bench comparison study. 

Some material manufacturers had begun to share their findings on 5G requirements and, based on the requirements of high-frequency PCBs, it looked like silver was the way to go. To make a long story short, high frequencies travel on the surface, and silver is practically as good as copper at precluding signal loss. Signal loss was the main problem for 5G applications as the component will stop working. 

Compensating for insertion loss requires a more sophisticated material, or heat-resistant components, or signal boosters, all of which lead to higher costs to accommodate ENIG.  

To read this entire article, which appeared in the June 2022 issue of PCB007 Magazine, click here.


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