Issue link: https://iconnect007.uberflip.com/i/1529411
56 PCB007 MAGAZINE I NOVEMBER 2024 Why was it more reliable? Holden: Tin-Nickel has a unique property of instantaneously forming an impenetrable oxide. It may have something to do with its metastable state in that it's atomically more like stainless steel, which you can't plate. Carano: Correct, you cannot plate stainless steel. Tin-nickel acts like stainless steel in many ways. It has this impervious oxide. You cannot get gold, solder, or copper to diffuse into tin- nickel. It resists other metals diffusing into it. It's called solid state diffusion. If you put tin directly over copper, aer a period, even if the circuit board or the part just sits on a shelf, the tin and the copper will diffuse together to form a copper-tin metallic. Nickel and tin-nickel do not dissolve. e benefit is they won't form an intermetallic. I liked tin-nickel for PCB manufacturing because you didn't have to strip it. You could put it directly onto copper on a circuit board and, as Happy said, either add immersion gold or immersion tin for solderability protection. Right now, we do solder mask over bare cop- per because you want a flat surface. We strip the tin off, a process that not only costs money, but has high environmental costs due to tin waste. Why not just put the tin-nickel down, and have that act, not just as an etch resist, but something to put your components on? You've eliminated a whole step, saved time, and cre- ated less waste. That makes sense to me. Carano: We developed this in Youngstown, Ohio, when I was with Electrochemicals. We had developed a higher pH/more stable tin- nickel solution that was easier to use and more solderable. In its final form, it had this gor- geous rose-pink finish. We took it to a furni- ture manufacturer in Youngstown that had chrome and nickel-chrome components for its furniture rails and told them we wanted some of their parts so we could plate them. e owner looked at us incredulously, but we brought them back, and found that it cost Typically, if your chemistry is out of bal- ance—if you have more tin in the solution than you have lead—you will plate more tin. It's pre- cisely why there are so many alloys out there. In that respect, tin-nickel is pretty amazing. Happy Holden: When I was at Hewlett-Pack- ard, Dr. Morton Adler from Bell Labs called me and said he wanted us to fab his boards using tin-nickel, because he knew we were using it. I plated the boards with tin-nickel and added a gold strike. is thin gold was cobalt-hardened and the final thickness was still only 3–5 micro- inches when we finished. About a month aer I sent them back to Adler, I got another call. He said, "What the hell did you send me? We've completed our reliability testing and your 3–5 microinches on tin-nickel outperforms our 120 microinches of plated gold on our contacts and other relays." Happy Holden during his Hewlett-Packard days.