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Tin Fusing Problem

We have a process where we plate nickel over the copper circuits, then we plate pure tin over the nickel. The tin is then fused at a temperature of approximately 480 degrees F. The problem shows up after the fusing process in the form of a white film on top of the shinny fused tin. We have changed fluxes several times, no help. In the beginning we tried about thirty different tin-nickel combinations before we found two solutions that would allow a bond between the two metal that wouldn't de-wet during the fusing process.

My questions: Does anyone know what the white haze is? Organics? Stannic? Have you seen this before? Do you think the problem is in the plating solution or is it a problem with the fusing process' preclean? We use a HCl system for cleaning proir to the fusing process. I have heard that some of the tin oxides actually expidite fusing. They act as wetting agents. Would we be better off if we forget the pre-cleaning? I don;t want to just try this as our product is very expensive and scrap is deadly. Is there a better temperature to fuse at than 470-485 degrees F? Could the problem be in the oil that we are using in the Hot-Oil fusing process? I have also heard that the Socket Industry uses a pure tin plating bath that gives great results during fusing, does anyone know what chemistry is used by them?

Any help would be appreciated.

Dennis Johnson
- North Jackson, Ohio

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The alkaline stannate bath produces a very pure tin. Does the problem go away with fresh peanut oil?

What kind of nickel do you plate? Don't use too much brightener in that bath.

I don't remember using HCl for cleaning prior to fusing. I believe we just rinsed after tin plating, then went right into the fusing oil when we were ready to fuse.

  
Tom Pullizzi
Falls Township, Pennsylvania

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This process is used in the printed circuits industry, so the panels being plated are covered with an alkaline strippable photo-initiated plating resist, which prohibits my using an alkaline bath. We are using a stannous sulfate bath. We also have to use a high temperature synthetic oil because peanut oil begins to breakdown, when maintained at the temperatures we need (460-485F). I believe pure tin melts at about 450F. But that is one of my questions, should we lower or raise our temperature?

Our nickel bath is a nickel sulfamate and we keep it compltetely matte. We add no brighteners although we do add wetting agents. The question of brighteners may come into play in the tin bath though, it has a two part system with a very vague instruction for additions: add so many ml of the two parts per liter of sulfuric acid. Could this haze be coming from over-additions of brighteners and wetting agents? I have been thinking it was stannic oxide, since all of my cleaning chemistry is acidic and stannic oxide is insoluble in acids, but organics would do it, wouldn't they?

How does the brighteners in the nickel affect the system?

Thanks for you help, Tom.

Dennis Johnson
- North Jackson, Ohio