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Tin-oxide deposits

Jason Nipper

#2739

Tin-oxide deposits | 13 October, 2000

A few weeks back we experienced a wave process problem where we received deposits on our pcb. At first we though it was dedrites but after an outside analysis of the deposits they were found to be tin. We have tried to remove the deposits with brushes, heat, and cleaning agents. So far the only thing that we have found that works is running the board back over the wave. The problem with this is that our product has a gas plasma display on the solderside that wraps around from the front. We currently do not have a fixture capable of supporting the display through the wave to maintain position, with the pcb depanelized. So I would like to consider this my last alternative.

My question to the forum is there any chemical availible that will remove the tin deposits without etching, delaminating,...etc. the pcb?

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#2740

Re: Tin-oxide deposits | 16 October, 2000

Jason: How did you determine that the deposits are tin oxide? What does this stuff look like?

Klein Wassink "Soldering in Electronics" (p. 221, quoting original work by others). States (in effect) tin (or solder) quickly forms a protective oxide layer a few hundred atomic layers thick, and the growth then effectively stops under ordinary room conditions.

SnO2 forms at room temperatures. Think of it this way - a layer of SnO forms on a new solder surface, once it gets thick enough that the Sn has a long diffusion path the outer surface begins to convert to SnO2 because of the abundance of O2 in the air. This SnO2 layer never gets very thick [because it is the stable form of Sn oxide], but it is very coherent and very insulating. The insulating nature of the oxide and its structure make it hard for fluxes to effectively remove it.

Dr. Tench and others have done additional work that expands on the information mentioned in KW "Soldering �". References with a bunch more detail are:

* "The Mechanics of Solder Alloy Wetting and Spreading" ISBN 0-442-01752-9, "Chapter 6: Oxidation of Solder Coatings" Dr. M. Tench * "An Examination of Artificially Aged PWB Surfaces Using Sequential Electrochemical Reduction Analysis" D. Hillman & M. Tench, IPC-TP-1060, October 1992.

Now, simple tin oxide is not a problem for most fluxes, if the oxide is not TOO thick. But you probably do not have simple tin oxide.

More complex tin compounds require an alkaline cleaner to COMPLETELY remove it. (Some organic solvents may partially, or largely remove it) The stannic tin oxide can be removed by either a STRONGLY alkaline cleaner, basically dilute caustic, or dilute hydrofluoric acid, or ammonium bifluoride (which really is half neutralized hydrofluoric acid). Ooops, I wonder if that stuff will affect gas plasma display on the solder side?

So, to your question, how do I remove oxide from solder? Well, you can use thiourea hydrochloride, often found in fab shop solder brightners, (and I think Wal-Mart sells it as Tarn-ex, next to Cascade in the PWB section), but what have you accomplished? You will very quickly form another even harder to solder to oxide coating.

The answer is not to eliminate tin oxide. It is to prevent it. Once the oxide is formed, in excessive amounts, it is difficult if not impossible to perform soldering operations, of any type, to effect acceptable solder joints. When the oxide is present, no type atmosphere removes it. Your flux didn't. Nitrogen won't as it only prevents.

Now, here�s the 2� question � why do you want to remove the stuff?

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Glenn Robertson

#2741

Re: Tin-oxide deposits | 17 October, 2000

Jason -

I have only one thing to add to Dave's excellent (as usual) information. Some time back we had a similar problem and we removed the dross bits with a soldering iron and solder wick. Yes, it's tedious and you have to look closely to be sure you find and remove all the dross. Maybe you have so much that this is impractical. Just a suggestion.

Glenn Robertsun

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Jason Nipper

#2742

Re: Tin-oxide deposits | 18 October, 2000

I know it's a tin residue because we have a Cheme in house who had a spectrum analysis performed in an outside lab.

The waves parameters were out of control long enough to effect a number of units that are, we�ll can I say substaincial in a fiscal sense. This happened in on of our non-domestic plants so corrective action was delayed due to a lack of experienced personel. As it so happens we originally didn�t attribute this residue to any serious problems. But soon after we realized that this thin layer was randomly providing an electricl path for a transistor to line therefore destroying the transistor and causing some hard to replicate issues. We tried gold leaf and still couldn�t reproduce the issue. The atomtomically thin layer of conductive tin residue is the answer. Now we are trying to remove this residue of tin from avery large number of pcb. The fiscal amount of scrapping these boards would be tremedous. But running them back through the wave would require removing and refixturing a Gas Plasma Display that we are afraid of overhandling and stressing the leads�etc.

Jason

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