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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