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On the weekend I designed and etched a tiny little 12V booster circuit for an OLED. To prevent the traces from oxidising in the future, I removed the leftover ink after etching and placed the board in MG Chemicals’ Liquid tin straight away - so the copper was fresh. I left it for about 2 minutes.

Upon soldering though, some of the tin evaporated off the traces. I was just wondering how this is possible? I thought it “plated” the traces, or chemically bonded to the copper.

Soldering was done at 350 degrees celsius and I just used a standard electronics no-clean flux pen.

Here’s what the board looked like before and after soldering. I’ve placed two arrows at notable spots where the tin has evaporated.

Dodgy soldering aside, the circuit works great FYI.

enter image description here

enter image description here

Just wondering why this happened, and how to prevent it in the future.

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    \$\begingroup\$ Picture looks like the tin is oxidized but still present. Also, that is way too much solder. \$\endgroup\$ Nov 30, 2020 at 0:50
  • \$\begingroup\$ @user1850479 is probably right, unless it's oxidized right off. I don't know how thick a layer an electroless tinplate like that can manage to put on. I'd scrape the oxidized area gently and see if it looks like tin or solder underneath -- or just leave well enough alone. \$\endgroup\$
    – TimWescott
    Nov 30, 2020 at 1:05
  • \$\begingroup\$ BTW: unless your goal is to do everything, kinda, or to learn how to be a PCB etching specialist, you may want to look at circuit board aggregators. My favorite (in the US) is Oshpark (www.oshpark.com), but if you live in a developed country you have access to at least one. \$\endgroup\$
    – TimWescott
    Nov 30, 2020 at 1:07
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    \$\begingroup\$ I wonder if what happened is that it "beaded" up when heated due to a) surface tension, and b) insufficient "wetting" of the copper. Thus, rather than "evaporating", the "tin" simply moved to other areas on the trace. Do you know if the "tin" is elemental tin? \$\endgroup\$ Nov 30, 2020 at 1:45
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    \$\begingroup\$ Search on "electroless tin plating" for more information. I'm absolutely no expert -- but there appears to be expertise on the web. I would expect that the copper would need to be clean-clean, and it's probably best to start from distilled water, not just whatever comes out of the tap. \$\endgroup\$
    – TimWescott
    Nov 30, 2020 at 2:54

2 Answers 2

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Several possibilities:

  1. It's just oxidized. Tin dioxide is clear/colorless, tin oxide is variously (yellow to red to gray) colored, and any copper present can oxidize brown as well. Moreover, these are likely very thin layers (100s nm) so that, rather than being opaque, they are transparent, and interference effects -- as well as some dyeing effect from their natural absorption colors -- come into play. You often see a faint rainbow sort of coloration on clean, heated copper. There could still be tin there, it's just hard to see under the oxide(s).
  2. The tin is completely gone. Having oxidized as above, the flux has dissolved that oxide, leaving the bulk metal beneath. However, this has the problem that, the trace is still very dark, and usually flux leaves a pink (clean copper) surface. Perhaps insufficient flux was present, unable to clear away all the oxides and only the initial oxides were dissolved?
  3. The tin simply diffused into the base metal. Metal atoms are able to move around; that's pretty obvious for low-melting alloys like tin, but it's less obvious for copper which remains solid during the process. Normally during soldering, at the interface between solid copper and molten tin, an intermetallic layer forms. Intermetallics are compounds of metals, crystalline, and generally hard and brittle. The layer is thin, a few µm. But electroless tin can also be quite thin (comparable thickness). On heating, the metallic tin might simply disappear into the copper surface. (These intermetallics are still silvery colored, but if the tin diffuses deep enough, a yellow to pink "bronze" layer could be left: see also the classic golden penny reaction which uses zinc in a similar way.)
  4. Unlikely in my opinion, but perhaps still a possibility -- @Math Keeps Me Busy's suggestion of the tin beading up. The electroless layer should be very thin indeed, so the beads would be microscopic, and probably oxidize very quickly anyway.

I'd say some oxidation is still needed to explain the final color, so the exact answer will be a mix of these.

On a practical note, since electroless tin is so thin -- going over it with solder is still a good idea (as can be seen!). You can wipe on fresh solder quite quickly, and wipe off excess with solder wick if necessary.

And in that case, the initial tinning isn't necessary at all: you need to prepare a very clean copper surface for plating, but clean copper tins beautifully with a little flux and fresh solder!

I find hand-made boards, tinned, are still flat enough not to be annoying to place components and reflow solder, at least with a hot-air machine and some tweezers to adjust parts as I go. In fact I usually leave a little excess solder on the board, and let that bond the components; only flux is required (not paste), and maybe a little touch-up work with the soldering iron.


A note on flux chemistry: these are either acids, complexing agents, or both. A "complex" is when a metal atom is relatively strongly bound to some molecules, through a more unusual kind of chemical bond (technically, metal d-orbitals to lone pairs of O, N, etc., among other mechanisms). Acids dissolve oxides by direct reaction (forming a salt, which may dissolve in a solvent, or melt or evaporate), and complexes can dissolve oxides by attaching to metal atoms (to much the same end).

Heavier organic acids, such as those found in natural pine resin, happen to be very well suited to the tin-lead-copper system; usually with a few modifiers to make them form less gunk, stay better in place, maintain reactivity, etc. Hence the usual "activated" rosin types. Some lead-free fluxes may be partly/entirely synthetic; they use the same mechanisms, but different chemicals that perform better at the higher temperature.

Oxidation, is very much a normal and natural part of soldering, and is why flux is mandatory for almost all work (in air). Soldering can in fact be done in inert gas or vacuum, given perfectly clean metals -- metals "like" each other very much, and are more than happy to spread out over and bond together, given the chance. The problem is contamination, usually surface oxides.


Edit: some additional information some may find of interest: Wettable Flank Plating (particularly pp.5-6), gives micrographs of the plating and interface layer. The baking step at 150°C for 8hr will be equivalent to ballpark 10s of s at 250°C (assuming an Arrhenius extrapolation is valid over such a range of temperatures), so similar changes can be expected during soldering.

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The copper needs a better kind of plating process, on a manufactured PCB a surface finish is applied to the copper. I don't know how you'd solve this problem on a homemade PCB, but it probably has something to do with the etching then plating. I never had this problem with similar machined copper PCB's that I've built before, so it could be something to do with the etching.

It could also be the flux, which could be working a little too well, and changing the flux might also prevent balling.

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  • \$\begingroup\$ You might be able to just lightly coat the copper with liquid flux, apply some solder, and use a heat gun to perform DIY Hot Air Solder Leveling (HASL). \$\endgroup\$
    – PStechPaul
    Aug 10, 2022 at 5:54

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