MGChemicals liquid tin - risk of tin whiskers?

[matthuszagh]

MGChemicals liquid tin is a commonly used way of adding a protective coating to the exposed copper of DIY PCBs. However, tin platings are subject to the tin whisker effect. Apparently, pure tin plating is more subject to this than non-pure platings (see https://nepp.nasa.gov/whisker/background/index.htm). Does this mean that the result of using liquid tin is very subject to tin whiskers?

If so, what are good alternatives to liquid tin plating of DIY PCBs?

[matthuszagh]

One thought I've had is to use a soldering iron with a blade tip to apply a flat solder layer on top of the copper. I haven't actually tried this yet, but it sounds like it would be relatively easy to do.

[T3sl4co1l]

This seems relevant: https://ieeexplore.ieee.org/document/6066954/similar#similar
You'll have to find a source / buy if it really matters.

AFAIK, annealing (bringing it up to soldering temperature) helps.

Mind that electroless tin is extremely thin, so it may oxidize through and/or diffuse into the copper (forming Sn-Cu intermetallics), leaving an unsolderable mess.  (On the upside, with no metallic tin left, it won't whisker. )  Coating with soldermask, or solder or electroplated tin (heavier than electroless), is best.

I don't have a problem leaving bare copper on proto PCBs, as long as it's not exposed to a corrosive environment.

Regarding NEPP, mind that much of the information is fairly old (circa when RoHS was introduced).  Not to say it's outdated or irrelevant -- it is good work, and very much applicable to the materials and processes that were around at the time, and have been for a long time.  That's the catch: RoHS processes have improved since then, so the risk with modern commercial components and processes may not be as severe.

(To be clear, at-home electroless tin is probably old tech, not really anything they can refine in the formula I think, so it will still be applicable to that.)

Also, keep in mind, NASA's in the business of spaceships; if you're just making one-offs, well, that's a very different thing.

Tim

[Bud]

One thought I've had is to use a soldering iron with a blade tip to apply a flat solder layer on top of the copper. I haven't actually tried this yet, but it sounds like it would be relatively easy to do.

It can be done but the board will not look as neat and nice as if liquid tin solution was used.

[rancor]

Solder covering the plating will mitigate any risk so if you are not letting the tin plated board sit around before building it shouldn't be a problem. You'd also want to make sure all pads had solder on them. My only other through would be to attempt ENIG plating the boards. 

[bdunham7]

If so, what are good alternatives to liquid tin plating of DIY PCBs?

Thin some SnPb solder paste with liquid flux to make it like thick paint.  Brush it on and toast to perfection.  If it gets lumpy, use a hot air gun to blow the excess around or off.  Homemade HASL! 

Don't laugh too hard, it makes a nice solderable board, although you may have to vacuum out any through holes with a solder vac tool.  If you get the amount right you can just tap the board a few times when you take it out of your toaster oven.  And I've never tried it with large or fine-pitch boards, this is more for the DIY PCBs you make from a laser printer transfer or resist marker and an etchant tray.  I imagine it will also work for the ones you make with the little CNC mill machines.

[matthuszagh]

Solder covering the plating will mitigate any risk so if you are not letting the tin plated board sit around before building it shouldn't be a problem. You'd also want to make sure all pads had solder on them.

True, although with DIY boards I'll still have a lot of area without solder (I probably won't add a soldermask, though I may eventually start doing that).

My only other through would be to attempt ENIG plating the boards.

That sounds hard...

[matthuszagh]

Thin some SnPb solder paste with liquid flux to make it like thick paint.  Brush it on and toast to perfection.  If it gets lumpy, use a hot air gun to blow the excess around or off.  Homemade HASL! 

Don't laugh too hard, it makes a nice solderable board, although you may have to vacuum out any through holes with a solder vac tool.  If you get the amount right you can just tap the board a few times when you take it out of your toaster oven.  And I've never tried it with large or fine-pitch boards, this is more for the DIY PCBs you make from a laser printer transfer or resist marker and an etchant tray.  I imagine it will also work for the ones you make with the little CNC mill machines.

Not laughing - this is a great idea! Should have thought of it. I considered getting a soldering pot to DIY HASL, but I'm not sure that would come out level and I'm not sure how easy it would be to level. Your method sounds better.