Archive of the former Yahoo!Groups mailing list: Homebrew PCBs

A bit of anecdotal info on liquid tin plating. Some decades ago at a
company I was with we were doing PCBs that we tin coated with a
commercial electroless solution. The coating seemed fine and we
thought, "Great!" We would screen print a large lot of boards, etch
them, tin plate, and put on the shelf for later part stuffing by hand,
then solder by hand dipping the bds in a solder pot.

Now the down side. Within a relatively short time, the tin coating
seemed to get dull, and the solderbility with rosin based fluxes dropped
drastically (even worse than the bare copper) leading to faulty
soldering. You can observe the same phenomenon if you've ever found
some old resistors and try to solder the leads. It can be a bad
experience.

I attributed the problem to oxidation of the thin tin coating (or
tin/lead coating of the resistor leads) from the electroless tinning
process . Those oxides just don't solder well.

The solution we began using:
There are liquid organic acid fluxes designed for PCB soldering. We
switched to those. But several important steps must be strictly followed.
1) Press the soldering side of the board with components onto a sponge
soaked with the acid flux.
2) Immediately dip solder the board.
3) Then immediately drop the hot, just soldered board into a tub of
water. (Do not wait or store the boards, the consequences can be severe.)
4) Blow off the water and rinse again and blow off again. (Removing
excess water is important if using tap water as we were.)
5) Oven dry.

At this point I can immediately hear purists screaming, "Acid flux???"
Just let me say a few things. We were doing RC filters that required
extremely good surface resistance of the PCB before we coated the board
with solvent thinned epoxy to obtain and maintain the high resistance.
The process above gave us better results: bright, solid, well filleted
solder connections and incredibly consistent, very good surface
resistance of the PCB assemblies.

(Just a side tip, the best thing I ever found for moisture resistance
at that time on those PCBs was a solvent based silcone wax. However
the down side of that was that it was always a bit tacky and these
boards could end up in dusty environments, so we decided not to use it
in production. This whole area of moisture vs. resistance has come back
multiple times in my design career and could be thread in another
discussion list.)

Regards,
Charles R. Patton