Homebrew PCBs

Dave,
I agree with your conclusion.  Spray coating the copper just after the 
resist has been removed is a good look and what I did with acrylic spray 
at times in the past. The epoxy we used made rework almost impossible.  
I haven't done much in the area of things that need re-work once built, 
but generally I understand that acrylic or urethane spray coats are also 
good and allow reasonable reworking.  I do know that there was (still 
is?) a patented PCB production process where after the resist was 
removed, silk screening added, the board was coated with a "protective" 
coating.  The coating was thin, and basically a flux, but it also acted 
as an oxygen barrier so the copper remained solderable for some period 
of time.   Personally I always felt this could be duplicated by using an 
alcohol/rosin solution.  (A cheap source of rosin is dance supplies.  
Dancers put rosin on their ballet shoes.)  Paint/dip your newly cleaned 
board with the solution and you will have pre-fluxed it and protected 
the copper surfaces from oxidation.  Rosin by itself is dry and 
non-sticky (be sure the alcohol is pure and not drug-store rubbing 
alcohol which often has some mineral oil as a percentage of the 
solution).  A disclaimer here -- I've never done this myself as my 
personal PCB needs have changed drastically from the days described 
above.  I make a one off prototype using toner transfer that I 
immediately assemble by hand or I send off the files and have a board 
house make a bunch.
Regards,
Charles R. Patton

things, On 8/20/2013 9:39 PM, David Pickering wrote:
>
> Thanks charles
> After reading your findings I may well not bother with it, just keep 
> to the copper surface.
> Normally when building equipment like metal detectors which are out in 
> all sorts of environments I spray the finished populated board copper 
> clad surface with electrical pvc spray.
> Will just spray all boards lightly when the projects are finished to 
> keep the shine there.
> If you need to do any soldering at a later date it doesn't present 
> much of a problem.
> Thanks again
>
>
> ________________________________
> From: Charles R Patton <charles.r.patton@... 
> <mailto:charles.r.patton%40IEEE.ORG>>
> To: Homebrew_PCBs@yahoogroups.com 
> <mailto:Homebrew_PCBs%40yahoogroups.com>
> Sent: Monday, 19 August 2013, 18:23
> Subject: Re: [Homebrew_PCBs] silver coating
>
>
>
> 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
>
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>
> 



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