Homebrew PCBs

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@...>
To: Homebrew_PCBs@yahoogroups.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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