Homebrew PCBs

I pulled this off a reply by Don Wilkins of the 
rec.crafts.metalworking site:

After the holes are drilled in a douple sided board (before etching), 
"Plated through holes are done by first sensitizing the inside of the
hole using stannous chloride which leaves a tad of divalent tin on the
surface. This is followed by a treatment with a palladium solution.
The adsorbed divalent tin reduces some palladium in solution to
palladium metal which sticks to the PCB (in the hole). The PCB then
goes into an electroless copper plating bath where the palladium
catalyzes the reduction of copper in the electroless plating
solutions. The result is a thin conducting layer of electroless copper
through the hole. Too thin to use as is. (Of course this plating
through is done before any of the copper on the circuit board is
exposed. i.e. the mask has not been developed)

Finally the board goes into a conventional plating bath to produce  a
thicker coat and make contact with the copper on the circuit. This is
a rough outline without the gory details."

If you can't plate the holes, the next best ways are to either solder 
a wire or plug on both sides (see earlier discussions on this site), 
or fill the hole with a silver compound that Radio Shack sells as a 
PCB repair kit (also repairs rear window defroster tracks).


--- In Homebrew_PCBs@yahoogroups.com, JanRwl@A... wrote:
> In a message dated 6/3/2005 1:46:17 P.M. Central Daylight Time,  
> kg4wfx@y... writes:
> 
> so how  does one plate a circuit board where traces are not all 
connected to 
> each  other?  By using a sheet 
> of metal pinned to the circuit board to make  sure all traces are 
tied to the 
> cathode?  <<
> No, Tony, you are visualizing the PCB being nearly DONE before  the 
holes are 
> plated through.  In reality, just the opposite takes  place:  
Before ANY 
> resist or etching is applied, the drille, cleaned, and  rinsed 
board is 
> "sensitized".  Some kind of magical fluid is applied so  that the 
walls of all the holes 
> become conductive.  The copper on both  sides is, of course, 
already so.  The 
> "blanks" for plated-through work are  also made with THINNER than 
usual 
> copper.
>  
> Then, the "whole mess" is plated with copper, hole-walls and  
copper surfaces 
> as well.  This gives nicely-thick copper.
>  
> Next, the copper is photo-sensitized so that when developed  only 
the "wanted 
> copper" is bare; the rest covered up.  Then the boards are  "solder-
plated".  
> Yep, a tin/lead concoction is electroplated on  that.  ALL the bare 
copper 
> becomes solder-plated, the traces on both sides,  and the walls of 
the holes.
>  
> THEN the resist is washed off and the boards etched in a  solution 
that 
> "eats" copper but does NOT "eat" solder.  Then rinsing again,  and 
some heat is 
> applied, JUST-enough to melt that rather-raggedy grey-
looking  "solder", but not 
> enough to loosen the traces from the epoxy.  This "shines  up" the 
traces just 
> fine.
>  
> Next would be any solder-mask (that green transparent stuff),  and 
> silk-screening, if wanted (white markings indicating what is 
what).  
>  
> Next would be preparation of an OUTRAGEOUS invoice . .  .
>  
> Got it?  This is how it was explained to me.  I  never was allowed 
to "watch 
> it be done", but I have etched a few "copper-only,  no PTH" boards 
at home.  
> Nasty!      Jan  Rowland
> 
> 
> 
> 
> 
> 
> 
> [Non-text portions of this message have been removed]