Homebrew PCBs

In a message dated 6/3/2005 1:46:17 P.M. Central Daylight Time,  
kg4wfx@... 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







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