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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