--- In
Homebrew_PCBs@yahoogroups.com, Adam Seychell <a_seychell@y...>
wrote:
>
> alan00463 wrote:
>
> > --- In Homebrew_PCBs@yahoogroups.com, Adam Seychell <a_seychell@y...>
> > wrote:
> >
> >>Sounds kind of odd to want to make an entire pre etched PCB, back to
> >>being conductive over its entire surface. Why not skip the etching
> >
> > step altogether so you have fully copper coated fiber glass material ?
> >
> >>Your question does not read as wanting to selectively coat the
> >
> > copper tracks with a conductive material since copper is already quite
> > conductive.
> >
> >>You post is also contradictory by mentioning the use of polyurethane
> >
> > as being an option, becuase this is not a electrically conductive.
> >
> >
> >>Maybe your better to explain what your goal is.
> >
> >
> > Sure, Adam. I want to etch the copper artwork ∗before∗ I have all
> > the components to populate the board with. So that is why I wanted
> > some substance to cover the copper traces immediately after etching so
> > they don't become oxidized. I was thinking it would be easier to
> > solder the (through-hole) components in place if I used an
> > electrically conductive material. Or should I use something I can
> > later clean off the copper traces with solder flux when ready to
> > populate the board?
> >
> > Anyway, I did some research this afternoon and discovered that of the
> > conductive substances nickel, tin, lead, and silver.
> > Nickel is a poor conductor of electricity.
> > Lead is a poison as well as a poor conductor.
> > Tin is also a poor conductor of electricity.
> > That leaves silver, which is an excellent conductor. Stefan, you
> > are right as usual. Thanks, Stefan for taking time to answer me.
> >
> > And yes, I realize it would have been necessary to interconnect all
> > the copper traces in order to electroplate a metal onto them. That's
> > an extra step that will be nice to avoid.
> >
> > Thanks to all who answered,
> > Alan
> >
>
> Plating the copper traces with another metal would cause no measurable
> increase in resistance, between an connection interface consisting of
> solder/nickel/copper and that of solder/copper. Obviously you know that
> solder has 12? times more resistance than copper!
> Protective plating finishes are typically no more than a few microns
> thick. Would you scrape of the tin from tin plated copper wires of
> resistor and IC leads in order to decrease resistance ?
It really depends upon the application. If he's designing a
circuit for microwave frequencies, then it may be desirable to
Silver plate everything (while taking the appropriate precautions
against Silver whiskers or electromigrated dendrite formation).
But, unless he is looking for a microwave application, then Tin
plating shouldn't cause any problems (excepting, of course, Tin
whisker formation).
> There are a few ways to avoid copper oxides. Simplest is to use a
common
> PCB lacquer that is sprayed on. this lacquer can be soldered through. I
> use it all the time, and think its great.
That's not bad for discrete soldering, but I'm not sure I'd
recommend it for a wave soldering system. It really depends on
his production methods (although, I think I'm pretty safe in
saying that he almost certainly won't be employing a wave
soldering bath).
> Another approach is to tin plate the copper, with an electroless tin
> formula, but these are expensive.
> There is also electroless nickel formulations but difficult to get hold
> of and probably even more difficult to make yourself. I've never made
> any electroless solutions. Electroless plating is a solution that
> deposits metal onto a metal without applying external electricity.
> Electroless plating has ability to plate on its own kind so the metal
> can build up continuously provided there is a supply of fresh metal
> ions. Search google for electroless
I have made my own electroless Tin plating solution, although I
certainly wouldn't recommend it to a beginner (or, for that
matter, even an expert without the appropriate protective
systems!). The solution I used to make was based on Stannous
Chloride, Sodium Chloride, and Sodium Cyanide (Yes, Cyanide!) [1].
It worked wonderfully, but was sensitive to air oxidation. And,
you don't even think about having a bottle of acid in the same
room, since a small splash in the electroless plating bath knocks
you dead in about three seconds ("What's that funny sm....<klunk>!").
[1] I'm intentionally not going to give the receipe here, due to
safety concerns (and, because I can't remember it exactly).
> You really don't have a choice to use anything else other than
> electroless plating since it will be unpractical to connect each copper
> trace to a power supply for a electrolytic plating bath. Electrolytic
> plating is inherently simpler, cheaper and more forgiving.
I've seen some designs, usually for gold plating, where each
and every trace was run to a header on an edge of the card.
This allowed the card to be electrolytically plated, and then
the header would be sheared off, electrically isolating all of
the traces. Of course, that made for a rather complex layout,
and it was only possible for relatively simple cards (Think of
small computer cards from 30 years ago, with SSI level TTL
chips.).
Of course, there are some concerns about what metals can safely
be plated over other metals. Gold has a bad habit of forming
intermetallic compounds with certain other metals (primarily
Aluminum, although I'm not a metallurgist), and may require a
passivation metal layer (Nickel?).
> Adam
Dave