Homebrew_PCBs

On Thu, 02 Mar 2006 16:24:08 +0100, lcdpublishing
<lcdpublishing@...> wrote:

> Hi Guys,
>
> I etched a board the other day using some new PCB material I got. I
> did the usual cleaning and polishing process that has been working
> good for me. However, I did have some toner transfer problems - and
> even etching problems on these PCBs.
>
> The best I could determine is that the suface of the copper is very
> slightly rippled - sort of like the threads in fabric - but more
> course. This seems to be the cause of the toner transfer problem -
> toner not sticking in these areas.

You are seeing the glass fabric of the PCB. This is normal, and varies
between PCB brands.
What do you use again to fuse the toner? I don't have problems with _very_
patternd boards.
To me this seems amplified by the light reflection, there doesn't seem to
be much depth to it.

>
> After etching, I also found that some small spots didn't etch all
> the way through - ie. a dot of copper is left on the board. I am
> guessing that this was a small void in the glass backing (FR4) and
> when the copper was applied, it was thicker in the area of the
> dimple - thus taking much longer to etch. Something like this, I
> don't believe there is anything at all that can be done to correct
> it so I am not too concerned at this time.

Well, usually they apply a foil of even thickness ;-)
Did you think they suqeegee molten copper onto it?

There probably was some "dirt" of sorts on the spot that acted as a bit of
a resist.

>
> Howver, the toner transfer process seems to require that you have a
> smooth and flat surface for good transfer and adhesion. If I had a
> micro scope, or some such device, I would look much more closely at
> the surface texture of the copper to validate this. With the naked
> eye, or even a loupe, I can't really see much deformation on the
> surface. I do suspect though, that even a deformation of .0001"
> would be enough to cause problems.

i can see the pattern well on some boards, but it never seemed to be a
problem.

>
> When pressing the paper onto the PCB, I use a paper based shop towel
> between the iron and the PCB to act as a pad and to also help
> accomodate surface irregularities. However, I am going to look for
> something that will work better.
>
> I guess what I am saying is this. The next time you get "Missed
> spots" with toner transfer, take a really close look at that area on
> the copper to see if it is in fact flat and smooth.
>
> Chris

Certainly requires further investigating, but i don't think the glass
pattern is causing your problems.

ST