Homebrew PCBs

From: Stefan Trethan
> The temperature doesn't seem to be a problem. i set the iron to
> highest on last try and the toner fused well. i maybe didn't soke
> it enough. but the most problem is i think that my printer puts not
> enough toner on the paper (also leaving pinholes). i also had this
> problems with ohp film (and partly because of this want to
> abandon using phototransfer process).

Laser printing on overhead projector film works very poorly.  The dark parts
aren't very dark, and if you try to make it darker, the transparent parts
pick up some toner, and you get pits in your copper traces.  Awful.

I've had much better luck with laser printing on vellum (thin,
semi-transparent paper, can be bought in artist supplies stores for a few
cents per sheet).  Put the vellum with the toner side down against a
photosensitive board and expose to light.  I got excellent results with 12
mil lines and spaces.  Got poor results with 8 mil lines and spaces (some
traces came out too thin or broke), but I think some more experimenting
might produce better results.

I tried printing on ordinary laser printer paper, but it is just too
opaque - I couldn't get the board to expose.

One other thing to watch for: fluorescent lights change their intensity as
they warm up and as they age.  So, if you turn on the light and expose one
side of the board, then flip it over and expose the other side for the same
amount of time, the two sides may not be equally exposed.  There is a
cumulative exposure meter circuit in The Art of Electronics by Horowitz and
Hill that might help with this.

Cheers,
- Jan