Re: making PC boards

Alan Marconett KM6VV wrote:

> Do you do double sided boards? Purple (?) sheet "negatives" from a
> laser jet? Then etch them? I've seen the CNC-type mechanical board
> etches like LPKF, and often thought about writing code to generate CNC
>
> "g-code" to cut boards on a Sherline mill. Anyone doing something
> along
> these lines?

I do double sides, but without the plated through holes.
No, laser printers are not accurate enough to register the two
sides together. Any board over 2" square will have serious
misregistration, such that holes drilled dead center from
one side will completely miss the pads on the other side.

I built my own laser photoplotter, which records images directly to
red-sensitive
silver litho film at 1000 dots/inch. The blacks are totally black,
about a log
density scale of 5 or so, and the clear is very clear (high contrast).
The films match up, over 10" or so, to within a few thousandths.
I align the films with a plexi sheet the same thickness as the board
between them, and tape to a piece of PC board scrap to hold the
alignment. I use boards coated with DuPont Riston dry film resist,
and expose in a fluorescent vacuum frame. Development is done
with a sodium carbonate solution, leaving a resist that is almost as
tough as solder mask. It will take extensive overetching without
much damage. The boards are etched in a heated spray etcher.

I drill the boards with a mod that mounts on the quill of my Bridgeport.

It is a Westwind air bearing drill spindle that now runs at up to
24,000 RPM. (With a better inverter I could go to 80,000 RPM.)

I've seen the LPKF machines, they seem to work, and do get rid
of the nasty chemical processes, that are also time consuming.
But, on the other hand, the etching process develops both sides
of the board at the same time, and it takes no longer to make a
board that is 10 times more complex.

Jon