Re: Limits of positive-resist circuit board making

From: Mac (foo_at_bar.net)
Date: 12/14/04 

Date: Tue, 14 Dec 2004 06:19:47 GMT

On Mon, 13 Dec 2004 16:09:05 -0800, grunt wrote:

> For those who make circuit boards using positive mask lithography,
> I have a fair assessment of how small one can make traces and details
> on a printed circuit board.
>
> The practical lower limit on the space between traces of copper is
> twice the thickness of the copper. For 1 oz (per Sq.Ft.) copper at 1
> mil, thats two thousandths of an inch.
>
> The lower limit on the width of a trace is the thickness of the copper,
> as above, one thousandth of an inch.
>
> Masks can be made by reducing over-sized artwork to black and white
> film (with a camera) from negative artwork.
>
> Experiment with shutter speeds for maximum contrast.
>
> In terms of application, one could fit up to 16 traces in the 20th of
> an inch space between dip pads, etc.
>
> - Geoff

If you can reliably make boards with 1 or 2 mil traces, you should go into
business.

As of two years ago, it was expensive to go with 4 mil traces. I heard an
engineer say that you could have a trace "neck down" to 3 mils in places
to do funny things with the impedance, but I never actually saw an example.

Two mil traces were not practical at all.

I don't think things have changed all that much.

--Mac

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