Limits of positive-resist circuit board making

Few commercial board houses will do 4 mil resolution, and some won't even do 6. 1 or 2 mil isn't practical on a board of any complexity... yields would be too low.

Most board houses start with 1/2 oz or even 1/4 oz copperclad to get the resolution, then plate it up. It's not uncommon these days to order "1 oz" layers and get something less.

Functional traces on 3 mil centers? Not likely.

John

copper,

You are dreaming! Most professional board houses stop at around 4/4, or maybe 3/3 if you pay the money, and there are many practical reasons why this is so. I can do 6/6 on my own home made boards with positive photo resist, but only JUST if I'm lucky, and usually the boards need some touchup. 8/8 no problem home made.

Dave :)

You do not know what you are talking about. Since years people do not use masks any more but have fine artwork exposed by Photoplotters. Those have resolutions of around 1/2 mil. I have never seen a boardhouse making 2mil structures either, maybe in future? It would require a very controlled process. I have been designing hybrids and could produce tracks that fine by directly lasering away the silver conductors, but not on PCBs.

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

To elaborate, 2 mil spaces and 1 mil traces can be done, but I have not observed any tangible yield rate - you'd have to try a few times. This isn't the sort of thing I'd expect normal board houses to do - but this is the sort of thing I'd expect a creative person could handle by the second or third try.

At the lower limit, it's not easy to do, but if the application (research apparatus, for example) calls for it, it can be done without much fuss or gear. Some failure rate is to be expected, as someone already noted, so board houses wouldn't bother trying. I was looking at test patterns and the very near miss at 1 mil / 1 mil / 1 mil ( trace / gap / trace ) detector loop I was trying to fab.

If you have money for film developing and time to mess with it, you could easily attempt 2 mil traces and 3 mil gaps.

The next time I use this technique, I'll make a test pattern at the limit and post a link to a picture of the board.

The point of this post is to convey that anyone who wanted to make 2 mil traces with 3 mil gaps could expect to do so without much difficulty, and enthusiastic persons could make boards with sections at the limit of 1 mil / 2 mil - but if you're sure I don't know what I'm talking about, your lower limit might be different in practice.

One 'trick' I employed is using UV LED light down a 30 cm x 10 cm blackened tube, the weight of which also pressed the glass and mask (film pieces) to the board. The glass is too good at absorbing UV, so acrylic plastic is better, but I got results. The advantage of the lighting scheme is to reduce the number of photons arriving off axis, as happens with the broad flourescent types.

backlit transparency (where the photo paper normally goes) and exposing pos. resist board (where the film normally goes) in an enlarger did not work, but showed good resolution potential if it was not for the UV absorption of the enlarger optics (aka lens).

- Geoff

Don't forget to take in consideration that inverse characteristic of capacitance...

-M

Naaa, when I do my own, I have 20mils/20mils for trackwidth and spacing everywhere, but can go downto 15mil/15mil on small areas. I found drilling below 0.7mm (28mil) uses too many drills. so I stick with them. My mask is a polyester foil coming out of the laser printer such that the black is on the side touching the pcb, meaning the top layer is mirrored

Rene

Ban Wrote;

Of course, you are absolutely right, I don't know what I'm talking about. Follow this URL to a photo of the board I made yesterday with a

- Geoff

not

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What about warped boards? What about unevenness in the board or photoresist surface? What about imperfections in the photo mask? What about developing? What about etching? What about dust and other contaminents? What about repairing these sort of tracks when they DO etch through or bridge?

Why would you want those sort of fine tolerances anyway if it's a)incredibly fiddly, b)not easily reproducible, c)would only work for a few test tracks (try doing a large complex board), and d)would take so much time and fiddling that it's not finacially viable. You could go to higher tolerances and get a professionally made multi-layer board, that's what any sensible person would do.

What do you want to have such fine tolerances on a double sided board?, just to prove you can? It's pointless, forget it. 1/1 or anything approaching that is a waste of time even trying. But if that's how you get your kicks, be my guest.

Dave :)

File is inaccessible. From your OP it was not clear you are a hobbyist. With todays technology BGAs with fine pitch (0.75mm) balls it is impossible for a hobbyist to make an appropriate board at home because you need microvias and multiple layers. What is the use of 1 or 2 mils conductors if you cannot produce even 2 layers with that repeatability, not to talk about 6 layers with buried vias. I have worked with photograpic methods(4:1) in the mid 80s and from then on only seen photoplotters. Sure you can also solder by hand, but with a reflow oven, printed solderpaste and -masks it is so much more easy to produce prototypes, and the price for PCBs and stuffing houses is so attractive that I wouldn't want to go the homebrew way any more. Sorry for the misunderstanding.

I read in sci.electronics.design that grunt wrote (in ) about 'Limits of positive-resist circuit board making', on Wed, 15 Dec 2004:

Access not allowed.