Professional board houses (but not small ones) use pattern plating after
negative print. They start with thin layer plain copper, leave the dry
film where is to etch copper, and copper plate up to desired copper
thick. Then they tin plate above the copper tracks. And they they remove
all the dry film before etch. This way they end with plain thin copper
where is to etch copper away and tracks that are higher levels of copper
and which are tin plated above and incredibly also on sides. After they
etch in alkaline spray baths (complex mixture of ammonia and
sulfuric/hydrochloride/nitric acids with phosphoric acid to don't etch
the tin) but the etchant only removes the really thin copper, the tracks
are already much higher. If over etch occurs you always see the correct
width from above.
Metallic resist is the only way to do plated holes bellow 0.5mm and thin
tracks bellow 0.3mm. But if is used tin it has to be removed because of
tin whiskers. This is done in simple hight concentrated NaOH bath (tin
can be removed and reused again over and over).
All this can be done at home with toner transfer and Peroxy-sulfuric
with phosphoric acid for tin resist, ammonia/sodium persulfate for
nickel resist or cupric chloride for gold resist.
The still remaining problem is hole activation for plating...
Regards,
Simao
On Sun, 2009-03-08 at 10:55 +0100, Stefan Trethan wrote:
> I think if you plate it up afterwards it will widen again. Besides,
> you need to contact all traces for plating so it is impractical for
> real boards.
> Thinner copper would definitely reduce the problems with underetching.
>
> Professional board houses have a number of factors working in their
> favour. They have high resolution imaging/plotting equipment so they
> can actually draw fine lines. Their photoprocess supports transferring
> those fine lines to resist using well collimated light and a number of
> other factors i probably never even considered.
> Then they have spray etchers which assure an even etch over the whole
> area, so they don't have to over-etch so much. Also, spray etching
> shows much greater directionality than immersion etching. The etching
> process is carefully controlled (chemistry, temperature, etc..).
> All in all, i think they can reduce the tolerances in just about every
> step compared to our homebrew work.
>
> I'm not saying fine lines are something to work towards. 8mil or
> 6.66mil is more than small enough, even for the boards i design for
> production. I would not want to make any smaller traces simply for
> electrical reasons. So i have no desire to push towards finer lines in
> my homebrew setup, merely a curiosity about how it would be possible.
>
> ST
>
> On Sun, Mar 8, 2009 at 7:54 AM, Jim <jhutch17@...> wrote:
>
> > If you start out with VERY thin copper (say 1/10 the thickness of 1
> oz
> > copper) ... lay down a 1.5 mil resist and etch it ... drill
> holes ...
> > activate holes ... then plate the copper up to the thickness of 1 oz
> > copper ... would this result in narrow traces? ... or is the limit
> in
> > the photo/printer part of the process? ... some how the professional
> > houses get very narrow traces and the edges look very good under
> > magnification!
> > Jim
>
>
>
>