Re: TI-H: PCB
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Re: TI-H: PCB
There are two basic photographic methods for making PCBs. One uses the
negative-resist boards, the other uses positive-resist boards. Both come
in a light-tight package, and both have a coating over the copper which
is sensitive to UV light. On the negative-resist coating, UV light will
cause the coating to "harden", and the plan is that you'll have "hardened"
spots wherever you want to have traces, pads, whatever. The rest of the
coating will remain "un-hardened" (probably the wrong term, but oh well)
so it can be removed. On the positve-resist boards, the opposite happens.
The board comes pretreated with a coating which is *softened* by UV light.
So you want shield it from the UV wherever traces and pads should be.
For the negative method, you have to make a negative of whatever PCB
pattern you want. Usually a design is generated (markers, pens,
computer/printer, whatever) and it is put on something clear. Then you
make a negative of it by using some photographic techniques. You have to
put the negative film below the original and then expose it for a certain
amount of time, then develop it and fix it, just like when processing
photos. Then the negative is put in a frame with the PCB and exposed for a
while. Next you have to do a developer thing on the PCB and then it can be
etched using FeCL3 or whatever etchant you like. This is the method I
tried first.. it flopped. Terribly. Probably because I have absolutely no
experience with any kind of photography..
Then there is the positive method. You make a 1:1 scale original on clear
plastic, just like for the previous method. It has to be pressed firmly
onto the positive-resist treated board (because otherwise you could get
shadowing, wavy lines, lines which don't transfer, etc) which is usually
done by pressing the board against some clear glass with the pattern
between the two. The positive-resist boards aren't as sensitive to
ordinary light as the negative-resit boards, but they should still be
handeled in very dim light. Get it all lined up, and go expose it to some
UV light source. Since sunlight has a fair amount of UV, I expose mine
outside the house under direct sun for about 10 minutes. Then you take it
inside, rinse off the resist which has been softened by the UV, and you're
left with a copper board and some resist lines in the pattern of whatever
was on your plastic sheet. Drop that in some warm FeCL3 for a few minutes,
and you've got a PCB.
In my experience, it seems that the positive-resist method is much easier
because I don't need to make a negative of my original artwork. I think
the boards are also a bit more tolerant of light before being exposed,
too, which is nice because it's easier to get the thing set up when you
can easliy see what you're doing.
On Fri, 7 Aug 1998, Richard Piotter wrote:
> So the photoresist is like a piece of film, and the transparency masks
> it, you then "fix" the chemicals with the developing solution and etch
> it. Sounds simple enough. what are the costs??? I think i saw some good
> deals in tech america and parts express. any recomendations though. I
> think I'll try it. you say 10 minutes is the aproximate exposure time???
> Is this a slow reacting film (to allow little bits of light to not
> seriously affect it, so you don't need a dark room, or is something like
> a dark room needed?)
--
Greg Hill
greg-hill@bigfoot.com
www.comports.com/link
References: