Homebrew PCBs

Bob,

Most of my boards are rather simple and single sided. I use dry film 
photo resist on FR-4 (glass epoxy) and CEM-1 (paper base)laminates. 
These boards are what is called "print and etch" in the industry, 
bare copper traces on the base laminate with no plating or solder 
coating. Sometimes I do use a silk screen to print a soldermask which 
is the green coating that covers everything except the pads that are 
soldered. 

A negative is used for exposing a print and etch board. The back 
ground of this negative is opaque black and the traces are clear to 
let the UV light through. The UV light will expose the traces through 
the clear part of the film negative but not through the opaque areas. 
The dry film areas on the board that didn't receive any light will 
wash away during development thus leaving the traces which are now 
covered with exposed resist and ready for etching. After etching a 
weak lye solution will strip off the dry film resist from the traces. 
The board can be scrubbed to clean the copper and drilled.

I make my films using a machine called a photoplotter. This machine 
takes the data from a CAD system and "draws" the image on 
photographic film for a perfect image in a positive or negative 
format. These films are known as a "phototool" in the industry. 

I have never used the laser or ink jet methods but know others that 
have had excellent results with these methods. I believe they use 
some type of translucent paper for printing which will allow UV light 
to pass through. 

Before photoplotters were popular we used large process cameras 
similar to what a printing company uses. The artwork in those days 
was made using narrow black tape and stick on pads. Most artwork was 
made at 2:1 scale and some designs were 4:1 scale. These master 
layouts were then reduced on the camera and a negative was shot. It 
was a major job setting up multiple images on a larger panel making 
sure that all layers were perfectly registered. Using a photoplotter 
and a CAM system made this job much easier.  In some cases we 
could "tool up" a job in 30 minutes that would otherwise take all day 
with hand taped artwork. 

I still have a process camera but prefer the photoplotter because the 
film is always perfect. If you look at a photoplotted film with a 
microscope you will notice that it has excellent film density. There 
are no gray areas. The film density goes from opaque black to clear 
and back to black. It's difficult to get even the best cameras to 
make films like this. 

While the plotter is making the films, the drilling machine is 
drilling the holes at the same time. Everything registers perfectly. 
Because of all this automation the bare board prices have been 
greatly reduced. Hope this answers some of your questions.

Tom





--- In Homebrew_PCBs@yahoogroups.com, "Bob Fitzgerald <newaag@y...>" 
<newaag@y...> wrote:
> Tom - 
> Thanks. As you can see, I'm not to savvy about how the pros do it.  
I 
> would imagine this process needs a very good negative. Do you use 
a  
> laser or inkjet printer? I really marvel at the quality boards one 
> can buy for relatively cheap these days, like computer motherboard 
> fully stuffed sans processor for 60-90. 
> Bob  
>  
> --- In Homebrew_PCBs@yahoogroups.com, "twb8899 <twb8899@y...>" 
> <twb8899@y...> wrote:
> > Bob,
> > 
> > Dry film photo resist is laminated to the panel using a hot roll 
> > laminator. The dry film resist that I use is .0015" thick and is 
> > developed in a 1% sodium carbonate (soda ash) solution. These 
films 
> > are negative acting and exposed with a UV light source. My light 
> > source uses a mercury vapor lamp which exposes the film in about 
17 
> > seconds. If you expose the film using a negative you will have 
the 
> > traces and pads left behind on the copper panel for etching. 
After 
> > etching the photoresist is stripped from the board using a weak 
> > sodium hydroxide or lye solution. 
> > 
> > Tom
> > 
> > 
> > --- In Homebrew_PCBs@yahoogroups.com, "Bob Fitzgerald 
> <newaag@y...>" 
> > <newaag@y...> wrote:
> > > What is the technique with dryfilm resist? It sounds like it is 
> > > laminated to the boards with heat? I just bought a dry mount 
> press 
> > so 
> > > I'm interested. I assume this is better than spraying boads? Is 
> > this 
> > > one of those products where it is not sold in small qty?
> > > thks
> > > Bob
> > > 
> > > --- In Homebrew_PCBs@yahoogroups.com, "twb8899 <twb8899@y...>" 
> > > <twb8899@y...> wrote:
> > > > I can help anyone interested in blank boards with dryfilm 
> > > > photoresist. I have a laminator and plenty of dryfilm resist 
on 
> > > hand. 
> > > > 
> > > > Also have a photoplotter that makes positive or negative 
films 
> > from 
> > > > Gerber photoplot data. Contact me off list if there is any 
> > > interest.  
> > > > 
> > > > Tom     twb8899@y... 
> > > >