Radra wrote:
> The purpose of my original post was to query the group regarding
> resolution limitations of the photolithography process. My attempts
> to fabricate a useable 1 mil (0.001 inch) track have proved
> unsuccessful leading me to ask "why"? So far I have received two
> comments (thank you): one related to collimation of the UV source and
> the other related to etching considerations.
>
> Let me provide a little more information regarding my goals and
> processes. My intent is to devise a simple "at-home" process which
> will produce PCBs with finer resolution and without the pin holes
> associated with the toner-transfer process. I am an RF engineer and
> need to produce good looking prototypes.
Me too, but i've been doing more CPU stuff than RF lately.
> I created a test pattern which I used to assist me in the development
> of the process. The pattern consists of 100, 50, 10, 5 , and 1 mil
> parallel lines and a grid made of 1 mil lines. My UV light source is
> a Blak-Ray B-100A long-wave source made by UVP. This source produces
> an intense spot beam. I position my artwork about 2 feet from the
> lamp and expose the dry-film PCB for 1 minute. Trial runs were made
> using exposure times between 10 seconds and 6 minutes; 1 minute
> proved to be satisfactory.
>
> My test runs demonstrated I could reliably produce 5 mil lines but
> not the 1 mil lines. Microscopic examination of the 1 mil line
> artwork revealed that the two parallel edges of the line tended to
> not have a clearly defined area between them. Thus when I used this
> artwork to expose the PCB, the resulting pattern on the PCB was not
> well defined prior to etching. It is then natural to wonder if my
> transparency artwork can be improved "at home" and, if so, will I be
> then able to fabricate a higher resolution product.
>
> Lastly, my goal is not to actually fabricate 1 mil lines as I don't
> need them. Instead my goal is to learn what can be reasonably
> accomplished "at home" and apply that knowledge to fabricate the best
> looking PCBs possible. So, setting aside etching considerations for
> the moment, my original question still stands, does anyone have
> suggestions regarding how I can improve the processes employed prior
> to etching?
You'll need to know the collimation characteristics of the light-source
and the thickness of your uv resist. If the rays at the pcb have angles
up to eg +/-20deg from normal and the film is 10um thick, then instead of
edges being cast as a sharp shadow, the edge-shadow will be dispersed over
+/-10um x Tan(20deg) = +/-3.6um, giving a fuzzy edge. For thicker tracks,
the effect is that jagged edges in the plot are magically smoothed, but
the width of the exposed track depends slightly on exposure and etching
time.
I use genuine epson inkjet transparency and ink in an old stylus 400 color
printer and it gives excellent pinhole-free results and is good for tracks
and spacing smaller than 8mil.