Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, Dave McD <davemac2@...> wrote:
>
> Chris and others,
>    
>   Won't increasing the emission angle of the LEDs increase the
undercutting effect and ultimately the resolution achievable when
exposing a board using these LEDs?  Since we are talking homebrew
boards, how much do we really need to worry about light collumation vs
the granularity (resolution) of the artwork?  Is there some sort of
rough formula?
>    
>   dave mc

There is a risk of more undercut. But I guess it depends on what kind
of mask you use and the detail required. I use a UV face tanner and
chalk paper printed in my laser printer for the mask. For a board with
small details (SMD stuff) I should not put the paper upside down
during exposure or I have undercut problems. Also I need to make sure
the PCB is pressed really flat to the chalk paper. But doing it right
I have no problems at all.
Some people have a problem with proper blackening of the masks and
claim that using two chalk paper or film layers on top of each other
is the solution. With that method I would imagine that a more
perpendicular lightsource is key. So grinding the LEDs to spread the
light, or putting a piece of chalk paper on top to get the same result
would be asking for trouble.

If you can afford more exposure time, but cannot not afford to invest
in more LEDs, I would suggest to place the LED's further away from the
PCB, rather than diffusing the light source.

Cheers,

Joop