Homebrew PCBs

--- In Homebrew_PCBs@yahoogroups.com, Adam Seychell <a_seychell@...>
wrote:
>
> I've been experimenting with some UV LEDs for building an exposure
unit. 
> I'm using the negative dry film photoresist and discovered to my 
> surprise that there is a limitation on minimum light intensity. If the 
> intensity its too low then no amount of time will cause photo 
> polymerisation of the resist.
> For example, one LED I tested at 15mA, 120mm distance did not cause any 
> polymerisation in 40 minutes. The same LED at 30mm distance, 20mA, took 
> only 30 second for polymerisation.
> 
> I found if the light cannot expose the resist within about 2 or 3 
> minutes then the photoresist do not seem to react at all. At boarder 
> line UV intensity, it partly polymerises leaving behind a soft matted 
> looking resist after development.
> 
> This imposes a design constraint on anyone who wants to make a LED 
> exposure unit must because it there will be a minimum density of LEDs 
> needed in the array. So far I've only experimented with single LED 
> exposures. I'll make a 9 x 9 array for testing.
>

Definitely very peculiar. I would expect the exposure time to go up
with the square of the distance. So a four times increase in distance
would result in a 16 times exposure time. In your case it should have
taken 8 minutes. 

Do you have any kind of material that fluouresces when you shine the
LEDs to see if there is uniform emission. I wonder if the UV could be
getting emitted in a ring pattern. If that were the case, when you get
too far away, the UV would not be hitting the resist.


I bought the Best HongKong LEDs as well. I received a notice from the
post office that I have to pick them up. When I get them I will try
the same kind of experiment to see what happens.