Homebrew PCBs

lcdpublishing wrote:
> Hi guys,
> 
> I got a chance to do some more experimenting with the dry film photo 
> process.  I did a "step" test doing various exposure times and found 
> out that the UV lamp I bought for this purpose simply isn't powerful 
> enough.  I have a regular light bulb of the right color range and 
> tried that with much better results - exposure time through 
> transparncy of only 3 minutes.  I have much more experimenting to do 
> to get it better, but was able to make my first board all the way 
> through the etching process.

Most people use those UV "insect zapper" florescent tubes, also known as 
black lights ("BL") which emit a bluey violet colour. Don't waste your 
money on filtered black lights used for special effects. You can get 20W 
compact florescent blacklight lamps too these days.

> 
> 1) When I used to have a black and white dark room, I discovered 
> that the developer would last a long time and could be reused over 
> and over again.  

There is no shelf life for sodium carbonate solutions, other than water 
evaporation. The Na2CO3 is obviously consumed during development, but 
its actually very small. The data sheet for my resists specifies a 
developer loading capacity of about 9.4E-6 m^3 of resist per litre of 
developer. For your average 40um dryfilm resist and double sided PCB 
with 50% trace coverage then that works out to be 0.23 m^2 of PCB area 
(485 x 485mm square) per litre of developer.


> 2) While doing the exposure / developer tests, I realized very 
> quickly just how tough this resist is.  I was scrubbing with a 
> scotch brite pad vigorously without removing the resist. 

That's normal. You strip it off in 2~5% NaOH, in 5 minutes at room temp. 
  Don't waste your energy scrubbing it off, just let the NaOH do its 
work and it will lift off before your eyes.