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Old 11th Jan 2017, 1:49 am   #2
joebog1
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Join Date: Jun 2015
Location: Mareeba, North Queensland, Australia
Posts: 2,704
Default Re: UV Photo-Resist Dry Film PCB Process

For what its worth!! I gave up on dry film 35 years ago. Its really designed for "factories" .
By that I mean a dedicated area with "artists" to do the work. I use aerosol spray positive photo resist and have good success. The sun is my light box !! Its a point source with the source 93 million miles away, so VERY accurate. Without " bleedthrough" around the edges of the tracks. I print my positives on matt clear film on a normal B/W laser printer.
I design the PCB,s using an ancient DOS program using Linux and DosBoxEmulator.
The program is called Protel Easytrax. It was freeware many years ago so I "guess" its free now. I still have an origional 3 1/2 inch floppy of it. ( hint)

To prevent the "dry spots" as per your photos wash the PCB stock with a powder cleanser normally used for bathroom use. rinse it with clean water, and dry it well, using a CLEAN soft cloth. If there are kinks in the film, the glue thats used will not adhere. Apply the film immediately the board stock is dry. BE VERY careful of dust!! the smallest dust particle will give a hairline fracture of track/tracks that are the very devil to find later.

I built a very expensive lightbox using Philips UV tubes specifically designed for PCB work, and gave up after many many attempts. The edges of the tracks are furry !! I was never able to get really tight edges to the tracks using a lightbox
The sun has HUGE amounts of UV ( especially in north queensland) and as I said is a point source.
When you print the negative/positive reverse the image as necessary, so that the "ink/toner ALWAYS is in contact with the board.
Gently heating the "developer" to about 70 degrees F will help in getting nice resist edges.
Agitate the PCB in the developer ( just like developing a photograph in days of old) to ensure "clean" developer is always washing across the board.

I still use ferric chloride for etching, and find that warming it up also helps in achieving nice clean edges.

Without boring the mods too much, you can PM me if you wish and I will try and write a how to do for you.
I found the biggest benefit was experience, BUT gave up on dry film as it is SSSOOOOO variable. Changes in temperature, humidity, cleanliness of copper film, all made huge differences.
Spray laquer works everytime.
As does the sun as my UV source.
If you are trying to make PCB,s commercially, I would suggest winning the pools first.

Hope I have been helpful

regards

Joe
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