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Old 27th Aug 2019, 7:26 pm   #12
David G4EBT
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Join Date: Feb 2003
Location: Cottingham, East Yorkshire, UK.
Posts: 5,737
Default Re: Micro-Porous film for PCB masks - Homecrafts closing down!

Quote:
Originally Posted by Oldcodger View Post
RE #7. David - I've tried one of these sheets ( Hobbycraft) as an experiment in my Epson 2010 ( which runs on non OEM ink). I suspect my first attempt led to an underexposed print, but I had to develop for a long time to remove the resist to find a decent sharp track layout.
When you say 'underexposed print' do you mean that the acetate mask turned out OK, but that when you placed the acetate mask on the UV sensitised laminate you didn't allow sufficient exposure time, so the developer wouldn't then wash away the (under)exposed areas of the PCB?

For some time now I've been using negative UV 'dry film' - not sure if that's what you're using. It requires only the clear track and pad layout to be exposed to UV. Any opaque area of the design which is not exposed is washed away by the developer. I've found that if the mask isn't completely opaque, if the exposure time is too long, UV gets through what should be opaque areas of the mask and exposes those areas, which won't then wash off with the developer.

A solid opaque mask is the key to it all. Like most I guess, on occasions I've applied two acetates on top of each other to darken the image, but that can backfire, as not only do the black areas become more opaque, so do what are supposed to be the transparent (track and pad) areas.

Generally, I've found that if I can print a good mask, when using negative dry film in a 2 x 8 Watt light box, about 45 seconds is the optimum exposure time - far less than with pre-sensitised positive resist laminate. If I get the exposure time wrong and the board doesn't develop as desired, I just clean off the UV film with acetone and apply another piece of dry film. It can be bothersome, but it's no big deal. and the UK dry film only costs pennies.

The fist pic below shows a negative mask of a PCB.
Second pic is the PCB after it's been developed, (with soda ash) but prior to etching.
Third pic is etched, with the dry film UV resist still in place.
Fourth pic is the PCB with the resist removed with acetone and the holes drilled.

I still enjoy making my own PCBs but it's easy to see why most people think that making their own PCBs are a bit of a faff and not worth the effort. I take the same view about growing one's own vegetables, but I have a friend who spends every waking hour at his allotment, then gives away most of what he (over)produces. Another chum's definition of a wonderful day is to sit under a large brolly in the rain for hour after hour on a riverbank fishing, to maybe catch a couple of fish then throw them back so he and other can catch them again. (Apparently fishing is second only to gardening as the most popular pastimes).

Funny old world eh - maybe I should see a doctor!?
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