Double sided PCB on single sided exposure unit?

[Llamarama]

Hello everyone, I need to make a double sided PCB however my exposure unit is only single sided. It has a piece of grey foam that holds the pcb in place once the lid is closed. I'm worried that the light may bleed through and double expose the side I expose first after flipping it over.

I've had a think and have come up with these;

1 - Expose one side, then flip, leaving the artwork attatched.

2 - Expose one side then flip over after placing piece of black paper between foam and board.

3 - Only remove one side of protective film, expose that side, develop, then flip and repeat.

4 - Leave exposure unit open and place block of black foam with a book or something to keep artwork flat.

5 - Something completely different

I have via rivets for the through board connections and pretty much everything else, I'm just double and triple checking because this is my last piece of photo etch PCB and I don't really want to go and buy more till next weekend if it can be avoided.

Thanks for any help - Mike

 

[KeepItSimpleStupid]

There is a trick I use to keep the mask flat against the board. I just use the surface tension of a drop of water. So there is no elaborate glass on the exposure side.

The difficulty is the registration of the front and back images.

I had available a photolighography lab that was not designed to do PCB's. The boards were small so they could be spin coated and baked to put the resist down.

The drilling of the registration holes prior to coating causes issues with the uniformity around the registration holes while spin coating.

A cut thumb tack worked for registration. I was operating in a "yellow room", a "darkroom" for UV. A drill wasn't available to me in the "darkroom".

A mixture of vinegar and salt also works to clean the oxidized copper.

 

[Pommie]

Unless you need these extremely quickly it really isn't worth making them yourself any more. At Dirty PCBs you get 10 double sided boards up to 100x100 mm for $25. No setup fees, upload your files on line and the boards arrive 1 week later.

Mike.

 

[Llamarama]

Thanks for the replies, the main problem is teh size of my boards, the smallest one I've made is 180mm by 110mm, and the largest one I have to make is 350mm by 140mm. I'd much prefer them to be made by someone else, but I can't afford the $120 that's the cheapest quote for the board. Even then I'd have to "Digitise" the artwork as its from the 80s, but that's simply a rainy day job.

I have however found a trick for aligning the artwork, drill the holes as usual, use wire wrap wire to align the mask then tape in place, expose and develop and etch as usual.

Also, it turns out the Foam in my exposure unit is opaque to UV, so problem solved for the time being

If anyone knows of a super cheap PCB manufacturer that will make just 1 or 2 oversized boards for less that $50, i'm all ears