Two sided PCB UV process - diyAudio
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Old 11th September 2013, 10:46 PM   #1
akis is offline akis  United Kingdom
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Default Two sided PCB UV process

Another question : Can I attach the transparencies on both sides of the PCB and then expose each side in the UV machine? Or is there a need to dip the board into the developer chemical as soon as it comes out of the UV machine?
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Old 12th September 2013, 04:28 PM   #2
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Expose one side first, then the other, and develop both of them together. Just make sure that both transparencies are aligned. Here's what I do: I attach the transparencies together (that's easy to do when there is no board between them), putting some sticky tape at the three of the four sides. Then slide the board between them and secure it with some tape at the fourth side. Note that the three first sides of the transparencies must be longer by at least one centimeter than the board. Careful not to scratch the printed (inner) sides of the transparencies as you slide the board in. Also, when you put the board in the developing container, make sure that the photosensitive material of the "down" side of the board is not scratched by the bottom of the container.
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Old 27th September 2013, 08:39 PM   #3
akis is offline akis  United Kingdom
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Many thanks! The UV light box is coming over from China or from Hong Kong. I have not tried using my laser printer yet on the special transparencies I have bought - I presume one printing pass will be enough?
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Old 27th September 2013, 11:20 PM   #4
DUG is offline DUG  Canada
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AnthonyK2

How do you through plate holes, or do you?
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Old 29th September 2013, 11:07 AM   #5
marce is offline marce  United Kingdom
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Tooling holes, punched in the artwork and drilled through the PCB material before imaging.
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Old 29th September 2013, 11:23 AM   #6
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Above advice is good.

Re management of vias: I tend to limit the number of vias, then either or use resistor pads as vias (soldering the resistor / transistor / whatever top and bottom) or solder in tinned copper wire through the vias if I can't arrange for a convenient component lead to do the job.

Works pretty well as long as you are disciplined in the number and placement of vias.

I have not heard of anybody doing "home plated through holes" but I would be fascinated to hear if it can be done!
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Old 29th September 2013, 12:36 PM   #7
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I do both sides separately.

Leave the mask on the one side until the first side is etched.

Drill the four corner holes and use them to key both sides together.

Mask the finished side with masking tape and carry on with the second side.

If you leave a few mm around the edge of the board you wont get any bleeding along the edges of the masking.

Getting the exposure right can be a bit frustrating. My laser printer doesn't produce a completely opaque image so I often print two copies and layer them.
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Old 29th September 2013, 12:56 PM   #8
akis is offline akis  United Kingdom
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I am slightly lost, why do we need to do the sides separately? I have not done it yet, but my idea would be :
1) print two transparencies
2) somehow affix them on the two sided PCB and make sure they align
3) expose in UV light
4) develop in photo resist liquid thingy
5) wash with water
6) dip in acid to remove copper
7) wash in water
8) dip in tin/silver solution if wanted
9) wash
10) drill
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Old 29th September 2013, 12:58 PM   #9
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Just cos its easier. It's awkard to get both sides perfectly aligned otherwise.

There are of course many ways to align both sides - you will find out which method suits you in time.

The above process looks very easy but believe me when I say it doesn't always go according to plan.

Last edited by KatieandDad; 29th September 2013 at 01:06 PM.
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Old 29th September 2013, 01:10 PM   #10
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If I use the "press and peel" process I do each side seperately as Katieanddad does. While I have not used photoresist for a while (mainly because I seem to have got on top of the press and peel process) when doing boards photographically I do both at the same time.

Tricks to watch for are:
- Protecting the PCB surface while etching (the bottom side can be scratched) I drill and mount nylon bolts in the PCB to ensure it does not actually touch the bottom of my etching dish.
- Getting the plots properly aligned is not trivial. Not the least because laser printers are NOT lab instruments, and you find distortion across large images. This seems to be fairly constant from print to print, but you need to watch out when doing double sided PCB's because when you flip the plots for the "other side" I usually find that there is some mis-registration across larger boards.
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