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I have a positiveresist PCB that I exposed the board to UV to transfer a layout for 5 min. I use UV box that I bought from Mega UK.

After that I put the board onto positive developer. As I did not have a measuring cup, I used a small cup to obtain NaOH to water ratio 1:10 (the datasheet says 20 gr onto 2 liter). Then I put the PCB onto the solution (temp. around 30 C as datasheet says) and waited some time, then it solved all photoresist film including the layout, then I thought maybe the solution was too strong and added more water (the ratio should be 1:20) then put a new board onto the solution, I saw the layout of the board in 15-20 sec then I removed the board and put it under a cold tap water, then the all layout washed away and I had a blank board.

Can you tell what I am doing wrong and how can I understand when to remove the board from the solvent?

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    \$\begingroup\$ Check that datasheet again. 20gr into 2 litre would be 1:100, so 1:10 would be just a bit too strong... \$\endgroup\$ – Brian Drummond Oct 1 '13 at 20:09
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What you say sounds much like you overexposed the photoresist - that is: You had the UV light on for too long. 5 min sounds like a lot (but it's been a while since I did my own PCBs). If I remember right, I used something like 0.5 to 2 minutes, but this of course depends on the type of lamp you use.

The right exposure time is way more critical than the right time in the developer.

Also, the concentration of the NaOH does matter: How small was your small cup?

A bit less likeley, but also a possible cause: The contrast of your film was too weak - that is: The black stuff wasn't black enough.

As a general guideline: I had to do a lot of trials (and errors!) until I knew all the specifics, and they are plentiful: Exposure time (much depending on the "strength" of the UV light), concentration of the developer, temperature of the developer, developing time, ...

Here's a good related answer (although it is about underexposure): https://electronics.stackexchange.com/a/63989/930

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