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Photoresist film pcb

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mel
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Username: mel

Post Number: 151
Registered: 05-2005

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Posted on Monday, 12 September, 2016 - 02:54 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

I have just spent a frustrating weekend trying to find out how to use plastic photoresist film to make a printed circuit board, I practiced until I was able to apply the film to the blank printed circuit board and get rid of the bubbles, so far so good! I then went on to expose the artwork of the circuit being constructed, using the ultraviolet light box that I have.
I tried various exposure times, from 10 seconds up to three minutes, there was a nice clear impression on the photoresist, but the problem I had was when it came to developing the board, I used sodium carbonate as the developer, the problem I kept having was that this developer wouldn't get rid of the areas that were protected from the ultraviolet light.
What would happen was that I would put the board in the developer and start to rub it with my fingers, and also rub it with a brush, but it just wouldn't get rid of the unexposed parts, I left it in the solution for up to 20 minutes without it affecting these areas at all, after this time then the whole photoresist itself would start peeling from the copper (just as if I had started to remove the resist with sodium hydroxide( caustic soda) after etching with sodium persulphate) This of course rendered the board useless. I tried quite a few times altering things as I went along but with the same result every time, so now I am at a bit of a loss, can anyone suggest where I may be going wrong please.
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james
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Post Number: 727
Registered: 02-2007

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Posted on Monday, 12 September, 2016 - 05:10 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Is the film definitely negative photoresist?
I think it usually is.
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james
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Post Number: 728
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Posted on Monday, 12 September, 2016 - 05:55 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Is your mask blocking the UV sufficiently (opaque enough)?
Does doubling up on your mask improve matters? i.e 2 layers of the same mask on top of one another.

Just suggestions
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james
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Post Number: 729
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Posted on Monday, 12 September, 2016 - 09:45 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Could you try a little experiment to check that your photoresist film is OK and that your developer is a strong enough solution?

Attach film to an offcut of board and see if your developer will remove the photoresist without bothering with a mask or UV exposure.

I would expect the sodium carbonate developer to remove all the negative photoresist as it hasn't been exposed to UV.

A possible way forward?

Cheers

James
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james
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Post Number: 730
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Posted on Tuesday, 13 September, 2016 - 04:43 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Looking around the web it looks like your developer concentration should be about 10g of sodium carbonate to 1 Litre of water.

After exposure let board rest for at least 5 mins before developing.

Cheers

James
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mel
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Username: mel

Post Number: 152
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Posted on Friday, 16 September, 2016 - 04:10 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi
I tried what you suggested James

Re:

(Attach film to an off cut of board and see if your developer will remove the photoresist without bothering with a mask or UV exposure),

& can confirm that the negative photoresist was indeed removed in about 5 mins, indicating that part was ok; I tried various other things as well.
At the moment I am leaning towards the theory that the UV light is somehow reaching the parts that are supposed to be protected from the UV light, so sort of semi hardening the resist enough to stop the developer getting rid of it because it hardened.
If I leave the PCB in the developer, for about 10 mins after it has been exposed rubbing it occasionally then the whole lot starts to peel off of the copper. I am also wondering if perhaps I am over exposing to UV light, I am currently exposing for 5 mins, but have found conflicting information regarding how long it should be exposed for, perhaps someone who uses the film to make pcbs could tell me how long they expose the artwork for please.
I use a home-made exposure box consisting of 2 x 8W UV fluorescent tubes about 280mm long & about 80mm under the glass.
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james
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Post Number: 731
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Posted on Friday, 16 September, 2016 - 06:32 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Hi Mel,

Could you perhaps try exposing a piece of resist film covered board with several large coins on it, the coins being a good substitute for an extremely opaque mask.
If that board develops OK then it would lead me to believe that your usual mask is not opaque enough and needs doubling up or tripling up.

Just suggestions to try as usual.

cheers

James
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james
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Post Number: 732
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Posted on Saturday, 17 September, 2016 - 07:33 pm:   Edit Post Delete Post Print Post    Move Post (Moderator/Admin Only)

Another possible way forward...

Using a 6 inch x 2 inch piece of board with film applied.
Produce a 6 inch x 2 inch "artwork" which is totally covered black all over.

Place the black "artwork" between your board and UV bulbs.
Take a piece of cardboard and place it between the "artwork" and UV bulbs leaving an inch at one end uncovered.
Expose for 30secs.

Move the cardboard up an inch and expose for a further 30 secs.
Repeat this exposure procedure until all 6 inches of board have been exposed through the black artwork.
This results in 6 strips on the board which have been exposed through the totally black artwork for time periods of between 30secs and 3 mins.

Exposure time

Now develop the board.

I would expect the strips towards one end of the board to develop and the strips towards the other end of the board to not develop due to the leaking of light through the black mask for upto 3 minutes.

From this test you can determine how long you can expose the board for through a black artwork and still develop the board giving you an indication of the required exposure time.
Hopefully this exposure time is long enough to harden the uncovered areas in a real job. If it isn't then you'll probably have to double up on your mask and repeat the experiment.

This gives you an idea of how to determine exposure time, you may have to adjust that 30 sec exposure interval in the test.
Experiment as necessary.

Cheers

James

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