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This is my first time making a PCB, and I ran into a wall. I have a design printed on two transparencies (with a laser printer), taped together; I've also done UV exposures in a commercial box-thing for (ridiculously) long times (i.e. 6-13 min), as well as the shorter recommended times. Whenever I put the board into a 1% NaOH solution after exposure, nothing happens (agitating and brushing don't help), even after several minutes. I thought there might be a problem with the UV exposure, so I drew something directly onto a board with a dry-erase marker and repeated the process; it worked. The boards and equipment are new, and I make a fresh solution each time. I'm really sorry if this is an obvious problem, but I haven't been able to work it out! Any suggestions are appreciated!

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  • \$\begingroup\$ What sort of photoresist do you have on your boards? Is your laser printer set to print as dark & solid as it possibly can? \$\endgroup\$ – brhans Jun 8 '15 at 13:25
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    \$\begingroup\$ Does the transparent film block UV? Try and cover part of the PCB with it, expose and develop and see if the not-covered part does get developed correctly... \$\endgroup\$ – og1L Jun 8 '15 at 13:27
  • \$\begingroup\$ Occam says the UV is getting through your photo resist in the exposure times used. (Or you had the board upside down in the other cases. You do not say how long you exposed the marker version for. SO - Get some sure to be opaque material and cover several places in a strip of darkened resist. Expose for a small time - no more than correct time. Develop. If opaque material blocks UV but not where there is only "resist" you need to recalculate. | How do you know strength of light used compared to that in instructions \$\endgroup\$ – Russell McMahon Jun 8 '15 at 13:43
  • \$\begingroup\$ I've always had excellent results with an HP inkjet printer and Mega Electronics JetStar Premium film. \$\endgroup\$ – Leon Heller Jun 8 '15 at 14:01
  • \$\begingroup\$ So what was the solution, Maggy? I was going to ask what exact kind of UV illumination was being used. \$\endgroup\$ – rdtsc Jul 21 '15 at 21:32
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This works

Carry out the basic check in my comments - added at end as a note.

Once you know the board isn't upside down, that the developer is OK and that the resist "sort of works" but want to zero in on best exposure time, do as follows.
Note that this test is for exposure times of 1 2 4 8 16 & 32 "time units". If each count is a second the totals are in seconds.
Scale as desired.
Adding extra steps is easy - follow the pattern.

See image at end.

The digram MAY be slightly hard on the brain until you get the point.
There is a single test strip of photo sensitive board covered by an UNMOVING strip of exposed / dark resist material.
The whole strip is the size and shape of the green rectangle at bottom (scale dimensions to suit - this large is far too wasteful).

Start at top.
Row A is one exposure scenario.
Place an opaque (here = black) cover over the whole strip except trhe left hand end.
Expose for 16 time units (I'll say seconds from now on).
The left hand square has had 16 seconds exposure. All others = 0.

Pull the opaque strip one square right = scenario B
Expose for 8 seconds.
The 1st squarte has now had 16+8 = 24 seconds. The second has had 8 seconds.

Pull the opaque strip one square right = scenario C
Expose for 4 seconds.
The 1st square has now had 16+8+4 = 28 seconds. The second has had 8+4 = 12 seconds. The 3rd has had 4 seconds.

Lather, wash, rinse, repeat ... 30 14 6 2

Again 31 15 7 3 1
This is starting to look familiar.

Finally
32 16 8 4 2 1 seconds.
Binary progression.

Develop.

Report back.

Image:

  • Green rectangle is actual strip used.

  • A B C D E F are subsequent exposure scenarios.

  • Black opaque layer is pulled right by one square per exposure.

  • Red numbers are cumulative exposure that each square has received.

enter image description here


FWIW: This is how one used to check basic photo exposure needs in the darkroom long long long ago before these newfangled digital cameras came along. I'm not complaining, mind


Notes: (From my prior comments)

Occam says the UV is getting through your photo resist in the exposure times used. (Or you had the board upside down in the other cases.

You do not say how long you exposed the marker version for.
SO - Get some sure to be opaque material and
cover several places in a strip of darkened resist.
Expose for a small time - no more than correct time.
Develop.
If opaque material blocks UV but not where there is only "resist" you need to recalculate.
Consider: How do you know strength of light used compared to that in instructions – it sounds like it's stronger than you are expecting.

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