How does photoresist attach to a PCB during manufacture?

[Siwastaja]

Post-exposure (of negative resists, not positive!) after development is a nice trick in your sleeve if you have to work with poor printer density. As you have to limit the original exposure time not to burn through the blacks (making unwanted areas "harden"), the exposed parts might be marginally exposed, so that they survive from development step but are not quite hard enough for etching. Post-exposure fixes that.

Post-exposure (or even heat treatment) is also needed with soldermask materials (liquid or dry film), in my experience. Hardening through the original artwork is not sufficient.

I had decent results with dry film soldermask, too, but rarely bothered with it. While OK for the job, it never was as good as in professionally made PCBs.

[shapirus]

Post-exposure (of negative resists, not positive!) after development is a nice trick in your sleeve if you have to work with poor printer density.

Do I understand correctly that it means soaking the board for some time in the development solution without brushing the developed photoresist off, and then exposing it to UV for an additional time?

[wraper]

Post-exposure (of negative resists, not positive!) after development is a nice trick in your sleeve if you have to work with poor printer density.

Do I understand correctly that it means soaking the board for some time in the development solution without brushing the developed photoresist off, and then exposing it to UV for an additional time?

First you get fully developed board with all unexposed negative photoresist removed. Then expose it to UV again to make photoresist more resistant to copper etching solution.

[shapirus]

First you get fully developed board with all unexposed negative photoresist removed. Then expose it to UV again to make photoresist more resistant to copper etching solution.

It doesn't align with the printer ink/toner density consideration that I was replying to: with negative photoresist, the areas covered with ink/toner are the ones that are developed and removed, and that is where the print density matters.

Hope I'm not confusing negative and positive PR here .

p.s. after re-reading the post that I was replying to another couple of times I get the idea, finally, now it makes sense to me. At least I think so.

[wraper]

First you get fully developed board with all unexposed negative photoresist removed. Then expose it to UV again to make photoresist more resistant to copper etching solution.

It doesn't align with the printer ink/toner density consideration that I was replying to: with negative photoresist, the areas covered with ink/toner are the ones that are developed and removed, and that is where the print density matters.

Hope I'm not confusing negative and positive PR here .

p.s. after re-reading the post that I was replying to another couple of times I get the idea, finally, now it makes sense to me. At least I think so.

It aligns perfectly. If you have poor photomask opacity, you must underexpose PCB because otherwise developer won't be able to remove "non-exposed" parts as they will be somewhat exposed. But it also means that exposed parts did polymerize only partially. So after developing, you expose it to UV again without any photomask to make it polymerize completely.

[Siwastaja]

With negative photoresist, ink/toner stops light -> resist remains "soft" -> is removed in developer.

Poor ink/toner -> light leaks through -> some amount of unwanted "hardening" happens -> you have harder times in developer, need to brush more or develop longer, possibly damaging the exposed parts too, etc.

If you compensate poor ink/toner by exposing for shorter time to avoid unwanted hardening, then those areas that should receive light might remain a bit too soft to survive the whole process - but just barely enough to survive the developer. After you are sure all the photoresist that needs to be removed is removed, you can post-expose the whole remaining thing (dry or wet, doesn't matter; but do wash the developer out properly) before etching.

When we transitioned to mostly using that expensive Epson inkjet which produced excellent blacks, I just increased the original exposure time by some 30-40%, no need for post-exposure tricks. The only issue was that the printer belonged to photography club which was in a different building and only I had access, so we did see those occasional laser prints from whatever printers, some of which were usable and some not. For low-resolution stuff (say down to 12/12 mils), printing the artwork twice and sandwhiching them together was a simple solution to poor density. We also had a magical bottle of expensive spray which turned so-so toner into really good pitch black, but we were never able to source the same thing for non-ridiculous price and the alternatives we tried as suggested on the interwebz did not work. I guess the product was made of unicorn pee.

[shapirus]

Yes I see. Got it now.

My printer is a photo-quality Epson L805 (known, I believe, as EcoTank in some markets), black areas seem to be quite good, so I'll give it a try. The Linux driver, however, is, as usual, crippled, and it doesn't allow to choose the desired ink density or resolution directly, instead it has the "draft/standard/high" print quality settings, and, what makes the biggest difference, a choice of paper types, which apparently sets the resolution and ink density under the hood. It looks like the "Epson Ultra Glossy" setting produces the best quality prints even on matte paper. Go figure.

Btw, as far as the print medium goes, it shouldn't necessarily be a transparent film. This isn't my experience yet, I'm just telling what I read elsewhere: you can print on regular paper (of those that work well with ink jet printing) and soak it with olive oil before applying to the future PCB. Oil doesn't affect the ink (which I've tried and can confirm) and makes the paper transparent to UV. It will also help it to adhere well to the photoresist, so covering it additionally with glass may not be necessary.

[Siwastaja]

I gave up with the linux drivers. Even if you use linux otherwise, a Windows machine is needed as a printer driver driver, go figure.

Some people report good results with certain types of paper, like tracing paper sleemanj mentioned; less dense, and more uniform density. Maybe soaking normal paper in oil would do the trick as well, I don't know. With normal paper there is too much structure in the paper; density differences (max - min) is directly subtracted from your density margin as you need to add exposure time based on maximum paper density.

Those expensive inkjet transparencies are engineered to hold good amounts of ink without pooling and I never had any issue with them. Worth noting, a "transparency" option on printer driver often just reduces amount of ink, the opposite you would like to do. You need to experimentally find what "media" setting works the best. Just the default paper setting is a good starting point.

The idea that oil helps the artwork stick to the board is excellent. The exposure system I built had a thin mylar plastic sheet permanently attached to a frame that can be lowered over top of the PCB+artwork, and a vacuum system which pulls the whole sandwich together.

I drilled the holes first (with dry film photoresist tenting over the holes - another benefit of the dry film instead of spray resist), so that alignment of the artwork was necessary. I can imagine with oil the artwork could keep in place by surface tension and would be easy to adjust. I don't know how oil would work if you have pre-drilled holes.

[jmelson]

Post-exposure (of negative resists, not positive!) after development is a nice trick in your sleeve if you have to work with poor printer density.

Do I understand correctly that it means soaking the board for some time in the development solution without brushing the developed photoresist off, and then exposing it to UV for an additional time?

I use Riston dry film from Think-n-Tinker.  I have a hot roll laminator made by Kepro that has two rubbery heated rollers that are pressed together.  The machine unrolls the resist and keeps even tension.  I expose the master artwork in a vacuum frame for one minute.  The light source is a bank of fluorescent black light bulbs.  I then let the exposed board sit for 10 minutes in the dark to allow the polymerization to complete.  Then, I develop in warm sodium carbonate solution for about 2-3 minutes with constant "agitation" by rubbing the board gently with fingertips.  Then, I wash the board in running water until it no longer feels "slimy".
Then, I inspect with a magnifier to make sure all the parts that are to etch have the resist washed off.  Etching is done in a spray etcher with 40-50 C ferric chloride for about 1-2 minutes.  You want the etching to be done as fast  as possible to prevent undercutting the resist.
Jon