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How does photoresist attach to a PCB during manufacture?
Posted by Richardcavell on 27 Mar, 2023 16:40 -
Hi,
I’ve been watching YouTube videos of people handmaking their own PCBs. In some cases, it seems as though the copper is put under a UV light with a mask in the way (artwork printed on transparent plastic). Then the board is immersed in “photoresist” fluid.
What happens to the copper under the influence of UV light? How does the photoresist fluid know where to attach to the board?
Richard -
You got it the wrong way. Board (all of it) is coated with the photoresist before exposure. Exposure to light softens or hardens (depending on the photoresist type; positive or negative) those parts of photoresist where the light hits. The liquid the boards are dipped into after exposure as you have noticed is called "developer", it simply washes out the soft photoresist. Hardened photoresist remains and protects the copper from acid during etching.
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Just to further clarify, the photoresist is like a spray paint. I have purchased it in rattle cans, just like Krylon or Rust-Oleum spray paint. You spray it on and let it dry - in the DARK, of course. It clings to the copper just like paint would. Then the board is ready for the exposure with a positive or negative photographic image of the circuit.
One more thing. Ordinary photographic film is not dense enough. The UV light will penetrate what are black areas for normal photographic purposes. A high contrast film is needed. Or a high contrast image from a laser style printer. Ink jet printers do not make a dense enough black. -
Just to further clarify, the photoresist is like a spray paint. I have purchased it in rattle cans, just like Krylon or Rust-Oleum spray paint. You spray it on and let it dry - in the DARK, of course. It clings to the copper just like paint would. Then the board is ready for the exposure with a positive or negative photographic image of the circuit.
Additionally to sprayed or dipped photoresist formulations, a roll-type photoresist is very common. It comes in big rolls, where protective film is removed, and photoresist laminated on the PCB using pressure and heat. (I have successfully used just an office laminator. I find that working with the laminated photoresist is much easier and more repeatable than the spray types where you have to get the spraying process just right, and it affects everything else.)QuoteOne more thing. Ordinary photographic film is not dense enough.
What do you mean by ordinary photographic film? Who has access to large format photographic film these days, even? If you could, I would place a bet that a standard B/W photographic film of any type would be dense enough, no problem at all. People did use exposed film to look at the Sun directly (not that I'd recommend doing it).QuoteOr a high contrast image from a laser style printer. Ink jet printers do not make a dense enough black.
It's usually the opposite. Laser printers tend to suck colossally in getting enough density, in fact I have never seen a high-quality laser-printed film, they vary between unusable and barely OK. There are sprays that improve the density of laser prints and they tend to work wonders. On the other hand, many inkjet printers can be configured (in the driver settings) to increase the amount of ink used, which will get you much better density than most standard office laser printers. But oh boy those inkjet transparency films are expensive.
When I did this PCB stuff regularly and wanted to do it properly, the only solution that really worked well was an expensive Epson photographic inkjet printer, those expensive inkjet transparencies, laminated dry film (negative) photoresist, point-source UV light, ½oz copper clad, and vigorously agitated (large amounts of compressed air) cupric chloride etching tank to minimize edge effects. That combination was enough to get 6/6 mil and near-100% yield, and demonstrate 4/4 mil with acceptable yield.
Another issue I noticed with laser printers was unrepeatable geometrical distortions, even within page, due to the heat used. (Cheap) inkjets also produce distorted images but at least it keeps the same for every page printed so you can compensate in the artwork (I just used GIMP shear effect and resize functions for this.)
Also, you usually don't need to work in dark, as the photoresist materials are designed to be only sensitive to near-UV. Just remember that fluorescent tubes release a bit of near-UV, and photoresist materials have a little bit of sensitivity for deep blue visible light as well. Sunlight is also no-no. You can simply reduce the amount of light, preferably some incandescent or low power warm white LED lights, which is still enough for you to work conveniently and safely. -
(Large format photographic film is not *entirely* gone these days!)
Just as a detail, I believe it's the developer that does the hardening, not the exposure. Then I would expect another wash of some kind to remove the unwanted coating. This is very similar to the dye transfer photographic process, which was the most difficult and complex, but also the most beautiful.
Anyway, what I was going to say... if you're using a material that isn't dense enough to control the exposure, you may be able to double it up. Use two layers and align them carefully, or tweak the printing process somehow, something like that.
If I was still doing that rather than having boards made for me VERY inexpensively, the first thing I'd do is some testing. Use all the advice about this or that light and material and time and such as a starting point, and the invest in some tests to figure out what's really going on, and deal with the frustration of bad results once and for all. A great help for exposure testing is a "step tablet," a graduated gray scale that you can expose through. Every step of .3 in density is the equivalent of cutting the exposure time in half, so you expose *once* at your longest estimate, and you get results for every time from there on down. Develop. Pick the best step. Use that time. -
There is still B&W large-format film available. Not cheap. Example below is for 8x10 inch, 4x5 inch is roughly 1/4 the price per sheet.
One source in the US: https://www.bhphotovideo.com/c/products/ci/335/N/4093113314?filters=fct_film-type_1893%3Ab-w-negative%7Cb-w-orthochromatic%2Cfct_sheet-size_3122%3A8x10in -
You got it the wrong way. Board (all of it) is coated with the photoresist before exposure. Exposure to light softens or hardens (depending on the photoresist type; positive or negative) those parts of photoresist where the light hits. The liquid the boards are dipped into after exposure as you have noticed is called "developer", it simply washes out the soft photoresist. Hardened photoresist remains and protects the copper from acid during etching.
It does not really soften/harden for the sake of the process (but it can be a side effect to some extent) but makes it susceptible (positive) or chemically resistant (negative) to developer. Also AFAIK in production alkaline etching is more common than acidic etching due to faster etching speed. -
You got it the wrong way. Board (all of it) is coated with the photoresist before exposure. Exposure to light softens or hardens (depending on the photoresist type; positive or negative) those parts of photoresist where the light hits. The liquid the boards are dipped into after exposure as you have noticed is called "developer", it simply washes out the soft photoresist. Hardened photoresist remains and protects the copper from acid during etching.
It does not really soften/harden for the sake of the process (but it can be a side effect to some extent) but makes it susceptible (positive) or chemically resistant (negative) to developer. Also AFAIK in production alkaline etching is more common than acidic etching due to faster etching speed.
Actually, I thought "soften" and "harden" the resist relative to the developer was a good description. Perhaps rather than criticise an accurate description that doesn't meet your need for physicochemical accuracy, you will enlighten us on the chemistry of positive and negative resists with regards to the photochemical reaction(s) and how that affects their reaction to the developers used.
I am interested. -
Negative photoresist polymerizes under UV exposure and it seems that opposite thing happens in positive one.
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Negative photoresist polymerizes under UV exposure and it seems that opposite thing happens in positive one.
What is the role of the photosensitizer? As for your second statement, what do you mean by "the opposite?" Are you suggesting that the same "polymer" that is formed in the negative resist is the actual resist in positive photoresists and "depolymerizes?" Come on. We need more detail. A few chemical structures would help.
Basically, you seem to be saying that a polymer is less soluble ("harder") than the monomer or other photochemical product in a developer. Isn't that what Siwastaja said? -
susceptible (positive) or chemically resistant (negative) to developer.
Speaking of which, what's a good chemical to use at home? Much preferably something non-toxic and easily disposable of. Baking soda? I heard people used it successfully, but it's required to find the right concentration.
Anything else?
(I've not yet tried making PCBs using the photoresist method, but want to, since I have an ink jet printer but don't have a laser printer, so I can't use the toner transfer method.) -
Polymerization is not about being harder/softer or whatever but about forming long chain-like molecules. IMHO words "harder" or "softer" used by themselves are extremely unsuitable when talking about chemical reactions.Negative photoresist polymerizes under UV exposure and it seems that opposite thing happens in positive one.
What is the role of the photosensitizer? As for your second statement, what do you mean by "the opposite?" Are you suggesting that the same "polymer" that is formed in the negative resist is the actual resist in positive photoresists and "depolymerizes?" Come on. We need more detail. A few chemical structures would help.
Basically, you seem to be saying that a polymer is less soluble ("harder") than the monomer or other photochemical product in a developer. Isn't that what Siwastaja said?Quotewhat do you mean by "the opposite?"
https://en.wikipedia.org/wiki/DepolymerizationQuoteA few chemical structures would help.
I'm not going to dig in detail how particular photoresists work and what exact chemical reactions happen. I'm not a chemist by trade (but not clueless in chemistry either), and process in detail won't mean anything to most of people here. And those to whom whom it means something, can find better info than I themselves. What I can say is that polymers are generally more inert than their monomers.QuoteBasically, you seem to be saying that a polymer is less soluble ("harder") than the monomer or other photochemical product in a developer.
Developer is not a solvent. It does not simply dissolve the photoresist. It chemically reacts with it, or eats it away in simple words. In a similar way that chemicals used to etch copper are not solvents either.
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There is still B&W large-format film available. Not cheap. Example below is for 8x10 inch, 4x5 inch is roughly 1/4 the price per sheet.
One source in the US: https://www.bhphotovideo.com/c/products/ci/335/N/4093113314?filters=fct_film-type_1893%3Ab-w-negative%7Cb-w-orthochromatic%2Cfct_sheet-size_3122%3A8x10in
Can you still buy xray film? I was able to get quite a lot of it for very little money back when everywhere was converting to digital. I haven't looked recently. It is essentially just monochrome photographic film. -
Baking soda does not work as developer, you need sodium carbonate or potassium carbonate for negative photoresists. Although you can make sodium carbonate yourself by heating baking soda for a while. For positive photoresists sodium hydroxide is usually suggested but it totally sucks to use and often results in tiny holes/partial etching in the traces. You need nearly perfect photomask as even barely exposed photoresist get affected by it. Also you need precise exposure time, solution concentration, temperature, and developing time for stable results. Some people use sodium metasilicate developer which is an order of magnitude more forgiving. Overall negative photoresist is much more forgiving and sodium carbonate as developer is much easier and cheaper to get than sodium metasilicate developer to make positive photoresist not suck to use. EDIT: I actually just found sodium metasilicate for a very cheap price https://www.circuitspecialists.com/products/posdev You need to look into MSDS to see what it is.susceptible (positive) or chemically resistant (negative) to developer.
Speaking of which, what's a good chemical to use at home? Much preferably something non-toxic and easily disposable of. Baking soda? I heard people used it successfully, but it's required to find the right concentration.
Anything else?
(I've not yet tried making PCBs using the photoresist method, but want to, since I have an ink jet printer but don't have a laser printer, so I can't use the toner transfer method.) -
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Additionally to sprayed or dipped photoresist formulations, a roll-type photoresist is very common. It comes in big rolls, where protective film is removed, and photoresist laminated on the PCB using pressure and heat. (I have successfully used just an office laminator. I find that working with the laminated photoresist is much easier and more repeatable than the spray types where you have to get the spraying process just right, and it affects everything else.)
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Nice to know. -
Negative photoresist polymerizes under UV exposure and it seems that opposite thing happens in positive one.
This polymerization is exactly what I called "hardening". I think it's both intuitive and technically correct, because... it is harder/softer (as a generic word, not a specialty chemistry term). (And a good enough description for a discussion like this.)
For example with the usual negative dry film solder mask (the laminated roll stuff), you can touch the coating with your fingernail and feel how it is soft when unexposed, and becomes harder (and changes its color) after being exposed. The unexposed part easily "dissolves" in the sodium carbonate developer; but exposed, polymerized parts require a strong NaOH solution to get rid of; the stuff is cross-linked enough so it's released in large "flakes". And as you say, this is not actually matter of solubility (only), but chemical or even just "mechanical" reaction. Note that some photoresists, those used in classical silk screen printing (think about T shirts etc.), are actually developed using water. The unhardened resist is washed off. This is hard to classify, it's partially solubility but clearly mechanical as well.
But the question about "hardening" is the same thing as those dentist tooth fixing stuffs; they are soft so can be put into place, in correct shapes, and then hardened using UV light. If you don't have an issue calling it "hardening" in dental context, I don't see a problem in photoresist context.
Now I can imagine it's much more tricky to engineer a positive formula. We did use the POSITIV 20 spray photoresist and it was developed with a weak NaOH solution, never tried the metasilicate thing. The concentration and temperature of the NaOH developer, as well as developing time, were critical; coating thickness and evenness were also factors. The dry film negative stuff is very forgiving in comparison!
Now note that the word "developer" is also used in photographic contexts and then it's chemically a totally different thing; not polymerization, but amplification of silver halide (e.g. AgBr) conversion into metallic silver. -
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What do you mean by ordinary photographic film? Who has access to large format photographic film these days, even? If you could, I would place a bet that a standard B/W photographic film of any type would be dense enough, no problem at all. People did use exposed film to look at the Sun directly (not that I'd recommend doing it).
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In spite of the common use of digital cameras, I have at least three, photographic film IS still available. Just not at the corner drug or grocery store or even WalMart.
And have you ever tried to use ordinary film for a PCB negative or positive? I have. It didn't work. Yes, people have looked at the Sun with it after maximum exposure and developing. That is NOT a good idea because too much of the Sun light comes THROUGH. Don't do it. But the very fact that you can see the Sun through it tells you that it is not completely opaque. And, for PCB exposure you do need something that is almost completely opaque. As I said, I tried it and it just didn't work.
I also tried ink jet transparencies. They are even worse than ordinary photographic film. They let even more through. Perhaps there are some makes/models that I have not tried, but from what I saw, you better get a money back guarantee before buying an ink jet for this use.
I have been making my own PC boards for many decades. Back in the day, I took the artwork to a local company that had a copy camera and asked for a high contrast negative or positive. They had special films that produced only black and white, no gray tones. That worked well. And the copy cameras were capable of exact reduction ratios so 2X or 4X artwork could be reduced to 1X with no problems. But I always included a scale to double check.
In the past I have been successful with some office copy machines. Others did not work well. And there have been numerous descriptions of the use of laser printers on the web. One popular method involves printing on a transfer paper and using heat to transfer the pattern directly to the PCB. I have not tried this. I have not tried home style, laser printers at all since the prices of PC boards has dropped so low as to make it not worth the effort.
Of course, there are multiple brands and types of photo resists. And things have probably changed over the years. I am not an expert, just a tinkerer with some experience. However, I do speak from actual experience, not just guesses. And if I don't know, then I say so. -
Speaking of which, what's a good chemical to use at home? Much preferably something non-toxic and easily disposable of. Baking soda? I heard people used it successfully, but it's required to find the right concentration.
Anything else?
(I've not yet tried making PCBs using the photoresist method, but want to, since I have an ink jet printer but don't have a laser printer, so I can't use the toner transfer method.)
My bet: source some dry film photoresist (I was able to get DuPont Riston Multimaster on Ebay some 10 years ago), get a cheap office laminator, and then all you need for developer is sodium carbonate. Basically this stuff is in most detergents/dishwasher powders. You really should be able to find it somewhere separately (also called washing soda), but if everything else fails, you can indeed convert from sodium bicarbonate (baking soda) by heating.
Dry film photoresist is forgiving and easy to develop; you can even gently brush it with a toothbrush (use hot air to soften the plastic and make a 90 degree bend on a toothbrush to make it a fancy specialized instrument) during development to loosen the unexposed ("soft") parts of the photoresist.
I think etching (and sourcing the etchant) is more difficult. -
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Also, you usually don't need to work in dark, as the photoresist materials are designed to be only sensitive to near-UV. Just remember that fluorescent tubes release a bit of near-UV, and photoresist materials have a little bit of sensitivity for deep blue visible light as well. Sunlight is also no-no. You can simply reduce the amount of light, preferably some incandescent or low power warm white LED lights, which is still enough for you to work conveniently and safely.
In my younger days I did photographic darkroom work. Yes, I know the photo resist is relatively insensitive, but pre coated boards do come wrapped in heavy, black plastic; at least the ones I bought did. So there is some danger of fogging, particularly if exposed for long periods. You can't just coat a PCB and then leave it laying around for a week. Why take chances. Darkrooms do have "safe" lights and I had some of them from that earlier photographic work. I purchased new filters for them and could safely work in the resulting, dim light.
I was never trying to provide complete, step-by-step instructions on the process. I was only clarifying your previous answer a bit. Trying to make it more understandable to a person who obviously knows very little of the process. My key point was that the photo resist was already adhered to the PCB before it was exposed. -
I also tried ink jet transparencies. They are even worse than ordinary photographic film. They let even more through. Perhaps there are some makes/models that I have not tried, but from what I saw, you better get a money back guarantee before buying an ink jet for this use.
The issue is that there isn’t one kind of inkjet printer. There are, by now, probably hundreds of ink formulations, but even with the same ink, different printer models lay it down differently. (Which is why there are examples of the same cartridge having different yields in different printer models!)
For this application, you definitely want pigment ink, not dye, and you need a printer that, with the right settings, is willing to lay down a lot of ink. But it can’t be too much, or it can pool up before the transparency’s coating can absorb it, and pooling causes all kinds of density unevenness as surface tension pulls it around as the coating absorbs it. You’d probably want to use the slowest, highest resolution mode, which lays down the ink in multiple passes using smaller droplets, which allows ink to absorb a bit before the next pass, reducing pooling. -
In spite of the common use of digital cameras, I have at least three, photographic film IS still available.
With film available, the question is: how do you then expose your artwork to the film? It's a chicken-egg problem.
In PCB industry, photographic film was used, because they used laser photoplotters that exposed the digital artwork on the film. Those photoplotters did not have enough power (or suitable wavelength) to do Direct Laser Exposure, that's a later thing; so film was used as an intermediate. But few of us mere mortals have those photoplotters at our homes.QuoteAnd have you ever tried to use ordinary film for a PCB negative or positive? I have. It didn't work.
Not only I have, without any problems, I have also measured the densities. I don't know how you exposed your film, and why did you use the film as a medium at all, but probably you
1) exposed it for too little time,
2) developed for too little time or using spent or aged developer,
3) used aged film which had fogging
4) had stray light etc. that reduced your contrast by causing "fogging", exposure of supposedly non-exposed areas
For example take a look at Kodak T-MAX 100 datasheet: https://imaging.kodakalaris.com/sites/default/files/files/resources/f4016_TMax_100.pdf , one can get density difference log D = 2.5, which is far better than any laser printer printout I ever measured (which usually varied between maybe 1.5 to 2.0). With a high-contrast print film you'd get D = 3.0, and you would get that more easily from narrower exposure (log H) range, that's for sure, but that does not mean one could not use a standard negative film if that's what you have, and for some strange reason would like to use photographic film as an intermediate step.
But quite frankly, I can't figure out why anyone would do that in year 2023. Time is better spent looking for a printer that does good job directly on transparencies (laser or inkjet), or if you want to DIY, do a Direct Laser Exposure thing, using a powerful UV laser to raster scan the photoresist-coated PCB. These have been a big thing in the industry during last decade or so.QuoteAnd, for PCB exposure you do need something that is almost completely opaque.
"completely opaque" is meaningless, please put it in numbers. In my experience, something like logD = 2.0 is well enough. The more you have, the more forgiving your process will be against variations (specifically exposure intensity/time). Around D=1.5..2.0 is what many laser printers and most inkjet printers produce with their default settings. My experience just is that in laser printer settings, the toner density adjustment does nothing, while inkjet printers respond better to the adjustments. It's just you need to use Windows and the official manufacturer's driver to get access to these settings.
Where to measure with laser prints is a big question. I noticed with practically all laser printers they have edge effects: larger black areas are less dense in the middle. With negative process, this sometimes meant you got fair isolation next to the traces, and thin traces were OK, but larger polygons or ground pours were crappy in the middle. Never saw anything like this with inkjets; the density is more even (unless the nozzles are clogged or ink running out, so that stripes appear. Yes, inkjets are pain in the ass and expensive to maintain, price per print skyrockets if you only need to print rarely, but always need good prints!)QuoteAs I said, I tried it and it just didn't work.
You failed to analyze the reasons why the test failed. This is the most difficult part in failing, and it's where all the learning happens. For me, things never ever "just don't work". There has to be a reason.
When I struggled with this question (how much density is needed), I just used density test strips (e.g. https://ndtsupply.com/film-viewers-densitometers/density-strips/ndt-density-step-tablets/ ). Expose through the test strips and simply see what is the minimum density difference between a properly developed and properly undeveloped area. Note that if you get for example D=1.0, then it means if you use D=1.0 films in production, your exposure time and process need to be 100% perfectly tuned each time. That's why you actually want at least D=1.6.QuoteI also tried ink jet transparencies. They are even worse than ordinary photographic film. They let even more through.
It's not about the transparencies, it's about the ink. Specifically, you need an inkjet printer which allows adjustment of print quality / ink density. I have tried something like 5-6 different inkjet printers and only 1 of them was unusable due to way too little density, and no functional adjustment in the driver. Do note though that budget inks (not approved by the manufacturer) usually ruin the whole thing, never had good results with those, even if they look OK-ish when printed on paper.
But discussing this is useless as we both have just anecdotal data over small number of printers. My point is, I have never seen an excellent laser print; the density is usually "just barely OK", causing trouble with gauging the correct exposure time. With inkjets, I have seen both "just barely OK" but also excellent prints.
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In my younger days I did photographic darkroom work. Yes, I know the photo resist is relatively insensitive, but pre coated boards do come wrapped in heavy, black plastic;
It would make zero sense for them to produce yellow plastic engineered to not pass any UV or deep blue. They can just use black plastic. The plastic might be also needed to prevent interaction with oxygen or moisture, I don't know.QuoteYou can't just coat a PCB and then leave it laying around for a week.
There probably is some "best before" date on the coated PCB from chemical viewpoint, too, even if stored in dark.
We had some people who were quite careless about this exposure thing, had strong unfiltered fluorescent lighting (like 2 x 55W just 1.5 meters above), and I was saying that maybe you should be careful. Because there was a disagreement, I ran a test and I don't remember the exact result but it was something like that a 30-minute exposure to that level of room lighting already fogged the PCBs enough so that it interacted badly with the worst laser prints (those with D<1.5), bringing yield down a bit, while sometimes getting successful PCBs. Such slight fogging is annoying; it adds up with all other process variations and the cause is not obvious. Yield problems suck!
I bought UV filter film (cheap roll stuff, directly applied on top of the fluorescent tubes; the kind which only blocks UV and does very little to visible light), and rerun the test, and the slight problem at 30 minutes was completely gone.
If you want completely trouble free working guaranteed, then you would obviously buy yellow filters specified for this exact task. But for casual work, it's really fine to just avoid high power fluorescent lights and use modest amount of LED light, say a few 5W bulbs, and avoid sunlight as completely as easily possible (small leaks are not a problem). Basically normal artificial living room lighting is fine; industrial level bright fluorescent lighting or sunlight not. -
I think etching (and sourcing the etchant) is more difficult.
Ferric chloride (FeCl3) is easy to get in any electronics hobby store here. It's a bit of a nasty stuff, so I guess I want to try some recipes based on citric acid and hydrogen peroxide, which are even easier to source and safer/cleaner to work with.
Getting the timing right is going to be the main problem, I believe. I'll pick some reasonable guide as a starting point and then adjust as necessary for my conditions. -
Ferric Chloride gives good etching results so it's really fine, just nasty. You want heating, and you need strong bubbling, but you really need that with any etchant for good results (and minimization of edge effects, something people routinely ignore; especially bad with HCl+H2O2 which is nearly unusable in any high-resolution work due to localized overetching).
Timing of the exposure can be found out with a single test PCB by partially blocking the light e.g. with a cardboard thing and give incremental steps by moving the thing during exposure. With a negative dry film, you will see you have enough exposure when the film does not start to deteriorate in the developer even when gently brushed. You want enough exposure so that the remaining photoresist is strong and "hard" after development. Finding a printer which does dense blacks pays out here, giving you process margin - allowing you to expose a little bit more without exposing through those blacks.
With dry film photoresist, development is really easy: just wash (+ gently brush) in the developer until you see all of the unexposed resist has been fully removed. Look closely, carefully, in enough light so that you see if tiny speckles still remain. When it seems completely ready to naked eye, develop some 30% of the elapsed time more. When properly exposed, some extra time in the developer does not hurt at all. This is different to POSITIV spray developed in NaOH - it was very sensitive to overdeveloping. -
My dry film tips: https://sparks.gogo.co.nz/dry-film-tips.pdf
Comments on debate points
Harden: In the lay term, dry film is definately more abrasion resistant when cured, I would call that "hardened"
Sodium bicarbonate: some people do indeed use sodium bicarbonate instead of sodium carbonate for developing, slower, but functional.
Laser printers & media: adjust settings to get darkest you can, use tracing paper, but not too thin or it will wrinkle with fuser heat
Other
After developing expose the developed board to (unmasked) UV for a few minutes to ensure it is fully cured, this will also highlight any areas that didn't quite develop which can be cleaned up before etching
You can strip after etching just leaving it to soak in "developer", if in a hurry use acetone instead, no need for hydroxide.
Nice as it would be, dry film resist doesn't work well as a soldermask. Dry film soldermask is available but harder to find because it's not used much in industry which mostly uses liquid soldermask, which is readily available but a pig to diy with.