Issue with sodium persulfate pcb etch.  - Page 1
Forum section: electronics / beginners

author: Fflint
author_rank: Frequent Contributor
timestamp: December 28, 2020, 01:57:42 am
content: Hi,I used ferric chloride etch in the past, but I wanted to test sodium persulfate. My first etch with it was great, second board etched on next day took 50% time longer, but also etched fine. The third board I tried etching immediately after the second never finished. I removed it from the solution after 90 minutes when I could still see copper between traces. Is this normal with sodium persulfate?I prepared 60g of sodium persulfate in 300ml of tap water (medium hardness). I have a magnetic heater/stirrer with temp probe so I could keep it at 40degC and agitated. I etched one test board (single side) 5*3cm witch took 35minutes. Solution was stored overnight in a glass jar (open) at 5 degrees C.Next day in the same solution I etched another test board of similar size it took 50 minutes. The third similar size board still had bits of copper between traces after 90 minutes of etching. As far as I could tell under a microscope there was no photoresist left between traces. Just that etching took 4 times longer...I'm pretty sure the temperature was the same 40degC during all 3 etches. Is this normal for persulfate etch? Do I need fresh solution for almost every board? If yes, I will probably go back to ferric chloride despite its disadvantages.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: December 28, 2020, 02:25:23 am
content: Aqueous sodium persulfate solutions decompose rapidly at elevated temperature compared to hardly at all at room temperature.e.g., see Page 10 ofhttps://www.peroxychem.com/media/241528/pxc089_persulfates_brochure_pdf_fnl.pdfI suspect you are seeing decomposition.You could try the "contact etch" method used with ferric chloride to save on etchant:https://www.pcbfx.com/main_site/pages/tech_support/no_etching_tank.html

author: David Hess
author_rank: Super Contributor
timestamp: December 28, 2020, 04:39:35 am
content: We used bubbling for agitation with sodium persulfate with the boards oriented vertically.

author: Psi
author_rank: Super Contributor
timestamp: December 28, 2020, 04:44:05 am
content: I gave up on persulfate, it was useless. FeCl works well but is messy and annoying to dispose safely.The peroxide and HCL etch solution is the best option.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 28, 2020, 09:09:40 am
content: Quote from: JohnnyMalaria on December 28, 2020, 02:25:23 amThank you for the link. It is very useful. Thermal decomposition is definitely a possibility. My heater/stirrer if set to 40degC from cold overshoots to 55-60 degC and then when temperature goes down to 40 it holds it there. However, looking at the chart on page 10 and considering the heating time was few hours at most my 20% solution shouldn't loose more than few % of its strength. Maybe this is how it works when weak... If this is the reason for its poor performance it should be possible to simply add powder to offset decomposition. It would be nice to have some way to measure its concentration.Quote from: David Hess on December 28, 2020, 04:39:35 amThanks for the info. Do you know if you reused etching solution for multiple boards, or mixed fresh each time?Quote from: Psi on December 28, 2020, 04:44:05 amWhy was it useless to you, was it because it has to be heated, or some other reason? I'm trying to give it a best shot because as you mentioned Fecl is messy (also it's spray is very corrosive to equipment). As for Hcl+h2o2, I currently have access to both, but hcl fumes are also extremely corrosive and high strength peroxide is nasty-nasty stuff. If I don't get acceptable results from persulfate I'll try it next under a fume hood. Can one use 3% peroxide, or does it have to be stronger?

author: sleemanj
author_rank: Super Contributor
timestamp: December 28, 2020, 09:29:40 am
content: Quote from: Fflint on December 28, 2020, 09:09:40 amYoucanuse 3% H2O2 if it's readily available to you, but you'll have an easier time of it using something stronger, especially if you plan to keep using the etchant (refreshing it with more H2O2 or bubbling) as you won't be watering it down so much. Standard hardware store HCl is fine, you don't need that to be particularly strong.

author: Psi
author_rank: Super Contributor
timestamp: December 28, 2020, 11:12:13 am
content: Quote from: Fflint on December 28, 2020, 09:09:40 amYeah, the high temp required was a significant part of the issues I had.The solution would cool down and stop working before the etch was done. It made things tricky and required active heating during the etch. To get any decent etching it had to be hot, like burn yourself hot.FeCl is messy but much easier to work with than persulfate. It's better if you warm it a little but a cold etch will work, it's just a little slow.If you have a spare microwave oven that you can dedicate for non-food use you can microwave a glass dish of FeCl and then etch many boards. Just be careful not to get it so hot that it fumes, the microwave oven is made of metal after allFor the HCL/Peroxide option you don't need anything special or concentrated.I used 30% HCL from the hardware store for pools and 6% peroxide from the chemist and that worked great.No heat needed, and as others have said 3% peroxide will also work.And the solution lasts forever compared with the other two options. (ok, not really forever, but an order of magnitude longer)

author: cdev
author_rank: Super Contributor
timestamp: December 28, 2020, 05:11:27 pm
content: I use hydrogen peroxide, wine vinegar and salt. And nail polish remover to clean off the black gunk at the end.

author: tooki
author_rank: Super Contributor
timestamp: December 28, 2020, 05:58:11 pm
content: 40C is a bit low. You'll do better at 50-60C.I've kept used sodium persulfate solution in a closed container for weeks before reusing and it was fine. Not sure why yours has failed so spectacularly.

author: StuartA
author_rank: Regular Contributor
timestamp: December 28, 2020, 09:25:35 pm
content: Giving away my age here, I started using ferric chloride in the 1960's and I just love the stuff. Using it takes me straight back to my boyhood. As the guy in the film (nearly) said, "I just the smell of ferric chloride in the morning"It works very well too, for home use.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 29, 2020, 01:47:21 am
content: Quote from: Psi on December 28, 2020, 11:12:13 amQuote from: tooki on December 28, 2020, 05:58:11 pmThis is interesting. I kept the temps below 45degC during etching because I found it repeated in few places online that sodium persulfate "works best between 40-45degC" and decomposes rapidly above. Also the concentration recommended was 20g of powder per 100ml of water. Looking for additional info I just found a site recommending 50degC and only 10g per 100ml.Based on your advice, and my experience of really nice first etch and rapid deterioration I suspect the solution has to be very strong to work in 40 degC and one can increase temperature to 50-60 to speed it up when weaker. However, I wonder if it doesn't underetch by a lot more in higher temps.Tooki, roughly how many times did you use your solution? Also if you remember, how much powder/water did you use to make it?I wonder if there is anyone here who still uses it? I've got a kilo of it so I'll be using it for a bit just so it doesn't go to waste. I'll be trying few more things including adding powder to this "slow" solution. Measuring pH in the beginning and later etc.

author: JohnnyMalaria
author_rank: Super Contributor
timestamp: December 29, 2020, 01:56:02 am
content: Just for the sake of it (and being a chemist by profession), I did some calculations...60g of sodium persulfate in 300 ml water is 60 / 238 x 1000 / 300 = 0.84 mol. But that's for Na2S2O8 (i.e., 2 SO4 per molecule)A 5 x 3 cm board of 1oz copper has about 4.58g copper = 4.58 / 63.5 = 0.072 mol.So, your fresh solution has about a 24:1 molar ratio SO4/Cu.

author: Psi
author_rank: Super Contributor
timestamp: December 29, 2020, 05:23:36 am
content: Thinking back, i may have been using ammonium persulfate and not sodium.It was a long time ago.but I do remember that 60C was not hot enough. it needed to be 80C+

author: digsys
author_rank: Unknown
timestamp: December 29, 2020, 06:10:44 am
content: Quote from: PsiYep, I used Ammonium persulphate waaaay back as well, bought it by the kg (weed killer?). Always preferred it to FeCl. Don't remember having to heat it up much, if at all, but I did have it constantly flowing and agitating. Never had an issue, as far as I recall.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 29, 2020, 09:28:25 am
content: Quote from: JohnnyMalaria on December 29, 2020, 01:56:02 amThanks. This confirms my "gut feeling" that the solution should be able to etch close to an order of magnitude more number of boards. Unfortunately the only research article on sodium persulfate etching I can find is in cnki -(Chinese National DB) and I can't find a way to access it. This is the link if anyone can try via a university library:http://en.cnki.com.cn/Article_en/CJFDTotal-BMJS200903022.htmThe article has no DOI so the usual methods (Sci hub et al.) don't work.The English abstract claims:QuoteWhich would mean over an hour and 10 min for typical pcbs with 35um copper thickness (1oz). I'm not sure why they're adding sulphuric acid to the mix.

author: Psi
author_rank: Super Contributor
timestamp: December 29, 2020, 09:57:16 am
content: Quote from: Fflint on December 29, 2020, 09:28:25 amWhen I etch a PCB it takes around 5min.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 29, 2020, 12:09:31 pm
content: Quote from: Psi on December 29, 2020, 09:57:16 amBased on previous posts I assume this is with HCl/H2O2. I'll try it once I've used all persulfate. BTW, do you know how is it with thin traces, for example 10mil/0.254mm? Does it underetch a lot?Regarding persulfate I found a US patent which claimsQuoteI don't understand what would be the chemical mechanism how those phosphate salts could "stabilise" persulfate. Also, I don't quite understand the need for the "proton donor" if pure persulfate has pH of 3 as measured by me...

author: tooki
author_rank: Super Contributor
timestamp: December 29, 2020, 02:00:12 pm
content: Quote from: Fflint on December 29, 2020, 01:47:21 amThe package I have (instructions linked onhttps://www.kemo-electronic.de/en/Hobby-School/Ae200-Etch-set-for-electronic-circuits.phpas "datasheet") says to use 100g for 5dl, i.e. the same 20g/100ml you mention, and to use it at 50-60˚C. In my experience that worked really well, taking just a couple of minutes to etch. At room temperature, it took aaaaages. Warming it back up in the microwave fixed it right up.Quote from: Fflint on December 29, 2020, 01:47:21 amSomeone else can correct me if I'm wrong, but I seem to recall reading somewhere that a slow etch will result in more underetch than a quick one. As such, I'd assume the hotter temps would result in a better etch.Quote from: Fflint on December 29, 2020, 01:47:21 amI think I used it 2 or 3 times? Then the container leaked and most of it dried up anyway, so I chucked it.Since I've just been making small boards that easily fit in a small rectangular tupperware, I just mixed 0.5dl with 10g of powder. Then I put the board in, cover it, and swish it around so it etches better.

author: cdev
author_rank: Super Contributor
timestamp: December 29, 2020, 02:09:15 pm
content: I use peroxide/vinegar/salt + a small air pump, and still slosh it additionally from time to time, because bubbles form, and I'd imagine they might cause irregular etching without some additional agitation. But a cheap pump helps a lot. I also use a dedicated tupperware dish and make very small boards.

author: Psi
author_rank: Super Contributor
timestamp: December 30, 2020, 01:12:21 am
content: Quote from: Fflint on December 29, 2020, 12:09:31 pmThe ~5min was with warm FeCl and warm HCL/H2O2.The one time I tried persulfate it caused more issues of tracks/pads shrinking from side etching.Not sure why, but I definitely noticed a trend towards having more under/side etch problems when you have to leave it in the solution for a long time.I never really tested HCL/H202 against persulfate directly.I started with FeCl and it worked great, tried persulfate and it was terrible, so went back to FeCl and then sometime later tried HCL/H2O2 and found it just as good as FeCl with some advantages in safety/disposal/longevity. etc...And it's clear enough to see the PCB while it's under the solution, so you can tell if it's done without having to pull it out every 30 seconds like you do with FeCl.

author: Fflint
author_rank: Frequent Contributor
timestamp: December 30, 2020, 10:13:43 am
content: Well, I did an experiment. I heated my "spent" solution(the one I etched 3 3*5cm boards in previously) to 60-65 degrees C. I managed to etch another 3*5cm test board in it,but it etched badly. Specifically thin traces (0.2) were under etched completely while in other places there were specks of copper still unetched.At the same time I made a fresh batch of solution and it etched a slightly larger board (10*4cm) in 43-45degC in under 20 minutes with almost no underetch. When finished the solution was only slightly blueish, but after few hours (in 15degC ambient temp) it became really blue same as the previous solution.So in conclusion I think it goes bad after few hours, but in the first hour or so it works fine. I might have some tri sodium phosphate in few days, if I still have persulfate then I'll test if it really makes it last as in that patent.

author: David Hess
author_rank: Super Contributor
timestamp: December 31, 2020, 03:57:11 pm
content: Quote from: Fflint on December 28, 2020, 09:09:40 amIt was long enough ago that I do not remember. I recommend a bubble tank (with a lid) though no matter which etchant you use but it is less practical for fewer boards.

author: Fflint
author_rank: Frequent Contributor
timestamp: January 01, 2021, 02:03:19 am
content: Ok, I made a solution as per the US patent 6, 156, 221. However I couldn't get sodium phosphate dibasic so I used tribasic as the patent claims it also works.The solution was:400ml water, 12g phosphoric acid 75%, 9.5g sodium phosphate dodecahydrate, 80g sodium persulfate. This was made first by mixing everything but persulfate. Heating to 40degC and mixing persulfate then.A board sized 5*10cm was then etched in 20 minutes in 41-43degC (active heating) in vertical position and with minimal agitation. After etching the solution was poured into a glass jar (stored unsealed to allow gas escape). I observed it few hours later and the color appeared exactly the same as when etching was done.Interestingly during etching almost all copper was gone after around 9 minutes. Only few stubborn specks bridging some tiny gaps (0.2mm) remained. It took another 11minutes for all those specks to go away. However, despite this extra 11 minutes I saw no underetch.I'll try etching another board in it in few days and we'll see if it makes persulfate last or is the patent BS ;-)

author: tooki
author_rank: Super Contributor
timestamp: January 01, 2021, 02:18:09 am
content: The way you've got areas that take forever to etch make me wonder: how did you prepare the resist beforehand?Was this photoresist or toner transfer?If photoresist: did you apply the photoresist yourself, or were they precoated boards?If you applied the photoresist yourself, or if you used toner transfer: For good results, it's critical to use scotchbrite or fine steel wool to get the surface absolutely free of oxides. Then it must be thoroughly degreased. I often do the scrub with dish soap, rinse with water, and then wipe down with IPA to be absolutely oil-free. From that point on, bare hands must never touch the board surface, and it should be processed promptly so oxides don't have time to form.If you used precoated boards, I wonder if perhaps the exposure and/or developing is not correct. The symptoms you describe indicate you might have areas with a thin coating of resist still left on them where it shouldn't be. Depending on whether it's positive or negative photoresist, I believe this can occur from under- or over-exposing, respectively.Overall, something still isn't right with your chemistry. It just doesn't make sense that you're taking upwards of 9 minutes to etch a board when mine do it in, like, 3.What copper weight are your boards? FWIW, I've been assuming standard 1oz (35µm). If you're using heavier copper weight, it will of course take longer.

author: Psi
author_rank: Super Contributor
timestamp: January 01, 2021, 02:46:59 am
content: Quote from: Fflint on January 01, 2021, 02:03:19 amThat sound OK.Maybe the persulfate I got was bad.The stuff I had wouldn't do anything at 40C, even after like 30min.I had to use a boiling water bath, and even then it only half etched before the water cooled down too much to sustain etching.

author: Fflint
author_rank: Frequent Contributor
timestamp: January 01, 2021, 11:15:25 am
content: Quote from: Psi on January 01, 2021, 02:46:59 amMaybe it was a mislabelled ammonium persulfate rather than sodium? No idea why anyone would intentionally sell one as the other as price seems similar. I read in few places (and it was mentioned in this thread) that ammonium persulfate requires closer to 70degC to work.Quote from: tooki on January 01, 2021, 02:18:09 amThis is with dry film photoresist from China (most likely DuPont Riston). It is possible that the film doesn't get developed away fully in few random spots. During developing unexposed film becomes this thick gel-like substance. Brushing the board speeds up removal of it. However it is possible for tiny bits to be left in corners etc. During testing previously I did see (under a microscope) tiny specks of the film semi-attached where they shouldn't be. I probably need to shorten exposure time even more.However, during etching I stopped few times (rinsing the board in clean water) and I looked at those copper specks under a (low magnification) microscope. They looked clean, but something must have been covering them or they would etch away with the bulk of copper.I would be inclined to blame the pcb quality, but those "specks" happen only when I use negative dry film photoresist. If I use positive photoresist in a spray form everything etches at the same time. As you said I need to tweak the process a bit more. I have another thread about my photoresist use in the manufacturing section.QuoteI prepare the boards myself. I wash them in dishwashing liquid using this dishwashing sponge that has yellow foam on one side and green scotchbrite like material on the other. Then I use stainless steel brush to roughen the surface. Finally I wipe it with acetone. I also use nitrile gloves during the whole process. My pcbs are standard 35um.

author: bitter_mike
author_rank: Contributor
timestamp: January 01, 2021, 12:56:09 pm
content: If I may offer my 2 cents:Your persulfate etch will be both more stable and more effective if you include some sulfuric acid (we typically use 2%, so 20 g/L). The rate at which persulfate decomposes increases considerably as you get higher in pH.You're also running this at very high concentration and removing a lot of copper. Persulfate etches are typically used as part of the electroless copper process to clean oxides from the board and minimize palladium catalyst adsorption to copper inner layers and for that purpose, one usually only etches away ~0.5 microns. That's not to say you can't use it to etch in your pattern, it's just that ferric chloride and ammonia/peroxide etches are more common for that in production, presumably because they're cheaper.In any case, add some sulfuric acid. It will work much better.Mike

author: Fflint
author_rank: Frequent Contributor
timestamp: January 01, 2021, 03:49:43 pm
content: Quote from: bitter_mike on January 01, 2021, 12:56:09 pmThank you. However, I already added phosphoric acid (as in that patent). If the role of sulphuric acid is to lower pH phosphoric acid will do the same. I'm not sure why the patent specified phosphoric as preferable. I happened to have it on hand so I've added it alongside sodium phosphate.Interestingly I etched a second 5*10cm board in the same 400ml solution and it took the same amount of time! So betwee the acid and sodium phosphate something did prevent the breakdown so far :-)I'm going to continue using the same solution and see when it stops. If it lasts I think this recipe may be a good alternative to pure persulfate. Probably the acid could be sulphuric too. However, if I was going to recommend this to others I would suggest trying acetic or citric for safety etc. (the patent mentioned them too).

author: tooki
author_rank: Super Contributor
timestamp: January 03, 2021, 07:38:32 pm
content: Quote from: Fflint on January 01, 2021, 11:15:25 amMy hunch is that roughening it with stainless steel is a bad idea. Scotchbrite is a great surface finish for this. Remember that you want a consistent copper thickness. Your brushing is likely creating a big difference between low and high areas, further destabilizing the etching process.

author: MNIN
author_rank: Newbie
timestamp: August 07, 2023, 07:32:01 pm
content: IBM patented a recipe for this about 20 years ago"A preferred etchant composition is 230 gm/liter sodium persulfate, 3 volume % phosphoric acid and 0.058 molar sodium phosphate dibasic."The etchant works great and will last weeks if you allow it to cool between runs.