How to best remove a OSP (Organic Solderability Preservative) layer on a PCB - Page 1
Forum section: electronics / general

author: Electr0nicus
author_rank: Regular Contributor
timestamp: August 02, 2021, 06:54:32 pm
content: HiI have a special question concerning the common OSP layer on PCBs.For a project we ordered a flexible polyimid based PCB with heating meanders on it. Due to a communication problem, the PCB- manufacturer coated the surface with a OSP layer. The downside is that those heating meanders can reach temperatures of 140°C continously operating, so this OSP layer starts to smoke and emitt a awful smell. After staying at this temperature for some time, so that the complete OSP layer has smoked off, a white residue is left on the PCB, which sticks very well to it. The second problem we have with this layer is, that on the back of the PCB, a layer of HT adhesive and on the front of the PCB a layer of 25µm Kapton Film should be laminated. If the OSP stays on the surface, it will smoke off in the laminate, which will cause delmination and destroys the product.So to cut a long story short: What is the best method to completely remove the OSP layer from the PCB before the lamination process?We have tried using Hydrochloric Acid in 38% concentration, which removes the layer completly with ease. But the problem is that this method is really crude and you can't avoid etching some thickness off of the copper layer. As the thickness (= resistance) of this heating meander is critical, this is not a really feasable method.So is there some sort of non accidic solvent, which can do the same thing. We have tried with Isopropanol alcohol, which practically does nothing. Acetone works better, but there is also a lot of rubbing to do, to get the layer OSP completly removed.Any suggestions would be highly appreciated!Thanks,Gregor

author: jpanhalt
author_rank: Super Contributor
timestamp: August 02, 2021, 07:32:13 pm
content: If you knew its composition it would be easier to answer.Wikipedia lists some of the common ingredients:https://en.wikipedia.org/wiki/Organic_solderability_preservativeplus maybe rosin.As you mention, acetone worked somewhat. Acetone/ethanol mixture might work better. That group of azoles described in Wikipedia might be soluble in a chloroform/toluene mixture or chloroform/acetone mixture. They can be weakly basic, so adding acetic acid might help. Chloroform might be hard to find.Another solvent system to consider is ether/dioxane and/or related monoethers of glycols (e.g., methyl cellosolve, ethyl cellosolve, butyl cellosolve). Cellosolves have the advantage of water solubility, so water can be added. In fact, butyl cellosolve is in a lot of home-use products, such as floor strippers. ZEP heavy duty floor stripper has butyl cellosolve. Adding a little water may help too.I would first try the cellosolves with or without a little (10%) water.And, acetic acid may help, particularly if you add water. That is, consider white vinegar instead of pure water.Going up the scale a bit further you get into dimethylformamide and dimethyl sulfoxide.Those are just guesses as OSP is not a specific coating, but rather a description of a coating type.

author: langwadt
author_rank: Super Contributor
timestamp: August 02, 2021, 07:36:12 pm
content: google suggest flux and soldering temperature

author: Electr0nicus
author_rank: Regular Contributor
timestamp: August 02, 2021, 08:17:33 pm
content: Thanks jpanhalt for your reply.As my chemistry knowledge is not that big, i think I have more questions than beforeCould you have a look on this type of paint stripper? Would that one have the ingredients to dissolve the OSP layer?It's in german, but on page 3 the main ingredients are listed. Maybe you can tell from those if it would work or nothttps://www.prosol-farben.de/media/produkte/261925-sdb.pdfangwadt: As the PCB should be laminated this method would not work.

author: jpanhalt
author_rank: Super Contributor
timestamp: August 02, 2021, 09:00:25 pm
content: It's hard to tell whether it will work. Benzyl alcohol is a moderately non-polar alcohol. The esters in the mixture are similar.1) It won't hurt the PCB. So, if you have it, I would try it.2) Azoles are polar, but organic. They are reasonably soluble in water, but not as soluble as highly soluble stuff. For example, sugar and salt. I suspect whatever is in OSP prevents simply using water.3) Its partial solubility in acetone is what led me to the solvents that have a polar end (hydroxyl group) and non-polar end (ether group). If you have absolute ethanol, acetone/ethanol may work fine, as that combines both properties.4) From what little I read, OSP is applied as a water solution, but once it dries, it is water insoluble. One might think of all the water-based coatings available today. They can be very hard to remove once dry. As you found, HCl removes it. I suspect concentrated lye would also work, but you shouldn't use that either.How many board do you need to clean?

author: Electr0nicus
author_rank: Regular Contributor
timestamp: August 02, 2021, 09:29:21 pm
content: Tranks jpanhalt for your reply. You helped a lot so far!I've found another type of paint stripper, which I also would try tomorrow.https://www.prosol-farben.de/media/produkte/287863-sdb.pdfThis one has no acids in it, so it should do the copper layer no harm.So to conclude I've three options to try:The paint stripper which i mentioned in my last postThe paint stripper from this postAnd a mixture of ethanol and acetone. I think 50:50 shoud do?Quote from: jpanhalt on August 02, 2021, 09:00:25 pm10 pieces, but each pcb is a whopping 600x300mm in sizeI'll report back tomorrow, what the results are. I've to go to bed now as it's half before midnight where I live

author: jpanhalt
author_rank: Super Contributor
timestamp: August 02, 2021, 10:25:18 pm
content: Good plan. The second paint remover has some good stuff in it, like dioxane (a cyclic diether), dimethoxy methane ( a straight chain diether), methanol, and dimethylamino ethanol (very good). The hydrocarbons will probably not help.Ethanol/acetone (75:25) is what I use as a very good flux remover. I also use absolute (100%) ethanol. But, your OSP has reasonable water solubility so it will probably not be as sensitive to a little water.An ultrasonic cleaner with a good detergent and warm water might also work.Please keep us updated.

author: jpanhalt
author_rank: Super Contributor
timestamp: August 03, 2021, 12:04:37 pm
content: Additional info:It occurred to me that N-methyl-2-pyrrolidone (NMP) might work. It is a "super solvent" and was once used as a component of several "green" paint removers. Unfortunately, it has since pretty much disappeared from the consumer market because of toxicity from high exposures.See:https://www.m-chemical.co.jp/en/products/departments/mcc/c4/product/1201005_7922.htmlhttps://p2infohouse.org/ref/38/37102.htmSo, i checked what replaced it in such products. Alkoxyethanols (i.e., cellosolves), including butyl cellosolve (aka 2-butoxyethanol), or benzyl alcohol. If your experiments don't work today, find a cleaner with one of those substitutes. I prefer the cellosolves.

author: Electr0nicus
author_rank: Regular Contributor
timestamp: August 16, 2021, 09:07:18 pm
content: Sorry for the late reply, but i had too much work to do lately, so i simply forgot about it.To sum it up:Ethanol and Acetone didn't work, or if it would worked, it would've been a very slow process. After letting it soak for 20min without much change i discarded this method.This Colorstrip 2011 paint remover, worked suprisingly well (this is the one containing formic acid). Put it on, and you can immediately see a change. Let it do it's stuff for 30sec. and then rinse it off with plenty of water. Done.You can see the difference, because the copper has a brighter color than before, and of course if you let it sit around to much it tarnisches quickly.The other paint stripper, was also a complete fail. This was a gooey substance which had to be applied with a brush. Did nothing or may be very slow work to the OSP layer, and i ended up with a huge messy workbench, as this gooey substance stuck to everything which it shouldn't have stuck to.