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How to best remove a OSP (Organic Solderability Preservative) layer on a PCB

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Electr0nicus:
Hi

I 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

jpanhalt:
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_preservative  plus 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.

langwadt:
google suggest flux and soldering temperature

Electr0nicus:
Thanks jpanhalt for your reply.
As my chemistry knowledge is not that big, i think I have more questions than before  ::)
Could 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 not
https://www.prosol-farben.de/media/produkte/261925-sdb.pdf


angwadt: As the PCB should be laminated this method would not work.

jpanhalt:
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?


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