Why would cleaning my OSH Park PCB result in residue left over?

Question

I recently bought a PCB from someone that was created by OSH Park. Other than the pictures online, this is the first board I've seen from them. Even though I didn't buy the PCB directly from OSH Park, it was very clean when I received it from the seller.

Anyway, after handling the board (with finger prints) and soldering all of the components, I decided to clean it with 99.9% pure isopropyl alcohol. I do this all the time with other PCB's. I use Kim wipes, Q-tips, etc. and gently wipe the boards. I allow adequate time for drying. But for some reason, much of the board has a sticky residue left over which really has me puzzled. Oh, and yes, my hands were clean before I started...there are no obvious contaminants floating around and my house is smoke-free. :-)

My other boards don't have this problem. Plus, the bottle of alcohol I used was brand new...even had to break the seal on it so I don't believe it was contaminated.

Any ideas what this could be? I wouldn't think it's the purple solder mask OSH Park uses. I've read lots of good things about them.

The board works great. Just really puzzled why it is so much harder to clean than the hundreds of other boards I use.

** EDIT **

The flux makes sense. I just remembered that I cleaned the board BEFORE and after I soldered the components. I don't normally clean them before I solder them. Soldering the components more than likely burns off most of the flux which is why I never noticed it when I cleaned the other boards after. I didn't know that about IPA and flux. Learned something new today! Thanks.

Answer 1

Sticky residue is normally the flux. IPA doesn't dissolve flux, it just makes it runny. So if you gently wipe it off, all you are doing is smearing flux everywhere. Once the IPA evaporates, the flux turns into a horrible sticky mess.

The solution is to add a bit more IPA to the board, wait a minute or so for it to soften the flux, and then using something like kitchen roll, wipe in small circles, changing which bit of the kitchen roll you are using as you do (so that you don't smear what's already been wiped up back onto the board).

For intricate places around ICs, you can use the same process, but rather than wiping the kitchen roll on the board, use something like a pair of tweezers to push the roll down on the board and move back and forth around and between pins.

You should find the areas that have cleaned well stop being tacky. If you find anywhere that is still a bit sticky, then simply repeat the process.

What is also worth doing before you start cleaning with IPA is to gently chip off any really large blobs of flux - you can usually do this with a blunt instrument and as long as you aren't jamming something into the board it won't do any damage to the solder mask. The advantage of this is it means less flux on the board that has to be wiped off after it has softened up, so less that will smear around everywhere.

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I've used OSH Park before, and never had an issue getting the boards clean with IPA and some elbow grease. As to why your particular board is being more stubborn to clean, it happens from time to time. Maybe you had ended up with more flux on the board, or when you were wiping off the IPA you used something that wasn't doing as good of a job of absorbing the grot. Could be many factors.

Answer 2

I agree with Tom that the residue you see now is probably Flux.

Dirty/sticky flux is often/usually limited to a subset, R, RMA and RA, or category 1.x.x fluxes, which are based on Rosin. These are the "classical" fluxes and are hard to dissolve in IPA or water. They do dissolve, but in the order of tens to hundreds of ppm in most cases, which is negligible if you don't own 55 gallon drums of IPA.

There are category 2.x.x and 3.x.x fluxes which dissolve completely in IPA and even water and leave a more salty/white residue.

Category 2 fluxes are weak Organic type acids or other corrosives. Most famous in the "old world designation" being FSW34, commonly used by EDSYN in about half of their no-clean solders, with full category designation usually being 2.2.3B, or Organic, Non Water Soluble (IPA will work), Non-Halide, Solid. AFAIK EDSYN is one of the last to use the FSW designation on newly produced stock.

Category 3 fluxes being very unwise to use with electronics, as they often get based on highly corrosive substances that are bad for you and long term bad for your board as well.

The difference between Cat1 and Cat2/3 can very easily explain why this PCB from a seller that orders at OSHPark and then probably home-populates leaves larger amounts of Rosin based residue, as many normal Fabs and Solder Pastes use Rosin-free for years, and many home/hobby/small-scale suppliers still most commonly sell Rosin containing solder wire.

For more information on flux types check this PDF of Stannol/SOS I just found.