Dried flux residue. Does it damage components? - Page 1
Forum section: electronics / general

author: Doctorkong
author_rank: Contributor
timestamp: December 19, 2018, 12:05:21 pm
content: Hello everybody, I'm new on this forum and last time I was here you helped me a lot with a question I had, so before it all I want to thank you so much.I'm back again because I have a question that maybe some expert like you can solve.I own a PCB of an old videogame cartirdge (it should be a 20 years PCB). It works fine and I opened it just to see if capacitors are ok and also battery. Everything is ok but I noticed that there are two spots in 2 points. It should be dried flux residue and I think these residues are there since the manufactoring. I attach a photo of these. These are in R14 and SL1Do you think these residues can damage the components or are them safe? They seem like a trasparent glue, no metal residues seems to be there.Thanks again

author: Siwastaja
author_rank: Super Contributor
timestamp: December 19, 2018, 02:31:27 pm
content: No problem in your photo.Some aggressive fluxes need to be cleaned. They have used suitable flux to do the rework. If they didn't, you would see the corrosion. This looks clean.Some fluxes could cause issues underneath SMD components, being slightly conductive, but this would be more relevant in high accuracy analog circuitry, which doesn't seem to be the case.

author: Ian.M
author_rank: Super Contributor
timestamp: December 19, 2018, 03:35:58 pm
content: In general R (rosin) and RMA (rosin mildly activated) flux residue and their synthetic resin equivalents are pretty much inert at room temperature and moderate to low humidity. More active fluxes fluxes can be highly corrosive, but some that are free of inorganic acids and halides leave safe residue if used in accordance with their manufacturer's instructions, which may involve following a proper reflow profile - if it hasn't been fully heated the residue may be unsafe. You can assume that any water washable flux will leave an unsafe residue if not thoroughly washed. One drop of diluted water washable flux left in contact with a pin, pad or via under an IC is likely to result in failure weeks, months or even years later.

author: Doctorkong
author_rank: Contributor
timestamp: December 19, 2018, 04:53:13 pm
content: Quote from: Ian.M on December 19, 2018, 03:35:58 pmThank you. How can I see what is the type of flux used in my PCB? Can it be possible to desume that from the photos? I suppose this flux was there since 1998 and the PCB has been used for around 70-80 hours at least. Can it be possible the corrosion hasn't been evident but it will be in the future?

author: Ian.M
author_rank: Super Contributor
timestamp: December 19, 2018, 07:15:00 pm
content: There's no way of telling with any degree of certainty unless you've got access to a gas chromatograph set up to pyrolyse solid organic samples + a reference library of fluxes from that era.There'a a couple of things you could do to narrow down the possibilities a bit: You could scrape some off, and under a microscope test what solvents dissolve a few grains of the resulting powder, but odds are, all that's going to tell you is that it isn't water washable. You could try reheating a small area of the residue, avoiding the solder, with a small clean soldering iron andd see if it smells like rosin flux.However it been there 20 years with no signs of corrosion, and from the amount present and its location, I suspect its from flux cored solder used to make the solder pad jumper SL1 so it pretty much has to be a mildly activated rosin or synthetic resin flux, as those were the commonest fluxes used in the cores of SnPb solder of that era.IMHO its not worth worrying about - just leave it alone, as its had no visible effect in 20 years, its unlikely to do anything significant in the next 40 as long as the cartridge continues to be stored in a dry room temperature environment. Bitrot will almost certainly kill the cartridge ROM(s) before that residue gives any trouble.

author: Doctorkong
author_rank: Contributor
timestamp: December 20, 2018, 09:33:12 am
content: Quote from: Ian.M on December 19, 2018, 07:15:00 pmThank you. So if the material is a R or RMA are them safe? I stored this item in a low humidity environment and no high or low temperature.

author: Siwastaja
author_rank: Super Contributor
timestamp: December 20, 2018, 11:27:51 am
content: Quote from: Doctorkong on December 19, 2018, 04:53:13 pmYour photo looksverytypical. So, we are assuming that the original factory had a clue what they are doing - this is true 99.9% of the cases.One of the flux spots is a solderable selection "switch", "programmed" at the factory, manually with a soldering iron. Every device has some of those pads soldered together to configure the thing (a region, or TV system selection for the cartridge?). Another one is some generic rework for whatever reason, possibly there was a wrong value of resistor they needed to replace.In any case, when factories do these manual fixes, they naturally use solder which doesn't require cleaning. Which is 99.9% of electronic solder wire, anyway.They would be very clueless, in a very peculiar way, to use solder wire with aggressive flux requiring wash to do this kind of rework. Had they done that, it would be highly likely it would show by now as a form of corrosion.

author: Rerouter
author_rank: Super Contributor
timestamp: December 20, 2018, 12:11:28 pm
content: If the flux has been in place for at least 6 months in a non controlled environment and you don't see any discoloration from non fluxes joints its a no-clean and safe flux,If the joint has turned black its generally electrolytic leakage,If the joint has turned green, its something that loves soaking up water, e.g. syrup residue.If the joint has gone blue, its an older flavour of plumbers flux, and the joint is probably a write off.Flux doesn't always need to be cleaned up, but depending on the type, some do trap trace amounts of water, generally not enough for corrosion, but enough to magnify parasitic capacitance's with its higher dielectric,So if your working with very high speed, or very low leakage environments, then yes all flux and traces should be cleaned off, otherwise, less of a concern.Edit: a nice way to think of it for high speed is, "What would go wrong if someone randomly sprinkled 1pF capacitors on my PCB?"