Strange effect after ultrasonic cleaning pcb - Page 1
Forum section: electronics / testgear

author: duzycinek
author_rank: Contributor
timestamp: August 19, 2023, 07:51:58 am
content: Hello, I got question about using ultrasonic cleaner to clean pcb after soldering. When I clean pcb soldered with Mechanic XGSP80 Sn63/Pb37 solder paste, effect was really good, but after I placed it in ultrasonic cleaner with: 50 Celcius, 15 min, distilled water with flux cleaning solution for PCB, solder joints became dark gray. On other boards soldered in factory, solder joins did not change colour. When im scratching joints, they are normal inside, but PCB looks very strange becouse of this joints colour. Anyone know what cause this problem?

author: tomud
author_rank: Regular Contributor
timestamp: August 19, 2023, 11:10:21 am
content: Quote from: duzycinek on August 19, 2023, 07:51:58 amCleaning agents should be matched to the fluxes used during soldering. Here it is not easy and when the wrong preparations are selected, various tarnishes may appear on the solder and on the PCB itself. However, I am unable to answer whether the deposits (chemical compounds formed in the reaction of the solder alloy with the cleaning agent) have any negative impact in your case. Here it is best to consult the manufacturer of the preparation used to clean the PCB. Unfortunately, water-soluble formulations are not suitable for all solders and fluxes. In my experience, pure isopropyl alcohol (IPA) is the most neutral. Though it's not perfect either.

author: DavidAlfa
author_rank: Super Contributor
timestamp: August 19, 2023, 11:28:12 am
content: Degreasers use to have caustic agents that will cause this.Probably it's made for rohs and it's attacking the lead.Also expect Mechanic solder to be absolute junk.I used IPA and the result was great, of course be careful, as this could cause explosive vapours.

author: robert.rozee
author_rank: Frequent Contributor
timestamp: August 19, 2023, 11:32:22 am
content: in the past, after ultrasonic cleaning, i've rinsed PCBs in mentholated spirits (purple coloured) then brushed clean with a paintbrush. the final effect is a thin gloss, as meths (in new zealand at least) has a small wax component.this was for short runs of boards, and there was no RF or other analog stuff involved. a safer approach may be the same, but using IPA instead of meths.(written before DavidAlfa's post mentioning IPA)cheers,rob :-)

author: tomud
author_rank: Regular Contributor
timestamp: August 19, 2023, 12:07:37 pm
content: The preparation available in Poland under the name (KT-5) is also suitable for manual cleaning. It doesn't smear flux like IPA, IMHO it's one of the better cleaners out there. I'm not 100% sure but most likely it's turpentine. Below is a photo of the bottle with this preparation...

author: duzycinek
author_rank: Contributor
timestamp: August 19, 2023, 12:08:52 pm
content: Great, thank you for your replies. Can you please reccomend me better solder paste?

author: tomud
author_rank: Regular Contributor
timestamp: August 19, 2023, 12:32:26 pm
content: Quote from: duzycinek on August 19, 2023, 12:08:52 pmAt home, for hobby projects (where RoHS does not apply to me), I use Mechanic paste XGZ40 (Sn63Pb37, Flux IPX3) - photo below and I have no problems with it. There is also no problem with washing in an ultrasonic cleaner (using IPA) - an example photo of boards soldered with this paste after removal from the washer below.As I mentioned, I would focus more on changing the cleaning agent in this case. Yes, IPA is a bit dangerous when heated, but you can test other water-soluble preparations. Currently, unfortunately, there is not much choice when it comes to lead-containing solder pastes, and this one is one of the better ones I have tested.

author: KungFuJosh
author_rank: Super Contributor
timestamp: August 19, 2023, 02:10:45 pm
content: I use Kester EP256 Solder Paste, No-Clean 63/37.I also usually use IPA, I haven't purchased a bath yet, though I have contemplated it a few times.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 19, 2023, 06:20:13 pm
content: Darkening can be chemical or mechanical. Or both. The sonic power might be too high, or duration too long. The cleaner might be inappropriate, in general (just soaking causes corrosion), or in combination with sonication.You didn't say what power and size of sonicator, nor what solution, so any statement about exactly what's happening is impossible.Tim

author: GigaJoe
author_rank: Frequent Contributor
timestamp: August 20, 2023, 02:16:17 am
content: does that specific chemical cleaner affect solder without ultrasound ? , if it darken without my guess chemical. do you have microscope to look at surface , can be mix of cavitation and chemistry together

author: Ian.M
author_rank: Super Contributor
timestamp: August 20, 2023, 05:07:37 am
content: Probably the best option would be to test how long the solder surface can remain in the cleaning solution without unacceptable discoloration. This is simple enough to do, though a little tedious.Prepare a min. 16 cm long strip of copper or copper cad PCB, evenly tinned with your solder, reflowed to smooth it, and clean off all flux with IPA or preferably proprietary flux remover and paper towel. Don't use abrasives as you want a smooth bright surface. Don't touch the clean defluxed surface. Suspend the strip over the cleaning solution in such a way that you can easily lower it a centimetre at a time, and re-secure it. Then its just a matter of lowering the strip into the cleaning solution, by one centimetre every minute, on the minute, and at the fifteenth minute, removing it and immediately rinsing it. You can then examine the strip to see how far up from the bottom the discoloration extends, and thus determine the maximum permissible time in the cleaning solution.