Drying PCBs After Ultrasonic Cleaning - Page 1
Forum section: electronics / general

author: CoolNamesAllTaken
author_rank: Contributor
timestamp: July 30, 2019, 06:35:22 pm
content: I recently purchased an ultrasonic cleaner and have been using it to clean flux and other nasties off of PCBs after assembly. I've noticed that some people recommend drying off PCBs with a hot air blower after they have been cleaned, since the ultrasonic cleaning can move water under components and into other hard-to-reach spots. I had the idea to use a food dehydrator to dry out PCBs after cleaning, but thought I should ask around to see if it is a good idea first. The food dehydrator I have is made of plastic, and I'm wondering if it creates a statically charged environment that might be bad for the PCBs when it's running. If this might be the case, does anyone have recommendations on what to do to try to dissipate static charge from the airflow? I could create some kind of wire mesh with a grounding wire, but I'm not sure if it would be effective or necessary. Looking forward to hearing your thoughts!

author: jmelson
author_rank: Super Contributor
timestamp: July 30, 2019, 07:03:33 pm
content: Much better is a blast of compressed air. You want to blow away the dirty water that is under the components, not let it dry slowly in place, leaving contaminants under the parts. This probably only matters in high voltage or high impedance circuits.Jon

author: james_s
author_rank: Super Contributor
timestamp: July 30, 2019, 07:15:29 pm
content: I've used that often in the past, but more recently someone told me that compressed air can create static charges which can damage parts. I don't know offhand how much truth there is to this.

author: CoolNamesAllTaken
author_rank: Contributor
timestamp: July 30, 2019, 07:54:28 pm
content: I looked up compressed air static after @jmelson's post, and it sounds like it can indeed be an issue but probably won't be too big of a deal if the board is properly grounded etc.I'm starting to think that there is a fair bit of conflicting information / black magic when it comes to PCB best practices :p

author: Gary350z
author_rank: Regular Contributor
timestamp: July 31, 2019, 11:03:47 am
content: Here's what I've done in the past.After any PCB cleaning that involves water, dip or spray the PCB with isopropyl alcohol, then air dry or blast with compressed air. The alcohol absorbs the water and then dries quickly leaving the PCB dry and clean.This seems to work good. Let me know what you think, or of any problems with this method.

author: CoolNamesAllTaken
author_rank: Contributor
timestamp: July 31, 2019, 06:20:51 pm
content: Thanks Gary! I'll try that. I may also give the boards a quick rinse with isopropyl before I put them into the ultrasonic cleaner to loosen up some of the flux. I've noticed that the 4 minute ultrasonic bath doesn't quite take care of the stuff that's dried onto the board. I know that IPA in an ultrasonic cleaner is a fire risk, but the few leftover droplets of alcohol on the board shouldn't be an issue when diluted into 2.2L of soapy cleaning solution.

author: jmelson
author_rank: Super Contributor
timestamp: July 31, 2019, 07:17:03 pm
content: Quote from: james_s on July 30, 2019, 07:15:29 pmFirst, if the board is wet, this is less likely to be a problem. Compressed air, by itself will not generate charge. it needs to have microscopic particles entrained in the air stream, these CAN charge up.Jon

author: wraper
author_rank: Unknown
timestamp: July 31, 2019, 07:38:32 pm
content: Quote from: Gary350z on July 31, 2019, 11:03:47 amSpraying and drying is exercise with zero result. Any contamination which was there, will still remain there. And if you blast with compressed air afterwards, it makes just as little sense. You could just blast with air without any IPA. And you also contaminate/breath air with IPA vapors.

author: CoolNamesAllTaken
author_rank: Contributor
timestamp: July 31, 2019, 08:04:41 pm
content: Quote from: wraper on July 31, 2019, 07:38:32 pmI will say that the scrubbing with IPA strategy + spraying off with compressed air makes sense. The IPA dissolves the fluxes and other contaminants, and then spraying it off removes it. If I let the alcohol dry onto the board after cleaning, I have found that the flux paste ends up spread across the entire surface as a thin and sticky film.

author: wraper
author_rank: Unknown
timestamp: July 31, 2019, 08:14:14 pm
content: Quote from: CoolNamesAllTaken on July 31, 2019, 08:04:41 pmIf your cleaning process involves water (done properly), there is nothing left to do for IPA. Everything that should be dissolved is already dissolved. If there is something what cannot be dissolved with water solution, you should clean it with organic solvent before placing PCB into water solution. Simply because that stuff will cover contamination which should be dissolved in water, thus preventing proper cleaning.

author: Gary350z
author_rank: Regular Contributor
timestamp: August 01, 2019, 12:29:48 am
content: Quote from: wraper on July 31, 2019, 08:14:14 pmThe purpose of the IPA after cleaning with water is to remove the water before it can cause corrosion, not to do any additional cleaning. The IPA absorbs the water and then the IPA evaporates very quickly leaving the PCB dry of water. The PCB is completely dry within a few seconds of spraying it with air. It seems to work good for me.

author: KL27x
author_rank: Super Contributor
timestamp: August 01, 2019, 12:56:05 am
content: ^Every chem lab does this to dry out beakers after washing them. I guess it's not so common sense.Only thing I would suggest to your algorithm is you blow the compressed air before rinsing in IPA, so's your IPA doesn't have to absorb so much water. Then blow it, again, after.

author: Tegra
author_rank: Newbie
timestamp: September 05, 2019, 05:40:35 am
content: I have found that even after a vigorous shake, there is still water under some components that can linger for days. This is obviously not good, as even distilled water is surprisingly conductive. Thus tests can fail and hidden corrosion, especially in vias may be taking place. I am of the same mind as others that alcohol does not automagically displace this water and can actually cause its own problems if left in these crevices.I now do a 2 minute clean in EC, shake off any excess and then “bake” the cleaned boards at about 90 degrees C for 30 minutes. I then assemble whatever parts that could not be washed and do a final testing. As I can easily place 2 boards in the oven so I am able to to a 15 minute cycle of wash/bake/assemble/test/pack. This keeps me motivated to meet my quarter hour cycle deadline so that ten boards can be done in about 2 1/2 hours.Tom

author: SteveyG
author_rank: Unknown
timestamp: September 06, 2019, 07:52:14 am
content: A PCB drying rack and a fan does the job nicely. No need to get too extravagant.

author: CJay
author_rank: Super Contributor
timestamp: September 06, 2019, 10:32:55 am
content: There's advice kicking around this forum about HP/Agilent/Keysight's method of cleaning PCBs after assembly/repair, if it's good enough for them...