Flux remover recommendation (Rohde & Schwartz HMP 2020 PSU clean-up) - Page 1
Forum section: electronics / repair

author: anotherlin
author_rank: Frequent Contributor
timestamp: July 18, 2022, 01:48:18 pm
content: Hi everyone,I have a Rohde & Schwartz HMP2020 PSU that works just fine. It's my go-to bench supply that I always use, never failed me.I've opened it for cleaning and out of curiosity. In fact, I can post some more photos if you guys are interested.Some of the soldering are a little bit ugly (see photos), and I think I can reflow them to make them more tidy.Actually, I've bought that unit from that forum, from a guy in England (toploser if I remember), with a good discount.I think it had some rework done on it, mainly replacing the power elements, hence the discount, otherwise everything else is from factory.It's working perfectly, at a very good price, so I'm very happy, and it's just "cosmetic" rework.When done reflowing, what would guys recommend as flux remover?Currently, I'm using KF branded flux remover (https://fr.farnell.com/kf/1019/nettoyant-flux-de-soudure/dp/3916613), that I actually bought from Conrad and not Farnell.It does work, the flux is dissolved but when the cleaning fluid dries/evaporate there is some very thin "spread" residue.So I would like to try something else this time. I'm considering the following options:* Mg chemicals 4140A* Electrolube FLU400DBI've already bought some isopropyl alcohol made by MG chemicals and I've very happy. Hence I'll be glad to try their other products.The Electrolube brand just seems as much reputable, if not more than KF, and is actually a bit cheaper (https://fr.farnell.com/electrolube/flu400db/aerosol-fluxclene-400ml/dp/725663).Also, there's the chemtronics brand that has a flux removal offfering.Do you have any good recommendation? What's your favorite? What's your secret trick for a perfectly clean PCB ?Best regards,

author: jpanhalt
author_rank: Super Contributor
timestamp: July 18, 2022, 02:52:05 pm
content: It seems you are a charter member of the keep fixing it until it's broken club. You are happy with your flux remover. Why not use it?There are dozens of threads here on flux removers. Have you looked at them?

author: anotherlin
author_rank: Frequent Contributor
timestamp: July 19, 2022, 07:33:11 pm
content: Quote from: jpanhalt on July 18, 2022, 02:52:05 pmLOL, you killed me with this oneMore seriously, come'on, it's a german engineered and made lab PSU.Look at the 2 photos, the soldering are really ugly.It would certainly deserve some attentionQuoteWell, not so happy with my current flux remover, it doesn't completely evaporate.Perhaps the kind of factory PCB cleanliness I'm looking for, can only be achieved using ultrasonic cleaning bath.But you're definitely right, I should have done my homework and use the search function to look at previous threads.I'll definitely have look.

author: Audiorepair
author_rank: Frequent Contributor
timestamp: July 19, 2022, 07:44:46 pm
content: I think this was the point made.Quote from: anotherlin on July 18, 2022, 01:48:18 pmEdit: So now you could have a piece of equipment that was working perfectly and didn't need any soldering, and now you have a piece of equipment with a lifted pad.But all pads now look great so you won't easily find it.

author: wraper
author_rank: Unknown
timestamp: July 19, 2022, 07:51:16 pm
content: Soldering is completely OK. By resoldering you won't make it any better or more reliable. Exactly opposite, you will introduce additional stress to component and PCB. So the only thing you can achieve by this is satisfying your OCD.

author: wraper
author_rank: Unknown
timestamp: July 19, 2022, 08:00:02 pm
content: Quote from: anotherlin on July 19, 2022, 07:33:11 pmLOL, flux remover does evaporate completely as long as it's solvent type. What does not evaporate is flux. You cannot simply dissolve the flux in solvent (what flux removers do) and expect it magically disappear. You need to remove the dissolved flux, for example by wiping with paper towel or special wipes.

author: jpanhalt
author_rank: Super Contributor
timestamp: July 19, 2022, 08:21:28 pm
content: OK, let's repeat the old story. I used to use absolute ethanol with a little (up to 25%) acetone. That removed most rosin fluxes very well. I used almost exclusively Kester #44. To clean a whole board (say 2" x3") was a little tedious compared to just doing a small board or repair. It wasn't that the flux wasn't removed so much as I wanted an uniform shininess with no dull spots. White residue was sometimes a problem with manufactured boards.After reading lots of recipes from Kester, I settled on a cheap duplicate, namely a cellosolve + a saponifier as recommended by Kester. (https://www.kester.com/Portals/0/Documents/FAQs/White_Residue.pdf) A large part of my uni training was in chemistry, so I bought ethyl cellosolve, which was relatively volatile compared to others, and ethanol amine. They worked well. Then I discovered that ZEP, which makes lots of cleaning products, had a "heavy duty floor stripper" with basically the same formula I had made, except it used a less volatile cellosolve (butyl cellosolve). Light dawned on the city dump. My fixation with low boiling constituents was irrelevant since the solutions are aqueous and washed in water.For doing whole boards, I use ZEP Heavy Duty Floor Stripper diluted 1:3 with water (1 part ZEP + 2 parts DI water). Ultrasonic for 2 minutes, then rinse in water and ultrasonic an addition 2 minutes in DI water. Result is that solder resist is so hydrophobic, the boards come out virtually dry. A blast with compressed air takes care of any residual water.Anything on the board needs to be safe to soak. So, I assemble such things is switches last and spot clean with ethanol/acetone.

author: wraper
author_rank: Unknown
timestamp: July 20, 2022, 07:51:21 am
content: Quote from: jpanhalt on July 19, 2022, 08:21:28 pmI like this quote from the document:Quote

author: tooki
author_rank: Super Contributor
timestamp: July 20, 2022, 03:36:44 pm
content: Quote from: anotherlin on July 18, 2022, 01:48:18 pmThis kind of “spread stuff around” cleaning is why I don’t trust some people’s kitchens and bathrooms: solvents do not destroy* contaminants, they only dissolve them. You still have toremovethe contaminant-laden solvent yourself or you just redeposit the contaminants!You have to either rinse in copious amounts of solvent, or you need to mop up the dirty solvent and repeat until no traces are left.*Yes, I’m aware that some things are actually neutralized by cleaners, like using acid to neutralize alkaline battery electrolyte, or using soap to destroy viruses (by breaking the lipid layer bonds). But in most cases, there are still reaction products you want to remove.

author: jpanhalt
author_rank: Super Contributor
timestamp: July 20, 2022, 04:27:02 pm
content: Quote from: tooki on July 20, 2022, 03:36:44 pmFinally, we agree 100% on something.That is a strong reason I switched to water-based removers for whole boards. Hydrophobicity of solder resist gets rid of most of the water, and water is cheaper than any organic solvent cleaner.

author: wraper
author_rank: Unknown
timestamp: July 20, 2022, 04:53:53 pm
content: Quote from: tooki on July 20, 2022, 03:36:44 pmYeah, for some reason quite many people on this forum (and overall) believe in what would make this ad from an old comedy true.

author: mindcrime
author_rank: Unknown
timestamp: July 20, 2022, 05:15:37 pm
content: Quote from: anotherlin on July 18, 2022, 01:48:18 pmThen why in the blue hell would you eventhinkabout changing anything inside this unit, vis-a-vis "cosmetic soldering". The odds that your "repair" will break something are about 1000000000000x higher than the odds that it will improve anything in any measurable or meaningful way.My advice would be to put it back together, and use Loctite Red on the screws, then pot them with epoxy from the outside, to reduce any temptation to open it up again. But that's just me.

author: David Hess
author_rank: Super Contributor
timestamp: July 20, 2022, 11:32:11 pm
content: For rework I use cotton swabs and lacquer thinner to remove flux.

author: anotherlin
author_rank: Frequent Contributor
timestamp: July 22, 2022, 10:24:08 pm
content: Ok, I've done my homework and looked around! It was worth it, I've learned a few things.To be honest, I actually never really paid attention to cleaning until recently. I've always been using "no-clean" solder and considered the results acceptable. Having a bit more experience in electronics, I now try to do things right when possible.Manufacturer's website usually have tutorials, white papers, etc. Chemtronics is particulary good in that regard, they have tons of resources about cleaning. This one is really good, I think it sums up just what I need to know:https://www.chemtronics.com/ultimate-guide-to-cleaning-electronicsAnd they have 2 short youtube videos which are really excellent:https://youtu.be/ZtRBv6aKCKchttps://youtu.be/swls2yukYjsThe main take aways are:"No-clean" flux should be ok without cleaning, unless you have specific requirements (avionics, medical, etc.), other flux (rosin) must be cleaned;You have to scrub, mop, then rinse with copious amount of flux remover/solvent and/or do some more handiwork to remove the dissolved flux residue.So in my case, my KF 1019 flux remover is probably just about as good as any spray can flux remover. My mistake is not rinsing enough and not doing the scrub/mop the residue enough.Actually the KF has such a strong chemical smell (dizying) that I thought it could really somehow breakup the flux particules and bond them with the solvent, so they evaporate together. I thought that professional product, not what you buy at the supermarket to clean your house, where capable of such feat. Silly me! That's just not physically possible. And I've been to university!You guys were just right at pointing me that there's some work involved. And it's not like TV commercial when you spray a magic product and your bath or kitchen is clean!There's actually quite a lot of good videos on YouTube on that subject, one that is quite good:https://youtu.be/FwjOrT3tIjwThe guy seems to work in the field and he has a lot of other videos on soldering.Last but not least, there's the search function of the forum, here are few good topics:https://www.eevblog.com/forum/projects/flux-removal-ultrasonic-bath/https://www.eevblog.com/forum/beginners/alcohol-when-cleaning-fluxhttps://www.eevblog.com/forum/repair/simple-method-for-removal-of-stubborn-flux-residueAnd indeed, seems that some of you guys (jpanhalt?) have been mixing their own cleaning solution!I will just stick for an off the shelf product.This week-end will be a good opportunity for some practice.

author: anotherlin
author_rank: Frequent Contributor
timestamp: July 22, 2022, 10:38:12 pm
content: Quote from: Audiorepair on July 19, 2022, 07:44:46 pmYou have to be really careless in your reflow (or desoldering) to manage to lift a pad, especially with a "modern" PCB.I don't understand why you guys are against reflowing/resoldering the power electronics on the photos.If they were from an alibaba or other cheap chinese equipment, people would scream at poor workmanship and lack of attention to details.I doubt the soldering on the photos would pass the visual QC check at any reputable manufacturer (let's says R&S, Keysight, etc.).Even if from a functional point of view, these are acceptable.Also, this just is bench PSU: a classic linear supply with a pre-regulator (off the shelf LTC3703 buck converter). This is about the simplest equipment of an electronics lab, both in theory of operation and construction. Nothing to be scared about breaking.

author: jpanhalt
author_rank: Super Contributor
timestamp: July 22, 2022, 11:12:34 pm
content: Here's what I believe to be the SDS for KF1019 by CRC:https://www.farnell.com/datasheets/3198766.pdfIf you have anything different, please let us know. Nothing new. Go for it. Obviously, you have a mindset that is not going to change.Have fun. Good day.

author: abdulbadii
author_rank: Frequent Contributor
timestamp: July 22, 2022, 11:33:02 pm
content: Imho any weak/mild alkaline, ie. base to neutralize, counteract the flux weak/mild aciditycommonly a slight soluble hydroxides- Baking soda NaHCO3 Sodium bicarbonate- Ammonium Hydroxide (NH4OH)- Ammonia (NH3)- Pyridine (C5H5N)with IPA fiinishing touch

author: wraper
author_rank: Unknown
timestamp: July 23, 2022, 08:34:03 am
content: Quote from: abdulbadii on July 22, 2022, 11:33:02 pmWhat? After using those you will need through washing with water if you don't want corrosion. Not that they are safe to apply to PCB to begin with. Nor maybe except Pyridine they will remove rosin based flux.

author: tooki
author_rank: Super Contributor
timestamp: July 23, 2022, 09:58:42 am
content: Quote from: anotherlin on July 22, 2022, 10:24:08 pmUmmm… not exactly.Yes, as multiple people here had already told you, you must actually remove the residue-laden solvent so it won’t redeposit. Rinsing is one way, mopping up the other. I used to rely more on rinsing, but have been switching more and more to mopping up.As for what fluxes need to be cleaned:1. No-clean flux can be left on ONLY if it has been FULLY heated. With the flux core in solder wire, or with a board soldered in a reflow oven, this is guaranteed. But if you are adding external flux (liquid, gel, or paste) for hand soldering, the flux that isn’t immediately within a joint will be heated enough to activate it, but not enough to neutralize it again. So when using external flux for hand soldering, cleaning is mandatory. Many no-clean fluxes are essentially rosin fluxes with low rosin content, others use various other compounds.2. Many rosin fluxes are safe to leave on. The datasheet for the classic Kester 44 flux core, for example, expressly says the residues are safe, using the same wording as datasheets for their no-clean flux cores.3. Water-soluble (organic) fluxes must be cleaned PERFECTLY, because even tiny residual amounts of residue are corrosive over time. Because of this, you absolutely must not use these fluxes on stranded wire or other joints where flux can wick into places where it cannot be cleaned.A note on external no-clean and rosin fluxes: the liquid forms of many of these are corrosive while wet, but safe once dry. So if you apply a small amount that can dry within a few minutes, no problem. But larger droplets that take hours or even days to truly dry out (or when seeped into nooks and crannies or into stranded wire) can cause corrosion. I did some testing with various fluxes and observed this with many. The same applies to paste/gel fluxes, but since they dry very slowly, you’re practically always going to have corrosion with many fluxes if left on.Note that in recent years, “no-clean water-soluble” fluxes have been put on the market. From the sound of it, they’re rosin-based fluxes that have somehow been modified to make them water soluble. What I don’t know is how well they hold up with time. Somehow, it feels to me that they’d almost have to be hygroscopic, and that in turn could make them reactive over time. But that’s purely gut feeling, and I have been meaning to give them a shot. Being able to wash a board in simple hot water would make things easier for sure!