Flux paste and conductivity - Page 1
Forum section: electronics / general

author: bodger
author_rank: Contributor
timestamp: August 17, 2018, 06:06:50 pm
content: So about a week ago I got around to soldering my first SMD IC on a SOP8 to DIP8 PCB, I used MG Chemicals Rosin Flux Paste #8342. The soldering itself was fine, in fact I was impressed at my 1st attempt. Unfortunately, the paste was conductive, even though I'd cleaned the board with IPA after soldering; I guess some of it remained under the IC (hence it didn't behave as expected).So my question is how common is it for flux paste to become conductive after it's heated? Also, it would be great if someone can recommend a flux paste or liquid that is non-conductive, preferably something that's easy to obtain in the UK.

author: thm_w
author_rank: Super Contributor
timestamp: August 17, 2018, 10:14:49 pm
content: If you like paste you can use 8341:https://www.mgchemicals.com/products/solder-and-accessories/fluxes/no-clean-flux-paste-8341But any no-clean flux will be fine, it depends on your preference. Some prefer the thin watery type or thicker pastes. 8341 is a lighter paste.8342 link:https://www.mgchemicals.com/products/solder-and-accessories/fluxes/8342-ra-rosin-flux-pasteInteresting that they say "Post soldering residues removal is much easier due to the organic acid base of the flux. Residue can easily be removed with Cat. #824 Isopropyl Alcohol or Cat. #4140 Flux Remover". Maybe they expect you do soak the board or something.

author: wraper
author_rank: Unknown
timestamp: August 17, 2018, 10:50:39 pm
content: Quote from: bodger on August 17, 2018, 06:06:50 pmNo, usually flux is somewhat conductive before heating. After heating most of them become non conductive. The issue with that is that hand soldering in not a proper heating which will sufficiently heat all of the flux like reflow. Not to say 8342 is quite aggressive flux, must be cleaned and is water soluble on top of all.

author: wraper
author_rank: Unknown
timestamp: August 17, 2018, 10:52:00 pm
content: Quote from: thm_w on August 17, 2018, 10:14:49 pmAccording to dataheet only warm DI water is needed:http://www.mouser.com/ds/2/265/tds-8342-1099484.pdf

author: bodger
author_rank: Contributor
timestamp: August 18, 2018, 08:45:37 am
content: Thanks for the replies, decided to go for some liquid no clean flux for soldering SMD parts in future and purchased some Topnik RF800 which comes with a little brush applicator.

author: electronics man
author_rank: Frequent Contributor
timestamp: August 29, 2018, 08:21:19 pm
content: Quote from: thm_w on August 17, 2018, 10:14:49 pmI have this flux it leaves a horrible sticky residue which is extremely hard to clean off the bored.

author: thm_w
author_rank: Super Contributor
timestamp: August 29, 2018, 09:10:33 pm
content: Quote from: electronics man on August 29, 2018, 08:21:19 pmWeird, haven't noticed that being a problem, temperature is higher maybe? Using isopropyl alcohol to clean it off, could try a flux cleaner solvent.

author: bodger
author_rank: Contributor
timestamp: August 30, 2018, 08:59:59 pm
content: Quote from: thm_w on August 29, 2018, 09:10:33 pmI've been cleaning the stuff with IPA 99.9% with a ESD safe brush, but it can still leave the board a little sticky. I really like this flux paste for through-hole components, I'm using it sparingly now on the board to avoid the sticky residue.

author: wraper
author_rank: Unknown
timestamp: August 30, 2018, 09:11:57 pm
content: Quote from: bodger on August 30, 2018, 08:59:59 pmIt's impossible to clean the board by just diluting flux with IPA. You can spread it over the board as much as you wish but it's still remains there, it cannot miraculously dissappear. You may not notice that you are spreading the flux instead of cleaning it if you use flux with low amount of solids like liquid flux but with tacky (gel) flux it becomes much more noticeable. If you use brush, you need something to actually remove the flux, say kimwipes. Starting from 1:00

author: tooki
author_rank: Super Contributor
timestamp: September 04, 2018, 12:35:47 pm
content: Quote from: electronics man on August 29, 2018, 08:21:19 pmI use it too and this hasn’t been an issue.