i'm soldering on a pcb.. but islands seem to leak current? [SOLVED] - Page 1
Forum section: electronics / beginners

author: htm
author_rank: Regular Contributor
timestamp: August 20, 2021, 07:26:05 pm
content: HiAfter testing the setup on a breadboard, i'm trying to solder some components on a pcb.But when testing the resulting pcb, i get false readings. After soldering the components on another location on the pcb i still had similar issues.I was really wondering what i'm doing wrong.I found that when reading the resistance of the Resistors while on the PCB they read very different from when taken off from the PCB (e.g. on pcb it reads 20K and when desoldered/removed it reads 47K)i've soldered using s39, and us a small tip which doesn't look like it's burning everything.I attached my PCB and shown in red some islands which are interconnected.(i haven't drawn all. but at least directly around the red marks, those islands aren't connected.)The resitance between the islands range from 0.5Mup to 10M. In very worst scenario, the resistance was as low as 200K?!Am i doing something wrong?

author: wraper
author_rank: Unknown
timestamp: August 20, 2021, 07:33:01 pm
content: Most likely you are using conductive flux or solder with conductive flux which is not suitable for electronics. Other possibility is that you are touching multimeter probes or pads with your fingers.

author: wraper
author_rank: Unknown
timestamp: August 20, 2021, 07:39:46 pm
content: I googled S39 and it's a water washable flux for plumbing. But you won't be able to wash it adequately without special equipment, such as ultrasonic bath. And it's better to not use it for electronics anyway. So just don't use it and get a proper flux.https://www.markssupply.ca/ASSETS/DOCUMENTS/ITEMS/EN/S-39_Data_Sheet.pdfEDIT: it contains zinc chloride, so even ultrasonic bath won't save electronics from it. Also it will destroy the tip on your soldering iron.

author: thinkfat
author_rank: Unknown
timestamp: August 20, 2021, 07:59:07 pm
content: It will also corrode your PCB eventually, if you don't wash it off meticulously.

author: wraper
author_rank: Unknown
timestamp: August 20, 2021, 08:04:40 pm
content: The issue with zinc chloride is that you won't be able to wash it from the PCB completely. No matter how meticulously you wash it, small amount chlorine ions will be stuck in pores of PCB material forever.

author: htm
author_rank: Regular Contributor
timestamp: August 20, 2021, 08:12:16 pm
content: Thanks everyone!i didn't touch the pads.Never realized that s39 is for plumbing only. Makes sense.. my dad had it with his gear for household repairs.i'll get proper flux.

author: MAS3
author_rank: Regular Contributor
timestamp: August 20, 2021, 09:49:19 pm
content: Most (me too) will use a solder with a flux resin core .It will do perfectly for your use, and saves you from messing around applying too much of some gooey flux and getting rid of the excesses afterwards.

author: htm
author_rank: Regular Contributor
timestamp: August 20, 2021, 10:21:22 pm
content: @MAS3Would you use a lead-free solder or a 60/40-lead solder ?Some say lead-free works as easy as 'with lead', and others say it's much harder to work with because of higher temperatures.

author: MAS3
author_rank: Regular Contributor
timestamp: August 20, 2021, 10:32:51 pm
content: I'm using lead solder, which is fine for a hobbyist.I can't agree with lead free solder being as easy to use as the leaded type.There's lots of different alloys used in lead free solder, with copper or even silver in it.But i didn't try every type there is, so i might have missed the perfect alternative.Temperature isn't that much of a problem to me, i can set it with my soldering irons, and the butane mobile one always is a bit too hot.

author: wraper
author_rank: Unknown
timestamp: August 21, 2021, 09:45:11 am
content: Quote from: htm on August 20, 2021, 10:21:22 pmIt's harder to work and way less forgiving. How much harder depends on particular type of solder and flux. Lead-free solder not only has higher melting temperature, it's also worse at wetting copper, does not flow as well.

author: dtodorov
author_rank: Contributor
timestamp: August 21, 2021, 10:00:46 am
content: I've had issues with ever so slightly conductive flux. What was even worse is that the flux seemed to be highly hygroscopic, varying it's conductivity. So immediately when soldering I took measurements was ok, but overnight it would change and leakages would appear. Really recommend resin-based fluxes with thorough clean-up.

author: htm
author_rank: Regular Contributor
timestamp: August 23, 2021, 07:47:51 am
content: -Update-I had an old resin core solder some where, so attempted it again.I first measured if the pads didn't conduct yet, which they didn't.But after soldering using the resin-core wire (not using s39 or other flux),again there was conductivity, both before and 'after rinsing and scrubbing the board with water and a toothbrush and drying overnight',picture 1picture 2picture 3i'm waiting for new solder to arrive, hoping it works out better.

author: Shock
author_rank: Super Contributor
timestamp: August 23, 2021, 09:35:18 am
content: Tap water is conductive as well, depends where you live as to how bad.Resin - Noyau Colophane means Rosin. Especially when heated it can become insoluble in water. But sometimes also goes hydroscopic and takes on water. In general water isn't good at displacing rosin flux as it is to rinse the actual flux remover off. Note that this is two distinct processes - clean then rinse.So after cleaning before the pcb dries you need to rinse (possibly even a few times), with 99.9 Isopropyl alcohol or deionized or deminerlized water. This washes oxides/contamination and flux residue off the pcb and won't (hopefully) add contaminants.Isopropyl can be used to clean and rinse rosin but it needs a bit of work. Many rosins used in electronics soldering wire are purposely non conductive so it's not typically a problem. But there is the chance your soldering wires rosin flux is problematic, so changing it or using a higher performing flux remover or saponifier may be necessary.Anyway a process of elimination.

author: MAS3
author_rank: Regular Contributor
timestamp: August 23, 2021, 09:25:38 pm
content: That solder you found, is a bit large in diameter: 1.5 or 2 mm.But also it is very old.Chemische fabriek Schiedam, i doubt they are still around, the phone number shows it's pre 1995 but it wouldn't surprise me if it were 35+ years old.I can't assume the solder "went bad", but i wouldn't use it any more.You can buy the isopropyl from a drug store like DA or Kruidvat.

author: htm
author_rank: Regular Contributor
timestamp: August 26, 2021, 02:16:01 pm
content: While testing the new solder, i found out that this connector was partly the culprit. Probably it was dirty of the s39..i've replaced it, and now the pcb is working as intended.(soaking the connector in alcohol overnight was enough to clean it)