Using solid solder wire without a flux core - Page 1
Forum section: electronics / beginners

author: JoeN
author_rank: Frequent Contributor
timestamp: November 22, 2016, 02:38:32 am
content: When I do SMD soldering usually I am using a chisel tip and pre-flux my pins and pads. I drag solder the pins. When I put solder on the chisel tip the flux in the solder flashes away almost immediately and it fouls the tip, which is normal. I was thinking, for this kind of soldering, wouldn't a solid solder wire be preferable? The tip will get dirty slower, all of the solder will actually be solder, and flux will not be wasted at the same rate, and fewer flux fumes as well. Anyone else do this? I've noticed at the hobby store there is no solid wire, but it is easy enough to mail order. Am I right on the benefits? I don't see any downside except another roll of solder to have around. Obviously, I would keep the normal flux core solder around for through-hole soldering.http://www.all-spec.com/Catalog/Soldering-Rework/Solder/Solder-Wire/14-6337-0031-16600

author: Zero999
author_rank: Super Contributor
timestamp: November 22, 2016, 08:51:27 am
content: I use flux cored solder and never pre-flux the pins and pads because they're already pre-tinned or gold plated which wets very easily.Another possibility is your iron is too hot, causing the flux to boil off too quickly, which would explain why you need to put flux on the pads.

author: JoeN
author_rank: Frequent Contributor
timestamp: November 22, 2016, 09:10:57 am
content: Quote from: Hero999 on November 22, 2016, 08:51:27 amYou are the first guy to tell me that too much flux is actually too much flux. Everyone else always says that too much flux is either the right amount or not quite enough.I don't know if you use a big chisel tip, but one thing that I do when soldering SMD discretes is to get a bunch of solder on the tip and place discretes (caps, resistors, transistors, diodes) with a tweezer and then apply the solder with the tip. Otherwise, to hold the part in place and apply fresh solder from a roll and solder it on requires three hands and I am one short. So I do the "bring the solder to the part" which is a terrible idea for through-hole parts but it works great for SMD parts if the pads are fluxed. When you have solder on the tip for over a minute to tack in several SMD parts, keeping the iron on a setting low enough for the solder not to burn off is not a possibility.

author: Ian.M
author_rank: Super Contributor
timestamp: November 22, 2016, 09:56:56 am
content: If you are carrying solder around on the bit, you need flux at the joint. However excess flux is a PITA to clean up and doesn't help with the actual soldering. Therefore the sane way to do it is with a smear of gel or wipe of liquid flux on the pad which provides just enough flux at the critical point where the solder is meeting 'virgin' copper or tin or gold plate. Even though fresh tin or gold plate will wet fairly well without flux, I don't want unwetted pockets of oxides in the joint so would never try to solder without flux to aid solder flow. Aged ENIG and electroless tin are both notorious for poor or erratic solderability, so unless your boards are new, failure to use flux to give yourself the best possible chance of a good joint is just plumb crazy.

author: Zero999
author_rank: Super Contributor
timestamp: November 22, 2016, 08:00:41 pm
content: There's still no need for separate flux, unless the board is really badly oxidised. If it's bare copper or tinned copper (gold doesn't oxidise) and looks lightly dull, then just re-tin the board before soldering.

author: IanB
author_rank: Super Contributor
timestamp: November 22, 2016, 08:11:27 pm
content: Quote from: JoeN on November 22, 2016, 09:10:57 amI have observed the recommended approach is to tack the part down first. Since the leads of the part are pre-tinned, if you hold the part in place and press a pin down onto a fluxed pad with the tip of the iron it will stick to the pad. Do this to with pins in opposite corners and the part is now held in place and won't move. To be extra secure you could add atinybit of solder to the pad before tacking the part down.Once the part is tacked in place you now have both hands free to solder all the pins properly, one hand to hold the iron and one to hold the solder. Start soldering from the opposite corners to the ones you tacked down so you don't unstick them when you first apply heat.

author: Ian.M
author_rank: Super Contributor
timestamp: November 22, 2016, 08:20:51 pm
content: Quote from: Hero999 on November 22, 2016, 08:00:41 pmGold doesn't oxidise but its such a thin layer that the underlying Nickel can. The Gold fully dissolves in the solder, which then has to wet and bond to the Nickel layer.Seehttp://www.pcbdesignschool.com/2012/11/25/when-enig-doesnt-solder/