How To Control Flux When Hand Soldering?

Phil
Welcome to Board Talk. This is Phil Zarrow and Jim Hall of ITM Consulting. Today we have a hand soldering problem.

Jim
It comes from P.D. Do you know any tools or materials that will allow for precision application of no-clean flux for hand soldering operations? We are looking for something that has more precision than the tradition flux pens and don't want to use flux bottles if we cannot control the amount of flux applied. Any information assistance will be greatly appreciated.

Well, first off, I'd like to say bravo and kudos to you P.D. for realizing that you should not be using flux bottles.

Even flux pens are under question. It is really important to control the amount of flux, particularly no-clean flux, that we apply for hand soldering. The principle reason is that if we're not going to clean off the flux we've got to make sure that any of the residues left after the hand soldering operation are safe.

All no-clean fluxes work on the same principle. They start off with active chemistry that is deactivated by the thermal heating cycle regardless of whether it's reflow, wave or hand soldering.

And of course with hand soldering and liquid flux it's a huge problem. Too much flux moving away from the joint, not being properly heated. If it's not cleaned off the board, you now have no-clean flux materials that are still active because they have not been heated sufficiently to deactivate them.

So, the traditional solution is to not use any additional flux, either a bottle or a pen, but to use cored solder wire. The basic mechanism is that flux only comes out when you heat the wire, therefore all of the flux will be reasonably well heated and therefore deactivated properly so that the residue you leave is safe.

You can get solder wire in different diameters, and you can also get different concentrations and amounts of flux within the wire. That is the traditional book technique that is used, but we thought of some other ones, haven't we, Phil?

Phil
Yeah, one other possibility would be to use a very small syringe of solder paste. First of all, you're going be using the same formulation, or potentially the same formulation you're using for your reflow soldering, mass soldering methodology.

And second, you know the ratio's going to be right. Typically doing this using say a 50 gram syringe or larger, but 50 grams is easy to use at the hand soldering locations, should probably accomplish that. 

Jim
Remember, because the flux is mixed with the solder it will all be heated when you melt the paste. You don't have the issues with liquid flux or the stuff from flux pens where liquid flux can move away from the joint and not be heated properly.

Phil
Very good. Well, hopefully that answers the question of P.D.

Jim
Remember, whatever you do, don't solder like my brother.

Phil
Don't solder like my brother, and keep the kids away from the flux bottles.

I find the flux paste in the syringe a perfect solution, but you need to make sure your dispensing tip is very small so as not to dispense excess amounts.

I would emphasize the point that not using additional flux is preferred. I would add that using a flux specifically engineered for rework, not what is used on the wave/selective is highly recommended.

If you are able to use a no-clean paste flux this can be applied using a very small pointed probe. You can get pinpoint accuracy using this method and only apply flux to the lead and the pad.

I will often use cotton tipped applicators. These help place minimal amounts of flux and apply it accurately.

I am curious as to why not use flux bottles. Do you mean using a bottle to dispense it or to not use the flux bottles that you press down on to get the flux? I use the kind where I press down on and get a small amount of flux on the cotton applicator. Am I doing something that I shouldn't be?

I keep the bottle clean, so I don't think I should be worrying about contamination, but should I?

I have found a tool for the hand soldering flux problem that you two talk about in this issue. That is the (BONKOTE) BON-102 brush pen only puts flux where you apply it. Great tool, check it out!

Another tool to consider is gel fluxes. For example, Nordson has a line of paste flux dispensed from a syringe: http://www.nordson.com/en/divisions/efd/products/solder-paste/fluxplus-paste-flux.

I prefer using solder with water soluble flux inside. It can easily be rinsed off if on a large area and for smaller spaces, a moistened paper towel on the tip of a tweezer works wonderfully.

No-clean flux for hand soldering is a misnomer and it requires cleaning if used. Best avoid, the run away flux can become active at higher operating temperature - > 40 deg C and cause surface insulation resistance or electrochemical migration failures.

If you have solid wire solder, another method is to dip the wire into the flux you want to use and then solder the component. This gives you a controlled amount of flux while without getting the flux all over the board.

I work in an engineering lab and have faced the same problem. Thanks for the "very small syringe of solder paste" idea. It sounds like a very workable solution.