Flux "Testing" and Soldering Methods

Recently I watched in amazement a YouTube video of someone testing a large collection of soldering fluxes. I applaud the poster for testing the evangelical claims for certain brands of flux, but please use a technique that is meaningful. 

The first failure is trying to drag solder with what is clearly stale solder on the tip. See how the outside of the iron has slag on it? It wasn't cleaned before the test. And the hollowed tip exacerbates the problem because it holds a pool of stale solder. The exact opposite of what you want and the only tips I have like that are in my workshop bin. No wonder the resulting solder joints were so poor. 

The technique is also dubious. I never put a pool of solder on the tip to drag solder because the flux burns off. As an example of how far you can take this fresh solder approach you can, after tacking the part down, hold your fine solder wire so that it lies alongside the row of pins and pads. Then push the tip of the iron along the row of pins which melts the solder at the same time as the tip wipes across the pins. The result is fresh flux already in the solder doing it's job.

But yes, flux makes it easier and neater when drag soldering. Just not the way this was done. Stop and consider how much flux you need. Solder already contains flux, say 3%. Slapping on a teaspoon of flux is just ludicrous. My normal technique when drag soldering is to use a small paintbrush, trimmed to make it stiffer, and flux sitting in a suitable container. See below where the flux at the bottom of the container came from one of those cheap Chinese flux syringes. I am also trying a eyelash brush normally used for makeup because these are really cheap in a pack of 50. But even the most careful application of flux will still be more flux than is needed when drag soldering.

Sometimes I use a modified technique with a thicker solder wire. Stick the wire up into the flux and then as you push the tip along the pins pull the solder wire along the row of pins in front of the pool of solder forming on the tip. It takes a bit of practice but it avoids having to apply flux to the board first.

Any flux I have used was intended for electronics use and worked fine. Even rosin dissolved in metho works but I don't like how everything feels tacky afterwards because I'm clumsy. If you're slapping on gallons of flux like the video shows then yes you will find at times the fumes troublesome. But get your technique right and fumes become much less of an issue, if at all.

Ok, lets move on to soldering some surface mount parts. I was testing an Aldi butane soldering iron as a hot air gun and it worked well. I have three different butane hot air guns now and they have the advantage over the soldering station hot air gun in that there is no air flow. Something to keep in mid if your surface mount parts are blowing off the board at the slowest air setting!

I use a reflow technique based on wire solder and flux, not solder paste.

Step 1 - melt some solder onto the pads

Step 2 - brush some flux on

Step 3 - position parts - close enough!

Step 4 - Apply heat. I usually sit the board on my hot plate and use my soldering station hot air gun. But today it was just the butane powered hot air "pencil". The large inductors were too difficult so I ended up doing those with a soldering iron afterwards. on the hot plate they would have been easy to solder.

Step 5 - Touch up. I was rushing this because the whole purpose was to test the hot air pencil I brought so I didn't take as much care as usual. Not enough solder on a couple of joints but easily fixed with the iron.

Before Touch-up.

So nothing very challenging with this board but the hot air Aldi special worked very well. I'll buy another one this weekend. It works better than my Iroda butane powered soldering iron in this application and it doesn't blow parts around.

 

I repeated this with a USB charging connector which are difficult to solder with an iron though it can be done. Missed the ground pad on the connector but I show this because you can see what happens with too little solder. No pre-tinning of connector and be careful if you try to clean the pins (I don't) - once they bend you create problems getting them flat on the pads when soldering.

 

Let's do it again with more solder on the pads this time:

Final result after a quick clean with metho and cotton wool ear bud.

So there you have it - flux is your friend but you need far less than the you tubers use. And be critical when watching video's because all you're watching is opinion and ego. It's rarely the right way to do things. As someone who has used a soldering iron for over 50 years now I hope these couple of idea's get you thinking.

Footnote:

If you're buying flux I found that Temu is a good source. It's cheaper than other online sources though a bit annoying at times. Something like the following is what I use since I don't need another container to squirt the flux from a syringe into.

And if you do click the picture I might get some sort of referral bonus. While I have my doubts I'll get anything, if I do then I will stop the ads on this blog.