A simple op amp device which will tell how high is the resistance of your flux.
I did not invent it. Another person from a Russian forum made it and published the schematic and the results of his tests.
I will not provide all the results because most of the fluxes are local to Russia. You should check the flux you use.
The idea is really simple: make a simple op amp device and test resistance of soldering flux inactive and activated. The testing procedure is also simple: solder two output wires of the device to two parallel tracks on a stripe board, put a lot of flux between and onto the tracks, so the tracks are visually connected by the flux.
How it works
This is the original explanation from the author
“Make use of +5V source (it could as well be +12 or +15). Using R2 and R3 set the input voltage. Mine is +5V and I got 0,45V. You can take any other R2 and R3 and make any other input voltage. Op Amp (mine is LM358) is used as non-inverting amplifier which has the flux resistance and 1.5M resistor in its feedback. It could be another large resistor, not exactly 1.5M.
Take two parallel tracks with some distance between them. I had two tracks 2.5mm apart. BTW many flux manufacturers specify flux resistance for a certain distance between tracks. Then solder two wires to the tracks using the tested flux. Use a lot of flux, so it is everywhere. Solder the other ends of the wires to points flux1 and flux2. BTW it is notices that some fluxes have more or less resistance when wires are connected one way (flux1, flux2) and then vise versa (flux2, flux1).
The output of the amplifier is Uout = Uin*(1+Rflux/R1). The more the flux resistance between flux1 and flux2 the higher Uout. Maximum output with +5V source is around 3.8V. If you have 0.45V at the input then considering max output at 3.6V (just in case) we have max gain around 8. That means if Rflux 7 times more than R1 the output will be maxed (3.8V). So, if flux1 and flux2 are not connected you should see 3.8V. The less Rflux the less the output voltage (closer to the input).
Now you need the source and the voltmeter. First test the flux without cleaning it at all. This will tell you what will happen to your board if you don’t clean the flux at some places. Then clean the flux and see how things are.”
I do the test a little bit different. I solder the wires using rosin based flux, clean it very very thoroughly with some solvent and the put some fresh tested flux. See what it gives me on the output. Then I activate it with a soldering iron and check again what I have on the output. Many fluxes exhibit different resistance depending on whether it was activated or not.
I have made a PCB layout based on the schematic. Here is it in sprint-layout 5 format: flux-tester.lay
This is my tester in action.
Results of the author
Open circuit voltage is 3.82V
Tested fluxes are: ASAHI CF10 and the other local Russian fluxes (skipped).
"CF10 showed 3.82V. So, it has at leas 10M resistance”
I have tested the usual rosin flux and two more local Russian fluxes.
My schematic has 3.87V for open circuit.
Rosin flux showed 3.87V
One other flux showed 0.54V (that’s around 180K before cleaning and 3.87V after cleaning). The last one (gel flux TT) showed 1.05V (1.4M), after cleaning with hot water 3.45, after cleaning with solvent: 3.87V.
So, test you flux before you use it! Also, put it on a cleaned and polished piece of copper and leave for at least 20 days to see what it does to the copper. I have seen so many bad fluxes lately especially Russian and Chinese ones. So, test it before using it or you may find you devices nonworking in several months.
I look for a good substitute for rosin. Frakly, I hate rosin! Don’t get me wrong, rosin is very good electrically (rarely creates problems if ever at all). It smell good. But, it’s fumes are carcinogenic and it is cleaned well only with ethyl alcohol or strong and smelly solvents. Further rosin can burn and spoil the good look of a pcb and it is hard to pick from narrow places. So, I’d like to have soldering flux with the following features:
- Does not corrode copper at any time
- Easily cleaned
- Makes very good wetting
- Does not flow everywhere (preferably gel)
- No halides!
- Nice or neural smell
- Not very expensive
- Can be stored for at least one year
- Huge resistance in inactive and active form
I have some fluxes that seem to satisfy my feature list. I have ordered a 30cc bottle of Interflux IF 8300 and will test it soon.