I made a PCB to scale a +-100V signal into a 0v to 3.3V signal that I could read using an ADC on a microcontroller(see link for idea.) I used this solder paste flux to help with the soldering.

After soldering 16 of these, a bunch resistors were shorted. I got some tips from a friend that is good at soldering (use a microscope and go slowly) and got them all working.

Now though every few days a voltage divider will either get stuck at 3.3V or ground or somewhere in the middle. If I re-solder the resistors it'll work again. Even if I just reflow the solder on the resistors it will start working again.

  • Any idea why they keep breaking after a few days?
  • Any ideas how I can get them permanently working?

The resistors are all 0603 and 0805 footprint.

FYI: I used two 150k ohm resistors instead of one 300k resistor because I was worried about a 100V drop across just one.

Schematic of voltage divider

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    \$\begingroup\$ 1. You should be using solder with flux - you shouldn't have to add flux at all. Not even if you are using lead free solder. \$\endgroup\$ – JRE May 10 '20 at 22:24
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    \$\begingroup\$ @JRE nothing wrong with adding extra flux when it comes to SMD components, especially larger pin counts. You can't really get enough flux out of the solder when there pads are small and you're using a fine tip. It just has to be the right flux, preferably no-clean. \$\endgroup\$ – Tom Carpenter May 10 '20 at 22:39
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    \$\begingroup\$ Nothing good about flooding things in flux, either. Especially when it leads to situations like this where a novice hears "use lots of flux" and uses the wrong thing. Especially since no extra flux is needed. I solder 0201 sized capacitors by hand, using only the flux in the solder. \$\endgroup\$ – JRE May 10 '20 at 22:42
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    \$\begingroup\$ As well as being corrosive your acidic (Zinc chloride) flux is ionic so it will conduct electricity fairly well. \$\endgroup\$ – Spehro Pefhany May 11 '20 at 0:57
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    \$\begingroup\$ @JRE It's as wrong to say "you never need extra flux" as it is wrong to say "always add lots of flux". They're both extreme oversimplifications and they're both incorrect some of the time. I mean, sure, maybe there are cases where you don't need extra flux, but it can sometimes sure make a job faster, easier, and better. I think it really depends on the type of work you're doing - if you're not doing a lot of repairs or rework then you've probably not had need of flux like someone might who is doing more of that type of work. \$\endgroup\$ – J... May 11 '20 at 15:21

This seems to be plumbing flux, which often is higher in corrosive components than electronics fluxes. Considering solder wire comes with a rosin core, you'll probably be better off without that flux, as such fluxes have the tendency to corrode electronic contacts over time.

I've never head of that happening within weeks, however.

So, my hypothesis is:

  1. you got a "cold" solder join, meaning that there's no wetting of the contacts on both sides. (you can google image search for "cold solder joint")
  2. You have a large capacitance on the OutCh1 side that you don't show. After having charged that to a specific voltage, it takes long to discharge it (depends on how shortly after changing InCH1 you look)
  3. Whatever you have on the output actually latches up.
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    \$\begingroup\$ Seems likely it is the plumbing flux. Just in case, for the large capacitance idea the OutCh1 feeds into a mux. After the mux it goes through an op-amp buffer and then to the ADC pin of a microcontroller. I'm not very familiar how capacitance builds up. Does that sound like large capacitance could be an issue? \$\endgroup\$ – Ryan W May 10 '20 at 22:46

As others have pointed out, your issue will be one of using the wrong type of flux. Plumbing flux is highly corrosive and will damage your board and components. It should not be used for soldering.

Below is an example of why you should not use plumbing flux. The board was a DIY PCB without a solder mask that was tin plated. Because of the lack of solder mask I required a large amount of flux to solder the board to keep shorts from forming. This was before I knew any better about fluxes, so I used the plumbing flux I had to hand.

The picture shows the board after about a month. It had corroded to the point that several areas had actually been stripped entirely of the tin plating, and the terminals of the resistors and capacitors have corroded. Fortunately it was a temporary test board that lasted long enough to serve its purpose.

Corroded PCB

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    \$\begingroup\$ Yikes! I want this board to last a long time. I will try to clean off the plumbing flux \$\endgroup\$ – Ryan W May 10 '20 at 22:48
  • \$\begingroup\$ This is my "photo of the day"! Perhaps it should be included as part of the usage instructions on the plumber's flux, indicating how it can be used for building temporary boards with "accelerated decrepitude" :-) \$\endgroup\$ – SusanW May 13 '20 at 20:07

Your flux is probably eating away everything.

Reading on the page you linked for the flux properties:

  • cleans and fluxes copper piping
  • Compatible with all common plumbing solder alloys
  • Good for large diameter copper piping

This is a plumber's flux. Reading the Wikipedia page on flux, we find that…

Plumbing and automotive applications, among others, typically use an acid-based (hydrochloric acid) flux which provides rather aggressive cleaning of the joint. These fluxes cannot be used in electronics because their residues are conductive leading to unintended electrical connections, and because they will eventually dissolve small diameter wires.


Ryan, please note the following:

  1. It's clear from the picture that you have used an acidic flux.
  2. I am sorry to say that people think soldering is an easy job. Just fix, solder and a soldering iron! SOLDERING IS AN ART!
  3. Even by chance when you use acid base flux (not at all recommended) a thorough cleaning is required using IPA.
  4. Even with this cleaning after a few months corrosion will take place.
  5. Other points to be noted are- the 0603 and 0805 packages do not get soldered properly when done manually. Only in the edges it contacts the baseboard.
  6. With this type of contact added corrosive flux every time you touch up each point the soldering joint is disturbed- either a dry joint or high resistance joint.
  7. There may be no problem with your design at all.
  8. With all the above the now no. of times you touch up the pad/ track also starts peaking off. You are into more complex situation.
  9. Looking at the board condition which is pretty bad, you have no other option than mounting all components on a fresh bare board with fresh set of components. If you more tech info on professional soldering you are free to contact me. I have little over 40 years experience in the area of manufactiring PCB assemblies and equipment.
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    \$\begingroup\$ Soldering is a skill, not an art. It takes practice to do it, but it is in no way a mysterious "art." \$\endgroup\$ – JRE May 11 '20 at 15:07
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    \$\begingroup\$ 0805, 0603 (and 0402 and 0201) can be soldered just fine by hand with a standard soldering iron. \$\endgroup\$ – JRE May 11 '20 at 15:08
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    \$\begingroup\$ OP did not post a board. Are you basing this answer on what other people did? You might want to read up on how this website is structured. \$\endgroup\$ – pipe May 11 '20 at 23:28

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