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I want to solder a circuit that has a few tiny 0.1uF SMD capacitors.

Is there any way to test each one during soldering so to be sure that it is soldered ok?

My multimeter does not do capacitance testing.

Here is a pic of a resistor plus LED I soldered. My handmade PCB and my inexperience in smd soldering makes anything a lot less tidy and easy.

enter image description here

EDIT: for me the easiest way is to use a sawtooth or pulse oscillator and an oscilloscope probe.

enter image description here

enter image description here

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    \$\begingroup\$ visual inspection is usually good enough. \$\endgroup\$
    – Andy aka
    Dec 7, 2022 at 11:12
  • \$\begingroup\$ I suspect that this might be some home made PCB? In which case the main problem might be that the vias aren't plated through. It would be easier and give better result to just buy some 0.1'' experiment board with plated vias and then mount on top of that, with wires as traces. \$\endgroup\$
    – Lundin
    Dec 7, 2022 at 13:04
  • \$\begingroup\$ (That being said, soldering over breadboard is definitely the way to go. No matter how bad your soldering skills are, the result will be better than that of the average breadboard.) \$\endgroup\$
    – Lundin
    Dec 7, 2022 at 13:09
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    \$\begingroup\$ I think the underlying problem here is poor soldering technique leading to very hard to evaluate joints. With all that excess solder it's very hard to see what's happening. Good solder, flux and chisel tips are extremely inexpensive and not that hard to use, so I would look into what is going wrong and fix it. Perhaps a few dollars worth of flux or a 1.2mm chisel tip could avoid this entire problem. \$\endgroup\$
    – user1850479
    Dec 7, 2022 at 14:59
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    \$\begingroup\$ A common mistake is using too thick solder wire. Somewhere around 0.5mm is probably what you should aim for here. For things like 0805 or smaller, then 0.25mm. \$\endgroup\$
    – Lundin
    Dec 8, 2022 at 7:50

3 Answers 3

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For home use, using X-Ray is mostly out. And ripping the device off to check if it has wetted completely is not rally a great option too. Measuring the capacitance won't help you (the value of doing this in cicuit is quite questionable anyway if you don't know exactly whyt you're doing)...

But there is one testing strategy that's old, well understood and can by done by basically everyone. It's called "visual inspection". All you need for this is:

  • Know how of what a good solder joint in the used technology should look like
  • functioning eyes
  • maybe a magnifying glass or a microscope
  • good light

I assume you have functioning eyes, so we won't need to discuss that any further. If you need a magnifying glass is a question of your eyes and the size of the components / solder joints. Good light is not something defined. I found daylight trough the window great to work with most of the time. Sometimes a LED flashlight helps to get the light at the right angle to see more contrast.

There is a standard called IPC-A-610 that defines industry criteria for what constitutes a good solder joint. Buying that standard is overkill for home use.

So let's look at what constitutes a good solder joint:

  • Component and footprint are aligned properly
  • pad is completely covered with solder
  • solder does only cover the pad
  • surface of solder is smoth (no holes or bubbles)
  • if you're using leaded solder, the surface must be shiny. Leadfree has matte finnisch
  • No cracks in solder
  • right amount of solders gives a concave curved surface from pad to component

"13 Common PCB Soldering Problems to Avoid" is a quite comprehensive list of how to detect bad solder joints optically (and how to prevent them). But be aware that they are using leaded solder. Forms and geometries stay the same but you won't get a nice an shiny surface with lead free solder.

Also, IPS has some demo material for training online.

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  • \$\begingroup\$ Thanks. My eyes are quite good. Not wearing glasses. But the capacitor is so small the blob of solder is triple its size... So I appreciate your comments about visual inspection which is quite obvious but risky. I do not think x-rays are required an oscillator and a oscilloscope is one method I think that could quite easily test if the connections are there... \$\endgroup\$
    – John Am
    Dec 7, 2022 at 11:37
  • \$\begingroup\$ @JohnAm "Ocular inspection" also known as "look at it", in combination with knowing the IPC standard requirements is indeed the way to go. Forget about measuring, that doesn't tell you jack. Cold joints in particular might work 100% fine until a later point in time where they don't. I can already tell you just by looking at the low res picture that the soldering is questionable on almost every joint and there's just no way in h*** this solder job would pass IPC certification. Poor wetting, too much solder, incorrectly cut component legs, several potentially cold joints... \$\endgroup\$
    – Lundin
    Dec 7, 2022 at 12:54
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    \$\begingroup\$ @JohnAm Professional assembly contractors either use manual visual inspection in microscope, and/or for larger batches "AOI" machines (automatic optical inspection). \$\endgroup\$
    – Lundin
    Dec 7, 2022 at 12:55
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    \$\begingroup\$ @JohnAm -- re: "the blob of solder is triple its size" -- the blob of solder is too big. Try finer solder. \$\endgroup\$
    – Pete Becker
    Dec 7, 2022 at 14:08
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    \$\begingroup\$ @JohnAm look at this page for how solder joints should look: seeedstudio.com/blog/2021/06/18/… \$\endgroup\$
    – kruemi
    Dec 7, 2022 at 14:41
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You wouldn't normally try to measure the component itself to see if it is properly soldered.

You'd normally just look at it. That's "visual inspection."

Here's a couple of photos I made a couple of years ago of a 1206 sized capacitor:

enter image description here

enter image description here

You can see that both ends are soldered to the PCB. They are slightly over-filled (too much solder) but are electrically fine.

If you really feel that you must test the connections, use the resistance setting on your multimeter and measure from some point on the trace to the metal part of the capacitor.

Like this:

enter image description here

If you were to solder an SMD part on the indicated pads, you could check the resistance between the red points and the blue points to see if the part were connected at both ends. You'd measure to the top of the part, of course, rather than the pads on the board.

A good close look is all you should need, though.

The photos were made to approximate what you would see while soldering.

Hold your hand about three or four inches from your eye, then move your head back and forth from the monitor until your fingers just cover the fingers in the photo - the SMD part will appear in the proper proportions.

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  • \$\begingroup\$ The PCB I use is handmade, and visual inspection does not always work for me. I just soldered a SMD 10k resistor attached to a smd LED. Testing to check that the LED worked took me a while and with visual inspection it wasn't always clear. With many SMD parts I really need to test each connection so to have confidence that everything is properly soldered and the circuit will work. Otherwise when the circuit fails to work there is a bit of anxiety when I seek to find the problem. \$\endgroup\$
    – John Am
    Dec 7, 2022 at 11:21
  • \$\begingroup\$ @JohnAm: You might want to take a look at how to solder SMD parts. \$\endgroup\$
    – JRE
    Dec 7, 2022 at 11:46
  • \$\begingroup\$ Thanks, I will check it out \$\endgroup\$
    – John Am
    Dec 7, 2022 at 11:47
  • \$\begingroup\$ In this case, it won't pass inspection as you can't really tell if it has wetted ok underneath the solder blobs. The blob might cover the pad without wetting against it for example. A component like this should be fixed by dotting each joint with a flux pen, apply some pressure on it with tweezers and then swiftly reheat each side of it. Excess solder will end up on the tip or on the solder mask from where you can remove it. \$\endgroup\$
    – Lundin
    Dec 7, 2022 at 13:01
  • \$\begingroup\$ @Lundin Thanks I will check what a flux pen is and how to use it. \$\endgroup\$
    – John Am
    Dec 7, 2022 at 13:32
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You might be able to put the multimeter in resistance measuring mode and test the capacitor. If so, you would expect to see the resistance increasing rapidly from zero to insulator as the capacitor charges. Of course, this depends on what other bits of circuit are connected. So if this is a viable option, place the capacitors first and make sure they are not connected to each other as far as possible.

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  • \$\begingroup\$ A correctly soldered cap will have ESR x and a cold soldered cap will have ESR x. So what exactly is the point of measuring? To check if you soldered it at all? \$\endgroup\$
    – Lundin
    Dec 7, 2022 at 13:07
  • \$\begingroup\$ @Lundin yes my problem is to check if there is a connection, not if the capacitance has changed or anything \$\endgroup\$
    – John Am
    Dec 7, 2022 at 13:28
  • \$\begingroup\$ @JohnAm For larger components like cables you can just use a multimeter with diode test/beep, across both surfaces. But again this doesn't detect bad joints, it only detects completely botched attempts of soldering. \$\endgroup\$
    – Lundin
    Dec 7, 2022 at 13:40
  • \$\begingroup\$ @Lundin Soldering is quite difficult for me. It has taken a lot of time to improve a bit. Now I feel that I have learned how to use the soldering iron. For SMDs perhaps the tip of the iron is too large. Anyway, thanks for the valuable info \$\endgroup\$
    – John Am
    Dec 7, 2022 at 13:42
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    \$\begingroup\$ @JohnAm For small SMD work use a fine tip, I personally prefer the bent ones as you can hold the iron more natural that way. Straight tips might cause awkward angles when you need to reach somewhere close to other components. \$\endgroup\$
    – Lundin
    Dec 8, 2022 at 7:57

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