My soldering skills are poor. In soldering a ceramic capacitor to bridge my motor terminals, I appear to have damaged it. Now when I test continuity it shows as a shorted. I'm pretty sure that means the capacitor is shot.
Is my diagnosis right? If so, any hints on keeping capacitors cool when soldering?
How did you test it? Whilst soldered to the motor? Have you got another motor around you can measure? Without a capacitor my meter shows only 4 ohms between terminals on a small motor, so you expect it to look like a "short" whether or not the capacitor was there.
It doesn't sound good to me. Capacitors can look like they're shorted when checking for continuity, but only for a brief period of time while they're charging. If it's a steady short then it's probably dead.
To keep them cool while soldering, use the lowest temperature that will do the job and even more importantly, use a soldering iron tip big enough to transfer the heat fast enough to do the job. Apply flux prior to soldering to help things along. Make sure the soldering iron tip is clean and shiny, hence tinned with solder.
If you can't solder the joint in 3-5 seconds, give up, clean up, and try again.
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Thanks for the responses.
It does seem like measuring the continuity across the motor terminals leads to error. I measured another similar motor (without a capacitor) and it indicated continuity too. So I'm hoping my capacitor is good.
But I do need to work on my soldering. Thanks for the tips on that. I'll follow the 3 to 5 seconds rule, and hope that leads to better results in the future.
What size are the ceramics you are soldering? There are two failure conditions for a ceramic when hand soldering: thermal cracks and flex cracks.
Thermal cracks occur between the layers of the ceramic material and the dielectric. In the layers (visualize in the middle of the ceramic) crack, opposing electrodes can come into contact with each other. You'll see this hand with hand soldering because the entire ceramic material is not at the same heat level.
Flex cracks always (always) occur after the capacitor has been mounted to the board. Or in the case of hand soldering, as you mount one side of the capacitor at a time. If one pad is attached and then you press down on the cap while soldering the other pad, you are likely to crack the ceramic layers. These cracks occur around the termination area.
Unfortunately, neither of these failures are visual. You can't just "look at" the capacitor and see if either happened. You have to remove them from the board and cross section them. The cool thing is, one you do cross section them it is pretty easy to see what kind of crack caused the failure.
Depending on the size of the capacitor (SMT 1210s or larger, for example) it is prudent to heat up the entire board area to reduce these thermal stresses.
The capacitor is just a 0.1uF ceramic. I attached it across the terminals of a motor. I don't think there was any mechanical stress during the soldering. But there may have been too much heat, leading to thermal cracks. Your information on capacitor faults was very interesting.
As mentioned, though, I was mistaken in thinking that the capacitor is shorted. I have no proof of that, since the continuity test shows the same for the terminals of a motor that have no capacitor.
A motor will offer a very low resistance path... You would normally DE-solder a cap to test it. The same is true for a resistor... since there are many circuit reasons that a resistor.capacitor would not return a correct measurement if left in-circuit.
The capacitor is just a 0.1uF ceramic. I attached it across the terminals of a motor
Capacitor with leads is far less likely to fail to either of those conditions. The leads act as heat sinks and they provide excellent mechanical relief.
Your information on capacitor faults was very interesting.
I work for a capacitor company and this comes up quite often. Customers blame us for Flex cracks even though 100% of the time we have nothing to do with them. It is always user error. 
(Thermal cracks aren't always the customer's fault.)
pwillard:
A motor will offer a very low resistance path... You would normally DE-solder a cap to test it.
Of course in this case, if he desolders it, and then if it tests OK and he resolders it, then resoldering may be what does the capacitor in.
...any electronic tech will tell you... always have spares. Any concerns about a part being marginal, swap it out.
Soldering is like riding a bike.... daunting at first... easily mastered... everyone can do it with a little practice... and you never forget how.
Oh, and resistors make nice test subjects for "practice" soldering since they are cheap.