SMD Mount Bases – Do SMD Mount Bases Exist?

Very good idea to practice, practice, and practice some more on salvaged boards until you get competent with new tools. You didn't notice the solder had reached the molten state because there's so little of it used to solder components.

The typical lead-free solder has a melting point around 217 degrees C, so you'll have to get the leads and pads up to that temperature before trying to remove the component. The reason why you need a much higher temperature is because you want to get the solder joints to the melting point as fast as possible. If the hot-air gun is set to a much lower temperature, it'll take a longer time to reach the melting point. The longer the time to raise the temperature of the leads/pads will increase the overall temperature of the component, possibly past the point of destruction. So the technique is to heat it up fast, remove the part and hot-air gun, then put the part where it can cool off.

Now, if you're removing a part that's already fried due to some other reason, no worries then. Just don't damage the board by overheating the pads and causing them to lift off. If that happens, your headaches are just beginning on this repair.

It sounds like they are ceramic caps, in which case they are unpolarised, so you can put them either way round - here is a photo of a typical ceramic cap (no markings):

If they look something like the one below (with markings), then it will be a polarised tantalum (the dark line indicates the + side):

If the schematic does show a polarised cap but it does turn out that you have been given unpolarised caps, then I would mention it to the vendor so they can correct it.

If they really are unmarked polarised (extremely unlikely) then a possibly destructive method of testing for polarity is to gradually apply a current limited voltage (e.g slowly up to ~25% of rated voltage, limited to ~10mA) in both directions across the cap whilst measuring current - if polarised and the wrong way round you should start to see a steadily rising current flow. Can be done with a bench power supply, and put a shield of some sort over the cap just in case it decides to detonate ;-)

I tested with the bench supply, above ~7V across reverse polarity with a 100uF/35V aluminium electrolytic, the leakage current rises above 1mA (measured using bench display) and quickly starts to accelerate upwards.
I also just tested this with a multimeter in series with the bench supply (more sensitive than the bench supply measure) measuring current across the same capacitor:

• Using 5V with correct polarity produced ~1uA leakage.
• With 5V and reverse polarity the leakage started at around 25uA and gradually got higher, after around 30 seconds it was at 50uA.
• Even at 3V it was reasonably obvious which way round it was - the reverse leakage was at least twice that of the correct polarity.

I don't think this sort of low voltage testing should do the capacitor any harm. Here is an excellent study by NASA, who seem to think many of the reverse bias ratings are rather conservative. To quote part of the summary:

Some lots of 35 V and 50 V rated capacitors survived 200 and even 8900 hours of reverse bias testing (RBT) at voltages up to 40% of rated voltage (VR). However, the survival rate was not 100% and the behavior was judged to be lot related. The conclusion made by G. J. Ewell that the existing manufacturer guidelines are extremely conservative concurs with the results of the testing performed at Hughes in 1988. In that work it was shown that some capacitors could withstand reverse voltage up to 25% of VR with very little degradation occurring below 15% of VR. In all cases healing began to occur after 5 minutes of the application polarity being corrected. These experiments suggested that while solid tantalum capacitors can survive substantial reverse bias without failure, this behavior significantly varies from manufacturer to manufacturer.