Flux plus needing clean pads. You can’t just take the old caps off and solder. Use either a sucker or solder wick to clean ALL the old solder off the pads. Then, use flux and the iron tip to clean any crud that is there till the pad is completely shiny (may require more sucking/braid as more crud is burned off). Once it is a totally shiny, smooth surface, then I generally place the smallest bit of flux and then solder on the pad, making a very tiny solder layer on both pads. Place the cap straddling the pads, holding with a tweezer or finger with a bit of down pressure. Add a tiny bit of flux to the leads on the cap. Then, touch the iron to the soldered pad surface and the lead on top of it at the same time. The lead should melt directly into the solder already on the pad in a second, after which you can remove the iron, then a second layer your finger. Repeat on the other side, and you should be done. If there is insufficient solder, add just a tiny bit on the tip of the iron, and touch it to the lead to melt it in.
The one thing you don’t want to do is use a massive amount/blob of solder; that can actually make a worse connection (and looks shoddy).
really makes you think...
