This thread reminds me of an experience I had. In the early days of PC board mounted vacuum tube equipment, electrolytic capacitors failed early and often. Many of these failures were caused by cheap parts operated at or above their maximum ratings. But this was not always the case. It turns out that the method of mounting and installing the parts was responsible for many of the failures.
Back in the 70’s when I was doing repair I discovered another failure mode. There was a cathode bypass in the vertical output section (6LU8 tube) of most TV sets that failed about once a year. Even the replacement parts failed. I tried different parts, higher voltage parts, caps with more microfarads, they all still failed. They didn’t short out or go open, they just slowly developed a high internal resistance (ESR) that caused the picture to shrink from the bottom. In an effort to figure this out, I actually cut open some of the failed parts, and a good one for comparison.
Most radial leaded electrolytics are mounted directly against the board. They are then soldered from the bottom. As you are soldering the lead, it gets hot. When the lead gets hot it expands. The lead on a capacitor can grow a bit while it is being soldered. If the part is pressed down tightly to the board while it is being soldered, what is going to happen when the leads cool off and they shrink back to normal length? There is already a force built into the board design that is trying to pull the leads out of the capacitor. Now, the capacitor is on a circuit board right next to a hot tube. It gets baked every time the TV is turned on, and returns to room temperature every time the TV is turned off. This repeated thermal expansion and contraction will eventually tear the leads loose from the aluminum foil inside the capacitor.
The simple answer to this was to leave some space between the capacitor and the PC board. For most capacitors 1.5 mm is enough. When installing a physically small capacitor into a space meant for a larger cap leave longer leads, such that there is no stress applied to the part. If you pre – bend the leads, do not put any bend in the leads closer than 2 mm from the body of the capacitor. For large snap – in capacitors (no leads, fat prongs) this is not a problem, just insert them as intended and solder.
But this brought me to another problem I had to deal with. If you space your large leaded capacitors up from the board, as described above, they become prone to damage if the board is subject to excessive handling or drop. Since the part is now free to bend slightly, the leads can be torn loose from the bottom of the cap. I had a solution for that too. I placed a piece of thick foam tape across the bottom of the cap, with the leads protruding through it. This allowed room for thermal movement, and provides a cushion for the part. I used double sided tape as a solution, without peel the backing from the bottom. Some high quality capacitors already have a soft rubber cushion built into the bottom of the part, so that this step was not absolutely needed any more, but I still did it anyway. After putting the piece of tape on the bottom of the cap, I insert it into the board in the normal fashion. Pushing the cap in until it is flush with the board and bend the leads slightly to keep it there until I solder it. I used this method for every cap who lived next door to the GZ34 tube and some hot resistors.
