When working with through the hole components, I have read that you should clip leads before soldering so the shock from clipping doesn't break the joint. On the other hand I've seen a ton of instructions for (more or less professional) DIY kits, where the picture depicted all components of a type being installed, then soldered, then clipped. The latter method is much faster and easier of course.
Electronic – Is it ok to clip component leads after soldering
componentspcb-assemblysolderingstability
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Best Answer
I was told to cut the leads to length before soldering. There was an IPC standard around on which basis we were taught that. I don't remember which standard it was exactly.
A reference I found is this document by IPC.
So to prevent parts from falling off, you bent the already trimmed leads over (in a direction where it doesn't cause problems). And after that you solder them.
It is probably taught that way just as @GummiV mentioned in the comment, that the force and shock of the trimming might cause the joint or the PCB to be damaged.
I actually managed to break of a fragile ring with the track once because I was trimming after soldering, so I think the advice of the IPC is quite sound.
There are also frames around which help you in the assembly of through hole boards. You can put the board in, put all your components in from the top, then put down a lid and turn the whole thing around to solder on the bottom side.
Looks something like this: