Silver solder flows via capillary action, unlike soft lead solder it cannot be used to fill gaps. Careful measuring and placement has to be performed to form a close fitting joint. The assembled joint should be carefully inspected before actually soldering. If there is any more than the smallest amount of light showing through the joint, then a reliable joint will not be made.
For capillary action to be most effective the surfaces to be joined must have a minimal amount of clearance. In commercial processes this is measured extremely accurately; in a less technical environment common sense and experience will act as a guide. Tubular parts should have a close fit whilst still allowing easy movement. For flat components, the weight of resting one on top of another will create the correct clearance. The microscopic surface imperfections of unpolished silver will allow capillary action. Care should be taken to not clamp components too tightly; no gap at all will prevent the solder from flowing freely.
