Alloy
Lead-free SAC305 is the default for exported products; tin-lead remains in use for legacy or exempt applications. The alloy sets the reflow peak and the joint appearance, so the profile must match it.
Powder size
Finer powder prints through smaller apertures but oxidises faster and is more prone to slump. Match the type to the finest pitch on the board rather than to the average.
Flux class
No-clean is standard; water-soluble is chosen when cleanliness matters, at the cost of a wash and dry step. Whatever the class, the residue must be compatible with the next process — especially conformal coating.
Practical rule
State the finest pitch, the assembly cycles and whether the board will be coated. Those three answers select the paste; the printer settings follow.
