PURPOSE: To prevent solder bridge between lead terminals of an LSI flat package, by installing isolating plates which are fixed on the upper surface of a surface mounting LSI flat package and inserted between lead terminals.

CONSTITUTION: A solder bridge preventive jig 1 has isolating plates 11, 12,...1n, 21, 22,...2n, whose thickness is equal to the gap between the lead terminals 31, 32,...3n, 41, 42...4n of a surface mounting LSI flat package 2. The number of the plates is equal to the number of the lead terminal gaps of a corresponding package 2. A main body retaining the isolating plates 11, 12,...1n, 21, 22,...2n has a shape along the external shape of the corresponding package 2. This preventive jig 1 is attached from above the corresponding package 2, and the isolating plates 11, 12,...1n, 21, 22,...2n are inserted into the gaps between the lead terminals 31, 32,...3n, 41, 42,...4n. By this constitution, the solder bridge between the lead terminals of the surface mounting LSI flat package can be prevented.