The analysis on bridge between leads of quad flat package in reflow soldering on printed circuit boards

The factors occurring bridge between leads of Quad Flat Package (QFP) have been studied based on potential theory. To find out the analytical property of bridge configuration between leads, it is assumed that the configuration of bridge is approximated by a solid of rotation, which the rotating curve is given as r+ax². The physical property of the bridge can be a good approximation compared with the bridge that is constructed by Laplace formula, and can describe the potential of occurrence of bridge accurately. The occurrence of bridge is one of bifurcation phenomena. Then, there is a branch point to decide to whether bridge occurs or not. It is proved that this branch point, represented as a saddle point of energy surface, is function of geometrical elements, such as width of lead and pad, and gap between pad and lead, etc., together with surface and adhesion tension of solder. It is also found that the most dominant factor of occurrence is the gap between lead and pad. There is the optimum width of pad and lead, which is the minimal condition of bridge occurring, independent of surface and adhesion tension of solder at given pitch of leads of QFP