Fundamental study on solder paste for fine pitch soldering

For fine pitch QFP soldering, solder amount and locations applied on pad must be precisely controlled. Solder paste has been printed individually on each pad for most card assemblies, but this printing method is confronted with process control difficulties. As a solution of these potential problems, a new type of printing method, in which many pads are covered with one bar (or belt) shaped paste pattern, is under evaluation. The author describes some experimental studies of solder paste suitable for this printing method and the feasibility for fine pitch soldering. An experiment on three kinds of solder pastes showed that low-melting solder containing SN-Pb-Bi eutectic solder was suitable for this method because of its good solderability and even solder amount on each pad after reflow soldering. These advantages seemed to result from partial melting of low-melting solder and bonding between solder particles during preheating in the reflow process. This was confirmed by scanning electron microscopy. With this printing method, soldering of fine pitch (0.5/0.3 mm) leads was successfully demonstrated