Void generation method yields low-void solder paste
Since the Waste Electrical and Elec- tronic Equipment (WEEE) and the Restrictions on the use of Haz- ardous Substances (RoHS) direc- tives came into effect in July 2006, the use of six toxic substances, including lead has become restricted. To date, solder manufacturers have completed the shift from using the conventional lead-con- tained solder pastes to lead-free solder pastes.
Compared to lead solders, the SAC (Sn3Ag0.5Cu), a typical lead-free alloy, has strong reactivity as well as high sur- face tension and melting point. However, a rise in the melting point has led to an increase in the pre-heat temperature and the oxidation of solder powder, causing many cases of unwetting and defective jointing. These defects have been almost solved as a result of the improvements in solder paste flux and the changes in mounting conditions. However, there are still no clues on the reduction of voids that tend to develop due to high surface tension. The voids occurring on a large pat may involve not only the decrease of jointing relia- bility, but also the defective parts due to poor radiation performance if the pat is used in a head sink, thus becoming a major issue. A host of customers have requested improve- ments and new developments to reduce the voids, even today when lead-free sol- ders have become more common.
This article will outline the M420 Se- ries, an ultra low-void solder paste de- veloped by KOKI Co., Ltd. using a new concept.