Monitoring the effects of storage on the rheological properties of solder paste

Solder paste is widely used as a bonding medium for surface mount assemblies in the electronics industry. The stencil printing of solder paste is a potentially low cost method of depositing solder paste onto printed circuit boards. It is very important to use the right paste, as a high proportion of all surface mount assemblies defects are related to the printing process. This is likely to continue with the drive to further miniaturization and the implementation of die-size devices. One of the key factors affecting solder paste print quality is its rheology. The rheological properties such as viscosity of solder paste are dependent on its “history” (i.e. paste storage). Key factors that influence solder paste viscosity have been identified over the years. In this paper, we examine the effects that storing solder paste at two different temperature and humidity conditions has on its rheological properties. Viscosity measurements at different storage intervals reveal about 60% increase for samples kept at room temperature conditions after 5 months. A model of the change in viscosity at both storage conditions is also presented. From this model, the shelf life of the solder paste can be determined. This model can also be used during quality control.