Voids are bullous inclusions in solder joints. They are not blowholes, which have their origin in the solder contracting during cool down. The possible causes for voids are many, and the study as to their origin is not yet completed. Organic inclusions in the surface metallization, flux- and other components of the solder paste, or oxides dissolved in the solder are suspected as their causes. Voids come to be through interaction between PCB surface, component metallization, solder paste and process management.