Rheological Characterization of Solder Pastes with Different Procedures

Solder pastes are strategic materials for the electronic packaging processes based on surface mounting technology (SMT). Solder paste printing is most critical step in SMT processing, due to several factors including the high sensitivity of paste rheology to its physical-chemical properties, the width and pitch of PCB pads and the complexity of screening process itself. During screening the squeegee movement induces paste rolling on the stencil surface and its flow through the stencil openings, depositing the paste on the PCB pads. Paste flow depends on the squeegee speed and pressure, the geometry of the stencil apertures and the snap off or division speed of PCB surface and the stencil under side. Some theories were developed in several research works at Dresden and Greenwich, which have given a lot of hints for mechanical, fluidic and rheological conditions during screen printing process. All investigations have shown the very complex influence of paste rheology and the necessity of a better paste characterization.