Correlating Printing Performance of Solder Paste with Its Rheology

Solder paste is a suspension of metal powder in the flux medium which is used for assembling various electronic components on printed circuit boards (PCBs). To apply solder paste on PCB, various methods like stencil printing, dispensing, dipping, pin transfer etc. are used as per suitability in a particular application. In all the applications, however, rheology of the solder paste plays a crucial role in transferring paste onto the PCB. Prior knowledge of rheological behavior of the paste can be of immense help in developing a solder paste for an application or to reduce defects in final assembly during high volume manufacturing. In surface mount technology (SMT), stencil printing is the most common process to apply solder paste on PCB. Rheological characterization of these pastes could be used to predict their printing performance. Rheological properties of any solder paste depend on various factors like flux chemistry, metal percent, powder size and its distribution, shape of particles etc. Rheology of a solder paste can be tuned or controlled by varying above factors to improve its printing performance.In this work, an attempt has been made to correlate rheological behavior of a no-clean solder paste with its printing performance. The study comprises the effect of metal percent and particle size on rheology and consequently on the printing performance. Techniques used for rheological characterization include shear rate dependent viscosity, oscillatory amplitude sweep test and step shear rate recovery test. Rheological studies were carried out with Kinexus Rheometer using parallel plate geometry. For printing evaluation, pastes were printed on test PCB and paste transfer efficiency was measured and correlated with rheological parameters.