Evaluating solder paste behaviours through rheological test methods and their correlation to the printing performance

Purpose - The purpose of this paper is to study the rheological behaviours of lead-free solder pastes used for flip-chip assembly applications and to correlate rheological behaviours with the printing performance. Design/methodology/approach - A range of rheological characterization techniques including viscosity, yield stress, oscillatory and creep-recovery tests were carried out to investigate the rheological properties and behaviours of four different solder paste formulations based on no-clean flux composition, with different alloy composition, metal content and particle size. A series of printing tests were also conducted to correlate printing performance. Findings - The results show that in the viscosity test, all solder pastes exhibited a shear thinning behaviour in nature with different highest maximum viscosity. The yield stress test has been used to study the effect of temperature on the flow behaviour of solder pastes. A decrease in yield stress value with temperature was observed. The results from the oscillatory test were used to study the solid- and liquid-like behaviours of solder pastes. Creep-recovery testing showed that the solder paste with smaller particle size exhibited less recovery. Research limitations/implications - More extensive research is needed to simulate the paste-roll, aperture-filling and aperture-emptying stages of the stencil printing process using rheological test methods. Practical implications - Implementation of these rheological characterization procedures in product development, process optimization and quality control can contribute significantly to reducing defects in the assembly of flip-chip devices and subsequently increasing the production yield. Originality/value - The paper shows how the viscosity, yield stress, oscillatory and creep-recovery test methods can be successfully used to characterize the flow behaviour of solder pastes and also to predict their performance during the stencil printing process.