The effect of solder paste particle size on the thixotropic behaviour during stencil printing

The thixotropic behaviour of different type lead-free solder pastes (Type 3: particle size: 20-45 μm; Type 4: 20-38 μm; Type 5: 10-25 μm) was investigated and compared. The viscosity decrease during the first stencil printing cycles is also described. The measurements were carried out with a rheometer using serrated parallel plates by sweeping the shear rate between 0.0015-100 s−1 with ten recordings per decade. Thixotropic behaviour of the solder pastes within the cycles of stencil printing was evaluated by including a “rest-period” between every rheological measurement to address the idle time between the printings. The “rest-period” was set to 15 s, 30 s and 60 s in this research The viscosity curve parameters η0 and η∞ were determined and were used to describe the viscosity change of solder pastes (from initial to stabilised state). Oscillatory stress sweep tests for different idle time periods were conducted in the stabilised state of the solder pastes. Significant difference was shown in the viscosity between the initial and the stabilised state of the solder paste; the η0 parameter decreased even to one thirds, and the η∞ decreased approximately to two thirds during the measurement cycles.