Thixotropic Rebuild Kinetics Governing Fine Pitch Stencil Print Yields
Thixotropic rebuild kinetics dictate stencil release and deposit stability, where tuning separation speed to viscosity recovery maximizes fine pitch print yield.
Print Deposit Slump
Dimensional Stability
Tendency of a printed solder paste deposit to spread beyond its original stencil-defined boundaries before reflow creates a risk of electrical bridging. This print deposit slump occurs when the rheological strength of the paste is insufficient to hold its vertical shape against gravity or heat. It is measured as a percentage increase in the base area of the solder brick.
High-quality pastes are designed to resist this deformation even at elevated room temperatures.
Viscosity Failure
Cold slump happens immediately after the stencil is lifted and the side walls of the paste lose their support. In print deposit slump, the flux chemistry and the particle size distribution determine the internal cohesion. Hot slump occurs as the board enters the pre-heat zone of the reflow oven and the viscosity drops further.
If the particles are too large, they cannot support the weight of the deposit as the liquid phase thins. Surface tension should ideally keep the material within the footprint of the copper pad.
Pitch Restriction
Excessive spread leads to shorts between adjacent pins on fine-pitch components like BGAs and QFPs. Print deposit slump is more prevalent in humid environments where the flux absorbs water. Limits are defined by the spacing between pads.
Proper storage of the solder paste mitigates this risk.