Surface Mount Stencil Aperture Design Principles
Optimizing surface mount stencil apertures demands balancing area ratios above 0.66, foil wall smoothness, and pad reductions for repeatable paste volume.
Paste Collapse
Dimensional Instability
Solder paste must maintain its printed shape and dimensions between the printing step and the reflow process to prevent solder defects. When paste collapse occurs, the printed deposits slump sideways, increasing in area and decreasing in height. This loss of structure occurs due to a drop in the viscosity of the solder paste, which is often triggered by elevated temperatures or excessive moisture absorption.
It leads to bridging between adjacent pads before the assembly enters the reflow oven. The collapse is tested by printing paste onto a test board and exposing it to controlled heat and humidity. Solder pastes with higher metal loads or larger powder particle sizes exhibit greater resistance to slump and slump-induced defects.
Rheological Behavior
The flux vehicle and the binder system must provide sufficient yield stress to hold the heavy solder alloy particles in suspension. If the yield stress of the paste is too low, gravity and temperature will pull the printed deposit downward. This behavior is more pronounced in fine-pitch assemblies where the gap between deposits is small.
Environmental Factors
High relative humidity in the assembly room causes the solder paste to absorb water, which degrades its rheological properties. Maintaining a controlled temperature and humidity environment prevents the paste from absorbing moisture. This environmental control ensures the printed solder deposits retain their shape during production pauses.