Characterizing Solder Paste Rheological Breakdown under Squeegee Forces
Excessive squeegee speed and downforce over-shear solder paste, reducing yield stress and causing severe deposit slump and bridging on fine-pitch components.
Squeegee Scavenging
Aperture Depressurizing
The unintentional scoop of solder paste from filled stencil apertures occurs when the squeegee blade bends slightly into the openings. This problem, known as squeegee scavenging, reduces the volume of paste left on the circuit board pads. It occurs most often in large apertures where the blade has less physical support from the stencil metal.
The resulting deposits are thinner in the center than at the edges.
Joint Reliability
Depleted solder volume reduces the mechanical strength of the finished joint after reflow. Thin paste deposits fail to form strong fillets on heavy components, making them prone to cracking under vibration. This defect is common in large power ground pads and big array packages.
Insufficient solder also increases electrical resistance across the connection.
Blade Selection
Using metal blades instead of soft polyurethane squeegees reduces scavenging defects. Metal blades do not bend into the apertures, maintaining a flat print surface across both small and large stencil openings. Fabricators also adjust printing speed and squeegee pressure to limit blade deflection.
These control measures ensure that solder paste volume remains uniform across all pads, regardless of their size or geometry, protecting the electronic assembly from joint failures that arise from inconsistent paste distribution.