Establishing Statistical Process Control Limits for Stencil Nano Coating Degradation Rates
Track solder paste transfer efficiency standard deviation to set statistical process limits that trigger stencil re-coating before yield collapses.
Squeegee Shear Wear
Blade Degradation
Mechanical degradation of printing blades occurs during high-volume solder paste deposition as the blade edge rubs against the stencil. The squeegee shear wear describes the rounded erosion of the printing edge of polyurethane or metal blades. This erosion alters the angle of attack and decreases the force on the paste.
Physical Mechanism
Friction and abrasive contact with the metallic particles in the solder paste generate this physical erosion over many print cycles. The squeegee shear wear occurs because the squeegee is pressed against the steel stencil under considerable force to clean the stencil surface during the stroke. This contact is especially severe when printing lead-free alloys, which contain hard metal particles that accelerate blade degradation.
The rounded edge cannot wipe the stencil cleanly, leaving a thin film of paste on the metal sheet.
Process Maintenance
Inspection of the squeegee edge at scheduled intervals prevents print defects from rising on the assembly line. When squeegee shear wear exceeds specified limits, the blades must be replaced or refinished to restore their sharp profile. Some automated printers utilize laser sensors to measure the blade straightness and edge radius before starting a new batch.
Implementing regular maintenance schedules for these blades reduces the need for manual underscreen wiping and ensures a higher first-pass print yield.