SMT Solder Paste Printing Parameters and Viscosity Evaluation Methods
Optimize solder paste print volume and yield by maintaining area ratios above 0.66, controlling squeegee force, and verifying viscosity stability.
Under Stencil Cleaning
Wipe Sequence
Automated removal of accumulated paste residue from the bottom surface of the stencil maintains print quality over hundreds of cycles. This under stencil cleaning process utilizes a fabric roll and a vacuum system to draw out excess material from the aperture edges. It prevents dry paste from contaminating future boards or blocking subsequent transfers.
Most printers are programmed to run this cycle every few boards or every fixed interval of time. High speed production depends on this maintenance to avoid manual intervention.
Solvent Transfer
Liquid cleaning agents are applied to the wiping fabric to break down old flux and facilitate the lift of the metal particles. During the execution of under stencil cleaning, the combination of solvent wetting and vacuum force clears the small crevices in the foil. If the fabric stays too dry, it can smear the paste rather than remove it.
Proper pressure ensures full contact with the complex bottom topology. This operation must happen quickly to avoid slowing down the assembly line.
Bridge Prevention
Contamination on the underside of the mask causes shorts by forming conductive paths between adjacent pads. Regular under stencil cleaning stops this horizontal migration of paste during the print stroke. If the bottom foil stays clean, the resulting deposits remain well defined and isolated.
Yield rates stay high when residue buildup is managed before it reaches critical mass. Reliable electronics assembly requires this step as a baseline process control.