Stencil Cleaning
Solvent-soaked fabric or paper rolls remove residual solder paste from the underside of a printing screen during high-volume surface mount production. An automated under-stencil wipe prevents paste accumulation and bridge formation on subsequent boards by cycling a clean portion of media across the bottom of the aperture plate at pre-programmed intervals. This mechanism relies on a vacuum or mechanical roller system to clear the debris that gathers during the transfer process.
Cleaning Parameters
Effective aperture clearing depends on the cycle frequency, the speed of the travel and the volume of solvent applied to the wiping material. Each specific stencil design dictates the necessity of the wipe to ensure that solder paste does not smear onto the board surface during the next print cycle. Excessive use of the solvent risks leaching the flux from the paste within the apertures, while insufficient wiping frequency results in solder bridging and short circuits.
Manufacturers establish these settings based on the viscosity of the paste and the pitch of the components mounted on the circuit assembly.
Maintenance Requirements
Operational stability of the system requires consistent tension in the wiping roll and regular replenishment of the cleaning media. Monitoring the consumption of these rolls provides data on the print process efficiency and helps predict when the wiping mechanism needs recalibration. Sensors detect the end of a roll to trigger an operator alert or a machine stoppage before the print quality degrades.
Precise control of the physical interface between the wipe media and the stencil face minimizes the wear on delicate aperture walls during the lifecycle of the printing tool.
