Plastic solder paste stencil for surface mount technology

Solder paste masks or stencils are an integral part of the manufacturing process for surface mount PCBs. This study examines the feasibility of a process for rapid creation of a solder paste stencil using thermoplastic material. CNC laser cutting of the stencil geometry is replacing traditional use of chemical etching on metal sheets to produce stencils. Laser cutting has been used to improve process speed, accuracy, and cost. This research attempts to continue to simplify and reduce costs in the stencil making process by proposing as the stencil material a common thermoplastic that can be cut easily and quickly using a low-power rapid-prototyping laser process. The effects of several variables on the success of the process are experimentally tested to determine a feasible solution. Various solder pastes and solder material characteristics are studied for their effect on the new material. Stencil dimensions, including thickness and aperture characteristics, are examined. Several key process factors are also varied in the tests to determine recommendations for settings such as print direction, alignment procedures, squeegee pressure and attack angle, print speed, and stencil release method. Subjective evaluation is also made of important qualities of the paste in reaction to the new material, including paste roll and consistency, paste volume, adherence, and instances of cold slumping. The resulting process is demonstrated by producing plastic stencils on a rapid prototyping laser housed in the Cal Poly laboratories. Cost, cycle time, and performance characteristics of the plastic stencil are estimated.