Printing materials stencil design

As with most processes, numerous variables affect the final product. For solder paste deposition using stencils, Chouta and Heck identified 39 variables relating to materials, 32 variables to processes and equipment, and 10 variables to personnel and environment. Among variables critical for the successful stencil dispensing of adhesives are stencil design, the adhesive material and its rheological properties, and the printing process. 

Amalu, E.H., Ekere, N.N. Mallik, S. (2011). Evaluation of Rheological Properties of Lead-Free Solder Pastes and their Relationship with Transfer Efficiency During Stencil Printing Process. Materials and Design, Vol. 32, (February 2011), pp. 3189-3197. 

In the solder paste printing process, defects typically are caused by poor alignment between the substrate and stencil, incorrect material selection (substrate, paste type and stencil design), or variations in the amount of paste deposited. Defect elimination relies on the engineer and operator to address these variables and monitor the process. 

Have the appropriate stencil or screen design and materials (e.g., thickness, aperture geometry, wall finish, etc.) for the circuit board product. Design considerations must also address the need to widen fine-pitch aperture openings slightly for printing Pb-free solder paste. 

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