Rheology stencil printing

Various adhesive application methods are available (dispensing, pin transfer and screen/stencil printing) and different adhesive rheologies are required in each case. 

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. 

The process parameters key to the stencil printing process include the squeegee type and hardness, print speed, print pressure, and print gap. An optimal stencil printer setup provides a clean sweep on the stencil surface and a repeatable solder paste deposition process. Shore A scale polyurethane squeegee hardness level between 85 to 95 is appropriate to achieve repeatable solder deposits for lead-free solder pastes. Print parameters such as the print speed, print pressure, and the print gap are adjusted to accommodate the variety of solder paste rheologies available. There are no specific equipment modifications necessary to stencil printer lead-free solder pastes. 

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. 

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