Evaluation of New SMT Stencil Materials

Chrys Shea
Shea Engineering Services, Burlington, NJ USA

Quyen Chu, Sundar Sethuraman
Jabil, San Jose, CA USA

Rajoo Venkat, Jeff Ando, Paul Hashimoto
Beam On Technology, Santa Clara, CA USA

High yields in the stencil printing process are essential to a profitable SMT assembly operation. But as circuit complexity continues to increase, so do the challenges of maintaining a successful solder paste deposition process. To help assemblers address the challenges presented by evolving technologies, stencil suppliers have provided a variety of options in stencil technology, including new foil materials, manufacturing processes and coatings.

A study was undertaken to quantify the effects of stencil material on paste deposition in high volume production processes. The experiment focused only on laser cut stencils, and compared the typical stainless steel, non-electro polished foils with electro polished stainless steel, fine grain stainless steel, and electroformed nickel. The DOE strived to maintain consistency of all other variables involved in the process, changing only the stencil material. The test vehicle design varied theoretical area ratios from 0.50 to 0.75 in 0.05 increments (actual area ratios varied between 0.48 and 0.77). Output variables were paste deposit volumes, which were expressed as transfer efficiencies based on measured (actual) aperture volumes.

The transfer efficiencies of the four materials are compared and performance differences are discussed. High magnification photographs of the aperture walls provide visual images of the wall topographies. The effect of electro polishing is shown and discussed.

The apertures in materials A, B and C were all slightly undersized, by up to 0.3mils. The apertures in material D were slightly oversized by a similar amount. All foils were cut on the same laser cutter; foil D was electro polished after cutting.

The two materials that provided the best transfer efficiency were materials A and D. Material D showed the most predictable response across the area ratio range of 0.57 to 0.73.

Materials A and D also showed the smoothest walls when observed at high magnification. A relationship between the surface roughness of the walls and the release performance of the stencil is noted, but not quantified. The aperture walls that appear smoother provided better solder paste release, especially at area ratios at or below 0.66.

A ridge was observed in the walls of the electro polished apertures; a separate study to optimize the electro polishing process will address the ridge formation and aperture size variance.