Study: nickel plays role in stencil paste release.

Flemington, NJ -- The nickel content of screen-print stencils appears to be the figure of merit when migrating to lead-free assembly, according to a DEK (dek.com) study on the impact of stencil materials on paste volume repeatability, paste-on-pad registration and process window.

In the study, paste volume repeatability was close to 90% for pure nickel stencils, with electroform only slightly ahead. After printing with nickel electroform stencils, as well as pure nickel and high-nickel-content stencils cut by a YAG laser, the experiment showed pure nickel electroform to be marginally ahead of pure nickel laser-cut. Both types showed better results than other stencils, including acrylic and stainless steel.

The research also shows that lead-free pastes using the Sn96.5Ag3.0Cu0.5 solder alloy continue to display differences in performance. The differences concern the effect of each paste's rheology on its aperture release characteristics, which impacts deposited volume repeatability.

Other factors affecting release efficiency include stencil aperture dimensions and aspect ratio.

DEK collected over five million data points for its experiments. The results show the importance of evaluating a solder paste extensively before adopting it for production. Some assemblers may need to consider migrating to a high nickel-content stencil supplier. Full results are available at dek.com/leadfree.