The nickel content of screen print stencils appears to be the figure of merit when migrating to lead-free assembly, according to DEK's 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 electro-form only slightly ahead. After printing with nickel electro-form stencils, as well as pure nickel and high-nickel-content stencils cut by a YAG laser, the experiment showed pure nickel electro-form 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 it is important to evaluate a candidate solder paste extensively before adopting it for production. Some assemblers may need to consider migrating to a nigh nickel-content stencil supplier.
 
The full results are available from DEK representatives or www.dek.com/leadfree.