The report reveals how aperture dimensions must increase to ensure adequate wetting forces and prevent tombstoning as paste-to-pad and component-to-pad offsets occur. Such offsets are unavoidable, implying that manufacturers must re-optimize stencils for Pb-free printing to ensure yield rates comparable to Pb-rich processes. According to the report, this issue is more prevalent with smaller components (0402s and below).
 
By testing 67 new aperture characteristics, including varying dimensions, aspect ratio, shape and stencil thickness, DEK identified the necessary changes in aperture characteristics. According to the team leader and author of the report, Clive Ashmore, the main concern is to ensure a higher volume of paste to compensate for the lower wetting forces exerted by Pb-free pastes. When placing small chip devices such as passive components, these wetting forces help retain the component during reflow. Slight misalignment between the component, solder deposit and paste leads to imbalance in wetting forces, which increases the risk of tombstoning.
 
Ashmore, Global Applied Process Engineering Group Manager at DEK Printing Machines, said, “A perfectly centred screen printing process will perform well whether Pb-rich or Pb-free paste is used. But such processes can only be sustained under laboratory conditions, which obviously cannot be provided in a production context. Assemblers must, therefore, take steps to ensure their stencils are optimised for Pb-free printing. Our findings show that continuing to use legacy stencils that meet the Pb-rich design rules will result in significantly higher numbers of defects.”
 
Understanding Stencil Requirements for a Lead-Free Mass Imaging Process is available at dek.com.