Latest lead-free research highlights new stencil design rules

The report, titled Understanding Stencil Requirements for a Lead Free Mass Imaging Process, reveals how aperture dimensions must increase to ensure adequate wetting forces and thereby prevent tombstoning as paste-to-pad and component-to-pad offsets occur. Such offsets are unavoidable in any production environment, implying that manufacturers must re-optimise stencils for lead-free printing in order to ensure yield rates comparable to lead-rich processes. The report also reveals that this issue is more prevalent with smaller components of size 0402 and below. By testing a total of 67 new aperture characteristics, including varying dimensions, aspect ratio, shape and stencil thickness, DEK's experimental team 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 in order to compensate for the lower wetting forces exerted by lead-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, who is Global Applied Process Engineering Group Manager at DEK Printing Machines, said, “A perfectly centred screen printing process will perform well whether lead-rich or lead-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 lead-free printing. Our findings show that continuing to use legacy stencils that meet the lead-rich design rules will result in significantly higher numbers of defects.”