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DEK Report Details Lead-free Stencil Design Rules
November 18, 2005 |Estimated reading time: Less than a minute
SAN JOSE, Calif. — DEK has published "Understanding Stencil Printing Requirements for a Lead-free Mass Imaging Process," a report detailing findings on screen printing using lead-free pastes that focuses on implications for stencil design rules.
The report, which tested 67 aperture characteristics, including varying dimensions, aspect ratio, shape and stencil thickness, shows that aperture dimensions must increase to ensure adequate wetting to prevent tombstoning as paste- and component-to-pad offsets occur. DEK's data shows that these offsets cannot be avoided in a production environment, requiring manufacturers to optimize stencils specifically for lead-free printing. The report also shows that this issue occurs more often in smaller components — 0402s and below.
Ensuring a higher volume of paste to make up for lower wetting forces of lead-free pastes is key, claims experimental team leader and global applied process engineering group manager for DEK Printing Machines, Clive Ashmore. When placing small chip devices such as passives, wetting forces help retain the component during reflow. Component, solder deposit and paste misalignments could lead to an imbalance in wetting forces — increasing the risk of tombstoning.
The full report, detailing experimental conditions, range of aperture characteristics and quantitative and qualitative results, is available at www.dek.com.