Paper: Stencil Design Resolves QFN 'Float' PDF Print E-mail
Written by Mike Buetow   
Tuesday, 11 September 2012 07:59

COLORADO SPRINGS, CO -- A new white paper details how to overcome assembly challenges of QFNs through proper stencil design and solder mask layout.

Authored by William E. Coleman, Ph.D, Printing and Assembly Challenges for Quad Flat No-Lead Packages also details proper stencil technology selection, specifically laser, electroform and nano-coat.

Quad flat no-lead packages are known to present several assembly problems. The QFN can float during reflow if there is too much solder. Aperture size is a problem because with aperture widths as low as 0.175mm and aperture lengths as low as 0.4mm there can be a problem with the percent of paste transfer.

Another challenge is the type of solder mask that’s employed on the printed circuit board. The article explores three types of solder mask designs – SMD where the pad opening on the board is defined by the solder mask; NSMD, where the pad itself defines the boundary of the pad and the solder mask is pulled back off the pad (typically 0.05 to 0.075mm per side); and NSMD–Window – and gives stencil design considerations and guidelines for each type.

QFN repair is also covered.

Coleman is vice president of technology at Photo Stencil.



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