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The Short Scoop: Stencils for QFNs

Quad flatpacks, no leads (QFNs) and dual flatpacks, no leads (DFNs) are becoming more popular component packages, in part because they are ideal for small, hand-held devices. As stencil manufacturers, our thoughts immediately turn to the question, how do we design and manufacture a stencil for these devices? What materials should be used? What types of coatings? How should the apertures be designed to get the best solder paste release? How can we accommodate the reduced footprint and keep-out areas? QFNs present many challenges to the assembly printing process, but the short scoop is that with proper stencil design, correct stencil technology selection (laser, electroform, and nano-coat), and consideration for the effects of  PCB soldermask layout choices, these challenges can be overcome.

QFNs have a very small form factor--they are typically 0.85 mm thick with a body size from 3 mm up to 12 mm. This allows for smaller and lighter packages. Most QFNs have a metal pad on the underside for grounding and heat conduction. The leads and ground plane conductor are flat and in the same plane on the bottom of the package. DFNs have a similar center metal pad, but have leads on only two sides. This ground plane provides excellent electrical conductivity and offers better heat sink thermal properties when compared to other SMT packages. All these advantages make QFNs a good choice for high density electronics. So let’s discuss the problems that arise when using these components and what you can do to handle the printing challenges associated with fabrication.Read the full column here.Editor's Note: This column originally appeared in the October 2013 issue of SMT Magazine.