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Ask the Expert
Helping our customers solve their printing problems is a fundamental aspect of what we do. If you have a printing issue, ask the expert for help. Bill Kunkle, our Manager of Stencil Technology, has been helping customers solve issues for over 2 decades! Check out the Shared Solution below for a helpful tip.
Ask the Expert: QFN Ground Slugs
A customer emailed that the center slug of reflowed 20 pitch QFNs did not have a good solder contact area, from the part to the board. For this stencil, MET had reduced the slug area to 64% of the exposed metallized area and used 4 mils thick SS. This was our normal plan, to allow the part to sit low enough that the perimeter leads would solder well.
In discussion with the customer, we learned that:
- • there is an array of large via holes in the slug area, and
- • the solder mask may be holding the QFN up off the board
We modified the design to increase the print area to 81% but the reflowed solder contact area was still not sufficient.
For the next stencil, we kept the slug area at 1 to 1 with the metallization. Our customer reported success: that the reflowed solder contact area was good.
Conclusion: The general rule to reduce the stencil QFN slug area to 50 - 60 % of the metallized area should be tempered by the presence of via holes and solder mask height. More or larger via holes means less reduction is needed. Solder mask vs pad height checking requires a sample bare board and appropriate microscopes or gauges.
Bill Kunkle
Manager of Stencil Technology
