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QFN void issue

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#55145

QFN void issue | 20 June, 2008

Hi all, any good recommendation for PbF paste application to reduce voids underneath the QFN thermal pad (stencil thickenss? opening? via hole? reflow profile etc)? We have tried few stencil opening design but no significant improvement as seen. There will be about 50% or above of voiding in the total pad area. Is is possible to reduce the voids below 50% or any industry standard/spec available for QFN voids? Thanks a million!

B. Rgds, Phil

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#55162

QFN void issue | 20 June, 2008

Back when I used to do a lot of these style devices we ran into the same problem you are describing. Use a pattern for the center pad area and keep the total coverage to about 68% of the pad area. Diagonal Stripes, tic tac toe, cloverleaf, dot array. The idea is allow better outgassing when the flux in the paste activates, for us this reduced the overall void effect. Also if you are running ramp,soak,spike profiles, find a method to approach the reflow temperature a bit slower, allow more time for outgassing. Good Luck.

There is a lot of information about this type of project at http://www.amkor.com/products/notes_papers/Board_Level_QFN.pdf

I do not work for Amkor nor do I receive any compensation for providing this information.

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