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Vacating Voids in QFN Packages
The SMT Quattro Peak S system when used with the Vacuum Plus N2 System module can help reduce voids in QFN packages by controlling the vacuum and nitrogen level used during production of packaged parts.

Surface-mount components rely on tiny housings such as QFN packages for their electrical and mechanical health and well-being. QFN, or quad-flat-no-lead housings, are leadless packages used throughout applications calling for surface-mount-technology (SMT) devices. With electrical contacts on the bottom side of the package, in a large connector area in the center of the package, QFN components can be mounted directly to the surface of a PCB. QFN housings are small, light in weight, plastic encapsulated, and provide good electrical and thermal performance. They are efficient and low in cost. They are outstanding in almost every way, except they are prone to voids.

The design of a QFN package makes it susceptible to voids. The small surface and smooth bottom side help trap solvent vapors and activator residues used in the fabrication process. An increased number of voids in the contact pad area usually will not compromise quality, function, or reliability. But the same cannot be said for the large area of exposed thermal pad in the center of the QFN. That portion of a QFN usually functions as a heat sink, or as the thermal path to a heat sink. The presence of a large amount of paste in the center of the bottom of a QFN can hinder the escape of volatile residues used during the reflow process, resulting in an increased number of voids in an area that is critical for thermal flow. A higher number of voids results in less contactable area for thermal flow, increasing the internal thermal resistance of the contact area and reducing thermal transfer for the QFN. As a result, the component will suffer a higher temperature rise during normal operation, with possible decreased electrical performance and diminished reliability and operating lifetime.

Counteracting Voids
Minimizing voids in a QFN package can help improve the reliability of the component within. Reducing voids requires attention to details during the packaging and solder reflow processes to avoid conditions that would be favorable to forming voids. For example, different solder pastes lead to different results in the number of voids, but by using the vacuum process with solder paste, some of those voids can be reduced. Also, different print patterns can yield different results in terms of voids. But even with different print patterns, a significant reduction in the number of voids can be achieved using the vacuum process. The use of nitrogen can also contribute in different ways to the number of voids, but the only way to really minimize the number of QFN voids is through the use of the vacuum process. The number of voids in a QFN package is also impacted by the pressure and temperature profiles applied during the packaging process. For example, compared to QFN packages produced at lower pressures, the number of voids was greatly reduced in QFNs at a subpressure of 300 mbar.

By using a reflow-vacuum-process during QFN packaging, it is possible to bring about a significant and reproducible reduction in the number of QFN voids. The Reflow Vacuum-Plus-Concept developed by SMT Wertheim helps minimize voids in QFN housings, by using the Vacuum Plus N2 System vacuum module that is positioned between the convection heating zones and cooling zones of the company?s SMT Reflow Soldering System. The programmable module makes it possible to program the parameters, such as vacuum level and time to vacuum, that can impact the number of voids formed within a QFN housing during the packaging process. The system provides touch-screen control to simplify operation and programming.


Contact: SMT Wertheim, D-97877, Wertheim, Germany +49-9342-9700 fax: +49-970-000 E-mail: r.faber@smt-werthheim.de Web: smt-wertheim.de

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