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The Impact of VIA and Pad Design on QFN Assembly



The Impact of VIA and Pad Design on QFN Assembly
This paper discusses the impact of the via size and design on QFN voiding and solder protrusion. Do small vias prevent the solder from flowing onto the other side?
Analysis Lab

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Authored By:


Jennifer Nguyen, David Geiger and Anwar Mohammed
Flex
Milpitas, CA, USA

Summary


Quad Flat No-Lead (QFN) packages has become very popular in the industry and are widely used in many products. These packages have different size and pin counts, but they have a common feature: thermal pad at the bottom of device. The thermal pad of the leadless QFN provides efficient heat dissipation from the component to PCB. In many cases, arrays of the thermal via under the component is used to dissipate heat from the device. However, thermal vias can create more voids or result in solder protrusion onto the secondary side.

This paper discusses our study on the impact of the via size and the via design on QFN voiding and solder protrusion. Do small vias prevent the solder from flowing onto the other side? How should via be designed? Which via type will have less voiding issue? A comprehensive experiment was designed in trying to answer these questions. Different QFN types, via design, via sizes, via pitches and stencil design were studied using three different board thicknesses: 1.6mm, 2.4mm and 3.2 mil thick.

Conclusions


Solder was seen flowing into the PTH via with no solder mask ring during reflow process regardless of the via sizes. The smallest via size was 0.20mm (8 mil) from the study. Smaller via didn't result in less solder protrusion on the secondary side. However, less solder protrusion was seen with larger via. Via size of 0.51mm (20mil) with no solder mask ring could result in no solder protrusion, depending on the process condition. Board thickness had no significant impact on the voiding and solder protrusion. Voiding depended more on the component type and design. Solder mask ring would help to prevent solder flowing down the via at the thermal pad. The solder mask ring results some unsoldered area under the QFN component, which looks like more voiding at the QFN thermal pad.

Initially Published in the SMTA Proceedings

Comments

Overall very interesting article. I didn't know 9mil drill can be used, usually, all drills come in 2mil increments, i.e 6, 8, 10... The solder mask ring, i.e via tenting on the component side, reduces the voids but also greatly reduces the solder applied on the thermal pad - so this not is a good practice especially today that VIPPO can be used to avoid this problem of solder mask wicking completely.
Guy, ePiccolo Engineering

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