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Analysis of Voiding Under QFN Packages



Analysis of Voiding Under QFN Packages
Paper covers results of experiments on QFN devices when the ensuing voiding level was calculated by x-ray inspection.
Analysis Lab

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Authored By:


David Bernard
Dage Precision Industries, Fremont, CA

Bob Willis
ASKbobwillis.com, Chelmsford, Essex, United Kingdom

Martin Morrell & Matthew Beadel
Artetch Circuits, Littlehampton, West Sussex, United Kingdom

Summary


The Quad Flat Pack No Leads (QFN) type of leadless package, also known as Land Grid Array (LGA), is rapidly increasing in use for wireless, automotive, telecom and many other areas because of its low cost, low stand-off height and excellent thermal and electrical properties. The implementation of any new package type always results in a learning curve for its use in design and processing, especially for the Process and Quality Engineers who have to get to grips with the challenges that these packages bring. In particular, the central termination of these QFN packages are prone, in most practical experience, to exhibit a high, or even excessive, level of voiding when seen under x-ray inspection.

Such excessive voiding can not only affect the package‟s thermal performance during operation, but it can also increase the stand-off height from the board making the QFN float higher on the solder surface. Such action can apply stresses to the device outer terminations causing them to no longer remain planar and affect joint quality. Therefore, monitoring central termination voiding provides a valuable method to qualify the presence of unsuitable stand-off heights which, in turn, may increase the propensity for open joints during production.

This paper will discuss the results of experiments being undertaken on identical QFN devices where the quantity of solder under the central termination was varied and the ensuing voiding level calculated by x-ray inspection. The results will be discussed in line with correlating this data as a method to provide a suggested upper limit for QFN central termination voiding so as to minimise the possibility of open joints in production.

Initially Published in the IPC Proceedings

Comments

I have done some similar things as discussed in this paper to keep the center ground pad in tow and the QFN package level. Instead of a solid ground pad, I have a grid pattern of maskless squares of about 0.8mm in size that sit on a solid copper pad. There are also unmasked vias to help with flux vapor venting. About 60% of the surface makes contact with the ground solder pad.

This helps assure the center area does not stand taller than the outside pads, as the solder gets wicked away, pulling the part downward as it cools. At least that has been my observation and my way to help produce consistent results, and to reduce trapped flux residues around the edges of a QFN package.
Andrew Perkins, Sr. Product Development Engineer, Datamatic, Ltd.

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