QFN failures are increasingly important as more devices are offered in this package style, sometimes exclusively. There are some notable manufacturing issues related to assembly processes for QFNs, and this post discusses two of the more common ones that cause failures.

Trapped conductive residue under the device

The capillary gap under assembled QFNs is typically very small and difficult to clean, so solder flux residue and moisture lead to corrosion and electrical leakage causing what initially looks like a device failure. The example below shows this type of condition and resulted in failure.

Conductive residue and corrosion product trapped under QFN device.

Another example shows several QFN signals bridged by conductive residue.

Conductive residue bridging several signals on the assembled QFN.

CCE mismatch induced warpage and subsequent failure of solder joints

The QFN package typically contains a relatively large volume fraction of silicon relative to other package styles, which creates a significant CTE mismatch between the QFN and the board. In addition, the solder joint height is minimal resulting in increased stiffness of the solder joints. These factors can lead to substantial residual stress in the solder joints. Any additional mechanical stress due to depaneling or ICT probes can overload the solder joints resulting in fractures. The example below shows a fractured joint that popped up indicating there was residual elastic strain in the QFN and PWB after assembly.

Fracture QFN solder joint.
Higher magnification view of QFN solder joint fracture.

The examples of manufacturing issues related to assembly processes for QFNs discussed above suggest a need for rigorous process validation for both solder reflow and cleaning processes.

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