To ensure that cars meet current safety and reliability requirements, the automotive industry requires original equipment manufacturers (OEMs) to perform 100% post-assembly automated visual inspection (AVI). In the case of quad flat no-lead (QFN) packages, solderable or exposed pins/terminals are less visible, making it impossible to verify that they are successfully soldered to a printed circuit board (PCB). The package edges have copper for the terminals and exposed, which are easily oxidized, which makes it difficult to wet the sidewall solder.
When using a QFN package, the sidewall solder coverage is between 50-90%. OEMs are bound to incur additional costs due to problems caused by incorrect assembly failures, as well as real failures caused by the apparently poor solder joints in the assembly process. Using an X-ray machine to check for high quality, reliable solder joints adds further cost or is simply not possible.
To address the problem of side lead wetting in leadless packages used by automotive and commercial component manufacturers, wettable flank processes have been developed. This process provides a visual indicator of weldability and reduces inspection time. The TI LM53600-Q1 and LM53601-Q1 automotive DC/DC buck regulators in DFN packages use a wettable flank process that is recognized by many of the largest automotive OEMs.
During the assembly process, TI used a special lead coating (SLP) as an additional step. During this process, the package was stepped and then re-plated with matte tin at half the side of the sidewall. See Figures 1 and 2.
Figure 1: Cross-section comparison between a standard QFN and a cut and coated QFN with wettable flanks
Figure 2: Partial cut of a QFN package and re-plating at half of the sidewall position - the image on the right is a partial enlargement
Tin plating provides a protective layer for the exposed copper. During PCB assembly, solder joints extend from the lower portion of the pad to the sidewalls to form an enhanced solder joint between the component and the board. AVI can now evaluate the appearance of solder joints on each side of the device. The presence of side welds indicates a higher probability of a higher solder joint integrity, but this inspection of the component failure rate per minute (PPM) is not guaranteed without x-ray inspection. Due to poor solder or PCB pad oxidation during printing, solder wettability (or shrinkage) is still present under the component, which is best estimated for other non-QFN devices using the same assembly method. Tinned PPM for PCB pads.
Figures 3 through 6 highlight the solder joints between a QFN leadframe and a PCB with a clearly exposed solder toe, which helps AVI and eliminates any incorrect assembly failures.
Figure 3: Side view of a standard QFN package
Figure 4: Weld toe on standard QFN
Figure 5: Standard lead frame package sidewall
Figure 6: Weld toe on a standard leadframe package QFN
In summary, you can see that there is no difference in performance or quality when using a wettable flank process. TI's LM53600-Q1 and LM53601-Q1 automotive DC/DC buck regulators include a reliable solder joint and are capable of passing the stringent 100% AVI requirements currently specified by the automotive industry.
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