The neverending quest to jam even more functionality into smaller packages has increased the popularity of yet another high performance device that is quickly gaining popularity among handheld product designers and manufacturers. It is the QFN: an acronym that stands for quad flat no-lead. Patented in 1999, QFN use has steadily increased in recent years, especially with the proliferation of smaller multifunction handheld devices. QFNs are just as the name describes: a flat plastic package with perimeter leads underneath the device and a large pad in the center. Basically, it’s a QFP (quad flatpack) with no leads; connections are made by soldering the perimeter lands underneath directly to the pads on the PCB. In addition to their small form factor advantages, QFNs offer excellent electrical and thermal performance.

While these devices provide clear benefits there are, of course, some challenges as well. At the package level, there are manufacturing hurdles to overcome such as issues with wire bonding on polyimide and the die to pad ratio effect on JEDEC performance. And, once the devices are made, the next challenge is assembling them onto the PCB and ensuring long-term reliability of the assembly. For the purposes of this discussion, we’ll focus on the assembly issues and how to best resolve them. Although the geometry of the QFN is, in part, what makes it appealing, it is also the cause of one of its greatest assembly problems: voiding. When you couple a QFN with a Pb-free process, the issue of voiding becomes even more problematic. Here’s why.

There are arguably many variables that contribute to the increased voiding characteristics of SAC alloy solder joints. Strictly speaking from a materials perspective, though, the problem has to do with the proclivity of SAC materials for volatile formation. SAC alloys form more gasses and these volatiles cannot escape as easily from a molten SAC alloy as they can from a conventional SnPb alloy. They have to travel a greater distance to escape and, therefore, become trapped inside the solder joint and form voids. When this condition is combined with the unique geometry of the QFN, voiding may become even more prevalent. Unlike BGAs, which have bumps, or a QFP, which have leads, the QFN provides no standoff, so there is nothing to absorb stress or permit volatile escape. What’s more, the pad in the center of the QFN, which is primarily used for thermal transfer, presents large area soldering challenges and, consequently, issues with voiding. Because there is such a large surface area and no standoff to allow volatiles to escape, these gasses may become entrapped and cause void formation. Though many would argue that some level of voiding is acceptable, our company’s stance has always been that reducing voids as much as possible is the best approach. Plus, with QFNs the voids aren’t just a problem from a mechanical perspective, but can result in thermal transfer impedance issues as well. This can lead to resistive heating and, if the voids are sizeable, hot spots can develop and may lead to thermal damage of the device.

Resolving the QFN voiding challenge may not be as difficult as it seems, however. Using a two-pronged materials-based and process-based approach, our company has successfully reduced the incidence of voiding in QFNs in both laboratory and high-volume production environments. Our work has revealed that modifications to the solder paste flux system can significantly reduce void formation. The flux’s solvent concentration and boiling temperature, flux content and flux activator concentration all play a role in volatile formation. By altering the flux system to reduce volatile generation, voiding is lessened significantly. Using this technique, low-voiding solder pastes have been developed and will, undoubtedly, enable the QFN to be the powerhouse package it was intended to be. A low-voiding solder paste in combination with optimized reflow profiles is clearly the best method for ensuring void reduction.

I would be remiss if I didn’t also mention the potential impact of varying the print patterns for the QFN’s center pad as another possible void reduction mechanism.  Depending on the size of the device, limited success has been realized through printing a pattern – such as a snowflake or cross – instead of covering the entire pad with paste, which may allow for some area through which gasses can escape.

While it has been proven that using a low voiding solder paste and an optimized reflow profile are the best routes to QFN void reduction, more detailed analysis of QFN voiding behavior is certainly warranted to fully understand this issue.

Renzhe Zhao, Ph.D., is a senior applications engineer at Henkel’s Electronics Group (henkel.com); renzhe.zhao@us.henkel.com.

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