Though 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 lead-free process, the issue of voiding becomes even more problematic.
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 where there are bumps or a QFP where there are leads, the QFN provides no standoff so there is nothing to absorb stress or allow for 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.
Resolving the QFN voiding challenge may not be as difficult as it seems, however. Henkel’s 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, Henkel has developed some innovative, low-voiding solder pastes that 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 we have successfully shown that using a low voiding solder paste and an optimized reflow profile are the best proven routes to QFN void reduction, more detailed analysis of QFN voiding behavior is certainly warranted to fully understand this issue.
