Voiding in bottom-terminated components is a big challenge in the industry today because when you have a significant amount of solder, you also have a significant amount of flux. Flux during reflow volatilizes and creates voids. One of the unique ways to reduce the voiding under a bottom-terminated component is to actually use Solder Fortification® Preforms. A Solder Fortification® Preform is basically a small pellet of solder typically in the size similar to an 0402 or 0201 component. What you would do is you would actually print solder paste as you normally do on your circuit board. Place the solder fortification preforms, place the QFN component on top, go through your normal reflow process.
The addition of those Solder Fortification® Preforms actually creates a slight angle to the QFN component when it's placed down. That angle allows those volatiles to more easily escape the molten solder during the reflow process. That ability for those volatiles to escape means the resulting solder joints are going to have much less voiding. For more information on Solder Fortification® Preforms and ways to Avoid the Void™, visit our website www.indium.com.