A PCBA was provided to SEM Lab, Inc. for evaluation of some QFN solder joints. This is a BSE SEM image of a microsection of the QFN.
The solder joint geometry appeared to be acceptable in this section. There was a uniform layer of Cu-Sn intermetallic at both the solder/pad and lead-frame/solder interfaces suggesting that the reflow profile was acceptable. The interfacial intermetallic layers also suggest that there was no solderability problem associated with the QFN or PWB pads.
This is a 2nd section plane that included the thermal pad under the QFN. The level of solder fill in the PTH-vias under the thermal pad was variable.
The “toe” of the solder joints shows an excess-solder condition possibly because of the balance of forces caused by solder surface tension between the signal solder joints and the thermal pad, which determines the solder joint height.
One possible solution for mitigating these issues is to fill the PTH-vias as illustrated in the image below.