Brief description of QFN package
QFN (quad flat no leads package) is a pin free package, which is square or rectangular. There is a large area of exposed pad in the center of the bottom of the package for heat conduction, and there are conductive pads for electrical connection around the package periphery of the large pad. Because QFN package does not have gull wing lead like the traditional SOIC and TSOP package, the conductive path between the internal pin and the pad is short, the self inductance coefficient and the wiring resistance in the package are very low, so it can provide excellent electrical performance. In addition, it provides excellent heat dissipation through the exposed lead frame pad, which has a direct heat dissipation channel to release the heat in the package. Usually, the heat dissipation pad is directly welded to the circuit board, and the heat dissipation vias in the PCB help to diffuse the excess power consumption into the copper grounding plate, so as to absorb the excess heat. The following figure shows the QFN packaging details. In fact, when we build the package, we still need to build it according to the datasheet of each chip.
QFN poor welding
A large pad in the middle is commonly known as exposed pad or thermal pad. I usually call it EP. In PCB layout, the via matrix is often punched to the ground for heat dissipation. Now we can see a picture of poor welding. It can be seen that the chip welding of this QFN package is not optimistic.