January 21, 2013, anysilicon
QFN package is probably the most successful package type today. Offering low price, excellent performance and small size, it is an ideal package for many applications.
QFN (quad-flat no-leads) is a plastic SMT package consisting of: a leadframe, single or multiple dies, wirebonds and a molding compound. The leadframe is the heart of the package and therefore the size and the dimensions of the leadframe determine the overall package performance.
The following diagram shows the necessary steps in the assembly process of QFN package.
All QFN’s are leadless packages where electrical connections are made via pads on the bottom
surface of the package. These pads are soldered directly to the PCB.
The standard QFN has a paddle (exposed pad) which enhances the thermal and electrical characteristics enabling heat transfer and RF applications.
Downbonds are wirebonds which are connected directly to the exposed pad, typically for better ground connectivity.
Punched and sawn QFN are slightly different, the illustration below shows the difference in the molding compound shape. The shape is a result of the different simulation process, and normally does not introduce any issues in the performance or in the target PCB soldering process.
In addition to the excellent electrical performance QFN provides superb thermal performance through the paddle (exposed pad), which allows a direct thermal conductivity for removing heat from the die.
It’s important to note that QFN package is relatively small thus the capabilities of dissipating heat without air flow are somewhat limited.
Thanks to the benefits of QFN package and the successful market penetration, most of the assembly houses provide QFN packaging services; however, they are using different names.
Read more about copper wirebonds benefits here