I’ve seen 3mm x 3mm QFNs that can output 5V at 800mA, which used to require a big TO-220 package and a heatsink. The silicon dice sits right on the thermal pad, thereby allowing for some impressive thermal performance. Most QFN and DFN parts have a large metal contact area on the underside, just inside the rows of signal contacts, as shown in the image above.
This allows for improved signal characteristics and excellent thermal properties. The smallest of these packages end up with very short lead wires. Physically small size and footprint aren’t the only advantages to a QFN. BTCs aren’t new, but they are becoming more and more common due to the proliferation of mobile and wearable electronics. They’re used whenever space is at a premium. This means that all of the leads are under the part. The QFN (Quad Flat Pack, No lead) and its diminutive cousin the DFN (Dual Flat Pack, No lead) are in a class of components referred to as BTC (Bottom Terminated Components).
It’s far better to design the proper footprint in the first place. We usually catch and fix them before they hit the machines, but if we don’t, then it’s time to call the rework folks in. In fact, incorrect QFN footprints are one of the most common design problems we see coming into our factory. A good “makes me sad” occurrence would be when PC boards with improper QFN footprints show up at my door. What better job for a hacker/maker than one that includes surface mount assembly robots in the building? Every now and then, however, something happens that makes me sad. With a little extra care, you may be able to shrink your board a little and save some of the fabrication costs. QFNs can be a bit intimidating, but they’re here to stay.
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