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Registered Member #29
Joined: Fri Feb 03 2006, 09:00AM
Location: Hasselt, Belgium
Posts: 500
You'll need 4 things to reliably solder QFNs. 1) Stereo microscope, magnifier (or extremely good vision) for placing the component. Alignment should be close..when the solder paste melts, surface tension will perfectly align the part if the footprint is laid out properly. 2) Solder paste (RoHS stuff is OK) 3) 200 deg C hotplate for preheating the PCB (this limits the high-temperature time your component experiences..reduces reflow time. 3) Hot air heatgun for actually doing the reflow.
Place the part, then preheat the pcb on the hotplate. Set the hot-air heatgun to about 450 deg C with moderate airflow. After the PCB heats on the hotplate for a couple of minutes, apply the hot air to the component. You should see the component "jiggle" a bit when the solder melts as surface tension corrects the alignment and the part is properly attached to the pins on the PCB footprint.
It is critically important to not apply too much paste. Use the microscope to verify a thin but even distribution of solder paste over the footprint area. Apply the part and rub it around a bit before aligning it to ensure a good distribution of paste and flux on the part and PCB footprint.
It takes practice, but after a few goes, you should be able to solder QFNs in one go without short circuits.. (always test for short circuits after soldering)..
If it doesn't work the first time, unsolder the part, clean the footprint and part with solder wick and repeat the process..
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Solder Pastes
