QFN (Quad Flat No leads) packages are the devil’s work. As the name implies there are no leads, just tiny pads on the bottom of the chip where it’s almost impossible to get a soldering iron or test probe to.
I found a couple of posts which suggest you can solder these bad boys with a soldering iron by positioning the chip, then ‘floating’ solder along the PCB trace to make the connection under the chip (with help from a lot of flux). I was able to use this technique to fix a broken connection, but not for the entire chip. This also doesn’t work if you need solder on the center pad.
- On a scrap piece of circuit board work out what distance you need to hold the hot air gun from the circuit board and for how long before the solder melts. For my setup (an Aoyue Int 909) with air temperature and flow speed set at 50%, this was holding the gun 1 inch from the PCB for 30 seconds.
- Apply flux to the PCB and tin each PCB pad, including the center. Its difficult to know how much solder to apply: too much and you’ll get shorts between pads, not enough and you’ll get bad connections. I aim for a very slight dome of solder. Make sure you have a particularly small amount on the center pad, as that is prone to shorting. If you’re etching your own board its also worth decreasing the size of the center pad on the PCB to reduce the probability of center pad bridges.
- Apply flux to the chip and tin the chip’s pads. Make sure every pad has small amount of solder on it – this makes it much more likely that you’ll get a good connection later. Be careful not to get too much on the center.
- Apply plenty of flux to the PCB, and position the chip.
- Apply heat with the air gun using the parameters you worked out earlier (1 inch and 30 seconds for me). Move the air gun in small circles to make sure the heat is applied evenly. The chip may move around with the air flow, so tap it back into position with the tweezers as necessary (or reduce the airflow). Note that you’re supposed to follow the manufacturer’s soldering thermal profile (usually 2 minutes at a low temperature to allow the internal parts of the chip to expand with the heat, followed by 30 seconds at a high temperature to melt the solder). This is worth doing if you have a thermocouple to measure the temperature near the chip (otherwise you risk frying the chip or the PCB, as I’ve done a couple of times).
- When the solder melts, tap the chip into position. Its important to tap down on the top of the chip a few times – this should cause the chip to snap into position because of the solder surface tension.
- When its cooled, check for shorts with a multimeter. If you’ve got a short, use the air gun to remove the chip, use desoldering braid to remove the solder, and try again. The first time I tried this it took me 4 attempts to get it working without shorts (amazingly the chip still worked).
- Check the connections visually. If you see a suspicious looking connection you can use your soldering iron and some flux to ‘float’ solder along the PCB trace into the join.
Here’s the finished product, a BQ24123 Lithium Battery Charger: