Quad Flat No-Lead Chip
This post builds on my previous post, which discussed hand soldering surface mount passives and QFP chips. As I mentioned last time, these are vital skills for any circuit board designer – the ability to modify or rework a board is integral to a time-effective development process.
Today I’m going to talk a little about hand soldering QFN chips – these chips are extremely small, which can be very useful for tiny, densely packed boards but makes them a little difficult to work with by hand. As a reminder, these posts are building on some excellent tutorials created by Curious Inventor. The relevant information for QFN chips can be found here.
Quad Flat No-lead (QFN) chips are like QFP chips but don’t have leads extending beyond the sides of the chip; instead, they have pads on the bottom of the chip. For these types of chips, however, my experience diverges from the Curious Inventor tutorial a bit. Their method certainly works but I have found it easier and more effective to use solder paste with the hot-air station as opposed to tinning the pads with actual solder. In terms of equipment, you will need good tweezers, a hot air source (either hot air gun or dedicated soldering rework station), solder paste and solder flux.
The technique that we have found works best involves the following steps:
- Clean the chip and circuit board pads and make sure that the chip can lie flush on the board.
- Apply solder paste on all of the circuit board pads using an applicator syringe (below). Make sure there is paste on all pads but use sparingly – too much could lead to shorts.
- Place the chip in the correct position on the board using tweezers. The solder paste may make it difficult to see if the chip is correctly aligned with the pads but use silkscreen markings on the PCB to index the chip’s location. Fortunately, surface tension effects with the large center pad will help the chip self-align as long as it is close to being in the correct position. Make sure that the chip is firmly pressed down on the board and is flush or as close as possible to it.
- Preheat the chip and board using the hot air gun to about 200 degrees Fahrenheit. Depending on your setup, this can be accomplished by varying the heat and flow rate of the gun or changing the distance between the chip and the hot air nozzle. The chip should remain in place without fixturing as long as the board is horizontal and the hot air is coming orthogonally from above.
- Turn up the heat to about 350 or 400 degrees. Once you see the solder melt, continue applying heat for up to 15 seconds more. You want to be sure that the solder has fully melted and reflowed to all of the pads but too long and you can overheat and break the chip and / or board.
- There will probably be excess solder along the sides of the chip. Apply flux and use a soldering iron and wick to remove this solder. Try to remove all accessible solder, not just visible accumulations.
- Verify that no shorts are visible with a loupe or microscope. Professional shops sometimes use x-rays to check underneath the chip and if this kind of equipment is available, it can be very helpful. However, in our experience it is not necessary.
QFN chip mounted on a PCB
I’d love to hear what works (or doesn’t) for others so feel free to add your two cents in the comments.