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REWORK STATION 1 CH 110V
SparkFun Electronics
How to Solder a QFN Component Using a Hot Air Rework Station
2021-08-16 | By ShawnHymel
License: Attribution Solder
Electronic parts with leads fully underneath the component body are tricky to solder by hand, as they have no exposed pieces to make contact with a soldering iron tip.
Common names for these parts are micro leadframe (MLF). If only 2 sides of the part have leads, it will often be called a small-outline no leads (SON), and if 4 sides have pads, the part is often named quad-flat no-leads (QFN). Other name variants (usually vendor-specific) can be found here.
In this tutorial, we’ll show you how to solder these parts by hand with a hot air rework station. You can watch me perform these steps in the following video:
Required Tools
You will need the following tools:
- Hot air rework station
- Soldering iron
- Solder
- Flux
- Solder wick
I recommend using “no-clean” flux and “no-clean” flux core in your solder, as it is difficult to reach underneath the QFN component to clean other forms of flux after you are finished soldering.
Required Parts
To practice this technique, I created a simple PCB in KiCad. The design files (including Gerbers) can be found here. I had the board produced by DKRed.
You will want a 16-pin QFN part with a 0.5 mm pitch to solder to this partiular PCB. If you’d like to check the operation of the board, I recommend ordering the following components:
Reflow Profile
I recommend finding a soldering reflow profile for your component and/or solder. As the particular LDO I used did not have a reflow profile, I looked for one for the solder I used: SAC305 lead-free, no-clean flux core solder. This document gives us a good starting point to use for this particular solder.
Here is the chart we will use to figure out how long to heat our PCB and component:
We want to heat the whole board up to about 150°C over the course of 1-2 minutes (known as the “soak zone”). This is to avoid thermally shocking the PCB and any other components, which might cause things to crack.
We then want to raise the temperature of the solder (just for the QFN component) to the melting point (217°C) in about 20-30 seconds.
We then want to hold the solder temperature between 225-250°C (reflow zone) for no more than 60 seconds while we work the part (e.g. make sure the solder makes good connection to the correct pads and the part is aligned).
Finally, we want to slowly cool the part and board off to prevent thermally shocking anything. We can do this by slowly moving the hot air nozzle away from the part for about 15-30 seconds to cool the part to about 150°C. We can then remove the nozzle completely and let the PCB air cool to the point we can safely touch it.
Pre-tin Pads
To start, carefully add a thin layer of solder to all of the pads on the footprint. You will want them to be as even as possible so that the part does not rock and lose connection to other pads.
You can use solder wick to remove areas with too much solder or to start over.
Thermocouple
You can attach a thermocouple near the footprint using some heat-resistant tape (e.g. Kapton tape).
Note that to accurately measure the temperature to compare it to your reflow profile, you would need to embed the thermocouple tip inside a blob of solder that’s being used to make a connection from a PCB pad to a component pad. However, many thermocouple probes are too large, so the best we can do is have the probe tip near our component. This will at least give us good ambient temperature readings for getting the board to 150°C.
Flux
Apply flux to all of the soldered pads.
Place Part
Carefully place the component on the footprint, doing your best to line up the part’s pads with the footprint’s pads.
Soak Board (with Heat)
Set your hot air station to about 300°C and an airflow of 50-60%. The exact temperature and airflow will require some experimentation. You want the temperature to be higher than your reflow temperature. With faster air speed, you can heat things up more quickly, but you risk blowing away parts. I found that 50-60% for my station worked well.
Hold the nozzle about 3-4 inches away from the PCB and draw circles in the air so that you soak the whole board in hot air. Do this for 1-2 minutes to slowly get the board temperature to about 150°C.
Reflow Solder and Work Part
Over the course of 20-30 seconds, slowly bring the nozzle to about ¼ - ½ inches away from the part while still making small circles (you want the hot air to reach all sides of the component). The solder should melt (reflow), and you can slightly tap the component to have it snap to the pads.
I like to slightly tap down on the center or corners of the part with tweezers to make sure it has fully seated into the molten solder.
Cool Board
Slowly pull the nozzle away from the board over 15-30 seconds, still drawing circles to apply heat to the whole board.
Once the board has reached 150°C, you can remove the hot air altogether to let the board air cool. Be careful, as it’s still likely hot to the touch!
Inspection and Rework
Use a microscope or loupe to inspect the solder job on the sides of the component. Even if you can’t see the entire connection, you can still look for gaps (no solder) or bridged solder joints.
If you find any bridging, you can rework the part by repeating the above process (don’t forget to apply more flux!). You can also add flux to the side with the bridging and drag a hot soldering iron across the pads to (hopefully) break the bridge.
I do not recommend using solder wick, as you run the risk of removing too much solder under a pad and breaking the connection.
Once you are done, you can use a multimeter to probe the connections (if test points are available on the PCB) to look for shorts.
Solder Other Components and Test
Once you are happy with the soldering job of the QFN part(s), you can solder all the other components and test the board! For this particular board, I supplied VIN with 5V and measured 3.3V, 2.5V, and 1.8V on the outputs.
Recommended Reading
I hope this helps you get started soldering some of the trickier QFN parts by hand! My preference is to use my hacked reflow oven, but if you don’t have access to one of those, this hot air technique might work. Because this method is time-consuming and hard to inspect, it’s only something I would do for prototypes (i.e. not production units).
There are other ways to tackle assembling QFN parts than what I’ve shown in this guide. For example, Nate from SparkFun published a method a few years ago for pre-tinning the pads on the part instead of the PCB.
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