I took the plunge and decided to upgrade the processor in my Devo7e transmitter to a STM32F103RET6. This upgrade is not yet officially supported by the DeviationTX team yet, but I figured it was doable as the upgrade doesn’t really affect the code much and I’m fairly comfortable with compiling from source code. In this post, we’ll look at the hardware end of the upgrade.

Desoldering and resoldering a SMD processor isn’t really that hard and the potential for damaging your board can be minimised if you take a few precautions that I’ll be outlining here.

You can choose to upgrade to either a STM32F103RCT6 (256kb Flash/48kb SRAM) or a STM32F103RET6 (512kb Flash/64kb SRAM). In my case I went for the STM32F103RET6 as I had a spare lying around on a development board from another project.

Hardware needed

Some tools you’ll need for this:

• Hot air gun
• Soldering iron
• Solder
• Solder Paste
• Soldering Flux (liquid no clean type works best)
• Soldering Wick/Braid
• Kapton Tape
• Aluminium Foil
• Aluminium Tape (optional but recommended)
• Isopropyl Alcohol
• Toothbrush
• Magnifying Loupe
• Tweezers

Protecting the main board

Since you are here and thinking of attempting the processor change, I’ll not go through the steps involved to remove the main board from the transmitter chassis. Suffice to say, it must be removed or you could seriously damage the chassis.

The first thing that we do is to tape a few pieces of Kapton tape to hold down and protect the other SMD components that are near to the processor that we want to desolder. This prevents us from accidentally knocking off an adjacent component during removal and installation of the processor.

It is important to only use Kapton tape and not some other tape as the area will be under high heat. You really don’t want a molten plastic mess on your board.

Next we use pieces of aluminium tape to shield the immediate surrounding area around the processor followed by aluminium foil to shield the rest of the board. At the end of all this, you only want the processor and it’s pins visible and exposed.

Removing the processor

Now we are ready to use the hot air gun to remove the processor (this can also be done with a solder rework station, but if you have a rework station, you really don’t need to be reading this).

Turn on your hot air gun to the lower setting and hold it about 10cm or so above the exposed processor. After about 1 minute, use your tweezers to try to gently lift the processor. It may come off or may take longer. This step takes patience, constant checking and a bit of gentleness so that you don’t accidentally damage anything. Eventually the solder will melt and you can easily lift off the processor. If for some reason, it still won’t budge after 3 minutes, you may not have a hot air gun that can get to the required temperature (about 280 degrees celsius). If that’s the case, find another hot air gun.

After you remove the processor, use the soldering wick/braid and your soldering iron to clean up the pads. Applying some flux at this stage makes the cleaning process go a lot smoother. Don’t do it for too long as you run the risk of burning the board, just clean it up of any bumps that the previous solder left so that it is easier to position and align the new processor.

Solder on the new processor

Let things cool down a bit before you start this part. We basically want to apply a small amount of solder paste to the pads. We don’t need much so I generally dilute my paste with a bit of liquid flux to make it easier to spread over the pads. The extra flux also makes the solder flow better during heating. Don’t go crazy with the paste here cause too much paste can create a mess of solder bridges that then needs to be cleaned up.

With the paste on the pads, gently place the processor directly from the top with your tweezers. This will minimise the paste getting pushed around to unwanted areas. Check that the big dot on the processor is in the corner that is diagonally cut off on the board. This is pin 1. After it is down, gently nudge it around with your tweezers to get all the pins properly aligned. Use your magnifying loupe to check and recheck until you are sure that every pin is aligned. This step is very important so do not rush this.

If you find that the soldering paste has obscured your view, use a toothpick to scrape some away so that you can see the pads aligning with the pins.

After making sure all the pins are aligned, we will again use the hot air gun. Hold the hot air gun (on low setting cause we don’t want to blow the processor off the board) about 10cm above the processor. Watch the solder paste and look for it to start melting. As it melts it will start to draw itself onto the pads and pins. Look to make sure that this has happened for all 4 sides then remove your gun.

Check & clean up

Let things cool down again and inspect all the pins with your magnifying loupe to check again for pin alignment and solder bridges (2 or more pins shorted together by excess solder). If you find any, try applying some flux and dragging your soldering iron over those pins. This can help to clear it up. If there is just too much solder, then apply some flux and gently use your soldering wick/braid to wick the excess solder away. Check and recheck this, cause any shorted pins can damage both the new processor and other components when power is applied.

Once you are happy with your work, remove the aluminium foil and Kapton tape. Apply some isopropyl alcohol to the processor area and use your soft brush to clean the area up. Check in the corners under the processor to make sure no solder paste was left behind there. Once this is done, you can get ready to flash the Walkera bootloader onto the new processor.

Read this post for instructions on flashing the bootloader.