First motor test has been a success!

I tested each of the six motor outputs and all of them are working. In the video, I have a simple PWM program running that increases the speed to 40% of the motor's fill thrust, and then decreases it to zero. You can hear the motor changing speed in the video. it never fully stops because there is a only a tiny delay between zero, and the speed increasing again.

The last log is downright easy compared to what we must accomplish now. 

Soldering a large 44 pin QFN package might've seemed difficult, but soldering the QFN IMU sensor is a different league.

I might be exaggerating the difficulty because I am new at this, but the MPU-9250 is tiny compared to the microcontroller. Luckily the pin spacing is the exact same as the ATmega at 0.5mm.

For a good reference, the MPU-9250 is 1/8 the size of my fingernail.

Yep that tiny black spot above my fingernail is the sensor.

Things to look out for:

Soldering the MPU-9250 needs special attention. Looking at the manufacturer's guidelines it says that the ground pad should not be soldered in projects that undergo a lot of strain. I would say a drone that is meant to crash into things and survive is a strainful situation! Keeping this in mind, DO NOT put solder on the ground pad(the exposed pad directly underneath the sensor package).

In addition, the application guidelines from before states that "Package stress can be introduced from thermal sources during soldering or reflow processes. Uneven thermal expansion of packaging materials (e.g. sensor package) and cooling during the assembly process introduces this stress."

This is very important! Uneven heating and cooling could cause inaccurate sensor readings, so make sure the package is being EVENLY heated during the hot air rework. 

Once you're done using the hot air station, do NOT use a soldering iron to fix bridges between pins because it would introduce heat to a specific point on the package. Instead, apply a good dose of flux directly onto the package and reheat it using the hot air station. This should remove any bridges.

I am going to have an horrendous time soldering this PCB. I have soldered SMD passives before, and I find them generally pretty easy, but I have never soldered QFN before, and this is my first time. I will document my journey of soldering QFN components here. Make sure you have a fine tip soldering iron, and a hot air station. The one I ordered was extremely cheap, and was a Hakko knockoff from Amazon. Here's the link for people who are interested.

First Attempt:

My first attempt was a total failure. I tried to go the cheap route without solder paste and stencils, and used the video below to help me. 

It looks so easy right? Wrong! I only soldered components essential for the microcontroller, and the microcontroller itself. My ISP programmer couldn't even recognize it because the pins weren't all soldered correctly. I desoldered the microcontroller and kept trying over and over, reballing the QFN pads every time. I finally got the ISP to recognize and program the microcontroller. Huzzah!

My victory ended there however, because I soon discovered I could not reprogram through the USB, and the sample LED blink program wasn't working. I reasoned it was once again bad soldering because the schematics were perfect.

---

Second Attempt:

Realizing going the cheap route wouldn't work, I ordered myself some solder paste, and hunted for cheap stencil manufacturers.

After some digging I found a website called oshstencils which despite similar naming is NOT related to OSHPark. The order process was incredibly easy, and the stencil itself was quite cheap. Only $5.63!, in fact the shipping cost more than the stencil itself with USPS Priority Mail costing $7. 

With the combination of a hot air station, a solder stencil, and a tube of solder paste, I got the microcontroller working on the first try! Everything worked perfectly including the USB and outputs.

Once the hard part is done, soldering the passive components is pretty easy with a fine tip soldering iron, and the SOT 23-5 packages in the form of voltage regulators are very easy to solder as well.

I actually received the PCBs quite a while ago, but didn't bother to update project log so I will now.

Just received the glorious Oshpark gold and purple PCBs.

This thing already looks like a nightmare to solder. I might go through how I did it in my next log.