Soldering components to the PCB and teaching YOU how to solder Part 2

A project log for The UnbreakaBLE Micro Drone

Use any Bluetooth 4.0(BLE) device to control this agile little modular drone.

Anshul SanamvenkataAnshul Sanamvenkata 07/24/2017 at 04:130 Comments

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.