SSF glove update: hadokenIMU

As always, prototypes have mistakes.

The first glove IMU prototype had a couple of showstoppers. In particular, the KiCad symbol for a USB connector has the D- and D+ lines reversed. I was also using a standard USB pinout, not the 5 pin connector that mini- and micro-USB has. With these problems fixed, and a few other modifications such as moving the MPU-9150 sensor chip out from underneath the metal radio shielding, it was time for another prototype run.

Using a stencil for one board was a lot of work, so this time I used a solder paste syringe. Even though the needle was not as small gauge as it should have been, applying the solder paste was much easier and more consistent than my attempt at a stencil with card stock and paste that wasn't warm enough. Unless I'm making many boards, this is the method I'll use in the future.

The assembly method was the same as last time: apply solder paste, place parts with tweezers, use the hot plate to reflow the solder, clean up bridged joints. This time there were far fewer solder bridges, and finding them was easy using a USB microscope.

Fixing them was as simple as wicking away the extra solder with copper braid, applying some flux, then just touching each solder joint with an iron.

In the end, I assembled two boards. On the first board I left the MPU-9150 chip out; I wanted to make sure the rest of the board was tested before committing an expensive chip. Initially I had the same problem as I did on the first prototype: the ATMega32U4 would accept an Arduino bootloader via ICSP, but it would not work over USB. On the first prototype, this was due to the D- and D+ USB lines being reversed; in this case, the 32U4 pins weren't soldered well enough. Adding solder to dry joints fixed the problem. Once this was solved, I tried adding the MPU-9150 using a hot air station, but ended up lifting the pads on the board.

Where did the pads go? On the bottom of the QFN chip!

The second attempt included the MPU-9150 as part of the hot plate reflow process. Fixing the solder bridges on the QFN pads involved the same process as on the larger components and wasn't difficult.

The good news is that the hadokenIMU board works. I tested the sensor with the example programs from Jeff Rowberg's I2C Device Library, and it communicates data back to the Arduino program over I2C without any problems. (Note: the MPU-9150 is pin-compatible with the MPU-6050, and the example program there will work with the 9150.)

The assembled IMU board is just smaller than the 1000mAh battery it uses:

The design files have been uploaded to the project repository on GitHub, details and documentation will be posted soon.