Using a reflow oven (custom made), I was able to assemble the blue tooth watch board with the system on chip (SoC) part, lithium ion battery charger, LEDs, connectors, etc. This board is the main board doing the signal processing and Bluetooth 4.0 Low Energy radio transmission of the heart rate data to a smartphone.
The crucial part to assembling a board with QFN package parts is the solder mask stencil. The QFN parts are 0.5mm pitch (0.5mm from the middle of lead to another) which makes it almost impossible to solder without a reflow oven or a reflow heat gun.
I obtained the stencil from pololu.com. For one sheet with 1.23″ by 1.23″ board size total cost was about $30 with shipping. Although what you get seems like a transparency paper with some holes, it is definitely worth the time saved during assembly.
Read More: Prototype of Pulsar Heart Rate Watch Board