Because two of the components are QFN-32 packages, it's important to obtain a paste stencil and use it properly to insure the correct amount of paste is applied exactly in the right spots.

Note, that it is incorrect to apply paste to the entire exposed pad on the bottom of a QFN package. Too much paste will result in a "bubble" under the chip that will lift it up from the board, making it more difficult for the solder on the pads to reach the pads on the package. Too little paste and the package won't be able to "float" enough during reflow to insure that the surface tension on the pads can properly align the package. In general, the correct amount is somewhere between 40% and 70%, but that will vary depending on how thick the paste stencil is. I strongly recommend either a steel stencil or a 3 mil kapton stencil. Place the QFN devices and the USB connector carefully with a vacuum placing tool. The remaining components are not critical.

Make sure the paste stencil doesn't have apertures over the DB9 edge-mount connector pads. Those will be hand soldered later.

Take the DB-9 male connector with PC pins and gently pinch the two rows of pins slightly closer together. The row spacing is 2.84mm, but the standard FR4 board thickness is 1.6mm. Your goal is to pinch the pins so that the board is centered between the two rows of pins. Place the board between the pins, so that the pins are neatly centered on the pads. Try and place the board up against the connector as much as possible. Solder pin one on the top side and then check the board's alignment. Look at the board and connector from the side and insure that the board is straight and perpendicular to the connector. Turn the connector over and solder pin 6 (opposite corner from pin 1). Check the board and connector to insure they're straight and square and then solder the remaining pins.

After reflow, check the USB connector pins and the QFN pads to insure there are no bridges. If there are any, clear them with an iron, or if necessary, braid.

The device is electrically complete at this point. To test it, connect it up to a computer and make sure that it is properly enumerated. Open the serial port with a terminal program and send characters over it. Every character should result in a blink of the TX led. Next, short pins 2 and 3 of the DB9 connector and transmit more characters. Both the TX and RX LEDs should blink simultaneously and you should see the character you transmitted come back to you.

This step is reserved for the creation and installation of the TBD 3D printed case.