1Tin the ground solder pad under the battery holder BT1.
Tin the ground solder pad under the battery holder BT1. This step is required in order to ensure a good connection when the battery is inserted. All that should be required here is an even distribution of a thin layer of solder on the square ground pad under BT1.
2Tin the 4 USB plug pads.
By tinning the 4 USB pads, we ensure a solid connection to the USB socket. Although only the 2 outer pads are actually used by the PCB to provide voltage, it is a good idea to tin the middle 2 pads as well.
3Solder power status LED indicator.
We first want to solder on the PWR status indicator LED (LED3), as well as the 470 ohm series resistor (R4) in order to test that our connection to the USB port is solid. Once these two components are soldered, test them by inserting the USB connector into an available USB port on a computer or hub and check that the power indicator LED (LED3) is lit.
If LED3 lights, move onto the next step. If it does not light, recheck that the USB plug is properly tinned as well as the solder joints of R4 and LED3.
4Solder the rest of the SMD components
Now that we know there is adequate power to the circuitry, solder the rest of the surface mount components (C1, C2, LED1, LED2, R1, R2, R3, U1).
5Solder on the through hole battery connector BT1.
Before soldering the battery connector BT1, ensure that the square ground pad under the connector is tinned and clean. I like to take an acid brush and a bit of isopropyl alcohol to clean any remaining flux from around the pad. Then insert the connector through the holes in the PCB and press firmly down. Flip the PCB over and solder in the component BT1.
6Clean PCB and prepare for first use
As with all PCBs that have been hand assembled, it's a good idea to clean off any remaining flux from the surface. I generally do this with a stiff acid brush dipped in isopropyl alcohol.
7Test finished PCB with a discharged LIR2032.
Insert a depleted LIR2032 or appropriate rechargeable 20mm coin cell into battery holder BT1 and plug the USB PCB plug into an available USB port. The green power indicator should light, as well as the "charging" status LED (LED1). Once the coin cell is completely charged, LED1 should turn off and the "charged" status LED (LED2) should light, indicating that the coin cell is now completely charged and can be removed from the battery holder.