The first step is to ensure that I have all the parts for the board. Since ideally you would only want to reflow the board once. I organized this in my project book as shown below. This makes it easier to find all the parts when it is time to place them on the board. Also the parts will not get mixed up! (The 0.1 uF, 22 pF and 4.7 uF caps all look the same!)
Then I realized that I am missing a key component: 3.3 V Zener diode. Ordering just a single part from Digikey is quite expensive and I will have to postpone the board assembly yet again. Luckily I had a through hole 3.3 V Zener in my parts inventory. So I made a choice to do the reflow without the Zener, and then afterwards solder in the through hole Zener to the surface mount pads.
The next step is to put solder paste on the pads. I am using Chip Quick no clean lead free solder paste (SMD291SNL-ND). The solder paste came with a syringe and a nozzle. Ideally to put solder paste on a board you have to make a stencil first. Getting a professionally made stencil for a board can be quite expensive. So I decided to do it the hard way by putting solder paste manually on the pads using the syringe.
I had no idea how difficult it is to put solder paste onto a pad with a syringe. The paste didn’t really stick at all. You have to make sure not to put too much solder paste on a pad. If you do then you run the risk of making solder bridges. Too little solder paste and it won’t make a good connection. Having never done this before, I had a high risk of making one of or both of those two errors. Solder bridges on pins can easily be corrected later so I wasn’t too worried except for two components: The ADXL345 accelerometer and the HSMF-C165 bi-color LED. All the pads of these chips are located underneath the component. I only had one chance to get it right with these two.
With care and patience I put solder paste on all of the surface mount pads.
Once solder paste is applied to the pads it is time to place the components on the board. I must say this part is quite fun. Specially putting the resistors and capacitors. Once placed on the pads the solder paste holds the component in its place. The ATmega328p took a little aligning and the ADXL needed quite a bit of nudging to get it in place. Once all the components are placed on the correct pads (also in their correct orientations; watch out for those polarized caps!)
Once all the components are placed in the pads I put the board carefully in the reflow oven and ran a previously prepared custom reflow profile. After about 10 minutes the board was ready to be taken out of the oven.
Initial inspection showed that there were no solder bridges! All the resistors and capacitors showed solid connections with their pads. The only issue was the 3PDT switch, and this could have been totally avoided. The heat caused the plastic parts of the switch to melt. The switch was unusable.
So I removed the switch and manually soldered in another one in its place. After that I soldered in the Zener diode, the seven square LEDs that go on the edge, and the 150 mAh liPo battery. And then the moment of truth; Did the accelerometer and the bi-color LED make it? Did all the parts survived the heat inside the reflow oven?
They did! I was able to upload the bootloader to the ATmega328p and program it. The bi-color LED was working, which also meant that the LiPo battery charge circuit was also working.
I am very happy that the first try on my home made reflow oven was a success. The next step for the Gyro’clock project is to make a case for it. That is another completely new avenue for me to explore. Until next time!