Reflow soldering is the process of soldering using a paste comprised of flux and solder metal. Surface tension is used to move the parts to their final locations. This project has some particular challenges.
- Flex PCBs are sensitive to high temperatures. Minimum heating cycles and lower temperatures should be used when possible. Most components are placed with a low temperature solder paste. The solar panels are placed with no paste, only a layer of tacky flux, the solder balls on the bottom of the solar panels are used to connect them to the PCB.
- The use of small parts necessitates more than typical attention when placing parts. The clock crystal for example is a 0.9×0.6mm part with 4 BGA pins on the bottom!
- Special layouts, especially for the 7 segments, are used to allow parts to touch each other. Very non-standard, but required in this design. These parts will attract each other during reflow! This makes rework very difficult.
- The high density dual sided flex is very difficult to apply paste to. Only one side can be applied with a stencil, as the second side cannot be placed flat. Luckily only the LEDs on the top side need to use paste, as the solar panels use their own solder balls. Currently the LED paste is applied by hand with a syringe. Potentially a jig could be made to allow placing paste on the top side.
The current procedure is as follows:
- Paste is applied via a stencil on the reverse side, the side with the super caps and the majority of the circuitry.
- Parts are placed by hand using a vacuum pen (which is a custom Milwaukee battery compatible vacuum pen of my own design.)
- This side is reflowed using a hot air station.
- The PCB is flipped, and flux is applied to the solar panel region. This area is heated with hot air to allow the flux to flow to a thin layer. This allows the parts to stick to the PCB (as well as make the joints flow nicely)
- Solder paste is applied using a syringe to the LED section.
- All the parts are placed using the vacuum pen.
- This side is reflowed using the hot air station.
- The whole board is cleaned extensively with IPA. I cannot use ultrasound, as the crystal manufacturer forbids it.