Making Double Side Boards in a Reflow Oven

I was planning to write up my next big gamble with the CPC2 build; double sided reflow boards. However, someone has done it for me!

With the addition of new components to my CPC2 design it’s starting to look very cramped if I want to keep the small form factor (the dongle part of my CPC2 dongle).

This is usually solved by putting some of the more flexible components, such as passive capacitors and resistors on the bottom of the board. I’d normally do this manually with a hot air pencil. But with the very large number of bypass capacitors that I’ve added to my latest design, hand assembly is not my preferred option, although still possible.

Instead I’m going to try to follow the lead of some of the PCB forums contributors, and bake both sides of the board.

The results seems mixed when both sides are baked using normal leaded solder paste. Usually it works, but sometimes not. However, it seems universally accepted that using lead-free solder on the bottom of the board and leaded solder on the top seems to be successful.

The higher melting point of the lead-free solder will ensure the components on the bottom of the board won’t drop off during the bake of the top side.

Additionally the bottom side of the board is a few degrees cooler than the top side as it’s facing away from the infra-red elements in the oven.

There’s a couple of things I need to do to ensure that this method is viable:

  • Place only high-temperature components on the bottom of the board, so no clocks or precision resisistors
  • Place only low-mass devices on the bottom. The solder has a good surface tension when reflowed, but adding a heavy, 20mm high-bright LED might overcome the surface tension.
  • No precision placed components. While I don’t expect the components will drop off, there will be some semi-liquid solder on the bottom side at the peak reflow temperature, and it’s quite possible they will jiggle during the second bake. An eMMC chip has 0.5mm pitch balls, so there’s no room for any jiggling.

So, that’s my plan. I’ve finished the update of the CPC2 schematic, and I’m laying out the board now. I expect I’ll be done in a few days, then a week of testing and I’ll send the gerbers to my favourite fab OSH Park.

Yes, it’s another shameless plug for a company that enables my hobby! Please support them by sending them your boards for fabrication so that I can keep sending them mine! You’ll also note in the linked Instructibles above, they also use both OSH Park and OSH Stencils. Coincidence….?

Stay tuned for the results in a few weeks.

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