I am getting a small pick and place machine with a reflow oven. I am wondering how can I reflow two-sided PCBs.

I have read many methods on the Internet like use two soldering pastes with different reflow temperature, using roaster oven, etc. I am not sure what the right method is.

How can I professionally reflow two-sided PCBs?

  • 2
    \$\begingroup\$ The lighter parts will be kept in place even when hanging top down due to surface tension. So if one side only has light components, you can reflow that first and it will be fine during the reflow of the other side \$\endgroup\$
    – tobalt
    May 2 at 5:09

There's no problem running it through the reflow process twice, once with parts on each side.

Things like resistor networks, resistors, capacitors, most chips will stay on fine. See this answer for a rule-of-thumb on component mass vs. pad area (30gm/in\$^2\$)- as I stated my standard 0805 is 4gm/in\$^2\$ so plenty of margin. No need of any fancy solder alloys.

Get the type of stencil printer that allows you to print paste on a PCB with components already installed on the opposite side, or make a jig.

There are also adhesives designed to hold parts into place, typically used when one side of the board must be wave soldered. They can be applied by pneumatic dispenser, or by stencil or screen printed (photo from the manufacturer):

enter image description here

Generally total assembly costs will be lower if you can avoid placing parts on both sides of the PCB, but sometimes that is not possible.

  • \$\begingroup\$ Thanks Spehro, But the problem here you assume all light weight parts on a single side. The two sides can have heavy weight parts. If we assume we have at least one part of more than 30gm/in^2 on each side of the board, relying on surface tension would not work here How you can reflow in this case? \$\endgroup\$ May 2 at 7:24
  • \$\begingroup\$ @HossamFattah Yes, in my experience that is the normal situation for SMT parts. Of course you can always install one or two later by hand for small production if that is only mostly true. \$\endgroup\$ May 2 at 7:26

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