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I'm designing a PCB for factory production and I'm not sure if placing SMD components on the bottom side of the board, which also contains THT components (pin headers and buttons, they're all on the top side though), will make the board manufacturable without a special process. Normally I would avoid this, but the board I'm designing is very space-limited, and this could save really a lot of space.

I understand that this is not a problem for hand-soldered DIY-level projects, but this board is factory produced and assembled.

Thanks for any suggestions.

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  • \$\begingroup\$ Have you considered switching the THT components out to SMD? Most pin headers have SMD versions. For push buttons, you may need to change styles. \$\endgroup\$ – DoxyLover Sep 7 '18 at 23:21
  • \$\begingroup\$ If course, but SMD pin headers have a larger footprint. I also have a few connectors that do not exist in SMD version, same for the buttons, which are very specific \$\endgroup\$ – DELTA12 Sep 7 '18 at 23:26
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    \$\begingroup\$ It is very possible they can support this. They have wave soldering apparatus which is very selectively applied. Sometimes they even use paste and reflow on the through-hole components. Depends on many things. Sending them the assembly files and getting a quote would be a good way to find out and get feedback. \$\endgroup\$ – mkeith Sep 8 '18 at 0:16
  • \$\begingroup\$ You may have to modify some footprints to make it work, so the sooner you engage with the assembly house the better. \$\endgroup\$ – mkeith Sep 8 '18 at 0:18
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You'll need to talk to your assembler, but this is not necessarily a problem. There are essentially five options:

  • Use pin-in-paste to install your through-hole parts. This is where solder paste is stenciled onto through-hole pads, then the through-hole parts are installed into the paste-filled holes, and your PTH and SMT parts can all be soldered in one reflow pass. The major downside is that the PTH parts have to be specifically designed to withstand this process which really limits your part options. The plastics used in many standard PTH connectors that are not designed for pin-in-paste will soften or completely melt at reflow temperatures.

  • Glue down the bottom side SMT components and wave solder them at the same time as the PTH parts. This is usually used on high volume production of low-complexity boards (such as power supplies), and is not advisable for fine-pitch parts, large ceramic capacitors, or other mechanically/thermally sensitive parts.

  • Wave solder using masks and jigs to protect the SMT components. This is highly dependent on the component placement, board geometry, and other factors.

  • Selective wave soldering, where a machine moves a small sort of fountain of molten solder around the board, so that through-hole parts can be soldered after all of the SMT parts have been installed. This is an ad for a machine manufacturer, but it shows the process fairly well: https://www.youtube.com/watch?v=p-VImd2yW5s

  • Hand soldering. Many assemblers have the equipment and personnel that make this a viable option for ~hundreds of boards, depending on complexity. Sometimes this is simply the best option for really fiddly jobs, and a good assembler will often create fixtures or jigs to help ensure accurate assembly even with hand soldering. It avoids the setup costs that the other processes require, so it's a good low-ish volume option.

Of course, additional process steps will increase the cost of assembling the board, but it's pretty common to have double-sided SMT and PTH on boards these days.

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  • \$\begingroup\$ Well thought out answer.+1 \$\endgroup\$ – Sparky256 Sep 7 '18 at 23:59
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    \$\begingroup\$ Great answer. Also paste in hole reflow may be viable. \$\endgroup\$ – mkeith Sep 8 '18 at 0:21
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    \$\begingroup\$ Not a problem to do, but most are more expensive than a one sided reflow \$\endgroup\$ – Scott Seidman Sep 8 '18 at 0:32
  • \$\begingroup\$ Good point about pin-in-paste, I've added that to the answer. \$\endgroup\$ – ajb Sep 8 '18 at 6:14

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