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I have a rigid PCB and a flex (4 layer) PCB. I want to bond them instead of using a connector (although if what I am expecting is impossible I will use a connector).

What methods are available for bonding a flex to the surface of a rigid PCB? Can we back them with solder like an SMT component and then bake them?

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This can be accomplished using soldering.

You basically create a set of pads on the rigid PCB which line up with a set of pads at the end of you flex PCB.

You can either join such that the pads extend right to the edge of the flex PCB and you solder with both sets of pads facing upwards. In this case the pins will be wired 1-to-1.

Soldered End on

In this approach the pads are both tinned, flux is applied, then a soldering iron used to drag the solder up from the rigid PCB onto the pads of the flex PCB to make a join. This is not the strongest way to join the boards, so some additional flexible glue between the flex PCB and rigid PCB might be called for.


A second soldering option is to have pads on the bottom of the flex, not quite at the end of the board, and top of the rigid PCB. Then the pads are joined with solder sandwiched between the pads like a QFN style connection. In this case the pins on one board are mirror imaged as they will be bonded n-to-1.

Soldered Flip-Chip Style

For the second option, to solder, tin both sets of pads and once cooled add some extra flux. Alternatively apply solder paste. Then line the pads up, and heat the joints applying gentle pressure.

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  • \$\begingroup\$ Can this be done with solder paste and then baking with the same process for SMT components ? \$\endgroup\$ Jun 4 '20 at 11:37
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    \$\begingroup\$ @LeoAtreides I'm not sure if it could, as you would need something to apply pressure to keep the flex PCB aligned. It might be possible to glue the flex PCB to keep it in place like they do with large SMD parts, and then put through reflow. You'd have to ask your assembly house on that one. \$\endgroup\$ Jun 4 '20 at 11:59
  • \$\begingroup\$ I will ask the assembly house if they can do it. If they can I will try and photo the process ( it will take me some time to get that though - these things don't happen quickly ). I am sure it would be possible to engineer a simple jig to hold the parts in place and apply some pressure during reflow. \$\endgroup\$ Jun 4 '20 at 12:03
  • \$\begingroup\$ We've never done this. Always built rigid-flex (or rigid-flex-rigid, or rigid-flex-rigid-flex-rigid) printed wiring boards as one assembly. With the solder approach outlined above, you need to take great pains to ensure that there is no stress on the solder joints by properly supporting both the flex and rigid boards. \$\endgroup\$
    – SteveSh
    Jun 4 '20 at 12:15
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We attempted to create a solder pattern and give it to pcba for reflow. The layout is shown.

Solder pattern for reflow as agreed with PCBA

The promise to make a reflow solution work was not met and we then send the parts to our assembly team. They also could not solder the parts. In honestly, it is possible to solder by hand but this is skilled soldering and is not suitable for mass production.

Fine solder points - beyond assembly line skill levels

We settled for a ffc component in the end, and we think that we got this right this time.

ffc connector - SMT - but hand soldered in this image The issue with connecting ribbons to rigid boards is a hard one IMHO.

So I would say that the answer is to use an ffc and then glue the ribbon. The solder work was too fine - and that had not been identified in the original question.

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