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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?
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.
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.
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.
We attempted to create a solder pattern and give it to pcba for reflow. The layout is shown.
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.
We settled for a ffc component in the end, and we think that we got this right this time.
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.
