Soldering SMD components on flexible PCB

Cindy Lin
Cindy Lin
Jul 27, 2015 · 3 min read

Fyber Labs produces rigid and flexible PCB designs. Small runs and our prototypes are done by hand. Here I demonstrate how we made the Light Square LED panel prototype.

General Process:
* Apply solder paste with stencil
* Apply epoxy (special to flexible PCB)
* Populate the components
* Reflow solder in oven
* Examine and test
* Rework solder and repeat test as needed

We take the Light Square freshly created from our earlier blog:

We utilize for our laser cut polyimide stencils. They provide a credit card like spreader/squeegee.

* Pre-apply solder paste to the stencil and add more as necessary.
* While assuring alignment of the pattern, make sure all cut outs are filled with solder evenly.
* Scrape the surface to help evenly fill the cut outs.

For this LED panel, we utilized lower temperature solder as the APA102C LEDs are very heat sensitive.

I am adding dots of epoxy to better hold the LEDs on the panel.

For flexible PCB, we recommend adding epoxy to help relieve strain on the solder during flex. We use a standard PCB epoxy that hardens during reflow. Be sure to not touch the solder paste that was just carefully stenciled on.

The next step is to place the components on their patterns. Without a silkscreen, you have to understand the circuit well to know where pin 1 on a component should be placed.

For this LED panel, they all follow a specific orientation with the PCB traces. Otherwise, we would print out an enlarged copy of the patterns and silkscreen to identify correct placement.

Always make sure to attach a properly grounded ESD strap when touching components.

Partially populated Light Square LED panel

Once finished, we reflow the board with the specific heating curve required for the solder type in our mini reflow oven.

Initial reflow result before testing

After reflow, we had to apply additional solder to many of the LEDs as our stencil ordered did not provide enough. Additional solder was also applied to repair a couple of over etched traces (This is common with DIY etching of large designs). Flexible PCB naturally resists solder, though solder may wet on to traces. To prevent that, you can temporarily add polyimide tape near the pads to be soldered. In addition, we added more epoxy to better attach the LEDs.

Test run after reworking. During rework flux residue may remain all over the board

During rework, we use a lot of thick gel flux. This helps to clear oxidization from the copper and helps the solder bound to copper quickly. Oxidization on DIY etched boards is very common. After reworking, flux residue should be cleaned off with specialized flux removers or isopropyl alcohol.

The end result!

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