Flex soldering is more complicated than rigid soldering. The basic principles for soldering components remain the same, but the flexibility aspect means dealing with different material handling and the flexibility aspect.
If you’re in a hurry, here’s a quick video with the highlights, however, if you want to know the details then read on!
SPECIAL CONSIDERATIONS/ RULES FOR FLEX SOLDERING
1- LOW SOLDERING TEMPERATURE
The basic rule is that you shouldn’t use the temperature profiles you’d use on a rigid PCB to reflow flex PCBs.
This is because the temperatures will most likely be higher than a flexible PCB can tolerate.
Apply too much heat on a flex PCB, and you risk blistering and delamination.
The typical reflow temperature for flex soldering oscillates between 330-400 C
Stick to the minimum temperature possible.
2- NO STACKING CONDUCTORS AND MULTIPLE LAYER BOARDS
Avoid it. Avoid stacking conductors on multiple layer boards. Doing so makes the flexible circuit thicker and creates an I – beam effect.
3- ENSURE PROPER HEAT RELEASE DURING ASSEMBLY
Any electrical device that doesn’t release heat properly is a danger; remember the exploding Galaxy Note 7? That wasn’t good.
A flexible circuit board, like all others, is an electrical conductor; therefore, electronic designers must ensure heat dissipates appropriately; they do this using components that have an excellent surface-to-volume ratio.
4- USE SMALL AMOUNTS OF SOLDER PASTE WHEN SOLDERING SMT RESISTORS AND CAPACITORS ON A FLEX PCB
The critical point here is the size; as you know, smt components are pretty small; that’s why there’s an automated process to mount smt components on PCBs; doing it by hand is tedious.
However, if you’re in a situation where you have to solder an SMT resistor or capacitor by hand, then make sure you get a needle with the proper gauge to apply the solder paste.
5- REMOVE SOLDERING RESIDUE
Then again, the issue here is dealing with tiny components.
Use low residue solder paste and a no-clean flux pen. Thoroughly clean the solder iron tip before quickly bringing it into contact with the components and substrate pad.
Do not forget to clean off the rosin residue from the solder joints if you’re not using a no-clean flux.
If you leave it there, it’ll most likely cause issues after some time.
Put electric tape over the front side of the traces.
6- PREVENT SOLDER PAD BENDING
The stress on solder pads on flex PBCs is more significant than on rigid PCBs. The pressure can cause fatigue and eventual breakage.
This is especially true for devices that will work under active flexing circumstances.
What you can do to reduce the risk of bending and breaking is to laminate the backside of the flex PCB; you can use
- Carpet tape
- Double back scotch tape
- Overhead transparency film
7- YOU HAVE TO POT THE SOLDER JOINTS
When soldering flexible circuits, you’ll need to pot solder the joints. This helps to prevent solder joints from flexing and breaking.
Avoid high T potting compounds; they may break solder bonds because of solder fatigue; they harden at higher temperatures and causes the coating to shrink rigid solid.
FLEX SOLDERING AND DESIGN FOR MANUFACTURING
Here’s the one big DFM consideration when working with flex PCB.
Solder is not flexible; therefore you shouldn’t subject them to the stress of bending.
Make sure the flex points are far away from the solder pads.
SMT ASSEMBLY AND FLEX PCB
Rigid PCBs can be easily held and placed onto a conveyor, but flex circuits are flimsy; this means they require modifications to make them compatible with the SMT machines.
The standard SMT procedure for flex PCBs requires a rigid surface which varies significantly in complexity and costs depending on the circuitry configuration.
The flexible panel is placed on the rigid surface, with the solder side down; openings have been milled out to expose the areas to be soldered.
As you can see, soldering flex PCBs is more arduous and expensive.
FLEX SOLDERING TOOLS (DIY)
- Standard soldering Iron
- Hot plate
- Standard solder
- Solder paste (low residue) eutectic, Sn43Pb43Bi14 or Sn63Pb37
- No-clean flux pen
- Hot air pencil
- Hot airbath
- Tweezers
- Tape
FLEX SOLDERING DIY STEPS
Image Flex Soldering DIY Tutorial
Tutorial by Wiliam Provancher
1. Solder Accelerometer in place
- Tape the flex circuit down and the accelerometer to line up with the circuit’s solder pads.
- Tack the accelerometer down at two points
- Remove the tape from the accelerometer and solder the rest of the pads (and perhaps retouch the original tacked solder joints).
2. Solder the SMT resistors and capacitors in place
3. Check continuity and sensor outputs
4. Clean off flux/rosin residue
5. Place dielectric tape over traces (front side)
6. Laminate stiffeners onto the backside of the circuit
7. Crimp-on flex-circuit connector
8. Double check sensor outputs
9. (Optional) Pot solder joints
FLEX SOLDERING DESIGN AND LAYOUT CONSIDERATIONS
- Bending, twisting, and flexing can put a lot of mechanical stress on the trace and solder in areas near bend or transition points.
- The adhesive system that bonds copper to the dielectric substrate has different properties than the epoxy adhesive system used for most rigid printed circuit boards.
- For a flexible circuit to flex, the adhesive system needs to allow movement.
- The adhesive system for a flexible circuit is “softer” and less cohesive than the adhesive system for a PCB
- When exposed to the high temperatures of the soldering iron, the adhesive system may soften a bit.
- To compensate for this, use “anchor spurs.”
FLEX PCB SOLDERING CONCLUSION
Flexible printed circuit boards are great because of their versatility; you can fold them to your desired shape, which gives you more freedom regarding product design.
The issue with flex printed circuit boards regarding soldering is that flexibility makes it a more arduous task.
Whether you plan on soldering by hand or SMT you need to learn how the rules change.
