10 Guidelines to Help You Successfully Flex PCB Soldering

1. Flex PCB Soldering Correct Temperature

The general rule is that you should never use rigid PCB temperature profiles to reflow flex PCBs. That is because these temperatures are likely to be much higher than flexible PCBs can tolerate.

Excessive heat during flex PCB soldering can lead to blistering and delamination. Where you do not know the proper reflow temperature profiles for flexible PCBs, it is best to seek guidance from a reputable PCB manufacturer. 

The typical single-sided flex joint requires a reflow temperature of 330 – 400°C. Generally, it is best to stick to the minimum temperature to minimize the chances of damage due to exposure to heat.

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2. Monitor Heat Release During Assembly

A flexible circuit board is considerably thin, so heat control is essential. But that is not the only reason. In all electrical devices, excessive heat is a legitimate cause for concern. Electrical appliances that do not release heat well can be dangerous. You may have heard that the cell phone battery exploded due to overheating.

Flexible circuit boards are electrical conductors. Engineers and designers must thus come up with a way to dissipate that heat. Failure to do so can lead to the outright destruction of the delicate circuitry in flexible PCBs.

So what do you do during flex PCB soldering and assembly to ensure that a flexible circuit board dissipates heat well? Well, you need to go for components that have an exceptional surface-to-volume ratio. Stick to compact designs to shorten the thermal path. This way, there is less heat buildup. 

3. Avoid Stacking Conductors On Multiple Layer Boards 

Many times during flex PCB soldering and assembly, manufacturers signal and return lines over each other. They do so on adjacent layers to reduce electromagnetic interference (EMI). It makes the overall flexible circuit thicker, which creates an I-beam effect. 

If you are going to use this method of EMI reduction, ensure that you stagger all the line pairs to reduce this I-beam effect. 

4. Remove Flex PCB Soldering Residue

Many beginners find this part of the flex PCB soldering challenge. The truth is soldering small contacts can be difficult. With practice, however, things should get more comfortable.

There are different techniques you can use when soldering the accelerometer. In this section, we will look at how to solder the accelerometer using rosin-core solder and soldering iron

In this technique, you will be using a flux pen during flex PCB soldering. The flux does several essential things. It cleans the surface oxides and etches them, averts oxidation of copper after heating, and boosts the flow of solder. Flux lowers the surface tension of the weld, ensuring that molten solder spreads and penetrates readily.

Always make sure that you use a small amount of solder paste onto the solder pad and accelerometer contact. Thoroughly clean the solder iron tip before quickly bringing it into contact with the accelerometer and substrate pad. The solder joint should form instantly. If it does not, dispense some more solder paste. Make sure that the paste bridges onto the pad from the copper solder. Dab the joint with the solder iron tip.

5. Soldering the SMT Resistors and Capacitors

Solder paste usually comes in large syringes or jars, so be sure to purchase a smaller needle that has the appropriate gauge tip. To solder SMT resistors and capacitors in place, follow the following steps:

• Start by cleaning the copper solder pads using the flux pen
• Dispense a little solder paste onto the capacitor pads
• Release the capacitors you require from the tape real
• Gently place the capacitor on top of the copper solder pads
• Tin the solder iron’s tip and clean it using a wet sponge
• Touch the solder pad and capacitor using the tip of the soldering iron

6. Checking Continuity and Sensor Outputs

This crucial step of the flex PCB soldering process can prove challenging for beginner PCB manufacturers. However, it’s quite straightforward once you get used to it. 

The first thing you need to do is check for continuity in the solder joints using a multi-meter. See whether there’s contact from the connector side to the accelerometer chip. Next, check for any shorts between the PCBs solder pads. See if there’s continuity from the copper traces to the adjacent contacts on the accelerometer. 

7. Cleaning Off Flux/Rosin Residue

The flexible printed circuit board (PCB) is a version of PCBs that have unique capabilities. Besides offering the same features of an ordinary PCB such as repeatability, high density, and reliability, flex PCBs have the flexibility and higher vibration resistance. These PCBs can assume three-dimensional configurations.

The flex PCB soldering process also involves cleaning off the flux or rosin residue from the solder joints. Of course, if you’re using a no-clean flux, then you may not need to worry about cleaning off any flux residue. Otherwise, the waste can corrode the solder joints and result in issues after a while. Don’t forget to place electric tape over the front side of the traces.

8. Prevent Bending of Flex PCB Soldering

Here, you can use either carpet tape or double-back scotch tape, as well as an overhead transparency film. 

Laminating the backside of the flex circuit helps reduce the concentration of stress at the edges of the accelerometer and connector. This stress concentration could cause fatigue and eventual failure of copper traces. 

To prevent solder joints from flexing, try placing a thicker stiffener close to the solder joints of the accelerometer. Given that there’s more stress concentration at each solder joint, it’s best to opt for a thicker stiffener, mainly if there’ll be active flexing on the accelerometer.

9. Potting Solder Joints

You’ll probably need to pot the solder joints as part of the flex PCB soldering process. Potting helps prevent solder joints from flexing. Dab 5-minute epoxy on the edges of the accelerometer to do so.

Keep in mind, however, that there may be a concentration of mechanical stress at the edge of the potting area. Some solder joints create electrical connections. Such solder joints also create high-stress levels that may lead to mechanical fatigue failures.

10. Flexibility for Stiffeners

Usually, flexible circuitry becomes evident during the processes of copper plating, pumice scrubbing, or etching. The exposure of this circuitry can cause dimensional changes. The design of flexible electronics, as we pointed out earlier, needs a significant amount of tolerance to allow for cover films and stiffeners. 

As you consider cover films, don’t forget to take out the adhesive squeezing after the lamination of the film dielectric. If you don’t do so, the processes of fabrication and inspection become harder. 

Summary

Printed circuit boards are famous for how versatile they are. They’re easy to fold, bend, and twist into the configurations you desire. The process of flex PCB soldering, however, can be quite complicated. To get everything right, manufacturers must understand the design rules of these circuits.

Are you looking for top quality flex PCBs for your electronic products? Talk to us at WellPCB today. We have extensive experience in the assembly of all kinds of printed circuit boards. Check out our tailor-made solutions today.