Flex soldering is not really something brand new, but the process of doing it has changed significantly over time. New techniques for Flex soldering are emerging regularly, which is making the process simpler and sometimes complicated.

For example, plastic soldering is a new trend, although it is more challenging than rigid soldering. 

Nevertheless, successful Flex soldering involves a number of steps, techniques, and tricks. It can generate useful results if done correctly.

If you are looking for detailed information on how to do a successful Flex soldering, you are at the correct place. So stay tuned with us and keep reading till the end.

What Is Flex Soldering?

What Is Flex Soldering

Flex soldering is the process of attaching components to flexible circuit boards. There are various methods of Flex soldering in the market, among which some are still outdated. When it comes to rigid printed circuit boards, most engineers are aware of the common steps. But the situation gets messed up in the case of flex circuit boards.

Flex boards are now becoming more common in the world of electronics. Hence, Flex soldering may give outstanding output if you can execute all the steps properly.

Top 8 Ways For Successful Flex Soldering

8 Ways For Successful Flex Soldering

The main issue lies in the fact that there is not so much information about Flex soldering. As its practice is less and requires expertise, engineers often try to avoid it. The core principles for the rigid board and Flex soldering are almost the same. However, there are a set of different considerations if you speak about Flex soldering. Below, we have explained some hacks of successfully conducting Flex soldering.

1. Maintaining Correct Temperature

Rule no. 1, you should never use high-temperature profiles for reflowing flex PCBs. The tolerating levels of flex PCBs are much lower than the high-temperature profiles.

2. Remove The Flex Soldering Remains

Heat control in “flex for the soldering” is essential. Since the density of the flexible circuit is extremely thin, it gets heated quickly. Otherwise, there are high chances of explosion owing to overheating. Hence, channeling out the heat by using components having an outstanding surface-to-volume ratio is beneficial. 

3. Solder the SMT Capacitors and Resistors

For soldering the DMT resistors and capacitors, all you have to do are:

  • Clean the copper solar pads keeping the flux open
  • Put some solder paste on the capacitor pads
  • Release those capacitors you need
  • Place the capacitor above the copper solar pads
  • Tin the iron tip and clean it with a wet cloth
  • Touch the capacitor and solder pad with the soldering iron.  

4. Refrain From Piling Conductors On Multiple Layer Boards 

The flex circuit becomes thicker when the engineers’ signals and return lines during flex soldering over each other. But this method is mandatory for reducing electromagnetic interference. So make sure you are weaving the line pairs for lowering the effect.

5. Regulate The Heat Release

How to Solder PCB is a very common question. But what’s more important is regulating the heat release during flex soldering PCB.

6. Potting The Solder Joints 

You require potting the solder joints during the flex soldering process as it helps to pause the flexing. But use it in a limited way as over-potting may cause high-level stress and mechanical failures.

7. Restrict Bending Of The Flex Soldering

To control the bending of flex soldering, you have to make use of double-back scotch tape and carpet tape. Also, the overhead transparency film is effective enough. Although, laminating the backside of the flex circuit can diminish stress concentration at the borders of the connector or accelerometer.

8. Verify The Continuity And Sensor Outputs

This is one of the crucial steps of the Flex soldering PCB mechanism, but it can be quite challenging at times. You have to:

  • Verify the continuity in the solder joints with the help of a multimeter
  • Check if there is any contact to the accelerometer chip from the connector
  • Now check if there us any shorts among the PCB solder pads
  • After that, check whether there is continuity to the adjacent contacts from the copper traces. 

Most Common Flex soldering Technology 

Most Common Flex soldering Technology 

While you implement unique soldering techniques to flexible keyboards, the Flex soldering procedure becomes advantageous to a great extent. Here we have jotted down some common Flex soldering technology that helps engineers deliver expected results.

1. Flash Technology

This has not yet gained major popularity, but it will slowly but surely. In this process, a high-powered homogenous xenon light flash is used. The light, on the other hand, solders the components to the copper tracks on the board. Make sure you are using a solder paste free of lead.

2. Low-Temperature Solder Paste

The practice of low-temperature solder pasting is fruitful for the assemblage of surface mounts on flexible boards. Also, for sensitive components, this technique works well.

4. Reflow Soldering

There is no easier process than reflow soldering. The best part about reflow Flex soldering is that it goes well with the flexible keyboards. This method includes the use of solder paste for creating temporary attachments.

Frequently Asked Questions (FAQ):

Q1. What Is Flex In Soldering?

Flex is an important component of the Flex soldering process. Flexible electronics, also recognized as flex circuits, is a technology to assemble electronic circuits. This assembly is successful only after mounting electronic devices on flexible plastic substrates like PEEK, Polyamide, transparent polyester film, etc. Moreover, you can also screen print the flex circuits on the polyester.

Q2. Which Side Of  Pcb Is Correct For Flex Soldering?

The sides of PCB also matter at the time of Flex soldering. Mainly, the bottom side of the PCB is the portion that connects the solder wave at the time of assembly. Being known as Solder sides, both the sides of the PCBs are populated, and during assembly, wave soldering is not required.

Q3.Where Is A Flex Sensor Used?

Flex Sensor is helpful in a plethora of research areas, including rehabilitation, security systems, as well as music interfaces.

Q4. What Is Flux In Soldering?

Flux is basically a cleaning agent that technicians use during and before the Flex soldering process of electronic products. This flux also safeguards the metal surfaces from re-oxidation at the time of Flex soldering. In this way, as the molten solder’s surface tension reduces, the soldering process becomes successful.

Summing It Up 

Flex circuit boards are gradually gaining popularity for their diversified features. Not only in terms of techniques and methods, but they are also easy to handle as well. Nevertheless, the entire procedure of Flex soldering seems to be complicated if you don’t have the proper knowledge. 

What’s your viewpoint about Flex soldering? Tell us in the comment section below; we can’t wait to hear you out. Do you have any questions about how to solder PCB? Shoot them also in the box below; we will be reverting back to you soon.

Read Also:

Micro Soldering and its future implications in the medical domain


Mashum Mollah is an entrepreneur, founder and CEO at Viacon, a digital marketing agency that drive visibility, engagement, and proven results. He blogs at MashumMollah.com.

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