PCB Assembly

What is Flex PCB Assembly: Advantages and Processes

Flexible PCBA

Printed circuit boards or PCBs are widely used in various electronic devices such as mobile phones, computers, electric vehicles and many more. They are the familiar green boards that physically support components to serve several functions like switching, sensing, and controlling. With the unstoppable progress in electronics, other types of printed circuit boards were also developed. Contrary to the green and rigid PCB that is made from FR4 (Flame Retardant type), flexible circuit boards appear as amber or orange color due to their polymer base material like polyimide. Flexible PCBs are soft and have replaced rigid PCBs in some important applications due to their lighter weight and flexibility.

What is Flexible PCBA?

Flexible printed circuit assembly or flexible PCBA is the method of assembling components into flexible boards which can either be single-sided or double-sided.  The base layer of a flexible printed circuit assembly product can be polyimide, polyester, or polytetrafluorethylene (PTFE). Polyimide is the preferred base material for reflow soldering as it performs better during high-temperature conditions. The circuit pattern is etched into the copper layer which is often laminated to the base material. The surface finish of the flexible circuit board can be a flexible solder mask or a polyimide cover-lay to shield the copper surface from oxidation.

Different types of flexible PCBs have different flexibility regarding the number of times they can be bent. Some PCBs can be bent one time only during assembly, while others can be bent several times during their product life. Single-sided flexible PCBA means that only one side of the PCB is mounted with electronic components while double-sided PCBA has components on both sides. Flexible PCBA are sometimes called Flex PCA for short.

What are the Advantages of Flexible PCBA?

PCB assembly flexible

Flexibility in portable device applications

Flexible PCBA is widely used for handheld devices like cameras, RFID tags and cellular phones. Since the material can be easily bent and folded, it is very convenient to use, unlike rigid boards where there are limitations to fit into miniaturized devices. Flexible PCBA product can conform to the shape and geometry of various surfaces.

Reduced Weight

An added advantage of flexible PCBA is its thin cross-section and low weight resulting in the device’s small overall form factor and low final weight.  Some of the devices that consist of FPCB are light-emitting diodes (LEDs), flow sensors, computer motherboards and electric motors. Flexible PCB assembly can reduce the weight and space by more than 50%.

Good Heat Dissipation

One of the challenges in printed circuit design is thermal management.  Flexible PCBA has better thermal conductivity than rigid FR4 substrate. This means that it can dissipate heat better when electronic components generate heat when carrying a high level of current.

Visual Appearance

Flexible PCB assembly contribute to the aesthetics of the final device making it more appealing to consumers.  For instance, vehicles nowadays are equipped with tiny cameras for an added safety feature. The cameras are tiny enough due to their flexible PCBA products that the cameras are well-concealed on or in the vehicles.

What are the Key Flexible PCBA Processes?

Flexible PCB assembly

The method of PCB assembly flexible is termed Surface Mount Technology (SMT). This is because components are directly mounted into the PCBs through a solder paste that acts as joining material between the contact pads and components. Below are the major steps needed to come up with a working flexible printed circuit board.

Design and Prototyping

Although design and prototyping are not part of PCB assembly flexible as they happen before the actual flexible circuit assembly, it is important to emphasize the need for a well-tested design and prototyping. During design and prototyping, the flexible PCBA product are subjected to reliability tests to determine their performance such as accelerated thermal and biased (with applied voltage) tests to verify if they can withstand the different types of stresses. Due to its thin substrate, flex PCBs are more prone to warpage and bending, which is why the design and reliability processes should consider such factors.

Solder Paste Application

The polyimide substrate undergoes a baking process to precondition the interconnect pads. FPCB baking removes the absorbed moisture during handling and storage. Only pre-baked FPCB can proceed with the solder paste application process. During solder paste application, a squeegee passes through the paste and distributes it on the openings or ‘apertures’ of the stencil containing the board patterns. The applied solder paste should have consistent and good volume and alignment to prevent excessive, lacking, or misaligned paste. Lead-free SAC (tin, silver, copper) is the most common type of solder paste being used with the restriction on lead-based adhesive materials.

Pick-and-Place

Pick-and-Place process involves a high-speed machine that picks the components from a feeder in reel or tray form and places them on the boards. The machine has a vision system capability that enables it to identify its correct XYZ position and achieve a well-aligned component. Even if there is slight misalignment during pick-and-place, the component has self-alignment properties during the succeeding reflow process. Flexible PCBA can incur process defects during component placement such as tombstoning, solder balling, and misaligned and missing components.

Solder Reflow

Since flexible PCBA products do not have stiff structures, this PCB type requires a special design carrier or fixture with retractors for easy release during reflow soldering. The carrier should be able to endure higher temperatures as well. These types of PCBs can withstand the additional thermal stresses with the shift from lead-based to tin-alloy solder paste. Compared to rigid type, thermal mass is lower which may imply reduced duration and temperature during reflow. This must be evaluated through experimentation to determine the optimized time and temperature.

Automated Optical Inspection (AOI)

The same with rigid boards, flexible PCBA also undergoes automated optical inspection to verify if components are correctly placed on the circuit boards. Automated optical inspection machines consist of cameras that can efficiently detect mispositioned or misaligned components and soldering issues like solder shorting and solder balling.

Automated X-ray Inspection (AXI)

To check the quality of the underlying solder paste, an automated X-ray inspection is also performed. This method uses X-ray technology to produce grayscale images of the boards depending on the absorption of different types of materials. X-ray machines are helpful in the immediate detection of reflow issues.

Testing

The final step is testing the circuitry if the FPCB can function according to test specs. Testing can either be done through in-circuit testing and flying probe testing. In-circuit testing is a more efficient method that makes use of bed-of-nails contact probes where the device is mounted to proceed with testing. Flying probe testing is the slower but cheaper version that has moveable probes that can test the test nets of the PCB. Flying Probe Testing is more suitable for prototypes and low-volume production.

Conclusion

By now, we have a better understanding of flexible PCBA processes. The numerous benefits of flexible PCB make it a popular choice for many market sectors like automotive, consumer electronics, medical and aerospace applications. Due to lightweight, bendability and reliability, flexible PCBA will continuously play a significant role in the electronics industry. Finding a supplier that has the technical capability to produce quality flexible circuit assembly products is vital in achieving a functional final product.

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