Flexible Circuit Introduction

  • Introduction:

 

A flexible Printed Circuit Board, commonly known as Flex PCBs consists of an insulating polyimide film with printed circuit patterns. Polyimide is an insulator, therefore only the circuit pattern conducts to complete the path. Just like the ‘solder mask’ of rigid PCBs, flex PCBs are covered a thin “cover layer”, which insulates the circuit from any Electromagnetic Interference. Flex PCBs are nowadays very common in smartphones and medical applications specially when the circuit is to withstand serious temperature changes while maintaining flexibility.

 

  • Flexible Printed Circuit Boards:

 

Flexible PCBs are considered "flexible" for many different reasons. The most obvious of them all is that their circuitry can be adjusted to match up with the product itself. This is beneficial specially when parameters such as sustainability, durability, low weight and flexibility are concerned. Conventional boards cannot meet the criterion of being durable, fragile and potent at the same time.

 

Flex PCBs outsmarts conventional rigid ones when it comes to product limitations. For instance, the size of the product can be reduced significantly using flexible instead of the rigid PCBs. They can be bent, rollover to adjust as per the core product. The product, as a whole can be made much more lighter with same components that that of rigid and heavier ones. Flexible boards however, are not completely flexibly. These PCBs do possess some rigid areas too, but the circuitry is mounted on mainly flexible part so to adjust as per the product. The rigid part is maintained for material support, therefore it kept to the minimum level possible.

 

1. Construction:

 

Flex PCBs likely to their rigid counterpart have been constructed using many different ways. Depending upon the technology, layers, and materials we have classified them as follows:

 

  • SINGLE-SIDED FLEX CIRCUITS:

 

Single-sided flex circuits (SSFC) consists of a single conductive layer of metal or metal filled polymer on a flexible dielectric film; usually polyimide. It uses a THT (Through Hole) mechanism for mounting the components, which means that components can be adjusted and changed using a single side. Single-sided flex PCB can be fabricated with or without shielding coating using an insulating film; however, the use of a shielding coating over circuits is the most common practice as it prevents the circuit mechanically and from any EMI. The construction and insulation of the single layer flex PCBs are explained in the diagram below:

 

Stackup of Single Sided Flex PCB

Figure 1 : Single Sided Flex PCB

 

  • SCULPTURED FLEX PCB:

 

Sculptured flexible PCB are a captivate subset of flexible PCBs, which involves a particular flex fabrication method that yields a flex circuits with copper conductors having different degrees of thickness along their length. The conductors are thin in flexible areas and thick at rigid areas. This approach includes selective etching of the copper foil to obtain depths in various areas of the circuit.

 

The sculptured flexible PCB technology, is often chosen to generate bare metal contacts such that it extends from edge to let plug-in connection. The increased area allows solder joints to be stable and more durable compared to the ordinary flex circuits.

 

  • Multilayer Flex PCBs:

 

A multilayer Flex PCBs consists of the same flexible circuitry with multiple layers. The layers are connected by means of the platting The layers of the multilayer flex PCBs are continuously laminated through the holes. These multilayer PCBs are similar to the rigid ones except the change in material, quality, features and cost. Multilayer flex circuits are expensive than that of their counterpart but ensure better quality. Following is the visualization of the multilayer PCB.

 

6 Layer Flex PCB

Figure 2:  “6-Layer Flex PCB”

 

The only rigid part is the one used for connectivity. The rest of the circuit board is flexible.

 

2. Applications:

 

Flexible PCBs found their applications in the following areas:

 

a)  Medical Equipment:

 

Flexible printed circuit boards are often used when reliability, adaptability and light weight products are needed as in the case of medical equipment. A  camera pill to swallow called     “Pill Cam” makes use of very thin flex circuitry, which must be properly insulated and durable. Patients after swallowing the pill, allows doctors and professionals to get an accurate view of tissues from inside the body. The pill needs to be extraordinarily small and must be flexible to move through the human body, therefore flex PCBs is a perfect choice for this unlike rigid and brittle ones.

 

b) Smart Phones:

 

The need of “smart” phones requires the mobile sets to be consisting of small components and flexible circuitry. Flexible PCBs therefore, play their vital role in circuits that are used in some eminent part of the circuits such as “Power Amplifiers”. This allows the phones to be smart, and light weighted.

 

c) Computer Electronics:

 

Electronics in mother board is the brain and soul of the modern day computers. The circuit design is to be implemented in a small, concise manner. The flexible circuit boards therefore finds their application to keep everything sustainable and small.

 

  • Comparison:

 

After a handful discussion on the construction and applications, it is now the time to compare the two technologies. The following table illustrates that.

 

 

                   Flex PCB                   Rigid PCB
                   Reliability                 Non-reliable
           Good Temperature Resistance     Not good at temperature changes
                  Flexible                             Rigid
      Accommodates any product design           The product needs to be adjusted
           Better for stress conditions             Cannot handle mechanical stress
                  Expensive                            Cheap

 

  • Conclusion:

 

Although the flexible PCBs have more features than that of rigid PCBs but from industrial point of view they cannot overcome the rigid circuit boards as they are very expensive compared to the simple conventional boards. However, for a small industry it is a perfect choice when light weight products, flexibility, sustainability and durability are concerned.