How are Flex Circuits Different from Printed Circuit Boards?

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In the world of engineering, the terms, flexible printed circuit board (PCB) and flex circuits are often used interchangeably. Many people, including engineers and designers, assume that flexible PCBs are nothing but a type of circuitry that are flexible or can be bent. But this is a misconception. While flex circuits and printed circuit boards can perform similar electrical functions, they are mechanically and structurally different. When designing circuit board shielding for any device, not understanding their differences may cause poor yields and field failures.

The Fundamental Difference between Flex Circuits and PCBs

The most noticeable difference between flex circuits and printed circuit boards is that flex circuits can be bent, folded and twisted, but the same cannot be done with PCBs.  To incorporate this unique attribute, flex circuits are manufactured with special materials, unique design rules, and production process.

Difference in Terminology

The electronics industry uses different terms for both flex circuits and PCBs. Flex circuits are also known as flex, flexible circuits, and flexible PCBs. On the other hand, PCBs are simply referred to as hardboards.

Difference in Material

Both flex circuits and printed circuit boards have similar construction, but the biggest difference between the two is perhaps the materials used for their production. If it is a single sided circuit, the components are:

  • A dielectric base layer
  • An adhesive layer
  • A copper or conductive material layer
  • A protective overlay layer (optional)

The Base Layer

For a PCB, the base layer needs to be rigid. So, the most common material used for the purpose is glass reinforced FR4. Though glass reinforcement gives excellent mechanical strength, thermal resistance and dimensional stability to the material, it provides very less or no bending ability. On the other hand, for flex circuits, the most common base material used is polyimide. Polyimide is a synthetic resin that is produced by linking polymer units with imide groups. This material is popular for its flexibility and heat-resistant properties but is less dimensionally stable. It does not have the same mechanical support for components as glass epoxy FR4.

The Adhesive Layer

Thanks to their rigidity, manufacturing of PCBs involves only a limited amount of adhesive. In fact, they require adhesives only for thermal and chemical properties. In comparison, flex circuits have an additional requirement for adhesives — to ensure flexibility and allow for bending.  Therefore, adhesives need to be chosen very carefully; what works for a PCB, may not be suitable for a flex circuit. Adhesives that are used to manufacture printed circuit boards may fracture or crack in flex circuits. A flex circuit needs a binding agent that has unique mechanical, chemical and thermal properties so that they can be stretched to some extent.

The Copper Layer

Two basic kinds of copper films are available, namely the Rolled Annealed (RA) and Electro Deposited (ED). The significant difference between the two is that the former is highly flexible and can be used for dynamic flex applications. In contrast, Electro Deposited copper cannot be said to have an excellent flexibility, and are mostly used in the production of printed circuit boards. The third type of copper film is also available, which is called High Ductility Electro Deposited (HDED) copper. Though HDED copper is electrodeposited, it is treated to have enhanced flexibility, though not to the extent of Rolled Annealed copper. Selection of copper is not an issue with printed circuit boards because they do not require flexing. But for flex circuits, selection of copper is critical to performance.

The Overlay Layer

Flex circuits, as compared to printed circuit boards, require different dielectric materials like laminated coverlay or flexible photo-imagable solder mask. The difference in the requirements calls for the use of materials with unique attributes and challenges. These materials should bend without cracking, breaking or delaminating even after continuous processing. The most commonly used flexible dielectric is laminated film dielectric bonded to the substrate with high pressure and high temperature, a substance overly unfit for rigid PCBs.

These differences in material make the design, fabrication, and tooling of flex circuits more complex as compared to printed circuit boards.

Difference in Design

There are considerable differences between creating a flex circuit and designing a printed circuit board. The malleability of flex circuits creates some unique issues concerning their robustness. Since flex circuits can bend, it should be ensured that bending does not break or crack solder joints at the point of bending. This is the reason for incorporating filleted pads and radiused corners in flex circuits. Also, substrates used in flex circuits are not as dimensionally stable as that of printed circuit boards. So, dimensional changes may also be required for flex circuits.

Difference in Fabrication

The steps of fabrication and equipment used for the purpose are similar to both printed circuit boards and flex circuits. , However, the difference stems from the fact that the two materials are handled differently during the production process..  It’s only natural given the fact that manufacturers are dealing with two completely different materials – a flexible substrate and a rigid board. Certain ultra-thin, flimsy flex circuits require special handling, thanks to their need for clamping, fixturing or fixing vacuum systems. Some manufacturers use a continuous reel to process flex circuits in high volume. This reel to reel processing system uses different material handling mechanisms. In contrast, PCBs are easier to handle, thanks to as compared to systems processing discretely sized printed circuit boards.

Difference in Tooling

While similar tools like photo tools, CAD files, and electrical test fixtures are used for both flex circuits and printed circuit boards, the major difference relates to their cutline definitions. Thanks to the near unlimited configuration opportunities, flex circuits can be created with many unusual shapes and sizes. Contrary to this, printed circuit boards are mostly rectangular, and may have some curved outlines or notches. For this reason, hard tool dies, and steel rule dies are used for flex circuits than for printed circuit boards. Another popular tooling option for flex circuit includes the use of laser definition of the circuit outline. Fixturing may also be required for handling flexible substrates.

One of the major advantages of flex circuits is that they reduce the weight, size, and amount of hardware required for electronic packages. If you have an application that requires circuit board shielding with multiple planes of interconnections, then flex circuits will work best for you. You can also use flex circuits for applications that require active flexing or bending for parts of the package.

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