What Are Flexible Printed Circuit Boards?

The world has seen a major digital technology revolution in the past two decades. The phenomenal success stories of personal computer, laptop, tablets and smart-phones have made these equipment a part of our daily lives.

Engineers designed complex flexible printed circuit boards to make these equipment reliable and user friendly. The traditional wire harnessing, costly and complicated wired assemblies were replaced by the much simpler and significantly cost-effective flexible circuits. Interconnection methods were revolutionized through the advent of Flexible printed circuit boards.

Flexible Printed Circuit Boards is made by bonding conductive traces on a flexible substrate. Flexible PCB is also called flexible circuits, flex circuits, flexible printed circuits or just flex. Flex circuits are printed circuit board that has the ability to flex or bend. The design, fabrication process and some other functional parameters will vary depending on the end use.

Fig-1 below shows a flexible PCB used in Hitachi TravelStar Portable Hard Drive. (Wikimedia Commons).

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Fig-1

Electronic design engineers are able to create innovative products due to the advent of Flexible Printed Circuit Boards. Flexible circuit boards will improve connectivity, add circuit density and enhance life of the electronic equipment and gadgets.

Types of Flexible Printed Circuit Boards:

(1) Single Sided Circuit – This type of flexible PCB has a single layer of the conductive metal traces bonded to on one side of a dielectric layer.

(2) Double Sided Circuit – This type of flexible PCB has the conductive metal layers on both sides of a single dielectric layer.

(3) Multi-layer Circuit – It is a flexible PCB in which several copper layers (conductive metal layer) are separated and enclosed by dielectric layers.

(4) Rigid Flex Circuits – They are multi-layer circuits which comprise layers that are hard board and flexible circuit.

Composition of Flexible PCB:

Flexible PCB is designed with a metallic layer of traces being bonded to a dielectric layer. Copper is one of the conductive materials used in the construction of a flexible PCB. However, other types of conductive materials can be used. The thickness of the conductive material used ranges from thin (<0.001”) to thick (>0.010). The dielectric layer which the conductive material is bonded to can either be polyamide or polyester in nature. The thickness of the dielectric layer is between 0.005” to 0.010”. Adhesives and vapour deposition is used to bond the conductive material to the dielectric layer.

Copper, which is commonly used as the conductive material in the construction of flexible printed circuit board, is usually coated with a protective layer. This is because the metal has the ability of oxidizing when exposed. The metal that is commonly used for this coating purpose is gold (due to its conductivity and durability in the environment). The dielectric material can also be used to prevent oxidation and electrical shorting in non-contact areas.

Numerous types of materials are used in the construction of flexible printed circuits. The decisive factor that determines the types of the materials to be used in the fabrication process includes:

  • Chemical and mechanical resistance requirement
  • Current capacity of the flex pcb.
  • Capacitance
  • Temperature
  • Nature of Flexing

What Are the Advantages of Flexible Printed Circuit Design?

Flexible printed circuit boards have a number of advantages over rigid PCBs.

Ease of Use: Design limitations are negligible. PCBs can be designed to fit any device shape. Range of motion allows Printed Circuit Boards to suit nearly any application. Less mass reduces risk in environments with regular vibrations. The errors that are found in standard PCB assemblies are reduced in flexible PCB. It also reduces weight through the elimination of additional wires, cables and connectors.

PCB Size is drastically reduced: Thinner and more lightweight than their rigid alternatives. It’s durable against motion and bending. HDI allows for the miniaturization of devices.

Rigid-flex PCBs: Blends flexible and rigid PCBs. Commonly formed with flexible circuits connecting several rigid flex boards.

HDI for Flex: Smaller package size increases the need of HDI. It allows additional space for other features on the PCB.

Cost Reduction: Total cost of installation is reduced. Flex circuits eliminate several steps within the production process and thus reduce overall manufacturing time and also reduces cost.

We’ll discuss more about Flexible Printed Circuit Boards in coming articles.