Unlike hard PCB plates that we generally associate with electronic circuitry, there are many flexible PCB’s and more so, due to the miniaturization and unique designs needed for human interface. A flexible PCB is basically a PCB that is printed on a flexible surface, as such, the materials used have to withstand a lot of wear and tear, unlike their static hard PCB counterparts.
Flexible PCB’s (FCP) are made using photolithographic technology. This reached by using very thin copper strips and laminating them between two 0.05mm layers of PET. These layers of PET are coated with a thermosetting glue.
There are seven types of FCP’s, and these are:
Single-sided flex circuits
Single-sided flexible circuits use a single conductor layer. This layer is either metal or a conductive metal polymer. These circuits are usually covered in a protective layer.
Double access/back bared flex circuits
Double access flex, which is also known as back bared flex, have a single conductor layer that is processed allowing access to certain features of the pattern from both sides.
Sculptured flex circuits
Sculptured flex circuits use different thickness of copper, thinner at flexible areas and thicker in rigid areas. Sculpted circuit boards are found in many applications, where the connecting cable is thin, and the board is rigid, combining both the ease of access from an FCP to the reliability of a solid-state PCB.
Double-sided flex circuits
Double-sided flex circuits have two conductor layers. This makes crossover connections easy for both sides. In some cases, there are plated through holes (military applications – Type V5) but are not found in most FCP’s. A lot of single-sided applications are actually fabricated using a double-sided substrate since they provide more flexibility and versatility in a design application.
Multilayer flex circuits
Some FCP’s have more than three conductor layers, these are known as multilayer flex circuits. The layers in these FCP’s are usually not bonded, making each layer flexible, in this instance, you will see three or more flexible layers connecting to one connector location. This is used where three-dimensional requirements are used to optimize for limited space.
These are not rigid flex construction circuits that use a hardened process to rigidize flexible circuits. These are hybrid circuits that use both rigid and flexible circuitry for a specific purpose. Originally popular in military applications, rigid-flex circuits are now popular in laptop applications. The rigid to flex circuitry is connected via plated through holes. These are a product of Compaq’s research in the 1990’s when trying to find a solution for miniaturizing laptops.
Polymer thick film flex circuits
Polymer thick film (PTF) flex circuits are actually true printed circuits. This means that the conductors are not soldered onto the circuit board but printed onto the design during fabrication. These are single conductor layers and are perfect for low voltage applications. These are found in many keyboards, and other peripherals that do not require too much voltage and provide a near to perfect thin layer option. These are also cost-effective products.