Issue link: https://iconnect007.uberflip.com/i/649323
58 The PCB Magazine • March 2016 Introduction Flexible printed circuits are defined by IPC [1] as: "A patterned arrangement of printed circuit- ry and components that utilizes flexible base material with or without a flexible cover lay." Simply put, a flexible circuit is a conductive pattern laid on a flexible dielectric film. Whilst early 1900s (metal on wax paper) and 1950s (etched metal patterns on insulating polymers or coated paper) inventions strived for replac- ing complex discrete wiring harnesses, today´s state-of-the-art FPC manufacture utilizes, for example, unreinforced polyesters (low cost), PEEK, polyimides (high reliability) as substrates for any conductive pattern [2] . Figure 1 shows a typical multi-layer FPC stack-up [3] . Within a flexible-circuit construc- tion, the dielectric film forms the base layer, with adhesives used to bond the conductors to the dielectric. In multilayer flexible circuits, ad- hesives are used to bond the individual layers together. Adhesives can also be used in a protec- tive capacity to cover the final circuit to prevent the ingress of moisture and dirt; hence they are termed cover lays (also cover layers) or cover coats. Most flexible circuits are passive wiring structures that are used to interconnect elec- tronic components such as integrated circuits, resistors, capacitors and the like; however, some are used only for making interconnections be- tween other electronic assemblies either direct- ly or by means of connectors. In other words, FPC offers the same design options as PCBs, with the added benefit of vibration resistance (e.g., airborne/space applications) and 3D con- figurations. FPC Technology A basic flexible circuit has three major com- ponents: conductor, adhesive and dielectric/in- sulator film. Conductors are usually electrodeposited or rolled-annealed copper foils with various weights/thicknesses. These copper foils are bonded to the dielectric film using an adhesive film or by applying an adhesive-less construc- tion technique. Adhesive-less construction has certain advantages over an adhesive base con- struction, such as thinner and more flexible cir- cuits with better electrical properties. However, this advantage is reduced when an adhesive base cover coat is used. When it comes to the choice of adhesive for FPC, acrylic and epoxy based adhesives are the most commonly used systems. Specific applica- tion requirements dictate which choice is the best. All three components, conductor, adhesive and dielectric, have specific roles in making a robust FPC. However, the choice of dielectric film is considered to be the most critical in a flexible circuit. Most commonly used dielec- tric materials are unreinforced polyester based substrates for low-end applications whereas for complex, high-reliable FPCs, polyimides are the materials of choice. The end application usually dictates the type of dielectric film used to build flexible circuits. Table 1 [4] shows key charac- teristics of the most commonly used dielectric films. Among the dielectric films listed in table 1, Polyimide is the most popular and the focus of this discussion. Polyimide (PI) Substrate Polyimides are a class of thermoset polymers with excellent flexing and electrical properties. It has superior resistance to high temperature, hence is an excellent choice for lead-free solder- ing conditions. The outstanding physical prop- erties (e.g., dielectric strength, stability on ther- mal impacts, flexural strength) and excellent chemical resistance of polyimides are a conse- quence of the highly crosslinked nature of the basic polymer (Figure 2) in its simplest form. direCt metallization system for flexible printed CirCuits Figure 1: Typical multilayer FPC stack-up.