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March 2017 • The PCB Magazine 59 includes variations such as ceramic, multilayer laminations, blind and buried vias, functional coatings, plating chemistries, hybrid circuits, buried passives, surface mount component as- sembly processes and a plethora of other tech- nologies. Rigid circuit boards are generally suit- able for applications where a static and planar wiring substrate meets the design requirements, but the inflexible form factor faces challenges in applications requiring out-of-plane circuit routing. Flex PCBs—The Second Wave of PCB Technology Although developed later than rigid circuit boards, flex circuits have also been in existence for many years. The advent of flex circuit tech- nology was a major breakthrough in electronic device design. This new form factor enabled the development of many of the electronic devices we are familiar with today. As with rigid circuit boards, there are countless variations of flexible PCB technology. This section will briefly review copper-clad polyimide because this material set has emerged as the dominant technology for solid metal conductor flexible circuits. Over the years, many types of thermoset- ting polymer films have been used for making flex circuits including fluoropolymers and flexi- bilzed epoxies. However, because of its combi- nation of mechanical, chemical, electrical and thermal properties, polyimide film has emerged as the prevalent thermoset substrate material for flex copper-clad film PCB constructions. While there are many types of polyimide avail- able from a number of suppliers, generally they all exhibit flexibility over a wide temperature range, with good electrical properties, excellent chemical resistance, superior tear resistance and high tensile strength. In a typical scenario, the polyimide film is clad on one or both sides with a copper foil. Rolled and annealed copper (known as RA cop- per) is the most common type of copper foil for PCB manufacturing. As with rigid PCB technol- ogy, the circuits are formed using photolithog- raphy and subtractive chemical processes. The copper foil is imaged and etched to form copper traces. Luckily, it turned out that copper also works well for flexible circuit constructions. As demonstrated by applied mechanics, bend- ing strains decrease linearly with thickness. The result is that any material, in sufficiently thin form, is flexible. Combining this mechani- cal principle with copper's ductile properties means that thin copper foils with the correct grain structure can bend repeatedly without cracking, within certain parameters. In addition to being bendable, flexible poly- imide based PCBs are generally thinner and lighter than their rigid counterparts, with the result that end products incorporating these flex circuits can also be smaller and lighter. And, as their name implies, flexible PCBs are designed to bend and twist, freeing product designers from the constraints of a two axis PCB layout. Flexible PCBs readily route circuitry, surface mounted components and interconnections out of the X-Y plane and into the Z plane. De- pending on the assembly process and end-use functionality of the device, flexible PCBs may be subjected to either static bending (bend and hold) or dynamic bending (repeated flexing) stresses. Engineers have a wide variety of mate- rial sets and design rules at their disposal to find the necessary combination of construction, substrate and conductor materials to achieve the desired bending performance. Polymer Thick Film—The Low-Cost Circuit Additive manufacturing is a hot topic today. Many people aren't aware that high-volume production circuits have been made using ad- ditive manufacturing technology for decades. STRETCHING BEYOND FLEX " In addition to being bendable, flexible polyimide based PCBs are generally thinner and lighter than their rigid counterparts, with the result that end products incorporat- ing these flex circuits can also be smaller and lighter. "