Solid circuit boards (known as “rigid”) have been with us for a long time and work well in most applications. However, modern CAD design has required circuit boards to adapt to more complex contoured shapes. Using CAD design software and modern manufacturing processes, almost anything can be made in almost any shape. So if a circuit board is rigid, how can they be used in something that isn’t flat?
This is where flexible circuits come into play. Also called “flex” circuits, they are made up of copper traces encapsulated with an insulating dielectric material made of polyimide. Polyimide is an excellent insulator that allows the circuit to bend, resist contamination and withstand high temperatures.
Not only are flex circuits beneficial due to their ability to bend and twist, but they are also much thinner than a rigid circuit board, making them an ideal solution where space is limited. Flex circuits are literally paper thin and can be sandwiched between other components or wrapped around obstacles. They also weigh very little which is useful in lightweight applications where every gram counts.
Another benefit to using flex circuits is the ability to reduce the number of connectors needed to connect one circuit board to another. Eliminating connectors means reducing the number of components used (which reduces cost) and increases reliability since fewer solder joints are needed. Environmental factors such as water, salt, and dust do not affect traces encapsulated inside a flex circuit but are historically connector killers.
Where are flex circuits used?
Curved TV’s and monitors (they are curved, after all)
Computer mice (they are odd shaped and have buttons on multiple sides)
Digital watches (some conform to the curved shape of your wrist)
DSLR cameras (there are buttons on almost every surface that are all connected together)
Fitness monitors (need to be as small, thin, and lightweight as possible)
Folding phones (flex circuits can be folded and bent many times without breaking)
Laptop screens (the monitor is connected to the main computer through flexible hinges)
Ear buds (not only need to be small and lightweight, but are oddly shaped)
Drones (obviously weight is a major factor in anything that flies)
Medical devices (things that go inside the body need to be small, lightweight, and reliable)
Sometimes a product requires a circuit board to be both rigid and flexible, which is accomplished by using a hybrid circuit board design, or “rigid-flex”. The rigid areas can be assembled using conventional techniques and components while the flexible areas go where a rigid circuit board can’t. It’s the best of both worlds for applications that require both rigidity and flexibility.
In the not so distant future, circuit boards will be transparent. Flex circuits exist now where the circuit boards are made from clear plastic materials (PET), but they are stiffer than normal polyimide flex circuit board material and they must be soldered with specialized equipment. However, improvements are being made so that one day we will be able to embed circuitry into windshields, cockpits, TV’s, phones, windows, glasses, goggles, and even coffee mugs!
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