Flexibility refers to the ability of an object to maintain its shape when it is deformed after losing its force. Rubber bands, for example, have good elasticity and will become longer after stretching, but the shape will return to its original state after the tension disappears, so the elasticity is very weak. Flexible board is a printed circuit made of flexible insulating substrate, which has many advantages that rigid printed circuit board does not have. The product is small in size and light in weight, which greatly reduces the volume of devices, and is suitable for the development of electronic products in the direction of high density, miniaturization, lightweight, thinness and high reliability. It has high flexibility, can be freely bent, wound, twisted and folded, can be wired in three dimensions, can be arbitrarily arranged according to the requirements of spatial layout, can change its shape, and can be freely moved and unfolded in three dimensions, thus realizing the integration of assembly and wiring. It has excellent electrical properties, high temperature resistance and fire prevention. Stable chemical change, good stability and high reliability. It has higher assembly reliability, provides convenience for circuit design, can greatly reduce the assembly workload, easily ensure the performance of the circuit and reduce the cost of the whole machine. By using reinforcing materials, its strength is increased to obtain additional mechanical stability. The design of soft and hard combination also makes up for the slight deficiency of flexible substrate in the bearing capacity of components to some extent. According to the data of IPC1TMRC in June, 1994, the output value of flexible circuit board was USD 400 million/year at the end of 1980s, and it is developing at the rate of 6-7% per year. 1994 is about1500 million USD, and 1997 is estimated to be1700 million USD. In recent years, due to the successful development and application of adhesive-free materials, flexible photosensitive film or liquid photosensitive solder resist suitable for flexible circuits. Flexible circuit not only ensures the quality and improves the qualified rate, but also is easy to be automated and mass-produced. In addition, the "light, thin, short and small" and three-dimensional assembly of electronic products have become necessary and key, such as PCMCIA cards, flexible boards, rigid-flexible boards and so on, which have been paid attention to by users. At present, although the flexible board is still in its infancy, its obvious advantages and potential ability make its position in PCB production and market more and more recognized and valued, so the output value of flexible circuit board will increase at an average annual rate of 20%.