Rigid flexible circuits can have circuit boards both externally or internally. They consist of multiple inner layers of flexible circuits. Advantages in rigid flexible circuits include high reliability and flexibility in design options. Rigid flexible circuits contain more materials than flat flexible circuits. The materials include rolled annealed copper, electro deposited copper, epoxy, acrylic, pre-preg, pressure sensitive, adhesiveless base material, FR-4, polyimide, polyester, solder mask, flexible solder mask, and photo image-able cover lay. Rigid flexible circuits are found in electronic products in many areas, such as, in the military and the medical field.