MULTI-LAYER FLEX CIRCUITS
Multilayer flexible printed wiring contains three or more conductive layers with plated through-holes, with or without stiffeners (see Figure above). Commonly, the layers are interconnected by plated through-holes, but this is not required since it is possible to provide openings to access lower circuit level features.
The layers of multilayer flex circuit may or may not be continuously laminated together throughout the construction with the exception of the areas occupied by plated through-holes. Discontinuous lamination is common in cases that require maximum flexibility and is accomplished by leaving the areas where flexing or bending is to occur unbonded. This will be discussed in more detail later.
Multi-layer flex PCB, like multi-layer Rigid PCB, utilizes the lamination technology which is important. Simplest stack up for Multi-Layer Flex PCB is a kind of 3 layer of single-sided Flex with copper shield structure on top and bottom sides. The most widely used structure in multi-layer Flex PCB is four layer Flex which is connected by plated through-holes with power layer and groud layer in the middle. With thin base material and light weight, good electrical performance like low dielectric constant, are advantages of Multi-Layer Flex PCB. Multi-layer Flex PCB with base material of polyimide has only 1/3 weight of rigid PCB of expoxy material.