1.3 multi-layerflexible PCB
Flexible Multilayer PCB Multilayer PCB, such as rigid as multi-layer laminate technology, can be made into multi-layer flexible PCB. The simplest multi-layer flexible PCB is covered on both sides in the single-sided PCB with two layers of copper shield formed by three flexible PCB. This three-layer soft PCB electrical characteristics equivalent to the coaxial cable or shielded cable. The most common multi-layer flexible PCB structure is four-story structure with a hole to achieve layer metal interconnect, the middle layer is generally the power and ground planes.
Multi-layer flexible PCB substrate film has the advantage of light weight and has excellent electrical properties, such as low dielectric constant. With a polyimide film substrate made of multi-layer flexible PCB, rigid epoxy glass cloth than the multi-layer PCB board light weight of about 1 / 3, but it lost its single-sided, double-sided PCB excellent soft can be flexible, most of these products are not required to be flexible.
Multi-layer flexible PCB can be further divided into the following types:
1) constitutes a multi-layer flexible insulation substrate PCB, the finished product can be defined as deflection: This structure is usually the many-sided or double-sided flexible PCB microstrip can be bonded to both sides of the side, but its center bonded together and the end part, which is highly flexible. In order to have the desired electrical characteristics, such as the characteristic impedance of the interconnection of rigid PCB and its matched, multi-layer flexible PCB components of each circuit layer, the surface must be designed to signal ground line. In order to have a high degree of flexibility, wire available on a thin layer, suitable for coating, such as polyimide, place a layer of thick laminated cover. Through holes that can be flexible circuit layers to achieve the required z-plane interconnect. This multi-layer flexible PCB assembly may be best suited for applications requiring flexibility, high reliability and high-density design.
2) substrate in the form of soft insulating layers PCB, its provisions can be finished at the end deflection: These multi-layer flexible PCB with soft insulating material such as polyimide film, made of laminated plywood. After losing in the laminate can be inherent flexibility. When the design requirements to maximize the insulating properties of thin films, such as low dielectric constant, dielectric thickness, lighter weight and can be continuous processing features, on the use of such soft PCB. For example, polyimide film multi-layer PCB insulating material than the rigid epoxy glass cloth light about one-third the weight of the PCB.
3) insulating substrate in the form of soft layers PCB, the finished product must be formed, instead of continuous deflection: the type of multi-layer flexible PCB is made of soft insulating material. While it with soft materials, but due to the electrical design constraints, such as resistance to the required conductor, requiring a thick conductor, or to the required resistance or capacitance, requiring the signal layer and ground layer thickness between insulation isolation, therefore, when it has finished forming the application. The term "formable" is defined as: multi-layer flexible PCB with components made of the ability of the required shape, and in the application can no longer flex. In the avionics unit inside wiring applications. At this time, requiring stripline or three-dimensional design of the conductor resistance is low, circuit noise, capacitive coupling or a very small and interconnected end to smoothly bent 90 °. Polyimide film material with a multi-layer flexible PCB layout to achieve this task. Because of high temperature polyimide film, there can be flexible, and the total electrical and mechanical characteristics of a good. To achieve this cross-section of all interconnect components, traces of which can be further divided into multi-part flexible circuit components, and together with adhesive tape to form a printed circuit bundle.