1.2 soft-sided PCB
Soft-sided PCB, with two conductors. The application of these double-sided PCB and advantages of soft-sided flexible PCB with the same, the main advantage is to increase the wiring density per unit area. It may be there, and there are no holes, no cover is divided into: a no holes, no cover layer; b no holes, there are cover layer; c have holes, no cover layer ; d with through holes, a cover layer. No double-sided flexible PCB cover fewer applications.
1.3 The multi-layer flexible PCB
Multilayer PCB Flexible PCB, such as rigid as multi-layer, 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 and the formation of three flexible PCB. This three-tier flexible PCB in the electrical characteristics equivalent to coaxial cable or shielded wires. The most common multi-layer PCB structure is the four-story structure, soft, with holes to achieve the interconnection between layers, the middle layer is generally the power and ground planes.
The advantages of multi-layer flexible PCB substrate film has excellent light and electrical properties, such as low dielectric constant. Substrate with a polyimide film layers made of soft PCB, epoxy glass cloth than the rigid multi-layer PCB board light weight of about 1 / 3, but it lost its single-sided, double-sided PCB excellent soft The flexible, most of these products are not required to be flexible in.
Multi-layer flexible PCB can be further divided into the following types:
1) constitute a multi-layer flexible insulation substrate PCB, the finished product can be defined as deflection: This structure is usually the number of single or double-sided flexible PCB on both sides of the microstrip end bonded together, but its center section and bonded together at the end, which is highly flexible. In order to have the desired electrical properties, such as the characteristic impedance of interconnect performance, and it matches the rigid PCB, multilayer PCB components of soft layer on each line must be designed in the ground plane signal lines. For highly flexible, wire can be a thin layer, suitable for coating, such as polyimide, place a layer of thick laminated cover. Holes between the layers of the flexible circuit to achieve the required z-plane interconnect. This multi-layer flexible PCB requirements best suited for flexible, high reliability and high-density design.
2) constitute a multi-layer flexible insulation substrate PCB, provides for bending the end of the finished product: These multi-layer flexible PCB assembly is soft insulating material such as polyimide film, made of laminated plywood. After the loss of the inherent laminated flexible. When the design requirements to maximize the insulation properties of thin films, such as low dielectric constant, dielectric thickness, lighter weight and energy characteristics of continuous processing time to the use of such soft PCB. For example, polyimide film multi-layer insulation material made of epoxy glass cloth than the rigid PCB PCB light about one-third of the weight.
3) constitute a multi-layer flexible insulation substrate PCB, the finished product must be formed, rather than continuous flexing: This multi-layer FPC is made of soft insulating material. While it with soft materials, but due to the electrical design constraints, such as the conductor to the required resistance, require a thick conductor, or to the required resistance or capacitance, required in the signal layer and ground layer between the thick insulation isolation, therefore, when it has finished forming applications. 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 deflection. Avionics unit in the application of internal wiring. At this time, required three-dimensional design of the ribbon cable or conductor resistance is low, capacitive coupling or a very small and in the interconnection circuit noise can be smoothly bent end into the 90 °. Polyimide film material using multi-layer flexible PCB layout to achieve this task. Because of high temperature polyimide film, a flexible, and always a good electrical and mechanical properties. To achieve this cross-section of all interconnect components, some of which traces can be further divided into multiple layers of flexible circuit components, and together with adhesive tape to form a printed circuit bundle.