Multilayer flexible circuits
We produce Multi Layer flexible circuits just like rigid multilayer PCBs, can be made into multilayer flexible PCBs by using multilayer laminating techniques. The most simple multi-layer flex circuits is a single-sided flexible PCB plus it’s both sides covered with two layers of copper shield to form a three-layer flexible PCB. The three-layer flexible PCB is electrically equivalent to a coaxial conductor or a shielded conductor. The most common Multi Layer flexible circuits PCB structure is a 4 layer flexible pcb structure, with plated through holes to achieve interconnection between the layers, the middle two layers are generally power and ground layer.
The advantage of the multilayer flexible PCB is that the substrate film is in light weight and has excellent electrical characteristics, such as the low dielectric constant. Multilayer flexible PCBs were made from polyamide film are about 1/3 weight than rigid FR4 multilayer PCBs, but they lose the flexibility compare with the single-sided and double-sided flexible PCBs, and commonly this kind of products are not required to be flexible.
Multilayer flexible PCB consists of 3 or more conductor copper layers on a flexible dielectric/polyimide film, normally connected with the plated through hole (see picture below). Multilayer flexible circuits can be produced with or without cover layers.
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Multi – layer Flexible Printed Circuit Fabrication
The manufacturing process of the multi-layer flexible printed circuit is very similar with the rigid multilayer pcb, however, due to the special nature of flexible materials and applications, it determines the design requirements and the production process are quite different from the ordinary rigid pcb boards.
Process flow
Substrate material – Pre-baking – inner pattern shift – etching — AOI Inspection – Punching Locating holes – Laminating – Drilling – Plasma Cleaning – Plated through hole – Outer pattern – AOI Inspection – pattern Plating – Etching – Power-on test – Overcoat protection – Surface coating – Outline
When to use multi-layer flexible circuits
• Required larger number of signals to be routed in the circuit
• Required localized areas where stiffeners and components can be added
• Required superior signal integrity
• Dense surface assembly
• Ground and power plane configuration
• Shielding applications and controlled impedance with shielding
• EMI/RFI shielding
The multilayer flexible printed circuit board can be further classified into the following types:
1.Make the Multi Layer flexible circuits based on the flexible insulating substrate, the finished product is defined as flexible.
This structure is usually combines the both ends of a lot of single-sided or double-sided flexible PCB together by adhesive, but the central part is not bonded together, which has a high flexibility. In order to get the desired electrical characteristics, such as the special impedance characteristics should match with the rigid PCB which are interconnected, and each circuit of the multilayer flexible PCB must be designed with signal lines on the ground layer. In order to have a high flexibility, a thin, suitable coating, such as a polyimide, may be applied over the conductor layer in place of a thicker overcoat layer. The plated through holes realize the interconnection between the z-planes of each flexible circuit layers. This multilayer flexible PCB is best suitable for high flexibility, high reliability and high-density design.
2.Make the multi-layer PCB based on the flexible insulating substrate, the finished product is not defined to be flexible.
This kind of Multi Layer flexible circuits are laminated into a multilayer board with a flexible insulating material, such as a polyimide film. Which loses inherent flexibility after lamination. This type of flexible PCB is used when the design requires maximum use of the insulation properties of the film, such as low dielectric constant, uniform thickness media, lighter weight, and continuous process ability. For example, multilayer PCBs made from insulation polyimide film are about one-third weight compare with the rigid FR4 multilayer PCBs.
3.Make the multi-layer PCB based on the flexible insulating substrate, the finished product can be specific shaped but not continuous flex.
Such multilayer flexible PCBs are made of flexible insulating material. Although it is made of a flexible material, it is required to have a thickness between the signal layer and the ground layer due to electrical design limitations, such as requiring a thick conductor for the desired conductor resistance, or for the desired impedance or capacitance Of the insulation isolation, so it has been specific shaped when it finished. “Can be specific shaped” means the multilayer flex PCB has the ability to be formed into a desired shape and which cannot be deflected in use. It is used in the layout of the avionics devices. In this case, the stripline or three-dimensional design is required to have low conductor resistance, minimal capacitive coupling or circuit noise, and can smoothly bending at the interconnection ends at 90 degrees. Multilayer flexible PCBs made from polyimide film materials fulfill this wiring task. Because the polyimide film is high temperature resistant, flexible, and has good overall electrical and mechanical properties. In order to achieve all of the interconnection of the cross-section of the part, the circuit part can be further divided into a plurality of multilayer flexible circuit parts and combined together by an adhesive tape to form a printed circuit bundle.