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Rigid-Flex PCB Lamination Technology

Rigid-Flex PCB Lamination Technology
Issue Time:2018-06-07

The flexible circuit board and the rigid board are pressed together to form Rigid-Flex PCB Board.

As with ordinary flexible boards, it requires independent drilling, plating, and etching. The difference is that due to the lack of glass fiber, it is thinner and more elastic. A board with less elasticity can be manufactured using PI and glass fiber as required. In the end, it is also necessary to press it together with a hard plate as a sandwich and make a panel.

The flexible circuit board is hot pressed together with the rigid circuit board and other flexible boards by adhesive. Each flexible board is not adjacent to each other. In order to maintain flexibility, each flexible board has a maximum of 2 copper layer. The flexible boards are separated by a rigid prepreg, a substrate, or a PI core adhesive sheet made of epoxy resin or acrylate adhesive.

Essentially, each rigid plate is individually cut in the filled portion of the flexible plate.

The following is the process of laminating the rigid and flex boards. Two two-layer flexible circuits are embedded in three rigid boards. The laminated structure is shown like the image 1 and image 2.

rigid flex pcb lamination

image1: Metal etching, plating, protective film and blank flexible board combined with glass epoxy rigid pcb board

A detailed flow chart of a plated-hole plated part with each flexible part and a rigid part plating

image2: A detailed flow chart of a plated-hole plated part with each flexible part and a rigid part plating

In the example shown in image 2, there are two pre-etched and cut flex circuits, each of which is double-sidedly plated. The flexible circuit has been embedded in the final panel, leaving a rigid circuit board under the flexible circuit for supporting the flexible circuit. This helps keep the flexible circuit flat during the soldering process. There are some potential hazards if the flexible circuit is not supported, such as bending or large cracks in the flexible circuit during the soldering process, especially in the reflow oven.

Solder masks can be made with a similar pressure sticker cover film, or with the aforementioned light-sensitive solder mask coating.

Finally, after the six-layer board consisting of the flexible board and the rigid board is pressed, the outermost (top and bottom) copper foil layers are connected together. It is usually drilled from the top layer to the bottom layer, and then plating is completed. It is also possible to use laser drilling for blind holes (from the top layer to the flexible board or from the bottom layer to the flexible board), but this increases the cost.

The final step is to print the top and bottom solder mask layers, silk screen layers and anti-corrosion gold plating (eg, nickel leaching gold) or solder uniform coating (HASL).

Although in principle it is possible to use a variety of laminated structures for rigid-flex printed circuit board, if you do not carefully consider production steps and material properties, its cost will be prohibitively expensive. One important consideration in designing a flexible circuit is to understand the pressure that the material can withstand inside the circuit when it is bent. After repeated bending, copper hardens and fatigue fracture occurs. One way to alleviate this problem is to use a single-layer flexible circuit structure. In this way, copper is at the center of the bend radius, and the compressive and tensile stresses absorbed by the film plate and the cover layer are maximized. Like the image below. Because polyimide has good elasticity, a single copper skin structure can have a longer life cycle in repeated movement than a multi-copper layer structure.

multi-copper layer structure

In the welding process, the rigid and flex pcb boards are put together. We place and solder the component on the rigid pcb. Some products require components to be welded in the flexible circuit board. In this case, rigid board are reserved below the flexible board to support the flexible part. The hard board is not glued to the flexible board and its contour is milled using a milling cutter of controlled depth. When the welding is completed, the worker can press it down with his hands.

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