With the advancement of bendable printed circuit technology, the research and development of rigid flex circuit boards have extensive applications. The durability and flexibility of rigid flex circuit boards, along with their strong resistance to harsh environments, make them more suitable than rigid PCB board for medical and military applications. Rigid flex circuit boards comprises flexible PCB and rigid PCB board, which are separately manufactured and then pressed together to form a single circuit board. Each rigid flex circuit board contains one or more rigid areas and flexible areas. The rigid PCB board provides high flatness and mechanical strength, while the flexible printed circuit allows bending and offers high flexibility.
Technical Problem of Rigid flex circuit boards
In the conventional structure of rigid flex circuit boards, the rigid PCB board is typically attached to both sides of the flexible board. During the production of rigid flex circuit boards, the circuitry on the flexible board section is typically prepared first. The flexible board's inner layer requires double-sided soldering, usually using materials suitable for double-sided boards in production. The materials for double-sided boards are sequentially from top to bottom: copper foil, adhesive layer, substrate, adhesive layer, and copper foil. To ensure conductivity between the upper and lower copper layers, industry standards usually involve drilling 0.3mm diameter holes in the copper foil surface and plating them with copper on the hole walls. This allows conductivity between the upper and lower layers, enabling double-sided soldering operations on the bendable board section. The process for creating bendable board circuits is as follows: mechanical drilling of copper foil - PTH (copper plating) - copper plating - circuit production - application of protective film - laminating protective film. Since the product has undergone copper plating and electroplating, the effect of copper inside the hole wall is difficult to confirm due to the hole diameter of 0.3 mm, and the conductive performance of the product is tested after the product is completely manufactured. Additionally, because of the uneven distribution of the electroplated copper, it will affect the product's performance.
Invention content of Flex Plus Rigid flex circuit boards
In the industry, before combining the flexible and rigid boards, the ineffective parts of the rigid PCB boards are usually removed to expose the corresponding areas of the flexible circuit board. This process causes a height difference in the combined flexible-rigid board. When creating circuits on the bendable board in the areas with height differences, the dry film cannot fully fill these parts, leaving air residues that cause circuit breakage, resulting in low product yield. Therefore, the industry generally adopts a method of vacuum pressing the dry film, where the air is completely removed before applying the dry film to avoid circuit breakage due to air residues. However, vacuum laminating machines are much more expensive than ordinary dry film machines, costing about ten times more. How to cut parts of the rigid PCB board without affecting the quality of the flexible board's dry film lamination using ordinary dry film machines is an urgent technical problem to solve.
Flex Plus's Rigid flex circuit boards technical scheme:
The objective of Flex plus invention is to provide a simplified process for manufacturing rigid flex circuit boards that eliminates the need for the copper plating and electroplating processes for the bendable circuit board, and uses ordinary dry film machines instead of vacuum laminating machines without affecting the quality of the dry film lamination process, while meeting production requirements at a lower cost.
This invention provides a rigid flex circuit board, including bendable circuit board and rigid PCB boards. The bendable circuit board is made of pure copper, with a protective film adhered to both the upper and lower surfaces, and the rigid flex circuit boards are bonded together by hot-pressing adhesive.
The production process of the bendable board is as follows:
Adhere a layer of dry film to the upper surface of the pure copper using a hot press, then expose, etch, and remove the film to complete circuit production, followed by hot pressing a protective film to protect the circuit.
The lower surface of the pure copper uses the same production process as the upper surface.
The purpose of achieving double-sided soldering is accomplished by differentiating the operations of the upper and lower protective films. The bendable board uses a structure of pure copper with upper and lower protective films. Areas not covered by the upper and lower protective films can be soldered, forming a complete circuit.
For the rigid PCB board areas that need to be removed, a mold is used to make an initial cut at one-third to one-half of the thickness of the rigid PCB board on the back side, creating an internal cut. The rigid PCB board is then hot-pressed and bonded with the bendable board. During the dry film lamination process for the combined rigid flex circuit boards, the ineffective areas of the rigid PCB board remain on the product, ensuring no height difference. After the dry film lamination and circuit production are completed, a second cut removes the ineffective areas of the rigid PCB board: the mold cuts from the front of the rigid PCB board, with the combined depth of the second and initial cuts being greater than the thickness of the rigid board. After two cuts, the ineffective areas of the rigid PCB board naturally fall off, and the rigid flex circuit boards are fully formed through normal product shape cutting.
The Advantages of Flex Plus’ Invention:
The bendable circuit board uses pure copper instead of double-sided board material, and the protective films are applied separately, eliminating the mechanical drilling, PTH (copper plating), and electroplating processes of double-sided boards. The circuit conductivity is entirely based on the original copper material without using additional electroplated copper. This method uses materials of pure copper plus double protective films, allowing product soldering at the openings of the upper and lower protective films, ensuring product performance. The manufacturing process is relatively simple, reducing material costs and simplifying the production process, improving product yield.
By pre-cutting internal cuts in the rigid PCB board material and performing a second V cut after the bendable circuit board protective film lamination, the rigid flex circuit boards can be produced using ordinary dry film machines. This reduces production steps, saves equipment investment, lowers production costs, and improves production efficiency.
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