Most designers have a particular fondness for the word “standard,” especially when it comes to flexible PCB thickness. Although there isn’t a strict standard defining the exact thickness of a flexible PCB, effectively managing it during the design process can significantly impact project costs. Flexible PCB, being a costly type of circuit, can greatly influence your overall project budget. So, how should we reasonably choose and manage flexible PCB thickness? Let’s dive in.
Standard Flexible PCB Thickness
In the industry, flexible PCB thickness typically ranges from 2 to 32 mils, with the specific choice depending on the application’s requirements for PCB flexibility and structural strength. For example, thicknesses of 4 mils or less are defined as ultra-thin flexible PCB, offering extreme flexibility, making them suitable for various wearable devices and implantable medical devices. On the other hand, flexible PCBs with thicknesses between 10 and 20 mils are more common, used in everything from consumer electronics to high-end aerospace applications, as they provide adequate structural support while maintaining moderate flexibility.
When considering PCB thickness, it’s important not only to focus on the standard thickness but also to consider the number of layers, as there is an inseparable connection between the number of layers and thickness, which also affects complexity and performance. For double-layer flexible PCB, a common thickness range is 14 to 32 mils, which helps achieve an optimal balance between durability and cost. If your design requires UV curing, a thickness of 10 mils is recommended, with at least one additional circuit layer added, bringing the total thickness to 18 mils to ensure better UV exposure.
For projects requiring a four-layer flexible PCB, the recommended thickness is 32 mils. This thickness is ideal for multi-layer structures, particularly in devices used in high-pressure environments. A four-layer flexible PCB with a thickness of 32 mils provides sufficient support and allows for more layers in the design, enhancing the overall performance of the circuit board.
Factors Affecting Flexible PCB Thickness
The influence of the PCB substrate material on thickness is obvious, and it also impacts overall performance. For instance, polyimide, the most common flexible PCB substrate material, typically has a standard thickness ranging from 0.5 to 5 mils, providing enough reliability and thermal stability to the PCB. Liquid crystal polymer is another main choice for flexible PCB substrates, with thicknesses ranging from 1 to 4 mils, it stands out due to its low moisture absorption and excellent dimensional stability. Aramid paper may not have the advantage in thickness, usually ranging between 2 and 10 mils, but it remains a popular choice, particularly in low-margin projects.
In addition, copper weight must also be carefully considered. Although copper weight becomes less critical in flexible PCB with fewer layers, it still impacts several factors such as minimum line width and spacing, current-carrying capacity, and impedance. For a design with just four layers, increasing the copper thickness from one-third ounce to half an ounce can add nearly 0.001 inches to the overall thickness. For a more substantial increase, from half an ounce to one ounce, the thickness may increase by more than 0.002 inches, approaching 0.003 inches, which significantly reduces the flexibility of the flexible circuit. For most electronic products, 8-ounce copper is an ideal range, balancing flexibility and performance in flexible PCB.
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