With the continuous growth in demand for electronic miniaturization, there is an urgent need to take measures to reduce the size of PCB while encapsulating a large number of components in tiny chips. Although flexible PCB have gained prominence in this trend, for some applications with stricter size requirements, the thickness of traditional flexible PCB seems insufficient. As a result, manufacturers are actively seeking technological means to further compress the thickness of flexible PCB. Through relentless efforts, ultra-thin flexible PCB have emerged, designed to achieve a 55% reduction in thickness.
What is an Ultra-Thin Flexible PCB
You are likely familiar with flexible PCB, which are circuit boards that can be bent and folded to fit into confined spaces, with their thickness being only one-fifth or even lower than that of rigid PCB. The typical thickness of rigid PCB ranges from 600 μm to 2400 μm. To reduce thickness, high-density interconnects can be used to compress it to below 600 μm. On the other hand, the typical thickness of flexible PCB is controlled to be within 125 μm, even for double-layer flexible PCB, the thickness does not exceed 320 μm.
You might wonder: Wouldn’t it be better to reduce the number of layers in PCB to achieve thinness?
While that could be a viable strategy, it’s not a comprehensive solution. Multi-layer PCB have multiple internal copper layers, meaning that circuits originally on the surface are moved internally, reducing the PCB’s surface area and increasing integration. Typically, designers optimize the layers, layout, and various aspects during the design phase based on PCB design guidelines. This implies that the final design might be the optimal balance point, and reducing the number of layers may not necessarily optimize the PCB.
The diagram below compares the thickness of a standard double-layer flexible PCB, a thin double-layer flexible PCB, and an ultra-thin double-layer flexible PCB. From this, you can observe that achieving ultra-thin flexible PCB is not through reducing the number of layers but rather by using thinner base materials and conductive materials, precision circuit processing, and other advanced manufacturing techniques without altering the circuit structure.
Do You Need Ultra-Thin Flexible PCB
The most significant feature of ultra-thin flexible PCB lies in the compression of board thickness. Leveraging this characteristic provides electronic devices with significant advantages in terms of size and weight, making it particularly beneficial for wearable electronics. In addition, another most eye-catching feature is the improvement of the bending performance of flexible PCB. The reduction in thickness allows ultra-thin flexible PCB to endure smaller internal stresses during bending, enabling them to withstand millions of bending cycles more easily. This makes them more adaptable to complex surfaces and bending shapes.
However, TechSparks believes that when using ultra-thin flexible PCB, it’s crucial to pay attention to their drawbacks, with manufacturing cost being one of the most significant concerns. Despite using fewer materials, the manufacturing difficulty of ultra-thin flexible PCB is higher, making them more prone to faults. These costs are then passed on to the customers. Additionally, the manufacturing process requires the use of more advanced and precise technologies, meaning manufacturers need additional facilities. In the end, you’ll find that the startup costs for projects using ultra-thin flexible PCB can be quite expensive.
Manufacturing Process of Ultra-Thin Flexible PCB
- Material Preparation
Ultra-thin flexible PCB do not substantially differ from traditional flexible PCB. For substrate materials, it is recommended to use traditional polyimide, which offers superior flexibility, heat resistance, and electrical performance. The thickness can then be selected based on project requirements.
- Photosensitive Film
This involves applying a photosensitive film to the surface of the copper foil to protect the desired circuit area. To ensure reliable coverage, this step needs to be carried out at a temperature of 110±10°C, with a pressure of 0.5±0.1MPa, and a processing speed of 1.5m/min.
- Exposure and Development
The semi-finished product covered with photosensitive film is fed into an exposure machine, where exposure to light causes a chemical change in the photosensitive film, forming the intended design pattern. Next, the PCB with the pattern is placed in a developer solution, dissolving the unexposed portion of the photosensitive film. During this process, the exposure energy is set at 100±20mj, and the developer solution concentration is 1±0.2%.
Etching involves removing excess copper foil, with the designed circuit protected by the photosensitive film. Control over the etching amount is crucial in the manufacturing process of ultra-thin flexible PCB. For example, 30μm of copper foil can be etched away, resulting in 5μm of ultra-thin copper foil. The anti-etching protective film safeguards the area around the copper foil, ensuring it remains at a thickness of 35μm. After etching is complete, remove the photosensitive film.
- Hole Drilling
While traditional PCB often use mechanical drilling, it is recommended to use laser drilling for ultra-thin flexible PCB projects. This high-precision drilling method can even create tiny holes with a diameter of 5μm on dielectric material as thin as 5µm, making it more advantageous.
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