1. Overview of Mini LED Flexible Light Boards
Mini LED flexible light boards are an emerging lighting technology that has developed rapidly in recent years. They use miniature light-emitting diodes (Mini LEDs) as the light source and mount them on a flexible printed circuit board (FPC). This combination brings together the high brightness, high contrast, and high reliability of Mini LEDs with the bendable characteristics of a flexible substrate, creating a high degree of installation flexibility and new design possibilities.
Mini LED is also known as a hundred-micrometer-scale light-emitting diode technology. It is a new LED display technology derived from small-pitch LED technology in recent years. Mini LED is considered an intermediate technology between conventional LEDs and MicroLEDs. Its pixel size and manufacturing difficulty fall between those of the two technologies. It also represents an improved version of conventional LED backlighting, with chip sizes generally ranging from 50 to 200 μm.
Mini LED flexible light boards have several key characteristics:
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High-density integration: Mini LED packages can be arranged at a density of 560 devices per meter or even higher. This dense arrangement reduces the visible pixelation associated with conventional LEDs and produces smoother, softer illumination.
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Excellent flexibility: Double-sided electroplated FPC substrates can provide a minimum bending radius of approximately 5 cm, allowing the light boards to conform to various curved installation surfaces.
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Rich color performance: Four-wire or five-wire designs support full-color mixing and independent white-light adjustment. They can reproduce up to 16.7 million colors and provide a color rendering index of at least 80–90 Ra, together with high color saturation.
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Low-voltage, high-efficiency operation: These light boards typically use a safe DC 24 V supply. Power consumption is generally 15–18 W/m, resulting in energy-efficient operation and low heat generation suitable for extended use.
Mini LED flexible light boards are now used in television backlighting, cabinet light strips, suspended ceiling lighting, advertising light boxes, stage decoration, and creative DIY applications. As the technology matures and costs decrease, Mini LED flexible light boards are becoming an important development direction for next-generation lighting and display systems.
2. Comparison of Mini LED Substrate Types
The following figure shows the types of LED backlight boards currently produced in volume.
Substrates widely used in Mini LED backlighting include aluminum-based PCBs, FR-4 PCBs, glass substrates, and PI flexible substrates. Each option has different advantages, disadvantages, and application areas.
| Item | Aluminum-Based PCB | FR-4 PCB | Glass Substrate | PI Flexible Substrate |
|---|---|---|---|---|
| Advantages | Good thermal dissipation, high-temperature resistance, and low cost | High process maturity, multilayer routing capability, and high structural strength | High circuit precision, good warpage and thermal expansion characteristics, and support for very large sizes | Flexible, bendable, and ultrathin |
| Disadvantages | Lower precision than FR-4 | Warpage in large-size boards | Susceptible to breakage; high initial investment before the process matures; thin circuit layers | High cost, lower precision, substantial dimensional expansion and shrinkage; assembly becomes difficult at excessive sizes |
| Primary Backlight Applications | TVs | TVs, monitors, and automotive displays | TVs, monitors, and automotive displays | Laptops and tablets |
| Notes | Currently the best choice for single-layer boards | Market acceptance is limited because of the high cost | Promoted primarily by panel manufacturers, but with high cost | Offers unique advantages for thin products and products with highly curved surfaces |
3. Conventional PCB Manufacturing Process
Conventional PCB fabrication involves a complex and highly mature process. A multilayer board can be used to illustrate the standard manufacturing flow.
The primary process steps are as follows:
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Inner-layer circuit fabrication: The copper-clad laminate is cut to the required dimensions and then subjected to pretreatment, dry-film lamination, exposure, and DES processing, consisting of development, etching, and stripping. This process requires high dimensional accuracy and uniform etching.
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Lamination: Copper foil, prepreg, and brown-oxide-treated inner-layer circuit boards are pressed together to form a multilayer board. Brown-oxide treatment increases the bonding strength between the copper surface and the prepreg, helping prevent interlayer delamination.
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Drilling: A precision drilling machine creates through-holes in the PCB, establishing conductive paths between layers. The drilling machine's positioning system must be controlled accurately to ensure that all hole locations are correct.
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Hole metallization: The nonconductive surfaces inside the hole walls are metallized to provide a foundation for subsequent plating. If the multilayer PCB includes buried or blind vias, each individual layer must be drilled and plated before lamination. After drilling, the inner walls of the holes must be plated.
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Outer-layer circuit fabrication: The required circuit pattern is transferred onto the outer-layer dry film through exposure. Copper is then electroplated to the thickness specified by the customer, forming the required circuit structure.
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Solder mask and surface finish: A protective solder mask is applied to the PCB surface to help prevent short circuits during soldering. A surface finish, such as electroless nickel immersion gold (ENIG), immersion tin, or OSP, is then applied according to the requirements of the product.
4. Roll-to-Roll Flexible Circuit Board Processing
Roll-to-roll flexible circuit board processing uses a different manufacturing approach and can employ PET substrates with lower material costs.
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Circuit patterns are formed on the flexible substrate through embossing and roll-to-roll processing. This method supports large-format material and provides high processing efficiency.
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Compared with conventional PCB fabrication, the process does not use an etching step and is therefore more environmentally friendly.
The basic process flow and parameters are shown below.
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Tool fabrication: The tool contains the pattern information for the circuit layer.
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UV adhesive embossing: UV adhesive is embossed onto the substrate in a roll-to-roll process.
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Circuit-layer fabrication: The circuit layer is formed using roll-to-roll processing.
Throughout roll-to-roll production, the flexible substrate is affected by temperature and mechanical stretching. To maintain dimensional stability, the substrate must be characterized for thermal expansion and shrinkage at elevated temperature, as well as for its inherent elongation behavior. The resulting data can then be used to apply dimensional compensation during circuit-board fabrication.
5. Mini LED Flexible Light Board Manufacturing Process
The manufacturing flow for Mini LED flexible light boards is shown below.
Because the process uses a low-cost PET substrate, its high-temperature resistance is lower than that of a conventional PCB. Special requirements therefore apply to SMT reflow soldering.
Reflow Soldering
When a PET substrate rated for 180 °C is selected, low-temperature solder paste should be used, and the peak reflow temperature must not exceed 180 °C. Particular attention must also be paid to testing the push-pull strength of the LEDs, soldering reliability, and temperature rise during operation.
Protective Adhesive Process
COB Mini LED structures also require encapsulation to protect the LEDs. Two options are available:
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Low-temperature adhesive dispensing: The curing temperature must be controlled below 180 °C.
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Hot pressing of adhesive film: This method is currently limited to small sizes of 16 inches or less. It provides front-side adhesive protection, but the module cannot use a reflector sheet.
6. Mini LED Flexible Light Board Applications and Challenges
One implementation uses a PET substrate, a roll-to-roll circuit process, and a POB Mini LED configuration.
The required soldering dimensions for the Mini LEDs must first be calculated. The most suitable flexible substrate material must then be verified and selected, with particular attention to its thermal, physical, and dimensional stability.
Thermal Rise
During circuit design, the traces should be made as wide as practical and the circuit-layer thickness should be increased where appropriate. Differences between separate zones should also be minimized. A suitable low-temperature solder paste should be selected, and the peak reflow temperature must be controlled. When the light board is attached to the backplate, suspended areas should be avoided so that heat can be dissipated uniformly.
Assembly
The flexible light board must be attached to the backplate with double-sided adhesive. This assembly operation is difficult and does not allow repair after bonding. A possible future approach is to first attach the flexible circuit board to the backplate and then evaluate the feasibility of performing component placement afterward.
Reliability
When the flexible light board is curled, the LEDs and circuit layer are susceptible to cracking or fracture. After push-pull testing, the LEDs may detach, while the pads and circuit layer may also peel away from the substrate.
