Analysis of Black Level Control in Mini-COB LED Displays

Posted by

Tingting Zhao

11/18/2025

On 11/18/2025

Dongguan HCP Technology Co., Ltd. – Wang Zhoukun

Abstract:
Mini-COB LED displays, with their advantages of high integration, high contrast, and low power consumption, have been widely used in commercial displays, security monitoring, and high-end consumer electronics. As a core factor influencing the visual performance of Mini-COB LED displays, black level directly determines image uniformity, contrast, and color reproduction. Based on the mechanisms that affect display performance, this paper systematically examines the key aspects of black level control in Mini-COB LED displays, including material management, design control, and surface treatment processes of the display. A full-process black level control methodology is proposed to provide technical guidance for the industry to improve overall display quality.

Keywords: Mini-COB LED display; black level control; PCB materials; surface treatment; display uniformity

1. Overview

1.1 Development Status of Mini-COB LED Displays

Mini-COB (Chip on Board) LED displays are a new type of display technology developed from COB packaging. By directly mounting micron-level LED chips onto a PCB substrate and applying encapsulation protection, these displays achieve high-density pixel pitches ranging from 0.5 mm to 2.5 mm. Compared with traditional SMD (Surface Mount Device) displays, Mini-COB displays offer several technical advantages, including higher pixel density, seamless splicing, improved resistance to impact, and reduced glare, making them a mainstream solution in the high-end display market.

With the wider adoption of 5G and 8K ultra-high-definition video technologies, the demand for enhanced visual performance in Mini-COB displays continues to increase. Among the key image-quality indicators, black level uniformity stands out as a critical factor that directly affects product competitiveness. “Black level” primarily refers to the color of the display substrate when the screen is off and the performance of dark scenes when it is on. The level of black level control directly impacts the display’s contrast, color purity, and overall visual comfort.

1.2 Core Influence of Black Level on Display Performance

1.2.1 A Key Factor in Enhancing Contrast

Contrast is the ratio of brightness between bright and dark scenes under certain ambient lighting conditions. The depth of the black level determines how well dark scenes are represented. High-quality Mini-COB displays must deliver a uniform deep-black appearance when powered off to minimize ambient light reflection and prevent unnecessary brightness buildup. If the black level is too light or shows noticeable variance, dark-scene luminance increases, contrast decreases, and image depth is compromised. For example, when displaying dark-content images in a low-light environment, poor black level uniformity can result in a “gray haze” effect, which significantly diminishes the viewing experience.

1.2.2 Foundation of Image Uniformity

Mini-COB displays typically use multiple modules assembled to create large-format displays. Uniform black level within each module and across modules directly affects the overall image integrity. When black level varies across different areas, issues such as “seam color mismatch” or localized whitening may occur, disrupting the continuity of the image. In high-definition display applications, the human eye is very sensitive to color differences — deviations exceeding ΔE > 0.5 can be visually detected. As a result, strict control is essential to ensure full-screen black level uniformity.

1.2.3 Critical Support for Accurate Color Reproduction

Black level affects not only dark-scene performance but also the accuracy of color presentation, as reflective light can interfere with displayed colors. If the substrate black level contains tint variations or reflects too much light, the reflected light will mix with the emitted light from the display pixels, causing color shifts. For example, a bluish-black substrate may cause red content to appear purplish, while a yellowish-black substrate can reduce the purity of blue content. Only through precise black level control can a display accurately reproduce true color information and meet the requirements of professional-grade display environments.

1.2.4 Direct Impact on Visual Comfort

The glossiness and uniformity of the black level play a significant role in visual comfort. Excessive reflectivity can cause glare, leading to eye discomfort during extended viewing sessions. Meanwhile, inconsistent black level across the display forces the viewer’s eyes to continuously adjust focus, increasing visual fatigue. High-quality black level control must deliver low reflectivity and high uniformity to ensure comfortable long-term viewing — a particularly important requirement in use cases such as security monitoring and conference displays.

1.3 Industry Challenges and Current Status of Black Level Control

Although the industry is increasingly recognizing the importance of black level control in Mini-COB LED displays, multiple technical challenges still persist. Differences in material selection and manufacturing processes among manufacturers often result in inconsistent black level performance between products. Additionally, black level is influenced by many factors, including material characteristics, design strategies, and processing methods, making full-process control complex.

Some small and mid-sized manufacturers lack comprehensive control systems, leading to issues such as shallow black levels or noticeable color inconsistencies, which restrict the overall improvement of industry product quality. Therefore, establishing a scientific and systematic black level control framework, along with comprehensive testing methods and well-defined quality standards, has become an urgent requirement for the continued development of the Mini-COB LED display industry.

2. Substrate and Ink Control

Materials are the core factors that determine the black-ink performance of Mini-COB displays. The properties of PCB substrates and inks directly affect the depth, uniformity, and stability of the black color. This chapter discusses the key control points for PCB substrate and ink management—from material selection to quality inspection—to lay the foundation for black-ink control.

2.1 PCB Substrate Control

As the core carrier of Mini-COB displays, the PCB substrate’s color, gloss, and stability have a direct impact on black-ink performance. Substrate control must begin with material selection, performance testing, and storage conditions to ensure that the substrate characteristics meet the required black-ink standards.

2.1.1 Substrate Selection Standards

Mini-COB displays require PCB substrates with high blackness, low reflectivity, and high stability. Currently, commonly used materials in the industry include high-Tg substrates such as Nanya FR-4 NP-175 and Shengyi FR-4 S1000-2M. These materials have excellent heat resistance and mechanical strength and can achieve high blackness through special manufacturing processes.
During selection, attention should be focused on the following indicators:
First, the natural color of the substrate—avoiding bluish, yellowish, or other undesired hues. The blackness value should reach 0.5 ΔE (CIE Lab color space standard).
Second, the gloss level: according to international gloss measurement standards (ISO 2813/ASTM D523), the surface gloss of the substrate should be controlled within 10–20 GU (Gloss Unit at 60°) to reduce ambient light reflection.
Third, stability: the substrate must not show discoloration or yellowing under high-temperature and high-humidity environments to ensure long-term black-ink consistency.
In addition, substrate flatness also affects the uniformity of subsequent ink coating. Substrates with a warpage rate of <0.5% must be selected to prevent ink-thickness variations caused by surface unevenness and the resulting color inconsistencies.

2.1.2 Substrate Quality Inspection Process

Establishing a strict incoming inspection mechanism is crucial for ensuring black-ink consistency. The inspection process should include the following steps:

Appearance inspection:Use visual inspection combined with a magnifier to ensure that the substrate surface has no scratches, stains, or color differences and that the edges are free from burrs.
Blackness and gloss testing: Using a colorimeter and gloss meter under a standard light source (D65), randomly select 10 samples per batch and test 5 points on each sample. Blackness fluctuation must be ΔE < 0.5, and gloss fluctuation must be <20 GU.
Environmental stability test:Place samples in an 85°C / 85% RH environment for 1000 hours. Blackness change must be ΔE < 1, with no visible yellowing.
Mechanical performance testing: Test bending strength and impact strength to ensure the substrate meets processing requirements and avoids damage-induced color abnormalities.
Non-conforming batches must be returned to the supplier and are strictly prohibited from entering production. A supplier quality database should be established to track and evaluate substrate performance from different vendors.

2.1.3 Substrate Storage and Pre-Treatment

Storage conditions directly affect substrate performance and, as a result, black-ink consistency. Requirements include: maintaining an ambient temperature of 20–25°C and relative humidity of <50%; avoiding direct sunlight and humid environments; using sealed packaging with desiccants to prevent moisture absorption and yellowing; and limiting stacking height to no more than 1.5 meters to avoid deformation caused by excessive pressure.
Before use, substrates require pre-treatment: baking at 120°C for 2–4 hours to remove moisture and prevent bubbles or pinholes during ink coating; and performing micro-etching to remove oxide layers, enhance adhesion, and maintain uniform surface roughness (Ra = 0.8–1.2 μm). Poor adhesion may cause ink peeling and affect black-ink stability.

2.2 Ink Control

Ink is the direct carrier responsible for black-ink appearance in Mini-COB displays. Its color, hiding power, and adhesion play a decisive role in black-ink performance. Ink control should focus on selection, formulation optimization, and quality inspection to ensure the ink meets required standards. Common matte-black ink models include SR-500MK50, PSR2000H, and H-8100LDIMBK004.

2.2.1 Ink Selection Principles

Inks used for Mini-COB displays must meet the requirements of high blackness, low reflectivity, high hiding power, and aging resistance. The industry mainly uses black photosensitive inks. Their key performance indicators include:

Color characteristics: Blackness L < 12*, with hue values a* and b* kept within ±1, ensuring a pure black appearance.
Gloss: Surface gloss after curing must be ≤15 GU (60°) to reduce reflection.
Hiding power: At an ink thickness of 15–20 μm, hiding power must be ≥98%, preventing substrate color from showing through.
Adhesion: According to ASTM D3359,the adhesion rating must be >4B to ensure durability.
Aging resistance: After 1000 hours of UV exposure (300W mercury lamp, 30 cm distance), blackness variation must be <0.8 L*, with no cracking or discoloration.
Process compatibility: Ink must be compatible with PCB substrates and support photolithography, with development residue <0.1%.
Selection must also consider compatibility with COB encapsulation materials and soldering temperatures to prevent process-induced black-ink defects.

2.2.2 Ink Formulation Optimization

Ink formulation directly affects performance and must be optimized for Mini-COB application scenarios. Key optimization focuses include:

Pigment selection and ratio: Use high-blackness pigments with carbon-black content >30%, combined with dispersants to prevent pigment agglomeration and uneven color.
Resin system optimization: Use an epoxy–acrylic hybrid resin system to enhance adhesion and aging resistance while reducing curing shrinkage to prevent cracking.
Matting-agent addition: Add 5%–8% silica matting agent to reduce surface gloss and minimize reflections.
Additive adjustment :Add antioxidants and UV absorbers to improve long-term aging resistance and prevent yellowing.
Orthogonal testing should be used to balance all performance indicators—for example, maintaining blackness without reducing adhesion caused by excessive carbon-black content.

2.2.3 Ink Quality Inspection and Usage Control

Ink must undergo strict quality inspection upon arrival, including:

Appearance inspection: Ink must be uniform, without sedimentation, clumping, or color irregularities.
Performance testing: Use a colorimeter to test blackness and hue, a gloss meter for gloss level, and coating testers to measure hiding power and adhesion. Aging chambers should be used for durability tests.
Process performance testing: Test the ink’s viscosity (1000–1500 mPa·s at 25°C) and thixotropy to ensure compliance with coating-process requirements; test development speed (60–90 seconds at 25°C in a 1% Na₂CO₃ solution) to avoid incomplete development or overdevelopment.
During use, strict ink control is required: the ink must be thoroughly stirred for 30 minutes before use to ensure uniform pigment dispersion; the operating environment should be maintained at 20–25°C with 40%–60% relative humidity to prevent performance fluctuations caused by environmental changes; opened ink must be used within 48 hours, and any remaining ink must be sealed and refrigerated to prevent deterioration.
Meanwhile, an ink batch-traceability system must be established to record the usage of each ink batch so that the root cause can be quickly identified if black-color abnormalities occur.

3. PCB Design Control

PCB design directly affects ink-coating uniformity, substrate reflectivity, and compatibility with subsequent processes, making it a core element of black-ink control. This chapter discusses key control points at the design stage from three dimensions: PCB stack-up, top-layer design, and inner-layer design, providing a design guarantee for black-ink consistency.

3.1 PCB Stack-Up Design

PCB stack-up design must balance mechanical strength, heat dissipation, and black-ink performance. A reasonable stack-up reduces substrate deformation, optimizes the ink-coating environment, and improves black-ink consistency.

3.1.1 Stack-Up Layer Count and Material Matching

Mini-COB display PCBs typically use a 4-layer, single-step or 6-layer, double-step stack-up. The core design principles are as follows:

Layer count selection:Determine the number of layers based on display resolution and circuit complexity, avoiding over-design that increases substrate thickness and weight. Layers should be symmetrically designed (e.g., 4 layers: top layer–inner layer 1–inner layer 2–bottom layer) to ensure even stress distribution and minimize warpage. (See Figure 1: 4-layer single-step stack-up; Figure 2: 6-layer double-step stack-up).

Core and prepreg matching: Use black substrate materials and black prepreg to improve PCB light-blocking performance (Figure 3). Select prepreg with the same material as the substrate to ensure consistent thermal expansion coefficients, reducing deformation caused by thermal stress during lamination. The resin content of the prepreg should be 60%–85% to ensure interlayer adhesion strength and avoid delamination.
Figure 1 4-Layer, 1-Step PCB Stackup

4-layer, 1-step; Board thickness: 2.0 mm ± 0.1 mm; L2–L3 Buried Vias
Layers	Material Type	Typical Layer Thickness (µm)
SR	Solder Resist	25
L1	1/3 OZ + Plating	13
L1	Prepreg 1027	43
L2	1/3 OZ + Plating	22
L2	Core 1.80 T/T Copper-Clad	1780
L3	1/3 OZ + Plating	22
L3	Prepreg 1027	40
L4	1/3 OZ + Plating	13
SR	Solder Resist	25
	Total Thickness (µm)	1983

Thickness control: Total substrate thickness is determined by pixel pitch, typically 0 mm, with top-layer copper foil thickness of 1/3 oz (12 μm), ensuring circuit load capacity while minimizing copper foil impact on black ink.

A well-designed stack-up reduces substrate deformation during processing, providing a flat surface for subsequent ink coating and preventing ink-thickness variations caused by uneven substrate surfaces, which could lead to black-ink inconsistencies.

3.1.2 Heat Dissipation Design and Black-Ink Balance

Mini-COB displays have a high density of LED chips that generate significant heat during operation. Poor heat dissipation can raise substrate and ink temperatures, causing yellowing or color changes. Therefore, stack-up design must balance heat dissipation with black-ink performance:

Inner-layer heat-dissipation copper design: Place complete copper layers near the top inner layers, with copper coverage >80%, and use vias to conduct top-layer heat to the inner layers, lowering the temperature around LED chips.
Via layout optimization: Evenly arrange vias around LED chip pads (via diameter 0.075–0.1 mm), and fill vias to prevent surface unevenness that could cause reflection and affect black-ink performance.
Material thermal conductivity optimization: Use PCB materials with thermal conductivity >0.8 W/(m·K) to improve substrate heat dissipation and reduce temperature impact on ink performance.

Through heat-dissipation design, the substrate temperature during LED operation can be controlled below 60°C, preventing ink aging and blackness fading or yellowing.

3.2 Top-Layer Design

The PCB top layer is the direct carrier for ink coating, and its design directly affects ink uniformity and black-ink quality. Key design points include:

3.2.1 Surface Flatness Design

Copper pattern optimization: The top-layer copper circuit patterns should be evenly distributed to avoid excessive local copper density differences (copper coverage variation ≤10%), reducing surface unevenness caused by uneven copper thickness during lamination.
Solder mask opening design: Opening size and spacing should be uniform, with edges rounded (radius >0.005 mm) to avoid ink accumulation or voids during coating. Opening areas are determined by LED chip size, typically 0.025 mm larger than the chip, ensuring die-attach accuracy while preventing excessive copper exposure and reflection.
Edge design: PCB edges should have consistent spacing and routing direction on all sides. Empty areas can be filled (FILL) to maintain pattern consistency (Figure 4). A process edge of ≥5 mm should be reserved for positioning during processing and removed in the final product.

3.2.2 Ink Thickness Control

Ink thickness should be designed according to top-layer flatness, typically 15–20 μm, ensuring complete coverage of the substrate and copper foil while avoiding cracking or peeling caused by excessive thickness. Excessive ink thickness can also affect die-attach yield.

3.3 Inner-Layer Design

Although PCB inner layers do not directly contact ink, substrate transparency and mechanical/thermal properties indirectly affect black-ink stability. Key design points include:

3.3.1 Inner-Layer Pattern Uniformity Design

Inner-layer copper patterns should be evenly distributed to avoid localized copper loss or concentration, ensuring even pressure during lamination and minimizing warpage. Especially in display partition gaps, dummy lines and copper fills should be placed, with consistent routing directions and fill patterns. Copper coverage should be >70%, and the difference in copper coverage between symmetrical layers should be <5%. (See Figure 5)

4. Surface Treatment Process of Mini-COB Displays

The surface treatment process of Mini-COB display modules is a key step in black-ink control, including T-coating, film-lamination, and post-lamination coating processes. By precisely controlling each process parameter, the depth, uniformity, and stability of black color can be ensured.

4.1 T-Coating Process

The T-coating process involves applying a layer of insulating protective glue composed of epoxy resin and black pigment onto the PCB surface. After baking and curing, a uniform black film is formed (see Figure 6).

The core purpose of this process is to optimize display performance: on one hand, it improves uneven blackness on the PCB surface, preventing noticeable color differences at the copper line edges under black-screen conditions caused by refraction, thereby eliminating the “mosaic” effect that affects display integration and premium perception. On the other hand, it unifies the screen’s surface reflectivity. Even if the LED chips emit consistent light color, differences in reflectivity can lead to brightness and chromaticity deviations during full-screen display, addressing uneven color distribution issues.

To achieve high-standard black-ink consistency, specialized instruments are used during COB module surface treatment for blackness classification. Since slight color differences may still occur within the same batch of materials in actual production, optical properties of raw materials are measured using specialized blackness testers, and materials are grouped according to precise standards, ensuring that the blackness effect of each module remains as consistent as possible.

4.2 Film-Lamination Process

The film-lamination process is indispensable in PCB surface treatment. Its primary function is to protect the Mini-COB display’s LED chips from physical damage, chemical corrosion, and contamination from dust or other impurities during subsequent processing and use. Films have different light transmittance. Commonly used films include 50 μm PET combined with 150 μm OCA material (see Figure 7).

The lamination is performed using hot-press lamination, which combines heat and pressure to tightly bond the film to the top-layer surface. The process generally involves: first, cleaning the top layer after T-coating to remove oil, oxides, and other contaminants, ensuring a clean and flat surface; next, placing the film onto the top layer and rolling it under controlled temperature and pressure using hot-press rollers, so the adhesive film fully contacts and bonds to the surface. The temperature is typically 80–120°C, with pressure adjusted according to the film type.

During lamination, several issues may affect black-ink control and product quality. For example, poor adhesion of the dry film to copper foil is common. Causes may include: the top-layer surface is not clean, containing oil or oxide layers that hinder bonding; bubbles forming due to excessive lamination temperature causing volatile components in the photoresist to vaporize and form pockets between film and PCB; uneven or scratched roller surfaces preventing uniform pressure; insufficient roller pressure; or surface scratches or pits on the board. Solutions include: adjusting lamination temperature to the standard range; regularly inspecting and maintaining roller flatness, avoiding hard or sharp tools when cleaning rollers; appropriately increasing roller pressure; selecting high-quality boards and minimizing scratches or pits in earlier processes.

4.3 Post-Lamination Coating Process

The coating process involves applying a layer of material onto another surface to improve properties, enhance protection, reduce wear, and improve surface quality. Coating processes are widely used in automotive, construction, home appliances, medical, aerospace, and other industries. Typical coating steps include: surface pre-treatment, coating selection and mixing, application via spraying/brushing/rolling, curing/baking, surface treatment (e.g., sanding, polishing), inspection, quality control, and handling.

The main benefits of coating include improving corrosion and chemical resistance, preventing oxidation and UV damage, extending lifespan and appearance, and enabling functionalities such as water resistance, scratch resistance, reduced friction, and improved surface blackness (see Figure 8, top-right shows post-lamination coating effect).

The coating uses epoxy glue mixed with nanomaterials, sprayed onto the module surface and baked, addressing issues such as uneven blackness and uneven lamination surfaces.

Different curing methods significantly affect blackness and product performance. Thermal curing involves baking to induce crosslinking of resin components; it cures relatively slowly but produces a harder, more wear-resistant coating. UV curing uses specific wavelength ultraviolet light to initiate polymerization via photoinitiators, achieving rapid curing at lower temperatures, suitable for temperature-sensitive materials, though cured coatings may be slightly less flexible.

Incomplete curing can reduce coating protection, causing peeling or cracking and affecting product reliability. Over-curing may make the coating brittle, reducing flexibility and impact resistance, negatively affecting performance. Therefore, in production, curing conditions must be precisely controlled according to coating material and product requirements to achieve optimal black-ink effect and product performance.

5. Conclusion

In summary, PCB materials, design, and surface treatment processes play an irreplaceable role in achieving precise black-ink presentation and enhancing product quality. Accurate blackness not only improves the overall aesthetics of LED displays under black-screen conditions, showcasing a high-end, refined quality image, but also ensures color uniformity and accuracy during full-screen operation, providing users with an outstanding visual experience. Meanwhile, high product quality is key to guaranteeing stability, reliability, and service life, directly affecting market competitiveness and user satisfaction.

Looking ahead, with continuous technological advancements and increasingly diverse market demands, PCB surface treatment processes are expected to achieve new breakthroughs and developments in multiple areas. In terms of improving black-ink accuracy, more advanced testing technologies and equipment are anticipated, enabling more precise measurement and analysis of blackness, further optimizing production processes, reducing color variation, and achieving higher standards of black-ink consistency. Additionally, innovations in materials science may introduce new surface treatment materials with superior optical properties and stability, contributing to enhanced depth, uniformity, and stability of black ink.

Regarding product performance improvement, surface treatment processes will evolve toward higher efficiency, environmental friendliness, and energy savings. For example, developing more advanced automated production equipment and processes can increase production efficiency while reducing labor costs and production errors; exploring green and environmentally friendly surface treatment technologies can minimize environmental impact; optimizing coating structures and performance can enhance PCB corrosion resistance, wear resistance, and electrical performance to meet the demands of electronic products in various complex environments. Moreover, as electronic products trend toward miniaturization, lightweight design, and higher performance, PCB surface treatment processes must continue to innovate to adapt to these new trends, providing strong support for the upgrading and iteration of electronic products.

Blog