COF FPC for Smartphones vs. TV Panels | SAP Semi-Additive Process, ALD Technology & Manufacturing Differences

COF FPC for Smartphones vs. TV Panels | SAP Semi-Additive Process, ALD Technology & Manufacturing Differences

What is the difference between producing a COF FPC carrier board for smartphones and a conventional TV panel COF FPC? Where are the processing difficulties? This article explains the fundamental manufacturing differences — including the shift from standard subtractive etching to SAP semi-additive processing with ALD (Atomic Layer Deposition) technology — and why smartphone COF FPC represents a significantly higher technical barrier than TV panel COF.

COF (Chip On Film) is a packaging technology in which the display driver IC is directly bonded onto a flexible printed circuit film, replacing older TCP (Tape Carrier Package) formats. COF FPC is widely used in LCD and OLED display panels to connect the driver IC to the panel electrodes. While TV panels and smartphone displays both use COF FPC, the manufacturing processes and technical requirements differ dramatically — driven by the smartphone's demand for ultra-narrow bezels, high pixel density (PPI), and compact IC pitch.

1. TV Panel COF FPC: Standard Subtractive Etching Process

The COF FPC traditionally used in the TV field is actually not very different from ordinary FPC in production and processing. Apart from having finer line width and spacing than standard FPC, TV panel COF FPC is still produced using the standard subtractive etching method.

In the subtractive process:

A copper-clad flexible laminate (typically rolled-annealed copper on polyimide, with copper thickness of 9–18μm) is used as the starting material.

A photoresist (dry film or liquid resist) is laminated and exposed to define the circuit pattern.

After development, the unwanted copper is etched away using chemical etchant, leaving the desired circuit pattern.

The resist is then stripped, and the board proceeds to surface treatment, coverlay lamination, and finishing.

The minimum line width and spacing achievable with the standard subtractive method on FPC is generally above 15μm (approximately 0.6mil) — sufficient for TV panel COF applications, where the driver IC pitch and panel electrode pitch are relatively coarse. TV panel COF FPC typically has line/space of 15–25μm and does not require the extreme fineness demanded by smartphone displays.

2. Smartphone COF FPC: SAP Semi-Additive Process with ALD Technology

The COF FPC carrier board for smartphones is produced in a completely different way from the standard subtractive etching method. It is produced using an additive method derived from semiconductor chip manufacturing, known in the industry as the SAP (Semi-Additive Process).

The reason is straightforward: the minimum FPC line width and pitch produced by the standard subtractive etching method are generally above 15μm, which is essentially inadequate for the finer circuit requirements of smartphone COF production. Smartphone full-screen displays — with ultra-narrow bezels (often <1mm), high PPI (400–600+), and fine-pitch driver ICs — require COF FPC with line/space of 5μm or finer, far beyond the capability of subtractive etching.

2.1 Origins of SAP Technology: From SLP to Smartphone Mainboards

The SAP semi-additive processing technology primarily originated from SLP (Substrate-like PCB) carrier board manufacturing. However, in terms of smartphone industry application, Apple was the earliest to use this process at scale in the production of iPhone mainboards.

Before that, when Apple, Samsung, and LG were developing new OLED display devices, they adopted a semiconductor process called ALD (Atomic Layer Deposition) — an atomic-layer-level thin-film deposition technology — for the encapsulation of OLED devices. ALD thin-film encapsulation (TFE) not only controls the encapsulation layer thickness to less than 0.1μm but also greatly improves the OLED device packaging yield and increases the service life of OLED products by several times, by providing a near-impermeable barrier against moisture and oxygen.

After ALD technology matured in OLED device encapsulation, Apple extended this technology to the production of iPhone PCBs. Recent generations of iPhone PCB mainboards have all been produced using the semi-additive method incorporating ALD technology — enabling any-layer HDI / SLP constructions with line/space down to 20–30μm and finer, far exceeding the capability of standard subtractive HDI.

2.2 ALD + SAP Extended to Smartphone COF FPC

After full-screen display technology began to be applied on smartphones, this ALD-based semi-additive processing method was also introduced into the production of COF FPC carrier boards. Except for Apple's iPhone XR (which used an LCD display with COF process), most of Samsung's full-screen OLED smartphones have also adopted the COF process — requiring ultra-fine-line COF FPC produced via SAP with ALD.

The driver for this shift is the smartphone's extreme form-factor requirements:

Ultra-narrow bezels: The COF FPC must bend tightly around the panel edge, requiring extremely fine pitch and thin substrate.

High PPI displays: Higher pixel density means more panel electrodes per unit width, requiring finer line/space on the COF.

Fine-pitch driver ICs: Smartphone display driver ICs have much finer bump pitch than TV driver ICs, requiring finer COF inner-lead bonding pads.

Thin profile: Smartphone COF FPC must be ultra-thin (often 25μm or thinner polyimide substrate) to fit within the tight stack-up of the display module.

3. Detailed COF FPC Production Process (ALD + SAP)

Specifically, the production of COF FPC substrates using the ALD + SAP method generally follows these steps:

Step 1: Punching / Via Formation

First, COF FPC still needs to determine whether to punch holes (vias or tooling holes) in the substrate according to the drawing design. If required, this step is completed first — typically using mechanical punching or laser drilling, depending on hole size and volume.

Step 2: Cleaning

The FPC substrate (typically a polyimide film, 25–50μm thick, without pre-clad copper) undergoes necessary cleaning to remove surface contaminants, particles, and organic residues, ensuring good adhesion for subsequent layers.

Step 3: ALD Adhesion / Barrier Layer Deposition

After cleaning, the FPC substrate enters the ALD machine to process the coupling / adhesion layer. After processing, an ultra-thin coupling material — less than 1 nanometer thick — is formed, covering the FPC substrate uniformly and conformally.

The ALD layer serves two critical functions:

Adhesion promotion: It creates a chemically active interface between the inert polyimide surface and the subsequent copper seed layer, ensuring strong copper-to-PI adhesion — a major challenge in additive processing on flexible substrates.

Diffusion barrier: It prevents copper ions from diffusing into the polyimide substrate, which would degrade insulation resistance and long-term reliability.

This is the key differentiator from traditional FPC production: instead of relying on mechanically roughened rolled copper foil for adhesion, the ALD + SAP process uses an atomically thin, chemically bonded interface — enabling ultra-fine circuits on ultra-thin substrates.

Step 4: Electroless Copper Seed Layer Deposition

The subsequent process is broadly similar to traditional FPC production but uses chemical deposition rather than laminated copper. Electroless copper is deposited on the ALD-treated FPC substrate to form a "copper seed layer," with the copper layer thickness controlled to approximately 0.1μm.

This ultra-thin seed layer is just thick enough to be electrically conductive for subsequent electrolytic plating, but thin enough to be completely removed during the final flash etch without undercutting the fine circuit patterns.

Step 5: Patterning (Photoresist Lamination, Exposure, Development)

A photoresist is applied, exposed, and developed to shape the circuit pattern — defining where the final copper circuit will be plated. For ultra-fine-line COF FPC, this step requires high-resolution lithography (often using LDI — Laser Direct Imaging — with registration accuracy of ±5μm or better).

Step 6: Electrolytic Copper Plating

The electrolytic copper plating process is used to plate copper onto the exposed seed layer areas (where the resist has been developed away), building up the final circuit copper thickness — typically 3–8μm for COF FPC, depending on the application.

Step 7: Resist Stripping and Flash Etching

Finally, the resist is stripped, and a flash etching (also called "seed etch" or "differential etch") process is performed. Flash etching removes the thin (0.1μm) electroless copper seed layer from the areas between circuits — while leaving the thicker (3–8μm) electrolytically plated circuits largely intact. This completes the entire COF FPC substrate production process.

Because the seed layer is so thin, the flash etch is very short and controlled — minimizing lateral undercut and enabling line/space of 5μm or finer, which is impossible with the subtractive method (where etching through 9–18μm of copper causes significant undercut).

4. Key Differences: Subtractive vs. ALD-SAP for COF FPC

From the above process description, the biggest differences between COF FPC substrate production (ALD + SAP) and the traditional standard subtractive etching method can be summarized across the following dimensions.

Starting material: TV panel COF uses a copper-clad PI laminate with rolled copper foil of 9–18μm thickness, while smartphone COF starts from bare PI film with no pre-clad copper.

Conductive layer formation: The subtractive method relies on laminated rolled copper foil as the conductive layer, whereas the ALD + SAP method deposits the conductive layer through electroless copper seeding followed by electrolytic copper plating.

Minimum line and space: TV panel COF produced by subtractive etching typically achieves minimum line/space of approximately 15μm or coarser, while smartphone COF using ALD + SAP can achieve 5μm or finer.

Adhesion mechanism: Subtractive FPC depends on the mechanical roughness of the rolled copper foil for adhesion to polyimide, whereas ALD + SAP uses an atomically thin ALD nano-layer to create chemical adhesion between copper and PI.

Copper thickness uniformity: The subtractive method is limited by the tolerance of the starting copper foil, while the SAP method achieves highly uniform copper thickness through controlled plating.

Substrate thickness: TV panel COF typically uses 50–125μm polyimide substrates, whereas smartphone COF requires 25μm or thinner substrates to fit within tight display module stack-ups.

Equipment cost: TV panel COF uses standard FPC manufacturing equipment, while smartphone COF requires ALD deposition systems, advanced plating lines, and high-precision LDI imaging — representing a significantly higher capital investment.

Typical application: Subtractive COF FPC is used for TV panels and large-format displays, while ALD + SAP COF FPC is required for smartphone full-screen OLED and LCD displays with ultra-narrow bezels.

In summary, the core innovation is that instead of laminating rolled copper on the substrate as the conductive layer, the ALD + SAP process deposits copper via chemical plating to form the main conductive layer — enabling processing of very thin products and the formation of much finer circuits.

5. Industry Landscape and China's COF Supply Chain Challenge

Compared with Japan, South Korea, and Taiwan — where the semiconductor industry chain is very complete — the COF industry chain in mainland China can currently only produce COF FPC substrates for TV panels, all using the standard subtractive etching method. However, some manufacturers and research institutes have begun to introduce ALD machines to develop semi-additive production processes related to ALD technology.

In the COF bonding (inner lead bonding) equipment for smartphones, Japanese manufacturers still dominate, and other manufacturers are essentially still in the research and development stage. Therefore, when Japanese, Korean, and Taiwanese manufacturers have no intention of expanding capacity in the smartphone COF process, mainland Chinese companies that want to open up the COF industry chain and increase production capacity need coordinated planning and simultaneous breakthroughs across panel factories, IC factories, FPC factories, and related production equipment manufacturers — working together to achieve domestic substitution.

For domestic PCB and FPC manufacturers, building expertise in fine-line processing, advanced plating, and high-precision imaging is a prerequisite for entering the smartphone COF FPC market. Companies with experience in high-density interconnect (HDI), rigid-flex boards, and fine-line manufacturing — such as ONESEINE TECHNOLOGY CO.LTD, which produces HDI boards with minimum line width/spacing of 2.0/2.0mil (50/50μm) and rigid-flex boards — are building the foundational capabilities that can eventually extend toward advanced COF FPC production as the domestic supply chain matures.

6. Processing Difficulties and Technical Challenges

The main processing difficulties in smartphone COF FPC production include:

ALD process control on flexible substrates: ALD is typically a batch, high-vacuum process designed for rigid semiconductor wafers. Adapting ALD to roll-to-roll or sheet-fed flexible polyimide substrates — while maintaining uniform <1nm layer thickness across large areas and ensuring no pinholes or defects — is technically challenging.

Ultra-fine-line imaging: Achieving 5μm line/space with consistent etching and registration requires advanced LDI (Laser Direct Imaging) with sub-micron registration accuracy, high-resolution photoresist, and tightly controlled process environments (temperature, humidity, cleanliness).

Seed layer uniformity and flash etch control: The 0.1μm electroless copper seed layer must be perfectly continuous and uniform. The final flash etch must remove exactly the seed layer without attacking the circuit copper — a narrow process window that requires precise etchant concentration, temperature, and timing control.

Adhesion reliability: The ALD interface must provide sufficient copper-to-PI adhesion to survive flexing, thermal cycling, and humidity testing — especially critical for COF FPC that is bent tightly around the smartphone display edge.

Thin substrate handling: Processing 25μm or thinner polyimide films without wrinkling, stretching, or dimensional distortion requires specialized handling equipment and tight process control.

Yield management: With ultra-fine features, even tiny particles or defects can cause open or short circuits. Yield management — including cleanroom standards, inline inspection (AOI), and process control — is critical for cost-effective production.

7. Conclusion

The difference between smartphone COF FPC and TV panel COF FPC is not incremental — it is a fundamental shift in manufacturing technology. TV panel COF FPC uses the standard subtractive etching method with copper-clad laminates, achieving minimum line/space of ~15μm. Smartphone COF FPC requires the ALD + SAP semi-additive process, starting from bare polyimide film, depositing an ALD nano-adhesion layer (<1nm), an electroless copper seed layer (~0.1μm), and electrolytically plated circuits — enabling line/space of 5μm or finer on ultra-thin (25μm) substrates.

This technology transition — borrowed from semiconductor manufacturing and OLED encapsulation — was pioneered at scale by Apple in iPhone mainboards and has since extended to smartphone COF FPC for full-screen displays. While mainland China's COF industry currently lags in smartphone-grade COF production, domestic manufacturers are investing in ALD equipment, advanced plating, and fine-line imaging capabilities. As the supply chain matures, companies with strong HDI, rigid-flex, and fine-line manufacturing foundations — including ONESEINE TECHNOLOGY CO.LTD — are well-positioned to participate in and eventually domesticate this high-value segment.

For the broader PCB and FPC industry, the ALD + SAP approach represents the future direction for ultra-fine-line flexible circuits — not only for smartphone COF, but also for advanced packaging substrates, wearable electronics, AR/VR displays, and next-generation flexible devices where miniaturization and density continue to push the limits of conventional subtractive manufacturing.

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