What is an ABF substrate?

ABF (Ajinomoto Build-up Film) substrates are a type of high-performance printed circuit board (PCB) used primarily in advanced semiconductor packaging. They serve as the interconnect layer between the semiconductor die and the larger PCB, enabling signal transmission and power distribution.

Why are they crucial for modern electronics?

With the increasing complexity of semiconductor devices, especially in high-performance computing (HPC), AI processors, and advanced system-on-chip (SoC) designs, traditional organic substrates cannot meet the demand for finer line widths, lower signal loss, and higher integration density. ABF substrates have emerged as the preferred solution, offering superior electrical and mechanical properties required for modern chip packaging.

Key role in semiconductor advancements

ABF substrates play a vital role in facilitating advanced packaging techniques such as flip-chip ball grid array (FC-BGA), which is widely used in CPUs, GPUs, FPGAs, and networking chips. As the industry shifts toward more compact, high-speed, and high-efficiency semiconductor designs, ABF substrates remain at the forefront of innovation.

ABF Substrate Materials and Structure

Composition of Ajinomoto Build-up Film (ABF)

Ajinomoto Build-up Film is a specialized insulating resin film developed by Ajinomoto Co., Ltd. It is primarily composed of:

  • Epoxy resin – Provides excellent mechanical strength and heat resistance.
  • Filler materials – Enhances electrical insulation and dimensional stability.
  • Proprietary additives – Improves adhesion and compatibility with copper traces.

Typical ABF Substrate Layer Stack

ABF substrates follow a multi-layer build-up structure, typically consisting of:

  1. Core layer – A rigid base layer (e.g., BT resin) providing structural support.
  2. Build-up layers – Multiple layers of ABF resin and copper traces formed through a sequential lamination process.
  3. Microvias and interconnects – Laser-drilled microvias that enable high-density interconnections.
  4. Surface finish – Protective coatings like ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold) for improved solderability and reliability.

Key Differences from Traditional PCB Substrates

  • Finer line width/spacing – ABF substrates can support line widths below 10µm, compared to traditional PCBs which are typically limited to ~30µm.
  • Higher layer count – Allows for better signal routing and higher integration density.
  • Superior electrical properties – Lower dielectric loss and higher signal integrity, making them ideal for high-speed applications.

ABF Substrate Manufacturing Process

Overview of Build-up Technology

The ABF substrate manufacturing process is based on a sequential build-up (SBU) method, where multiple thin layers of resin and copper are laminated one by one. This enables high-density interconnect (HDI) designs with fine-pitch microvias.

Key Fabrication Steps

  1. Core Layer Formation – The process starts with a rigid core material, often made of BT resin.
  2. Laser Drilling of Microvias – High-precision laser technology is used to create microvias for interlayer connections.
  3. Copper Deposition & Circuit Patterning – Electrolytic copper plating fills the microvias, followed by photolithography and etching to define fine copper traces.
  4. ABF Resin Coating & Layer Build-up – Successive layers of ABF film are applied, laminated, and cured to form the multi-layer structure.
  5. Surface Finishing – Final treatments, including ENEPIG or OSP (Organic Solderability Preservative), enhance solderability and reliability.

Advanced Packaging Requirements (FC-BGA, etc.)

For flip-chip ball grid array (FC-BGA) packaging, ABF substrates must meet strict requirements:

  • Ultra-fine line/space capabilities (sub-10µm) to accommodate high I/O chips.
  • Low CTE (coefficient of thermal expansion) to prevent warpage and improve reliability.
  • Superior electrical insulation for high-speed signal transmission.

Applications of ABF Substrates

High-Performance Computing (HPC) and Servers

ABF substrates are widely used in data center processors, including server CPUs and AI accelerators, where high-speed data transmission and low signal loss are critical.

AI Chips, GPUs, FPGAs, and High-Density Packaging

  • AI processors from companies like NVIDIA, AMD, and Google TPU rely on ABF substrates for high-bandwidth memory (HBM) integration.
  • GPUs and FPGAs require high-density interconnections, making ABF substrates ideal for their compact and power-efficient designs.

5G, Autonomous Driving, and High-Speed Applications

  • 5G base station equipment benefits from ABF substrates' high-frequency capabilities and low transmission loss.
  • Automotive electronics, especially in advanced driver-assistance systems (ADAS), demand ABF substrates for reliability and performance.

Market Trends and Future Developments

Major ABF Substrate Suppliers

The ABF substrate market is dominated by a few key players, including:

  • Ajinomoto Co., Ltd. (developer of ABF material)
  • Unimicron, Nan Ya PCB, and Kinsus (leading ABF substrate manufacturers in Taiwan)
  • Ibiden, Shinko Electric, and Kyocera (top Japanese ABF substrate suppliers)

Production Challenges and Supply Chain Constraints

  • Capacity shortages – Due to rising demand from AI and HPC markets, supply has struggled to keep up.
  • Material cost fluctuations – The limited availability of ABF materials affects production costs.
  • Geopolitical factors – Trade restrictions and localization efforts impact supply chain stability.

Technological Advancements

  • Finer line width & spacing – Moving towards sub-5µm features for next-generation semiconductors.
  • Higher layer count substrates – More layers enable better power delivery and signal integrity.
  • Alternatives to ABF – Some research is exploring glass core substrates as a potential replacement.

Conclusion: The Future of ABF Substrates

Growing Demand Driven by AI, HPC, and Next-Gen Computing

As AI workloads, cloud computing, and edge devices continue to evolve, ABF substrates will remain a cornerstone of high-performance semiconductor packaging.

Potential Alternative Technologies

  • SAP (Semi-Additive Process) substrates – Can achieve similar fine-line capabilities.
  • Glass substrates – Offer superior dimensional stability and potential cost advantages, but face manufacturing challenges.

Long-Term Outlook

ABF substrates will continue to dominate the semiconductor packaging industry in the foreseeable future, with further innovations enhancing performance, reliability, and manufacturability. However, companies investing in next-generation materials and processes will play a key role in shaping the industry's evolution.