Full-Dimensional Comparison of HDI Core and HDI Coreless Boards: Stackup, Process, Characteristics, and Applications

In the conventional understanding of the PCB industry, multilayer boards are predominantly produced in even-numbered layer counts such as 2, 4, 6, and 8 layers. However, odd-numbered layer PCBs are not nonexistent—they exist exclusively within the HDI (High-Density Interconnect) board category, accounting for approximately 5% of the market. The fundamental distinction between the two product types lies in their stackup logic: conventional even-layer HDI boards employ a core-board (Core) design, while the niche odd-layer HDI boards rely entirely on coreless process architectures.

Even-layer HDI boards use traditional core substrates as the structural foundation, representing the industry's most mature and widely adopted solution. Odd-layer HDI boards, by contrast, completely eliminate the core board and leverage coreless manufacturing to achieve differentiated performance. This article systematically examines the technical boundaries and application scenarios of core-based and coreless HDI technologies from the perspectives of stackup, manufacturing processes, thickness limits, comparative advantages and disadvantages, and application landscapes.

I. Stackup Differences: The Skeletal Framework Defines Structural DNA

The essential difference between core-based and coreless HDI lies in whether a solid core board (Core) is incorporated to provide mechanical support, insulation isolation, and electrical foundation. This single difference subsequently gives rise to significant divergences in performance, processing, and reliability.

1. HDI Core Board Structure

Core-based HDI boards adhere to the conventional multilayer PCB architecture, utilizing a pre-patterned copper-clad laminate core (CCL Core) as the "rigid skeleton" of the entire board, serving dual roles as both the mechanical foundation and the electrical reference plane. On this basis, prepreg (PP) is used as the insulating adhesive medium, and additional layers are sequentially laminated on one or both sides of the core, with micro-vias and fine-line circuits fabricated to ultimately form a high-density interconnect multilayer structure.

A typical symmetric stackup is: copper foil – PP – core board – PP – copper foil. The symmetrical design effectively counterbalances lamination stresses, imparting excellent structural stability and warp resistance—the fundamental reason why core-based boards achieve high production yields and superior dimensional stability, naturally suited to even-numbered layer stacking logic.

2. HDI Coreless Board Structure

Coreless HDI boards entirely eliminate the solid core board, using only ultra-thin insulating dielectrics such as PP or ABF as the base material. The circuit layers are built directly through sequential lamination and build-up processes, with no rigid supporting skeleton throughout the entire process.

This structure breaks the conventional limitation that core boards only enable even-numbered layers, allowing flexible design of odd-numbered layer HDI such as 3-layer or 5-layer configurations. At the same time, by omitting the thick core substrate material, coreless boards achieve extreme thinness, making them a critical technological pathway for ultra-thin, high-end PCBs.

II. Process Comparison: Mature Continuation vs. Innovative Breakthrough

1. HDI Core Board: Mature Process, Excellent Compatibility

The manufacturing process for core-based HDI boards is an extension and upgrade of conventional multilayer PCB processes, with core steps fully inherited from mature technology systems and exhibiting strong compatibility with existing production equipment. The overall workflow is highly consistent with standard multilayer board production, requiring no large-scale equipment modifications for mass production—offering low investment cost, low risk, and controllable processes, which is why it remains the absolute mainstream choice among global PCB manufacturers.

For interlayer interconnection, core-based boards rely on mature processes such as mechanical drilling, laser drilling, and vertical copper plating filling, backed by deep technical expertise and stable yields, sufficient to meet mid-to-high-end precision interconnection requirements.

2. HDI Coreless Board: Carrier-Assisted, Complex Process

Due to the lack of rigid support, the ultra-thin substrate of coreless boards cannot directly undergo precision circuit fabrication and lamination. Consequently, a detachable carrier (Detach Core) must be introduced as temporary support—this is the defining characteristic of its core process.

During production, multi-layer circuit lamination and patterning are completed simultaneously on both sides of the carrier, building the target odd-layer structure (e.g., 3 layers, 5 layers). The carrier is then removed through debonding and depaneling processes, yielding the pure coreless ultra-thin HDI board. Compared to core-based boards, coreless manufacturing adds steps such as carrier bonding, debonding, and precision depaneling, resulting in a longer workflow and significantly higher demands on process precision and warp control.

III. Thickness Limits: Clear Differentiation in Thinness Capability

Constrained by the physical properties of core substrate materials and the processing capabilities of mainstream equipment, there is a notable gap in thinness capability between the two approaches.

Currently, the industry's minimum manufacturable core thickness is approximately 40μm, with stable mass-production thickness typically ≥50μm. Subject to this lower limit, the finished board thickness of a conventional 4-layer any-layer core-based HDI board is at minimum only about 170μm, encountering a natural bottleneck in thickness reduction.

Coreless boards, however, require no core support and, with the aid of detachable carrier technology, completely overcome thickness limitations. For example, a 3-layer coreless HDI can achieve a finished thickness as low as 115μm, delivering an exceptionally pronounced ultra-thin advantage that provides a viable solution for extremely lightweight end products.

IV. Pros and Cons: Rational Trade-offs Between Performance and Cost

1. HDI Core Board: High Reliability, Low Cost, High Volume Production Suitability

Advantages: The solid core board provides robust mechanical strength, excellent flexural resistance, impact resistance, and moisture-heat resistance, with controlled warpage and outstanding overall reliability, capable of performing in harsh environments. At the same time, mature processes, general-purpose equipment, high yields, and low marginal costs deliver significant economies of scale.

Disadvantages: Core thickness constraints prevent extreme thinness, and layer-count structural flexibility is limited, making it difficult to meet cutting-edge demands for ultra-high density and ultra-thin packaging.

2. HDI Coreless Board: Extreme Thinness, High-Density Interconnect, High-End Suitability

Advantages: The coreless architecture enables extreme thinness, and when combined with high-frequency/high-speed materials such as ABF and fine-line processes, can achieve smaller line widths/spacings, higher routing density, and lower signal loss, with superior high-frequency performance. It is an ideal choice for high-end high-speed digital and RF applications. Additionally, it supports odd-numbered layer designs to accommodate specialized circuit topologies.

Disadvantages: Lacking rigid support, the board is relatively soft and prone to warpage, with yield significantly affected by warp control. The need for additional carrier materials and dedicated process steps results in far higher costs for equipment retrofitting, material consumption, and labor compared to core-based boards, making it unsuitable for high-volume general-purpose products.

V. Application Landscape: General-Purpose Mainstream vs. High-End Specialization

In the current HDI industry, core-based boards occupy over 95% of the market share, serving as the absolute mainstay, while coreless boards remain a niche high-end supplement, with highly differentiated application scenarios.

1. Mainstream Applications of HDI Core Boards

Leveraging high reliability, cost-effectiveness, and stable mass-production capacity, they are widely used in cost-sensitive, reliability-critical, and volume-demanding fields, including smartphone motherboards, automotive electronics, industrial control, conventional communication modules, and consumer electronics terminals—constituting the cornerstone of HDI markets for consumer, industrial, and automotive applications.

2. Mainstream Applications of HDI Coreless Boards

Relying on ultra-thinness, high density, and high-frequency/high-speed characteristics, they focus on advanced chip packaging and computing/RF modules, with typical applications including AI GPU graphics cards, high-performance HPC server processors, 5G/6G base station chips, ASIC miner chips, and high-end memory packaging—ultra-high-precision domains.

VI. Technology Evolution: Bidirectional Integration, Expanding Boundaries

Core-based and coreless technologies are not simply substitutes for one another but rather two technical paths addressing different requirements, and their boundaries are continuously converging with advancements in materials, equipment, and processes.

On one hand, core-based HDI continues to push toward thinner profiles by adopting thinner core substrates and optimizing dielectric layer stacking, gradually narrowing the thickness gap with coreless boards. On the other hand, coreless HDI is steadily improving warp issues through auxiliary support layers, optimized lamination stress management, and improved carrier debonding processes, while progressively extending toward higher layer counts, larger panel sizes, and higher reliability applications.

VII. Summary Table of Key Differences

Conclusion

Core-based HDI boards represent a robust evolution of traditional PCB processes, with core values centered on high reliability, economic efficiency, and production convenience, making them suitable for the vast majority of general-purpose high-density interconnect scenarios and serving as the backbone of industry supply. Coreless HDI boards, in contrast, are a pioneering product developed for ultra-thinness, ultra-high density, and high-frequency requirements, focusing exclusively on premium chip packaging and computing/RF niche segments.

As substrate materials, precision manufacturing processes, and control methods continue to advance, both technologies will progressively address their respective weaknesses, and their application boundaries will continue to expand. Ultimately, a long-term complementary pattern will emerge, wherein "general-purpose scenarios rely on core-based, while high-end scenarios depend on coreless" solutions.