A Comprehensive Guide to High-Density Interconnect Technology

What is HDI PCB? Definition & IPC-2226 Standards

Core Definition of High-Density Interconnect PCB

Per the IPC-2226 international standard, a High-Density Interconnect (HDI) PCB is a printed circuit board characterized by significantly higher wiring density per unit area compared to conventional PCBs. Its defining technical parameters include:

• Finer lines and spaces ≤ 100 µm (0.10mm) for dense signal routing;
• Microvias with diameter < 150 µm, paired with capture pads < 400 µm (0.40mm);
• Connection pad density exceeding 20 pads/cm² to support high component integration;
• Adoption of blind, buried, and stacked via technologies to minimize signal path length and electromagnetic interference (EMI).

Compared to traditional PCBs, HDI PCBs deliver superior signal integrity, thermal performance, and space efficiency—critical requirements for modern electronic devices demanding miniaturization, light weight, and enhanced functionality.

HDI PCB Structural Classifications (IPC-2226)

HDI PCBs are categorized into three primary types based on microvia layer configurations and interconnect technologies:

• Type I HDI PCB: Features a single microvia layer on one or both sides of the core. Utilizes plated microvias and plated through-holes (PTH) for interconnection, incorporating blind vias but no buried vias. Designated by the construction notation "1+n+1" (n = core layers).
• Type II HDI PCB: Also incorporates a single microvia layer on one or both sides of the core, with plated microvias and PTH for interconnection. Distinguished by the inclusion of both blind and buried vias, maintaining the "1+n+1" construction.
• Type III HDI PCB: Requires at least two microvia layers on one or both sides of the core. Combines plated microvias, PTH, blind vias, and buried vias for complex interconnects. Identified by the "2+n+2" construction (or "3+n+3" for three microvia layers).

Critical HDI PCB Design Challenges & Engineering Solutions

Common Design Pitfalls and Production Impacts

HDI PCB design directly influences manufacturing yield, cost, and performance. Below are industry-prevalent design issues and their associated production risks:

HDI Design Challenges	Production Consequences	Engineering Solutions
Dielectric thickness excessive for laser vias	Extended laser drilling time, reduced productivity; voids in microvia plating (especially at the bottom); higher costs due to low yields	Maintain an aspect ratio ≤ 0.8:1 for laser-drilled microvias
Microvia size too small	Risk of microvia blockage by foreign materials; inadequate plating coverage (critical at the bottom); yield loss and cost escalation	- Use 100 μm microvias (aspect ratio ≤ 0.8:1) for copper-filled applications
		- Adopt 125 μm microvias (aspect ratio ≤ 0.8:1) when copper filling is unnecessary
Undersized capture/target lands for microvias	Target land overshoot (material burnout to adjacent layers); capture land breakage (non-compliant with IPC-6016 standards)	Implement start pads 200 μm larger than microvias; consult technical experts for ultra-tight geometries
Overly restrictive dimple requirements on copper-filled microvias	Increased production costs due to reduced yields	Specify a maximum dimple tolerance of 25 μm
Excessive overplating thickness for plugged vias (POFV/VIPPO)	Disrupted process flow; compromised ability to produce thin outer-layer tracks/small isolation	Align with IPC-6012 Class II standards; require minimum overplating thickness ≥ 6 μm
Multiple via sizes requiring epoxy plugging (buried/through vias)	Bubble formation and incomplete filling in plugged vias	Prioritize a single via size for plugging; if multiple sizes are necessary, restrict to a 0.15mm range
Microvia placement on SMD surfaces	Void formation in solder joints during reflow soldering; higher costs for copper-filled microvias	- Relocate microvias away from SMD surfaces when possible
		- If unavoidable, place microvias directly in pads with mandatory copper filling
Insufficient spacing between staggered holes/microvias/buried vias	Overlapping hole intrusion; poor plating quality; increased costs for remediation	- Maintain a minimum 0.30mm distance between microvias (0.25mm if space-constrained)
		- Example: 0.10mm microvia + 0.25mm buried hole = 0.475mm/0.425mm center-to-center distance

HDI PCB Design Guidelines (IPC Compliance)

To ensure design manufacturability and compliance, adhere to the following key guidelines (extracted from industry-leading design standards):

Minimum Track and Gap Requirements (Cap Plating)

Location	IPC Class	Base Copper (oz)	General Trace/Gap (mm)	Moderate Trace/Gap (mm)	Advanced Trace/Gap (mm)
Outer Layer (PTH Holes)	Class 2	1/3, 1/2	0.10 / 0.10	0.10 / 0.12	0.089 / 0.10
Outer Layer (PTH Holes)	Class 3	1/2, 1/3	0.12 / 0.12	0.12 / 0.12	0.09 / 0.10
Buried Inner Layer*	Class 2	1/3, 1/2	0.10 / 0.10	0.12 / 0.10	0.089 / 0.10
Buried Inner Layer*	Class 3	1/2, 1/3	0.10 / 0.10	0.12 / 0.10	0.09 / 0.10

*Valid for outer phases of buried cores requiring cap plating (typically for stacked microvias on buried holes). For designs tighter than specified, consult technical partners for project-specific reviews.

Copper Wrap and Cap Plating Standards

• Minimum copper wrap distance: 25 μm (IPC Class 1, 2, 3)
• Minimum copper wrap thickness: 5 μm (IPC Class 2, 3)
• Minimum cap plating: 12 μm (IPC Class 3); 5 μm (IPC Class 2)

HDI PCB Technical Capabilities & Material Considerations

HDI PCB manufacturing demands advanced processes and specialized materials to achieve tight tolerances and reliable performance. Key technical capabilities include:

• Microvia drilling (laser-based) and plating precision;
• Thin prepreg and copper-clad laminate (CCL) integration (thinner than conventional multilayer PCBs);
• Stacked microvia formation for high-layer-count designs;
• Epoxy via plugging (POFV/VIPPO) for enhanced mechanical stability and thermal conductivity;
• Impedance control for high-speed signal transmission.

Material selection is critical for HDI performance:

• Low-loss dielectrics to minimize signal attenuation in high-frequency applications;
• High-Tg laminates for thermal stability (critical for automotive and industrial electronics);
• Thin copper foils (1/3 oz, 1/2 oz) to support fine line etching.

HDI PCB Industry Trends & Application Areas

Key Design Trends Shaping HDI Technology

• Miniaturization-Driven Complexity: As electronic components shrink, HDI PCBs require higher layer counts, finer conductor widths, and narrower spaces—pushing the limits of design and manufacturing precision.
• High-Speed Signal Integrity: Demands for faster data transmission (e.g., 5G, AI, IoT) require HDI designs optimized for impedance control, reduced crosstalk, and minimal signal delay.
• Multilayer Integration: The shift toward "2+n+2" and "3+n+3" constructions enables greater interconnect density, supporting complex systems in compact footprints.
• Eco-Friendly Manufacturing: Industry adoption of lead-free plating, low-VOC materials, and energy-efficient processes aligns with global sustainability standards.

Target Application Industries

HDI PCBs are indispensable in high-performance electronic systems:

• Consumer Electronics: Smartphones, tablets, wearables, and laptops (require miniaturization and high component density);
• Automotive Electronics: Advanced driver-assistance systems (ADAS), infotainment systems, and electric vehicle (EV) controllers (demand thermal stability and reliability);
• Medical Devices: Diagnostic equipment, implantable devices, and portable medical tools (require biocompatibility and miniaturization);
• Aerospace & Defense: Avionics and radar systems (need ruggedness and high signal integrity);
• Computing & Data Centers: Servers and high-performance computing (HPC) systems (require high-speed interconnects and thermal management).

Best Practices for HDI PCB Production Success

Achieving reliable, cost-effective HDI PCB production requires two critical steps:

• Design-Stage Optimization: Adhere to IPC standards, leverage design guidelines, and collaborate with manufacturing partners early to address manufacturability risks.
• Partner Selection: Choose factories with specialized HDI expertise, advanced equipment, and a track record of delivering complex designs—ensuring compliance with technical specifications and quality requirements.