How to Design High Density Interconnect PCB

18 January 2026
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What is High Density Interconnect (HDI) PCB?

High Density Interconnect (HDI) PCB is an advanced circuit board technology that maximizes component density and interconnection density in compact footprints. Defined by IPC-2226 standards, HDI PCBs are characterized by microvias (≤150μm diameter), fine trace/space ratios (≥50μm), capture pads ≤400μm, and pad density >20 pads/cm²—enabling 30-50% smaller form factors than traditional multilayer PCBs.

Unlike standard PCBs, HDI designs leverage laser drilling, sequential lamination, and blind/buried vias to eliminate bulky through-hole connections, making them ideal for devices like smartphones, wearables, medical implants, and aerospace electronics. HDI technology supports rigid flex PCB, flexible PCB (flexible printed circuit), RF PCB, and multilayer PCB configurations, balancing miniaturization with high-speed performance and reliability.

Key advantages include reduced signal loss, lower electromagnetic interference (EMI), and improved thermal management—critical for applications requiring dense components and high-frequency signals (≥1GHz). Whether designing a flexible printed circuit for a foldable device or an RF PCB for communication equipment, HDI technology delivers the density and performance needed for modern electronics.

Core Via Technologies in HDI PCB Design

Vias are the backbone of HDI connectivity, enabling layer-to-layer signal transmission without sacrificing density. Below are the critical via types and their applications:

Microvias

Microvias are laser-drilled vias with diameters ≤150μm (6 mils) and aspect ratios ≤0.75:1 (per IPC-6016). They are limited to 1-2 layer transitions and occupy 70% less surface area than traditional through-hole vias, enabling tighter component placement. Microvias are the primary enabler of high density in HDI PCBs, used for BGA fanout routing, fine pitch component interconnections, and space-constrained flexible PCBs.

Laser drilling (UV or infrared) ensures precision, with UV lasers capable of creating microvias as small as 20μm—essential for ultra-dense designs like medical device PCBs and high-performance multilayer PCBs. Proper microvia design requires void-free plating and adherence to IPC-4104 material compatibility standards to prevent reliability issues.

Blind Vias

Blind vias connect an outer layer to one or more inner layers without penetrating the entire PCB stackup. Classified as Type II vias (0.15-0.50mm diameter) per IPC-2226, they eliminate unused via stubs that cause signal reflections, making them ideal for high-speed signals in HDI circuit boards. Blind vias are laser-drilled for precision, with depth control (±5μm) ensuring they terminate exactly at target layers.

Common applications include rigid flex PCB, flexible printed circuits, and RF PCBs, where surface space is limited and signal integrity is non-negotiable. When paired with via-in-pad designs, blind vias further increase density by allowing direct component lead connections to internal layers.

Buried Vias

Buried vias connect only inner layers, remaining fully encapsulated within the PCB stackup. They do not occupy outer layer space, making them critical for maximizing routing density in multilayer PCBs. Buried vias are mechanically drilled for larger diameters (>0.50mm) and laser-drilled for micro-sized applications, with IPC-6012 specifying plating thickness (≥20μm) for reliability.

Buried vias are used for internal power distribution, signal routing between inner layers, and reducing EMI in RF PCBs. They require precise layer alignment (≤±10μm tolerance) during lamination to avoid connectivity issues, making early manufacturer collaboration essential.

Stacked Vias

Stacked vias are microvias vertically aligned across multiple layers, creating direct vertical paths through the HDI PCB stackup. They require copper filling to ensure structural stability and electrical continuity—void-free filling (per IPC-6016) prevents cracking during thermal cycling. Stacked vias enable extreme density (critical for 0.4mm pitch BGAs and advanced microprocessors) but increase manufacturing complexity and cost.

Key considerations for stacked vias:

Layer alignment tolerances ≤±10μm to prevent open circuits
Compatibility with sequential lamination processes
Copper fill thickness ≥80% of via diameter for thermal conductivity

Skip Vias

Skip vias bypass one or more inner layers without electrical contact, creating flexible routing paths in complex HDI stackups. They are particularly useful in Every Layer Interconnect (ELIC) structures, where all microvia layers require unrestricted interconnection. Skip vias reduce the number of required vias by jumping non-critical layers, simplifying routing and reducing signal loss.

For example, a skip via from layer 1 to layer 3 avoids layer 2, eliminating intermediate vias and freeing up space for other connections. This makes skip vias ideal for high-density flexible printed circuits and rigid flex PCBs where routing flexibility is key.

Standard Blind Vias

Standard blind vias (Type I per IPC-2226) are blind vias with diameters ≥0.50mm, mechanically drilled for less dense HDI designs. They balance density and cost, offering a middle ground between through-hole vias and microvias. Standard blind vias are commonly used in consumer electronics where cost sensitivity outweighs maximum density needs.

Key specifications:

Aspect ratio ≤1:1 for reliable plating
Annular ring ≥0.10mm to prevent pad lifting
Compatibility with standard lamination processes

Critical Design Parameters: Pad Size, Trace/Space, and Planarity

Pad Size

Pad dimensions in HDI PCBs are directly tied to via size and component pitch. Capture pads for microvias range from 200-400μm in diameter, with via-in-pad designs requiring pads sized 1.5-2x the via diameter to ensure sufficient solder joint strength. For fine pitch components (e.g., 0.4mm pitch BGAs), pad sizes are optimized to 120-150μm to prevent bridging during soldering.

IPC-2226 specifies pad-to-via alignment tolerances of ≤±10μm to avoid drill wander, which can cause partial pad coverage and reliability risks. Pad material selection is critical—electroless nickel immersion gold (ENIG) is preferred for HDI PCBs due to its excellent solderability and corrosion resistance, while soft gold is used for wire bonding applications.

Trace/Space

HDI PCBs use ultra-fine traces and spaces to maximize routing density, with typical values ranging from 50-100μm (2-4 mils). Traces narrower than 50μm require current capacity calculations (per IPC-2221) to prevent overheating—for example, a 50μm trace carries 0.5A at a 10°C temperature rise on FR-4.

Space between traces directly impacts crosstalk, especially in high-speed designs. Maintaining a minimum space of 3x trace width for parallel runs >10mm reduces electromagnetic coupling, while 5x spacing is recommended for RF PCBs and high-frequency signals (≥3GHz). For impedance-controlled routing, trace width and space must be consistent across the board to avoid discontinuities.

Planarity

Planarity refers to the flatness of the PCB surface, critical for fine-pitch component assembly and multiple lamination cycles. Uneven surfaces cause poor solder joint formation, component misalignment, and long-term reliability issues. Manufacturers specify planarity tolerances of ≤0.1mm per 100mm board length (per IPC-6012) to ensure compatibility with SMT processes.

Key factors influencing planarity:

Material selection (consistent prepreg thickness and core rigidity)
Copper filling of vias (reduces uneven shrinkage during curing)
Lamination pressure and temperature control (uniform heat distribution)
Balanced stackup design (equal layer counts on both sides of the core)

HDI PCB Stackup Design: Layers, Materials, and Configuration

Stackup Fundamentals

HDI stackup design directly impacts routing efficiency, signal integrity (SI), and manufacturability. The number of layers is determined by component density, signal count, and power/ground requirements—ranging from 4 to 16 layers for most HDI applications.

Core stackup principles:

Symmetrical layer arrangement to prevent warping during lamination
Balanced copper distribution (≤10% difference between top and bottom layers)
Integration of dedicated power and ground planes to reduce EMI
Separation of digital, analog, and RF layers to minimize crosstalk

Common Stackup Configurations

HDI stackups use sequential lamination (build-up) or ELIC (Every Layer Interconnect) structures, with three standard configurations:

Configuration	Layer Structure	Key Advantages	Applications
0-N-0	Core layers (N) with no build-up layers	Cost-effective, simple manufacturing	Low-density HDI PCBs, basic consumer electronics
1-N-1	1 build-up layer on each side of core (total N+2 layers)	Moderate density, balanced cost/performance	Smartphones, tablets, mid-range multilayer PCBs
2-N-2	2 build-up layers on each side of core (total N+4 layers)	High density, supports stacked vias	Advanced wearables, medical devices, RF PCBs
ELIC	All layers interconnected via microvias	Maximum flexibility, ultra-high density	Aerospace electronics, high-performance computing

Each configuration is tailored to specific needs—for example, a 1-N-1 stackup is ideal for a flexible PCB in a foldable phone, while ELIC is used for mission-critical aerospace applications.

Material Selection for HDI PCBs

Material choice is critical for HDI performance, with key considerations including dielectric constant (Dk), dissipation factor (Df), thermal stability, and laser drilling compatibility. Below are the most common materials:

Material Category	Examples	Key Properties	Applications
Standard Dielectrics	FR-4 (Tg 140-170°C)	Cost-effective, good mechanical strength	General-purpose HDI PCBs, consumer electronics
High-Tg Dielectrics	Isola I-Speed (Tg 180°C), Nelco N7000-2 HT (Tg 170°C)	Improved thermal stability, low loss	High-speed multilayer PCBs, industrial electronics
High-Performance Dielectrics	Isola Tachyon 100G (Dk 3.0), Polyimide (Tg 260°C)	Ultra-low loss, high temperature resistance	RF PCBs, microwave devices, aerospace electronics
Copper Foils	Thin-profile (12-35μm) electrolytic copper	Precise etching for fine traces, flexible	Flexible PCBs, rigid flex PCBs, dense layouts
Prepregs	Laser-drillable (LD) prepregs	Uniform resin distribution, easy microvia formation	Sequential lamination, HDI build-up layers

Critical material requirements:

Dk stability (±0.05 over operating frequency) for impedance control
Tg ≥170°C to withstand reflow soldering (260°C for 10 seconds)
Copper adhesion strength ≥6 lb./in. (per IPC-4104) to prevent delamination
Compatibility with laser drilling (low resin residue, clean hole walls)

Signal Integrity (SI) & Power Delivery in HDI PCBs

Signal Integrity Challenges & Solutions

HDI PCBs face unique SI challenges due to high density, including signal reflections, crosstalk, and EMI. Proven solutions:

Controlled Impedance Routing:

Match trace impedance to source/load (50Ω for RF, 100Ω for differential pairs)
Use impedance calculation tools to determine trace width (e.g., 100μm width for 50Ω on 1.6mm FR-4)
Maintain consistent dielectric thickness (±5% tolerance) to avoid impedance shifts

Minimize Signal Reflections:

Use blind/buried vias instead of through-hole vias to reduce stub lengths (≤500μm)
Avoid 90° trace bends (use 45° bends or rounded corners)
Terminate high-speed signals (≥1GHz) with series resistors (50-100Ω)

EMI Reduction:

Route high-speed signals over ground planes to minimize loop area
Use differential pairs for critical signals (USB, PCIe) to cancel out EMI
Implement ground planes as shields between digital and analog layers

Power Delivery Network (PDN) Optimization

A robust PDN ensures stable power supply to dense components, preventing voltage droop and noise:

Low-Impedance Planes:

Use solid power and ground planes with minimal gaps (≤1mm)
Optimize plane thickness (0.5-1.0mm) to reduce DC resistance
Place power planes adjacent to ground planes to form a capacitor-like structure

Decoupling Capacitors:

Place capacitors within 5mm of component power pins (≤2mm for high-speed devices)
Use a mix of capacitance values (0.1μF, 1μF, 10μF) to cover 10kHz-1GHz
Select X7R/X5R ceramic capacitors for temperature stability

Thermal Management:

Incorporate thermal vias (≥0.3mm diameter) under high-power components
Use high-thermal-conductivity materials (e.g., aluminum core) for PCBs with >5W dissipation
Distribute high-power components evenly to avoid hotspots

Space Saving & Fine Pitch Component Integration

Space-Saving Techniques

HDI technology excels at space optimization, with these techniques delivering maximum density:

Via-in-Pad Design:

Place microvias directly under component pads (BGA, QFP) to eliminate separate via pads
Fill vias with copper or epoxy to maintain planarity and improve thermal conductivity
Reduces board area by 30-40% compared to traditional via placement

Sequential Lamination:

Build layers incrementally to enable precise via placement and trace routing
Eliminates through-hole vias, freeing up inner layer space
Supports complex stackups (e.g., 2-N-2) for ultra-dense designs

Component Miniaturization:

Use 01005 (0.4x0.2mm) or 0201 (0.6x0.3mm) discrete components
Adopt fine-pitch BGAs (≤0.5mm pitch) and CSPs (chip-scale packages)
Implement double-sided component placement to double density without increasing board size

Fine Pitch Component Design Considerations

Fine pitch components (≤0.65mm pitch) require specialized handling:

BGA Fanout Routing:

Use microvias to route signals from BGA pins to inner layers (1 via per pin)
Implement stacked vias for 0.4mm pitch BGAs to maximize routing channels
Maintain minimum trace width (50μm) for signal integrity

Solder Paste Application:

Use stencils with 0.1mm apertures for fine pitch components
Apply solder paste with 80-100% aperture fill to prevent bridging
Use no-clean solder paste to avoid residue buildup

Alignment & Tolerances:

Maintain component placement tolerances of ≤±0.05mm
Use fiducial marks (≥1mm diameter) for automated alignment
Design thermal relief pads for fine pitch components to prevent solder wicking

Laser Drilling: Precision for Microvia Formation

Laser drilling is the cornerstone of HDI PCB manufacturing, enabling microvias that mechanical drilling cannot achieve:

Laser Types & Capabilities:

Ultraviolet (UV) Lasers: Create microvias down to 20μm diameter, ideal for fine-pitch components
Infrared (IR) Lasers: Drill larger microvias (50-150μm) in standard dielectrics like FR-4
Excimer Lasers: High-precision drilling in high-performance materials (polyimide)

Drilling Process:

Ablation: Laser vaporizes dielectric material without damaging underlying copper
Depth Control: Precision stops ensure blind vias terminate at target layers (±5μm tolerance)
Deburring: Post-drilling cleaning removes resin residue for reliable plating

Advantages Over Mechanical Drilling:

Faster drilling speeds for microvias (up to 10,000 vias per minute)
No tool wear, ensuring consistent hole quality
Ability to drill non-circular vias for specialized applications

Laser drilling is essential for HDI PCBs, enabling the microvia technology that defines high-density designs—from flexible printed circuits to RF PCBs.

Ground Planes & Decoupling: Enhancing Stability & Reducing Noise

Ground Plane Design

Ground planes are critical for signal integrity, EMI reduction, and thermal management:

Design Principles:

Use solid, unbroken planes wherever possible (avoid gaps that create current loops)
Place ground planes adjacent to signal layers to minimize loop area
Implement separate ground planes for digital, analog, and RF signals (connected at a single point)

Ground Plane Optimization:

Ensure ground plane coverage under high-speed traces (≥90% coverage)
Use ground stitching vias (≤5mm spacing) to connect multiple ground planes
Avoid routing signals across ground plane gaps to prevent EMI

Decoupling Strategies

Decoupling capacitors stabilize the PDN by suppressing voltage fluctuations:

Placement:

Mount capacitors as close to component power pins as possible to minimize lead inductance
Use separate vias for capacitor power and ground connections to avoid shared inductance
Place one decoupling capacitor per power pin for high-density components (e.g., FPGAs)

Capacitance Selection:

Combine ceramic capacitors (high-frequency noise) and electrolytic capacitors (low-frequency stability)
Select capacitors with low equivalent series resistance (ESR) and inductance (ESL)
Match capacitor voltage rating to operating voltage (≥2x for reliability)

Impedance Control & Crosstalk Mitigation

Impedance Control

Impedance control ensures signals propagate without reflections, critical for high-speed HDI PCBs:

Implementation Steps:

Calculate target impedance using dielectric thickness, trace width, and copper height (IPC-2221 guidelines)
Maintain consistent trace dimensions (±5% tolerance) across the board
Use impedance-controlled materials with stable Dk values (e.g., Isola I-Speed)
Verify impedance with test coupons during prototyping, adjusting trace dimensions as needed

Common Impedance Values:

50Ω: RF signals, coaxial connections
100Ω: Differential pairs (USB, PCIe, Ethernet)
75Ω: Video signals, coaxial cables

Crosstalk Mitigation

Crosstalk (unwanted signal coupling) increases with density but can be minimized:

Physical Separation:

Maintain minimum spacing of 3x trace width for parallel runs >10mm
Increase spacing to 5x for high-frequency signals (≥3GHz)
Cross traces at 90° angles when possible to minimize coupling

Shielding & Isolation:

Use ground planes between signal layers to isolate traces
Route differential pairs together with consistent separation (1-2x trace width)
Implement guard traces (connected to ground) between sensitive signals

Manufacturability & Reliability

Design for Manufacturability (DFM)

DFM is critical for HDI PCBs to avoid production delays and defects:

Key DFM Guidelines:

Adhere to manufacturer capabilities (minimum trace width: 50μm, via size: 20μm, spacing: 50μm)
Maintain annular ring ≥10μm for microvias to prevent pad lifting
Design thermal relief for through-hole components to ensure proper soldering
Incorporate test points for AOI and functional testing (avoid placement under components)

DFM Review Process:

Submit design files for manufacturer DFM review before production
Address issues like insufficient spacing, incompatible via sizes, or unmanufacturable traces
Validate design with prototypes to confirm manufacturability

Reliability Considerations

HDI PCB reliability depends on addressing thermal, mechanical, and electrical stressors:

Thermal Reliability:

Use materials with high Tg (≥170°C) and low CTE to withstand thermal cycling
Implement thermal vias and heatsinks for high-power components
Test for thermal shock (per IPC-TM-650 Method 2.6.7) to ensure stability

Mechanical Reliability:

Reinforce high-stress areas (e.g., connector mounting points) with thicker copper or stiffeners
Use flexible materials (polyimide) for rigid flex PCBs and flexible printed circuits
Test for vibration and mechanical shock (per IEC 60068) for harsh environment applications

Electrical Reliability:

Ensure microvias are void-free and properly plated (per IPC-6016)
Test for dielectric breakdown (≥1500V/mil) and insulation resistance (≥10^12Ω)
Perform accelerated life testing (thermal cycling, humidity) to validate long-term performance

Manufacturer Collaboration & Testing

Manufacturer Collaboration

Early collaboration with PCB manufacturers is critical for HDI success:

Pre-Design Consultation:

Discuss stackup options, material availability, and manufacturing limits
Confirm minimum feature sizes (trace width, via size) and tolerances
Align on DFM requirements and quality standards (IPC-6016)

Design Reviews:

Submit design files for manufacturer DFM review to identify issues early
Collaborate on material selection to ensure compatibility with manufacturing processes
Validate prototype designs with manufacturer feedback before full-scale production

Testing & Validation

HDI PCBs require rigorous testing to ensure performance and reliability:

Standard Testing Methods:

Automated Optical Inspection (AOI): Checks for surface defects (missing components, bad solder joints)
X-Ray Inspection: Verifies internal structures (buried vias, stacked vias) for alignment and plating quality
Flying Probe Test: Tests for shorts, opens, and continuity without custom fixtures
Functional Testing: Validates PCB performance under operating conditions

Advanced Testing:

Signal Integrity Testing: Measures eye diagrams, jitter, and signal loss (per IEEE standards)
Environmental Testing: Subjects PCBs to thermal cycling, humidity, and vibration (per IPC-6016)
Reliability Testing: Accelerated life testing to predict long-term performance

HDI PCB Best Practices

Start with Stackup Planning: Define layer count, power/ground planes, and via architecture early—stackup decisions impact all other design aspects.
Use Microvias Strategically: Replace through-hole vias with microvias to free up routing space, prioritizing via-in-pad designs for fine-pitch components.
Optimize Component Placement: Group components by function (power management, RF circuits) to minimize trace length and crosstalk.
Leverage Design Tools: Use ECAD software with HDI-specific features (impedance calculation, automated microvia placement) to streamline design.
Prototype Before Production: Test prototypes for signal integrity, manufacturability, and reliability to identify issues before full-scale production.
Follow IPC Standards: Adhere to IPC-2226 (design), IPC-4104 (materials), and IPC-6016 (performance) for industry-compliant designs.
Balance Density & Cost: Evaluate stacked vs. staggered vias, layer count, and materials to achieve density goals without unnecessary expense.
Consider End-Use Environment: Design for temperature, humidity, and mechanical stress based on the application (aerospace vs. consumer electronics).

FAQ: Common HDI PCB Design Questions

Q: What is the difference between HDI PCB and traditional PCB?

A: HDI PCBs use microvias, fine traces, and sequential lamination to achieve higher density, smaller size, and better signal integrity than traditional PCBs, which rely on through-hole vias and thicker traces.

Q: When should I use an HDI PCB instead of a standard multilayer PCB?

A: Use HDI PCBs for compact, high-performance devices (smartphones, medical implants) where space is limited, component density is high, or signal speeds exceed 1GHz.

Q: What is the maximum density achievable with HDI PCBs?

A: HDI PCBs can achieve up to 20x higher density than traditional PCBs, with trace/space as small as 25μm and microvias down to 20μm in diameter.

Q: How do I choose between stacked and staggered vias?

A: Use stacked vias for maximum density (e.g., fine-pitch BGAs) and staggered vias for cost savings and higher reliability in harsh environments.

Q: What materials are best for high-speed HDI PCBs?

A: High-speed HDI PCBs require low-loss materials like Isola Tachyon 100G (RF/microwave) or Nelco N7000-2 HT (medium speed/loss) to minimize signal attenuation.

Conclusion

Designing high-quality HDI PCBs requires a balance of density, performance, and manufacturability. By mastering core concepts like microvia technology, stackup design, signal integrity, and material selection, engineers can create HDI PCBs that meet the demands of modern miniaturized electronics.

Following IPC standards (IPC-2226, IPC-4104, IPC-6016), collaborating with manufacturers early, and prioritizing testing and prototyping are key to overcoming common challenges. With the right approach, HDI PCBs deliver smaller form factors, better performance, and higher reliability—enabling innovation in consumer, medical, aerospace, and industrial applications.

Whether designing a flexible PCB, rigid flex PCB, RF PCB, or multilayer PCB, mastering HDI design principles ensures your circuit board meets the technical requirements of today’s most advanced devices.