What is the lamination structure of HDI PCB boards?

HDI PCB Lamination Structure

Smartphones are becoming increasingly thin, while smartwatches are becoming increasingly powerful. HDI (High-Density Interconnect) PCB technology is at the core of this trend. Compared to traditional PCBs, HDI lamination structure design enables the placement of more complex circuits in a smaller space.

As a PCB manufacturer with 17 years of experience, Topfast has witnessed numerous projects fail due to the selection of inappropriate HDI lamination structures, leading to cost overruns or performance failures. It is therefore crucial to understand the various lamination structures of HDI PCBs.

1. HDI PCB Lamination Basics

The essence of HDI boards lies in achieving high-density routing through build-up processes, which are fundamentally different from traditional PCB manufacturing. Traditional PCBs are like making sandwiches—all layers are laminated at once—while HDI boards resemble constructing skyscrapers, requiring layered construction.

Key Process Comparisons:

• Foratura laser: Creates micro vias as small as 0.05mm in diameter (human hair ≈ 0.07mm)
• Pulse Plating: Ensures uniform copper thickness in micro vias (<10% variation)
• Sequential Lamination: Typical parameters—170°C±2°C, 25kg/cm² pressure, layer-by-layer buildup

In a smartwatch project I worked on, switching from a traditional 6-layer PCB (5cm²) to an HDI (1+4+1) structure reduced the board size to 1.5cm² while adding heart rate monitoring—showcasing HDI’s magic.

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2. Detailed Analysis of Mainstream HDI Lamination Structures

1. Simple Single Lamination (1+N+1)

Typical Example: (1+4+1) 6-layer board

caratteristiche:

• No buried vias in inner layers, single lamination
• Blind vias formed by laser drilling on outer layers
• The most cost-effective HDI solution

domande:

• Entry-level smartphones
• IoT endpoint devices
• Space-constrained consumer electronics

Studio di caso: A Bluetooth earphone brand adopted (1+4+1) design, integrating Bluetooth 5.0, touch control, and battery management within an 8mm diameter space.

2. Standard Single Lamination HDI (With Buried Vias)

Typical Example: (1+4+1) 6-layer board (buried vias in L2-5)

caratteristiche:

• Buried vias in inner layers require two laminations
• Combines blind and buried vias
• Balanced cost and performance

Design Pitfall: Improper buried via placement caused a 15% impedance deviation in one project, necessitating redesign.

3. Standard Double Lamination HDI

Typical Example: (1+1+4+1+1) 8-layer board

Caratteristiche del processo:

• Three lamination steps (core + first buildup + second buildup)
• Enables complex interconnect architectures
• Supports 3-step blind vias

Vantaggi in termini di prestazioni:

• Suitable for GHz+ high-speed signals
• Better power integrity (dedicated power layers)
• 30% improved thermal performance

4. Optimized Double Lamination Structure

Innovative Design: (1++1+4+1+1) 8-layer board

Key Improvements:

• Shifts buried vias from L3-6 to L2-7
• Eliminates one lamination step
• 15% cost reduction

Dati del test: A 5G module using this structure achieved:

• 0.3dB/cm insertion loss @10GHz
• 12% lower manufacturing cost than traditional structures
• 8% higher yield

3. Advanced HDI Lamination Structure Designs

1. Skip-Via Design

Technical Challenges:

• Blind vias from L1 to L3, skipping L2
• 100% increased laser drilling depth
• Significantly harder plating

Soluzioni:

• Combined UV+CO₂ laser drilling
• Special plating additives for deep vias
• Enhanced optical alignment (accuracy <25μm)

Lesson Learned: A drone flight controller batch failed due to skip-via plating issues, causing $50k rework costs.

2. Stacked Via Design

caratteristiche:

• Blind vias stacked directly over buried vias
• Shorter vertical interconnects
• Reduced signal reflection points

Design Essentials:

• Strict layer alignment control (<25μm error)
• Resin plugging to prevent air pockets
• Additional thermal stress testing (260°C, 10s, 5 cycles)

4. HDI Lamination Structure Selection

1. Key Selection Factors

2. Industry-Specific Recommendations

Elettronica di consumo:

• Preferred: (1+4+1)
• Trace/Space: 3/3mil
• Blind via: 0.1mm

Elettronica automobilistica:

• Recommended: (1+1+4+1+1)
• Material: TG≥170°C
• Additional thermal vias

Dispositivi medici:

• Highest reliability requirements
• Low-void resin plugging
• 100% microsection inspection

5. Practical HDI Design Techniques

1. Via Optimization Principles

• ≤3 Vias in high-speed signal paths
• Adjacent via spacing ≥5× via diameter
• Double power vias

2. Stack-Up Golden Rules

• Signal layers adjacent to ground planes
• Route high-speed signals internally (reduces radiation)
• Tight power-ground plane coupling

3. Reliability Enhancements

• Add 0.1mm thermal via arrays
• Ground guards for critical signals
• 0.5mm no-routing zone at board edges

6. Future Trends

Tecnologie emergenti:

• Modified Semi-Additive Process (mSAP): 20/20μm trace/space
• Low-Temperature Co-fired Ceramic (LTCC): Ultra-high frequency
• Embedded Components: Resistors/capacitors inside boards

Material Breakthroughs:

• Modified Polyimide: Dk=3.0, Df=0.002
• Nano-silver Conductive Adhesive: Alternative to plating
• Thermal Graphene: 5× better heat conduction

A lab successfully prototyped a 16-layer 3D-interconnect HDI (1mm thick, 1024 channels), foreshadowing even more compact future devices.

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Topfast Recommendations

When selecting the appropriate HDI laminate structure, it is necessary to find the optimal balance between wiring density, signal integrity, manufacturing cost, and reliability. The simplest structure often provides the highest yield rate and lowest cost.