What is an HDI PCB? Its Structure and Manufacturing

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Introduction to HDI PCB

High Density Interconnect (HDI) PCBs are a type of printed circuit board that offers higher wiring density than conventional PCBs. They are designed to accommodate more components in a smaller space, enabling the creation of compact, high-performance electronic devices. HDI PCBs are widely used in various industries, including consumer electronics, telecommunications, aerospace, and medical devices.

Key Features of HDI PCBs

  1. High wiring density
  2. Smaller component sizes
  3. Increased signal integrity
  4. Reduced signal interference
  5. Improved thermal management

Structure of HDI PCBs

Layers and Materials

HDI PCBs typically consist of multiple layers of conductive and insulating materials. The most common materials used in HDI PCB manufacturing are:

  • Copper foil for conductive layers
  • FR-4 (Flame Retardant 4) as the base material
  • Prepreg (pre-impregnated) layers for insulation between conductive layers
  • Solder mask for protecting the outer layers
  • Silkscreen for component labeling

The number of layers in an HDI PCB can vary depending on the complexity of the design and the application requirements. HDI PCBs often have 8 or more layers.

Microvias and Blind Vias

One of the key features of HDI PCBs is the use of microvias and blind vias. These specialized vias allow for higher wiring density and improved signal integrity.

  • Microvias: These are small, laser-drilled vias with a diameter of 0.15 mm or less. They are used to connect the outer layers of the PCB to the inner layers.
  • Blind vias: These vias connect an outer layer to an inner layer without going through the entire thickness of the board. They help reduce the size of the PCB and improve signal integrity.

Trace Width and Spacing

HDI PCBs feature smaller trace widths and spacing compared to conventional PCBs. This allows for higher wiring density and more compact designs. Typical trace widths in HDI PCBs range from 0.075 mm to 0.15 mm, with spacing between traces as small as 0.075 mm.

HDI PCB Manufacturing Process

The manufacturing process for HDI PCBs involves several steps, each requiring precise control and advanced technology.

Design and Preparation

  1. PCB design: The first step is to create a detailed design of the HDI PCB using CAD (Computer-Aided Design) software. The design includes the placement of components, routing of traces, and the creation of microvias and blind vias.

  2. Design verification: The design undergoes a thorough review process to ensure it meets the required specifications and standards. Any issues or potential problems are identified and corrected at this stage.

  3. Gerber file generation: Once the design is finalized, Gerber files are generated. These files contain all the necessary information for manufacturing the PCB, including the layer stackup, drill files, and soldermask data.


  1. Material preparation: The base material (usually FR-4) is cut to the required size, and copper foil is laminated onto both sides of the board.

  2. Drilling: Holes are drilled into the board for through-hole components and vias. In HDI PCBs, laser drilling is used to create microvias.

  3. Patterning: The copper layers are patterned using a photolithography process. A photoresist layer is applied, exposed to UV light through a photomask, and then developed to create the desired pattern.

  4. Etching: The unwanted copper is removed using a chemical etching process, leaving behind the desired traces and pads.

  5. Lamination: Additional layers of prepreg and copper foil are laminated onto the board to create a multi-layer structure.

  6. Blind via and microvia creation: Laser drilling is used to create blind vias and microvias, connecting the inner layers to the outer layers.

  7. Plating: The drilled holes and vias are plated with copper to establish electrical connections between layers.

  8. Soldermask application: A soldermask layer is applied to protect the outer layers of the PCB and prevent short circuits.

  9. Surface finish: A surface finish, such as HASL (Hot Air Solder Leveling), ENIG (Electroless Nickel Immersion Gold), or OSP (Organic Solderability Preservative), is applied to the exposed copper areas to enhance solderability and protect against oxidation.

  10. Silkscreen printing: The component labels and other markings are printed onto the PCB using a silkscreen process.

Inspection and Testing

  1. Visual inspection: The manufactured HDI PCBs undergo a visual inspection to check for any defects, such as scratches, cracks, or contamination.

  2. Automated Optical Inspection (AOI): An AOI machine scans the PCBs to detect any missing components, incorrect component placement, or solder defects.

  3. Electrical testing: The PCBs are subjected to various electrical tests, such as continuity testing and insulation resistance testing, to ensure proper functionality and reliability.

  4. Microsectioning: In some cases, a microsection analysis may be performed to examine the internal structure of the PCB, particularly the quality of the microvias and blind vias.

Advantages of HDI PCBs

  1. Miniaturization: HDI PCBs enable the creation of smaller, more compact electronic devices, as they can accommodate more components in a limited space.

  2. Improved performance: The higher wiring density and use of microvias and blind vias in HDI PCBs result in improved signal integrity, reduced signal interference, and better overall performance.

  3. Cost-effective: Although the initial setup costs for HDI PCB manufacturing may be higher, the overall cost can be lower due to the reduced board size and improved yield.

  4. Reliability: HDI PCBs offer higher reliability compared to conventional PCBs, as they have fewer interconnections and shorter signal paths.

Challenges in HDI PCB Manufacturing

  1. Complex design: Designing HDI PCBs requires specialized knowledge and experience to ensure optimal performance and manufacturability.

  2. Advanced manufacturing technology: Manufacturing HDI PCBs requires advanced equipment and processes, such as laser drilling and high-precision plating, which can be costly to implement and maintain.

  3. Strict quality control: Due to the high-density nature of HDI PCBs, strict quality control measures must be in place throughout the manufacturing process to prevent defects and ensure reliability.

  4. Material selection: Choosing the right materials for HDI PCBs is crucial to ensure compatibility, performance, and reliability. This requires a thorough understanding of the material properties and their interactions.

Applications of HDI PCBs

HDI PCBs find applications in various industries and products, including:

  1. Smartphones and tablets
  2. Wearable devices
  3. Aerospace and defense systems
  4. Medical devices
  5. High-performance computing
  6. Automotive electronics
  7. IoT (Internet of Things) devices
Industry Applications
Consumer Electronics Smartphones, tablets, wearables, digital cameras
Telecommunications 5G networks, routers, switches, base stations
Aerospace and Defense Avionics, radar systems, satellite communication
Medical Devices Implantable devices, diagnostic equipment, wearable health monitors
Automotive Advanced driver assistance systems (ADAS), infotainment systems, electric vehicle power electronics

Future Trends in HDI PCB Technology

As electronic devices continue to become smaller, more powerful, and more complex, HDI PCB technology is expected to evolve to meet the growing demands.

  1. Increased layer count: HDI PCBs with 12 or more layers are becoming more common to accommodate higher component density and more complex circuitry.

  2. Finer feature sizes: The trend towards smaller trace widths, spacing, and microvia diameters is expected to continue, enabling even higher wiring density.

  3. Advanced materials: New materials, such as low-loss dielectrics and high-performance laminates, are being developed to improve signal integrity and thermal management in HDI PCBs.

  4. Embedded components: The integration of passive components, such as resistors and capacitors, directly into the PCB substrate is gaining traction in HDI PCB design. This approach saves space and improves performance.

  5. 3D packaging: The use of 3D packaging techniques, such as package-on-package (PoP) and system-in-package (SiP), in conjunction with HDI PCBs, is expected to increase to enable even greater miniaturization and functionality.

Frequently Asked Questions (FAQ)

  1. What is the difference between HDI PCBs and conventional PCBs?
    HDI PCBs offer higher wiring density, smaller component sizes, and improved signal integrity compared to conventional PCBs. They achieve this through the use of microvias, blind vias, and finer trace widths and spacing.

  2. What are the main advantages of using HDI PCBs?
    The main advantages of using HDI PCBs include miniaturization, improved performance, cost-effectiveness, and higher reliability. They enable the creation of smaller, more complex electronic devices with better signal integrity and reduced interference.

  3. What industries commonly use HDI PCBs?
    HDI PCBs are widely used in industries such as consumer electronics, telecommunications, aerospace and defense, medical devices, automotive, and high-performance computing.

  4. What are the challenges in manufacturing HDI PCBs?
    The challenges in manufacturing HDI PCBs include complex design requirements, the need for advanced manufacturing technology, strict quality control measures, and careful material selection. These factors can increase the initial setup costs and require specialized knowledge and experience.

  5. What are the future trends in HDI PCB technology?
    The future trends in HDI PCB technology include increased layer count, finer feature sizes, advanced materials, embedded components, and 3D packaging. These trends are driven by the growing demand for smaller, more powerful, and more complex electronic devices.


HDI PCBs have revolutionized the electronics industry by enabling the creation of smaller, more powerful, and more reliable electronic devices. Their high wiring density, use of microvias and blind vias, and improved signal integrity make them an essential component in a wide range of applications, from smartphones and wearables to aerospace and medical devices.

As technology continues to advance, HDI PCB manufacturing processes and materials will evolve to meet the ever-growing demands for miniaturization, performance, and functionality. By staying at the forefront of these developments, manufacturers can ensure they are well-positioned to meet the needs of the rapidly evolving electronics industry.