HDI PCBs: Design, Manufacturing, and Advanced Applications
RayMing manufactures and assembles high-density interconnect PCBs with blind, buried, and micro-vias, built-up laminations, and the highest signal integrity in the industry.
What is HDI PCB?
High-Density Interconnect (HDI) PCBs represent a cutting-edge approach to circuit board design and manufacturing. These advanced PCBs are characterized by their higher wiring density per unit area, allowing for more complex circuits in a smaller space. But what exactly sets HDI PCBs apart from traditional PCBs?
Key Features of HDI PCBs
HDI PCBs are distinguished by several key features:
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Finer Lines and Spaces: HDI PCBs typically have trace widths and spaces of 3 mils (0.076 mm) or less, compared to the 4-8 mils common in standard PCBs.
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Smaller Vias: HDI boards use microvias, which are typically less than 150 microns in diameter.
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Higher Connection Pad Density: HDI PCBs can support more than 200 pins per square inch.
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Increased Layer Count: While traditional PCBs often have 4-8 layers, HDI PCBs can have 30 or more layers.
Advantages of HDI PCBs
The unique structure of HDI PCBs offers several advantages:
- Miniaturization: HDI technology allows for smaller, lighter electronic devices.
- Improved Electrical Performance: Shorter signal paths reduce signal loss and improve signal integrity.
- Enhanced Thermal Management: More copper layers and better heat distribution improve thermal performance.
- Increased Reliability: Smaller, more numerous vias improve the board’s overall reliability.
Common High-Density Interconnect PCB Structures
HDI PCBs come in various structures, each suited to different applications and complexity levels. Let’s explore the most common HDI PCB structures.
HDI PCB (1+N+1)
The 1+N+1 structure is one of the most basic HDI configurations.
- Structure: One high-density layer on the top, one on the bottom, and N number of conventional layers in between.
- Characteristics: This structure offers a good balance between cost and performance for less complex designs.
- Applications: Commonly used in consumer electronics like smartphones and tablets.
HDI PCB (2+N+2)
The 2+N+2 structure offers increased density and performance compared to the 1+N+1 configuration.
- Structure: Two high-density layers on both the top and bottom, with N conventional layers in between.
- Characteristics: Allows for more complex routing and higher component density.
- Applications: Ideal for high-performance computing devices and advanced telecommunications equipment.
ELIC (Every Layer Interconnection)
ELIC represents the pinnacle of HDI technology, offering the highest possible interconnection density.
- Structure: Every layer is interconnected with microvias, allowing for the most complex routing possibilities.
- Characteristics: Provides maximum design flexibility and the highest component density.
- Applications: Used in cutting-edge applications like aerospace electronics and advanced medical devices.
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Material Selection for HDI
Choosing the right materials is crucial for the performance and reliability of HDI PCBs. The materials must meet stringent requirements for electrical, thermal, and mechanical properties.
Substrate Materials
- High-Speed Laminates: Materials like Rogers 4000 series or Taconic RF-35 offer low dielectric loss for high-frequency applications.
- Low-Loss Materials: FR-4 variants with lower dielectric constants are suitable for many HDI applications.
- Flexible Materials: Polyimide-based materials allow for flexible or rigid-flex HDI designs.
Copper Foil
- Ultra-Thin Copper Foil: HDI PCBs often use copper foils as thin as 3-5 microns to achieve fine line widths.
- Reverse Treated Foil (RTF): Offers better adhesion and finer line capability compared to standard foils.
Dielectric Materials
- Build-Up Films: Epoxy-based or other specialty films are used for the build-up layers in HDI PCBs.
- Low CTE Materials: Materials with a low Coefficient of Thermal Expansion (CTE) help prevent delamination in multi-layer HDI boards.
Microvias Types for HDI PCBs
Microvias are the cornerstone of HDI technology, enabling the high interconnection density that defines these boards. There are several types of microvias, each with its own characteristics and applications.
Stacked Vias
- Structure: Vias stacked directly on top of each other through multiple layers.
- Advantages: Saves space and allows for direct vertical connections.
- Challenges: Can be more difficult to manufacture reliably, especially for many layers.
Stacked Microvias
- Structure: Similar to stacked vias, but using microvias (typically <150 microns in diameter).
- Advantages: Enables the highest interconnection density.
- Applications: Used in the most advanced HDI designs, like smartphones and high-end computing devices.
Staggered Vias
- Structure: Vias that are offset from each other in adjacent layers.
- Advantages: Easier to manufacture than stacked vias, provides more reliable connections.
- Applications: Common in less extreme HDI designs where maximum density isn’t required.
Staggered Microvias
- Structure: Offset microvias in adjacent layers.
- Advantages: Combines the benefits of microvias with the reliability of staggered structures.
- Applications: Widely used in consumer electronics and automotive applications.
Via In Pad
- Structure: Microvias placed directly in component pads.
- Advantages: Saves significant space, allows for higher component density.
- Challenges: Requires careful design and manufacturing to prevent solder wicking issues.
Tips for HDI PCB Design
Designing HDI PCBs requires a different approach compared to traditional PCBs. Here are some essential tips for successful HDI PCB design:
1. Start with a Solid Stackup Design
- Consider signal integrity requirements when planning your layer stackup.
- Use power and ground planes strategically to improve signal quality and EMI performance.
2. Optimize Component Placement
- Place components to minimize signal path lengths.
- Consider thermal management in your component layout.
3. Use Microvias Effectively
- Leverage different microvia structures (stacked, staggered) based on your design needs.
- Be mindful of aspect ratios in microvia design to ensure manufacturability.
4. Pay Attention to Impedance Control
- Use impedance calculators to determine trace widths for controlled impedance.
- Consider the impact of microvias on impedance when routing high-speed signals.
5. Design for Manufacturability
- Consult with your PCB manufacturer early in the design process.
- Be aware of your manufacturer’s capabilities and design rules.
6. Leverage Advanced Routing Techniques
- Use techniques like via-in-pad to maximize routing density.
- Consider differential pair routing for high-speed signals.
HDI Capability
| Feature | RAYMING ´s HDI technical specification |
| Number of layers | 4 – 32 layers standard, 56 layers advanced |
| Technology highlights | Multilayer boards with a higher connection pad density than standard boards, with finer lines/spaces, smaller via holes and capture pads allowing microvias to only penetrate select layers and also be placed in surface pads. |
| HDI builds | 1+N+1, 2+N+2, 3+N+3,4+N+4, any layer / ELIC, Ultra HDI in R&D |
| Materials | FR4 standard, FR4 high performance, Halogen free FR4, Rogers |
| Copper weights (finished) | 18μm – 70μm |
| Minimum track and gap | 0.075mm / 0.075mm |
| PCB thickness | 0.40mm – 6.50mm |
| Maxmimum dimensions | 610mm x 450mm; dependant upon laser drilling machine |
| Surface finishes available | OSP, ENIG, Immersion tin, Immersion silver, Electrolytic gold, Gold fingers |
| Minimum mechanical drill | 0.15mm |
| Minimum laser drill | 0.10mm standard, 0.075mm advanced |
HDI PCB Manufacturing Process
The manufacturing of HDI PCBs is a complex, multi-step process that requires precision and advanced technology. Let’s walk through each stage of the HDI PCB manufacturing process:
1. Pre-production
- Design Review: Engineers scrutinize the PCB design to ensure manufacturability and identify potential issues.
- Material Selection: Based on the design requirements, appropriate materials are chosen for the substrate, copper foil, and dielectric layers.
- Tooling Preparation: Necessary tools and equipment are set up for the production run.
2. Inner Layer Processing
- Copper Cleaning: The copper foil is cleaned to ensure proper adhesion of photoresist.
- Photoresist Application: A layer of photoresist is applied to the copper.
- Imaging: The circuit pattern is transferred to the photoresist using UV light.
- Developing: The unexposed photoresist is removed, revealing the circuit pattern.
- Etching: Excess copper is etched away, leaving only the desired circuit pattern.
- Stripping: Remaining photoresist is stripped off, exposing the copper traces.
- Automated Optical Inspection (AOI): The inner layers are inspected for defects.
3. Lamination
- Layer Alignment: The inner layers are precisely aligned with prepreg (pre-impregnated bonding material) layers.
- Pressing: The stack is subjected to high pressure and temperature in a lamination press.
- Cooling: The laminated panel is cooled under controlled conditions to prevent warping.
4. Mechanical Drilling
- Through-hole Drilling: Larger holes are drilled mechanically for through-hole components and vias.
- X-ray Drilling: For multi-layer boards, X-ray is used to ensure accurate alignment of drill points.
5. Laser Drilling
- Microvia Formation: A laser is used to create small, high-precision microvias.
- Controlled Depth Drilling: The laser power and duration are precisely controlled to drill to the correct depth without damaging underlying layers.
6. Plating
- Desmear: The drilled holes are cleaned to remove any melted resin or debris.
- Electroless Copper Deposition: A thin layer of copper is deposited on the entire board, including in the holes.
- Copper Electroplating: Additional copper is electroplated to build up the required thickness in the holes and on the surface.
7. Outer Layer Processing
- Similar to inner layer processing, but includes:
- Pattern Plating: Additional copper is plated onto the surface traces.
- Tin Plating: A thin layer of tin is plated over the copper to protect it during etching.
8. Solder Mask Application
- Solder Mask Printing: A layer of solder mask is applied to the board.
- Curing: The solder mask is cured using UV light or heat.
- Legend Printing: Component designators and other markings are printed on the solder mask.
9. Silkscreen
- Screen Printing: Component labels, logos, and other markings are printed on the board surface.
- Curing: The silkscreen ink is cured, usually using UV light.
10. Surface Finish
- Depending on the requirements, one of several surface finishes is applied:
- HASL (Hot Air Solder Leveling)
- ENIG (Electroless Nickel Immersion Gold)
- OSP (Organic Solderability Preservative)
- Immersion Tin or Immersion Silver
11. Profiling
- Routing: The individual PCBs are cut out from the panel using a CNC router.
- V-scoring: For panels that will be separated by the end-user, V-grooves are cut to allow easy separation.
12. Testing
- Electrical Testing: Each board is tested for continuity and short circuits.
- Flying Probe Testing: For complex boards, automated probes test multiple points for correct connectivity.
- Functional Testing: Some manufacturers perform basic functional tests as specified by the customer.
13. Quality Control
- Visual Inspection: Trained inspectors examine the boards for visible defects.
- Automated Optical Inspection (AOI): High-resolution cameras scan the boards for defects that might be missed by human inspectors.
- X-ray Inspection: For HDI boards with buried or blind vias, X-ray inspection ensures internal structures are correct.
14. Packaging
- Vacuum Sealing: Boards are often vacuum-sealed in anti-static bags to protect from moisture and static electricity.
- Boxing: Sealed boards are carefully packed in boxes with appropriate cushioning.
- Labeling: Packages are labeled with appropriate identification and handling instructions.
This complex, multi-step process ensures that HDI PCBs meet the high standards of quality and reliability required for their advanced applications. Each step is crucial, and manufacturers often have proprietary techniques to enhance the quality and efficiency of their HDI PCB production.
HDI PCB Industries and Applications
HDI PCB technology has found its way into numerous industries, enabling advancements in various applications. Let’s explore some of the key areas where HDI PCBs are making a significant impact:
Consumer Electronics
- Smartphones: HDI PCBs enable the compact design and advanced features of modern smartphones.
- Tablets and Laptops: HDI technology allows for thinner, lighter devices with improved performance.
- Wearable Devices: Smartwatches and fitness trackers rely on HDI PCBs for their small form factors.
Automotive Industry
- Advanced Driver Assistance Systems (ADAS): HDI PCBs support the complex circuitry required for features like lane departure warnings and automatic emergency braking.
- Infotainment Systems: High-density boards enable feature-rich, responsive infotainment systems in modern vehicles.
- Electric Vehicle Control Systems: HDI PCBs play a crucial role in the efficient management of electric powertrains.
Aerospace and Defense
- Avionics: HDI technology enables sophisticated flight control and navigation systems in a compact form factor.
- Satellite Communications: The high-frequency capabilities of HDI PCBs are ideal for satellite communication systems.
- Radar Systems: Advanced radar systems benefit from the improved signal integrity offered by HDI PCBs.
Medical Devices
- Implantable Devices: HDI PCBs allow for miniaturization of devices like pacemakers and cochlear implants.
- Diagnostic Equipment: High-density interconnects enable more advanced, compact diagnostic tools.
- Wearable Medical Devices: HDI technology supports the development of sophisticated, unobtrusive health monitoring devices.
Telecommunications
- 5G Infrastructure: The high-frequency performance of HDI PCBs is crucial for 5G network equipment.
- Optical Networking Equipment: HDI boards support the high-speed, high-density requirements of modern optical networking gear.
- IoT Devices: The compact nature of HDI PCBs is ideal for various Internet of Things devices.
Industrial Automation
- Programmable Logic Controllers (PLCs): HDI technology allows for more compact, powerful PLCs.
- Robotics: Advanced robotic systems benefit from the high-density, high-performance characteristics of HDI PCBs.
- Sensor Systems: HDI boards enable the integration of multiple sensors in compact industrial monitoring systems.
Conclusion: The Future of HDI PCB Technology
As we look to the future, it’s clear that HDI PCB technology will continue to play a crucial role in the advancement of electronics across various industries. The ongoing trend towards miniaturization, coupled with the increasing demand for higher performance and functionality, ensures that HDI PCBs will remain at the forefront of electronic design.
Emerging technologies like 5G, artificial intelligence, and the Internet of Things are driving the need for even more advanced HDI PCB designs. We can expect to see further innovations in materials, manufacturing processes, and design techniques to meet these challenges.
The future may bring developments such as:
- Even finer line widths and smaller microvias, pushing the boundaries of miniaturization further.
- Integration of advanced materials like graphene or carbon nanotubes for improved electrical and thermal performance.
- Enhanced 3D integration techniques, blurring the line between PCB and semiconductor packaging.
- Advancements in flexible and stretchable HDI PCBs, opening up new possibilities for wearable and implantable devices.
As HDI PCB technology continues to evolve, it will enable the next generation of electronic devices, from more powerful smartphones to advanced medical implants and beyond. Engineers and designers who master the intricacies of HDI PCB design and manufacturing will be well-positioned to create the innovative products of tomorrow.