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HDI PCB
HDIs or High-Density Interconnect PCBs (HDI PCB) are advanced circuit boards with dense components and traces that enable complex and compact electronic devices. These boards use microvias, blind vias, and buried vias to connect layers, allowing for higher circuit density and smaller form factors.
If you are working on a project requiring high-performance, compact electronics, HDI PCBs may be your solution. Contact a reliable PCB manufacturer today to learn more about how HDI technology can help you achieve your design goals.
Core Advantages of HDI PCB
Compared to standard PCBs, HDI PCBs offer the following key advantages:
Smaller and Lighter: It achieves higher wiring density within a smaller area, thereby reducing the size and weight of electronic products.
Better Performance: Shorter signal paths mean less signal loss, lower latency, and improved signal integrity, resulting in faster and more stable device operation.
Higher Reliability: Using microvia technology results in thinner via walls, making them more reliable than traditional through-holes in high-stress environments.
Cost-Effective: Although the process is complex, HDI can integrate the functions of multiple boards, reduce layer count and overall volume when board layers increase, potentially lowering the overall cost.
Build-Up" Orders and Types of HDI
HDI’s performance is directly related to its process complexity, which is primarily distinguished by its “build-up order” (or “layer order”):
| Build-Up Order | Technical Characteristics | Application Level |
|---|---|---|
| 1st Order HDI | The simplest HDI process; blind vias connect only two adjacent layers (e.g., Layer 1 to Layer 2). | Basic smartphones, consumer electronics. |
| 2nd Order HDI | Blind vias can span one layer, connecting non-adjacent layers (e.g., Layer 1 to Layer 3); more complex process. | Mid-to-high-end smartphones, automotive electronics. |
| 3rd Order & Above | Vias connect across a longer distance; process difficulty and cost increase significantly. | High-end servers, AI accelerator cards. |
| Any-Layer HDI (ELIC) | The highest form of HDI, allowing free interconnection between any layers through stacked microvias, achieving maximum density. | Flagship smartphones (e.g., high-end iPhone mainboards), high-performance computing chip substrates. |
Simply put, a higher build-up order signifies more “intricate” routing, along with higher technical difficulty and cost.
Practical Applications
Consumer Electronics: The core mainboards of devices that pursue thin, light, and compact designs, such as smartphones, smartwatches, and tablets, almost all utilize HDI technology.
High-Speed Communication & Computing: With the development of AI and 5G technologies, there are extremely high demands for signal transmission speed and bandwidth. HDI effectively shortens signal transmission distance and reduces loss, making it a key technology in AI servers and high-performance GPU modules.
Automotive & Industrial: HDI also plays an important role in Advanced Driver-Assistance Systems (ADAS), in-vehicle infotainment systems, and Industrial IoT (IIoT) devices.
In summary, HDI is a key technology for achieving miniaturization and high performance in modern electronic products.
Design Highlights
Designing a successful HDI PCB is not just about drawing thinner lines and drilling smaller holes; it is a precise planning exercise involving stack-up structure, via types, line width/space, and material selection.
The following are several core points to consider when designing an HDI PCB:
1. Determine the Appropriate HDI Build-Up Order and Stack-up Structure
This is the most critical decision, as it directly impacts cost and manufacturability.
Build-Up Order Selection: Choose based on the chip’s pin pitch and escape routing density. Use 1st order if possible instead of 2nd order, because each increase in order significantly increases the difficulty of laser drilling alignment and via filling.
Stack-up Design: The core of HDI is the “sequential lamination” structure. Below is a typical 8-layer, 2nd order HDI board stack-up:
Top Layer (L1): Placement of main ICs and components.
First Inner Layer (L2): Typically a ground plane, providing the closest reference plane for signals on the surface layer.
Second Inner Layer (L3): High-speed signal layer.
Core: A thicker core board, with large power and ground planes in the middle.
Symmetrical Structure: The bottom layers of the board (L7-L8) also need an HDI structure symmetrical to the top layer to prevent warpage.
2. Refined Via Design
Vias are the essence of HDI and the most critical part of the design requiring careful attention.
Prioritize Blind/Buried Vias:
Blind Via: Connects an outer layer to an adjacent inner layer, mainly used for surface layer escape routing.
Buried Via: Connects inner layers to other inner layers, used for high-density interconnection within inner layers.
Via Dimensions: Typically, laser-drilled blind vias have a finished diameter of 4 mil (approx. 0.1mm) or smaller, with a pad diameter of approximately 10-12 mil. The minimum finished diameter for traditional mechanical drilled holes is usually above 8 mil.
Stacking Methods: There are three primary methods, with performance and cost increasing from low to high:
Staggered Via: Blind vias on adjacent layers are offset from each other. Offers high reliability and controllable cost.
Stacked Via: Blind vias on adjacent layers are aligned and stacked directly on top of each other. Saves space but increases process difficulty.
Copper-Filled Stacked Via: Used in Any-Layer HDI. The via is completely filled with copper. Offers the highest reliability but is also the most expensive.
Annular Ring: The via pad must have a sufficient annular ring around the drilled hole (typically recommended 5-6 mil) to ensure reliability for drill alignment.
3. Fine Line/Space and Impedance Control
Typical Capability: HDI designs typically require line/space of 3 mil/3 mil or smaller (e.g., 2.7 mil/2.7 mil), whereas standard PCBs are often above 4 mil/4 mil.
Impedance Control: Due to finer line widths, achieving target impedances like 50 Ohm single-ended or 90/100 Ohm differential requires reducing the distance from the outer layer to its reference plane. This is typically done by laminating very thin dielectric layers (e.g., Prepreg 1060, thickness about 2-3 mil).
4. Layout and Routing in Critical Areas
BGA Escape Routing Strategy:
For a 0.4mm pitch or smaller BGA, the surface layer can typically only escape the outer 2 rows of pins.
Inner Row Pins: Must have a blind via directly drilled to an inner layer for routing.
Multi-Order Escape: The innermost rows might need a “1st order blind via” to L3, then continue routing deeper using a “2nd order blind via” or a buried via.
Avoid Routing Across Splits: When high-speed signals change layers, the return current path must have a adjacent ground via near the signal via.
Power Integrity: Due to thin layers, coupling between power and ground planes is very good. However, avoid creating bottlenecks in high-current areas.
5. Material Selection and Process Compatibility
Materials: Standard FR-4’s fiberglass does not perform well with laser drilling. HDI commonly uses halogen-free, high-Tg materials with good laser drilling performance, such as Shengyi S1000-2M, Panasonic Megtron 6, etc.
Copper Foil: For inner layers, it is recommended to use RTF (Reverse Treated Foil) or VLP (Very Low Profile) copper foil to reduce signal loss.
Via Filling Process: Blind vias on high-order HDI require copper plating and filling (copper-filled vias). Otherwise, problems will arise when stacking vias or routing on top of them.
Need HDI PCB?
HDI PCB provides greater routing density, finer lines and spaces, and better signal integrity, making them ideal for advanced electronics such as smartphones, tablets, and computers. If you need a circuit board with high-density interconnections and improved performance, an HDI PCB may be the solution you’re looking for.