4 Main Difference Between HDI PCB and Ordinary PCB

HDI (High-Density Interconnect Board) is a compact board designed for small-volume users. Compared to ordinary PCB, the most significant feature of HDI is the high density of wiring, the difference between the two is mainly reflected in the following 4 aspects:

1. HDI PCB - Smaller and Lighter

HDI board is the traditional double-sided board as the core board, through the continuous accumulation of layers laminated from. This way by the continuous accumulation of layers of the board is also known as the accumulation of multilayer boards (Build-up Multilayer, BUM). Compared to conventional boards, HDI boards have the advantages of being "light, thin, short, and small".

HDI's electrical interconnection between the layers of the board is achieved through the conductive through-hole, buried via, and blind via the connection. Different from the ordinary multilayer PCB, HDI boards are designed with a large number of micro-buried blind vias.
 
Ordinary PCB usually is drilled by mechanical method, while HDI PCB drilled by laser. So the number of layers and aspect ratio of HDI PCB is often increased.

2. High HDI Wiring Density

HDI board's high density is mainly reflected in the hole, line, pad density, and interlayer thickness.

Micro-Through Via. HDI circuit board contains blind holes and other micro-conductor hole designs. There are main performances in the two aspects. At first, micro-hole and hole-forming technology in diameter of less than 150μm. Second, cost, production efficiency, hole precision control, and other aspects of the high demand. In traditional multilayer circuit boards, there are only through holes without tiny buried blind holes.

Refinement of line width and line spacing. Its main performance in the wire defects and wire surface roughness requirements are increasingly strict. General line width and line spacing of no more than 76.2μm.

The high density of pads. The density of welded joints is greater than 50 per square centimeter.

The thinning of the dielectric material thickness. It is mainly manifested in the trend of the thickness of the interlayer medium to 80μm or below, and the requirement of thickness uniformity is more and more strict, especially for the high-density plate and packaging substrate with characteristic impedance control.

3. HDI PCB with Better Electrical Properties

HDI PCB can not only make the end product design more miniaturized but also meet higher standards of electronic performance and efficiency at the same time.

The increased interconnect density of HID PCB allows for enhanced signal strength and improved PCB reliability. In addition, HDI boards have better improvements for RFI, EMI, static discharge, thermal conduction, etc. HDI also uses all-digital signal process control (DSP) technology and a number of patented technologies, with a full range of adaptable load capacity and strong short-time overload capability.

4. HDI PCB has Strict Requirements for Buried and Plug Hole

As can be seen from the above, both the volume and electrical performance of the circuit board, HDI are better than ordinary PCB. Where there are two sides to the coin, as high-end PCB manufacturing, HDI PCB manufacturing threshold, and process difficulties are much higher than ordinaryStack-up of HDI PCB:

PCBs. So more problems need to pay attention - especially buried holes plug holes.

The core pain point and difficulty of HDI manufacturing is the buried plugging hole. If the HDI buried hole plugging is not done, there will be major quality problems, including uneven board edges, uneven thickness of the dielectric, pads with pits, and other states.

• The circuit board surface is not flat, the line is not straight in the depression to cause the beach phenomenon, which will cause line gaps, broken lines, and other defects.
• The characteristic impedance will also fluctuate due to the uneven thickness of the dielectric causing signal instability.
• The unevenness of the pad makes the subsequent packaging quality bad and causes the collateral loss of components.

Comparing HDI PCBs and Traditional PCBsComparing HDI PCBs and Traditional PCBs

Stack-up of HDI PCB:

The stack-up of an HDI PCB is a critical blueprint that defines the arrangement and composition of its various layers. Here's a breakdown of the key components and how they differ from traditional PCBs:

• Core: Similar to traditional PCBs, HDI PCBs often utilize a rigid dielectric core material like FR-4. However, in some high-performance applications, advanced materials with improved thermal properties or higher electrical performance might be used.
• Signal Layers:  This is where HDI PCBs truly stand out. Compared to traditional PCBs with typically 2-4 layers, HDI PCBs boast a significantly higher layer count, often ranging from 6 to 12 or even higher. This allows for a denser routing of electrical signals within the limited space.
• Dielectric Layers: These ultra-thin insulating layers separate the signal layers and prevent electrical shorts. In HDI PCBs, the dielectric layers need to be even thinner to accommodate the higher layer count while maintaining a compact overall thickness. This presents a significant manufacturing challenge.
• Via Layers: Vias are the tiny holes that connect different signal layers, enabling electrical communication throughout the PCB. Here's where HDI PCBs truly differentiate themselves:
  • Via Types: Traditional PCBs typically use through-hole vias, which are drilled all the way through the board. HDI PCBs, on the other hand, utilize various via types to optimize space and performance:
    • Blind Vias: These vias connect internal layers without extending to the surface layers. This allows for cleaner surface routing and higher component density.
    • Buried Vias: These vias are completely encased within the core material, further maximizing surface area for routing.
  • Via Size: Due to the increased layer count and need for dense routing, HDI PCBs employ significantly smaller vias compared to traditional PCBs. This requires high-precision drilling techniques like laser drilling to achieve the necessary accuracy and maintain reliable electrical connections.

Surface Layers: Similar to traditional PCBs, HDI PCBs have top and bottom surface layers that provide connection points for electronic components through solder pads. However, due to the higher density of components often used in HDI PCBs, the surface design needs to be meticulously planned to ensure proper routing and component placement

Manufacturing Process of HDI PCBs:

The fabrication of HDI PCBs is a complex and precise process, requiring advanced technology and expertise. Here's how it differs from traditional PCBs:

• Lamination: Similar to traditional PCBs, multiple layers are laminated together using heat and pressure. However, the use of thinner dielectric layers and precise alignment of blind and buried vias in HDI PCBs demands a higher degree of control and precision during this stage.
• Drilling: Traditional PCBs utilize mechanical drilling for vias. However, the smaller via sizes in HDI PCBs necessitate the use of high-precision laser drilling to achieve the required accuracy and minimal hole size.
• Plating: The via walls in both traditional and HDI PCBs are metalized to create conductive pathways. However, HDI PCBs might utilize specialized plating techniques due to the smaller via size and potential challenges with aspect ratio (via depth vs. diameter).
• Patterning: This stage involves creating the desired circuit pattern on the surface layers. While the basic photolithography process remains similar to traditional PCBs, HDI PCBs require higher resolution photoresists and stricter control measures due to the finer line widths and smaller features.
• Surface Finishing: Both types of PCBs undergo surface finishing for protection and solderability. However, HDI PCBs might require specialized surface treatments due to the potential for exposed dielectric material from blind and buried vias.
• Electrical Testing: Similar to traditional PCBs, rigorous electrical testing is performed to ensure proper functionality and identify any defects. However, the higher density and complexity of HDI PCBs might require more advanced testing procedures to ensure all connections and signal integrity meet specifications.
• Assembly: The final stage involves soldering electronic components onto the surface pads. While the basic assembly process might be similar, the higher density of components in HDI PCBs can require more precise placement techniques and potentially specialized soldering equipment.

HDI PCB Main application areas

HDI PCBs are used in consumer electronics, telecommunications, automotive, medical devices, aerospace, industrial equipment, and IoT devices. They enable compact, high-performance electronic systems with improved reliability. HDI technology supports miniaturization, high wiring densities, and signal integrity, making it essential for modern electronics.

Conclusion

Not all board factories have the ability to do well in HDI, but NextPCB has been working on this for over 15 years. Today, NextPCB HDI has its own complete system, the entire process is not outsourced, spending a lot of money to purchase advanced equipment, and all quality acceptance standards have been used IPC2 standards, such as hole copper thickness ≧ 20μm, to ensure high reliability.