UHDI or Ultra HDI PCB? How Does Ultra High Density Interconnect Technology Work?

Ultra High Density Interconnect (UHDI) PCB is a printed circuit board developed further on the basis of high-density interconnect (HDI) technology. It further increases PCB density, allowing components to be placed closer together and thereby reducing overall board size. UHDI pushes the boundaries of PCB and semiconductor device manufacturing capabilities, representing an important advancement in miniaturization and integration, enabling electronic components and systems to achieve extremely high functional integration within a smaller footprint.

UHDI technology can achieve finer traces and smaller vias/microvias while significantly increasing interconnect density, thereby delivering higher signal integrity and lower PCB power consumption. UHDI line widths and spacing are typically less than 1 mil (0.001 inch), which also makes it necessary to shift the unit of measurement from mils to microns. A 1 mil trace equals 25 microns; therefore, UHDI can refer to traces and spacing on a printed circuit board of less than 25 microns.

According to the definition established by an IPC working group, UHDI PCBs generally have the following characteristics:

· Conductor/trace width and isolation distance below 50 µm;

· Dielectric thickness below 50 µm;

· Microvia diameter below 75 µm;

· Product characteristics exceeding the current IPC 2226 Class C standard.

UHDI PCB Design

UHDI PCBs can be rigid or flexible, and typically use multilayer structures to meet high-density interconnect requirements. As electronic products continue to miniaturize, printed circuit boards are shrinking not only in the X-axis direction but also in the Y-axis direction. Although multilayering is essential for maintaining signal integrity and reducing electromagnetic interference in UHDI PCBs, PCB designers can use fine-line routing to consolidate traces into fewer layers, thereby reducing the number of HDI buildup layers. Reducing HDI buildup layers can significantly lower PCB manufacturing costs and further offset some of the additional costs associated with acquiring ultra high density interconnect manufacturing capabilities.

High-performance substrate materials, such as Ajinomoto Build-up Film (ABF), BT epoxy resin-based materials, thin liquid crystal polymers (such as UltraLam), and resin-coated copper films (such as metallized polyimide, pure polyimide, and cast polyimide), can serve as alternatives to laser-drillable materials and thin reinforced FR4. These materials can be used in combination to build ultra high density interconnect structures. One typical combination is: using BT epoxy resin-based copper-clad laminate as the core board, employing conventional buried vias, and using ABF as the outer buildup layer to support fine-line routing. This buildup approach has been used in organic substrates in BGA packages, and the same method can also be applied to ultra high density interconnect PCBs.

Challenges in UHDI PCB Manufacturing

Every major technological advancement brings manufacturing challenges. UHDI is not only a major change but also a leap forward in technology. It represents a shift in the fundamental method of printed circuit board manufacturing, namely from traditional subtractive processes to additive processes. At the same time, it requires not only new manufacturing methods but also new manufacturing equipment, chemicals, materials, and inspection capabilities.

In the past, UHDI PCB manufacturing capabilities were usually cost-effective only at very high volumes, and therefore were mainly used in some high-computing consumer products and IC production with higher I/O counts. Today, as global access to these advanced capabilities continues to expand, more designers can use them at lower volumes and even in prototyping stages. This also means that many advanced components originally used in high-volume consumer devices can now be used at lower volumes. As a result, UHDI capabilities are gradually becoming accessible to low-volume manufacturers.

PCB manufacturers producing ultra high density interconnect boards need to meet stricter requirements in terms of equipment and manufacturing environment. To enable a factory to manufacture ultra high density interconnect boards, extensive upgrades are required, along with investment in equipment and processes during the manufacturing stage. The upgrades required to manufacture ultra high density interconnect circuit boards include:

· State-of-the-art laser direct imaging (LDI) equipment;

· Extremely clean production environments;

· More comprehensive and stricter testing;

· The latest automated optical inspection (AOI) equipment;

· The latest copper plating equipment and chemicals;

· New processes, such as mSAP (modified semi-additive process);

· Newer, cleaner, and more uniform materials.

Advantages of UHDI PCBs

UHDI PCBs offer significant advantages, especially in reducing the weight and size of electronic devices while maintaining high performance. Their ability to accommodate more components within a compact space increases component density, making them ideal for advanced electronic applications. In addition, UHDI PCBs improve electrical performance by shortening the distance between components and electrical paths, thereby reducing signal loss, parasitic capacitance, and inductance. This helps achieve better signal integrity and faster signal transmission. At the same time, their multilayer structures and complex routing capabilities provide greater design flexibility, making it possible to develop more innovative and more complex electronic products.

Applications of UHDI PCBs

UHDI is transforming numerous industries through "miniaturization without compromising performance." Major application areas include:

· Consumer electronics: Smartphones, tablets, and wearable devices achieve high performance in compact designs through UHDI.

· Automotive electronics: Advanced driver assistance systems (ADAS), electric vehicle (EV) power electronics, and infotainment systems use UHDI to improve reliability and efficiency.

· Medical devices: UHDI enables highly compact and precise medical electronic products, such as imaging systems and implantable devices.

· Aerospace and defense: UHDI technology enhances performance and reliability in mission-critical applications such as radar systems, avionics, and satellite communications.