High-Density Interconnect PCBs

What is a High-Density Interconnect (HDI) PCB?

High-Density Interconnect (HDI) PCBs are a type of printed circuit board (technology) characterized by high circuit density, produced using microvias and buried vias technology. With the continuous development of technology and increasing electrical requirements for high-speed signals, circuit boards must provide impedance control with AC characteristics, high-frequency transmission capabilities, and reduced unnecessary radiation (EMI). Adopting Stripline and Microstrip structures, multi-layering has become a necessary design. To mitigate signal transmission quality issues, insulating materials with low dielectric constants and low attenuation rates are used. Meanwhile, to accommodate the miniaturization and array packaging of electronic components, circuit boards are constantly increasing in density to meet demand.

Layer Stack Structures of HDI PCBs

Single-Build-Up 6-layer (1+4+1)

Simple Single-Build-Up Printed Boards

These are the simplest boards, with no buried vias in the inner multi-layer board, completed in a single lamination. Though they are single-build-up boards, their manufacturing is quite similar to the single lamination of conventional multi-layer boards. The key difference is that subsequent processes (such as laser drilling of blind vias) are required. Since this stack structure has no buried vias, it can be fabricated by using a core for layers 2 and 3, another core for layers 4 and 5, adding dielectric layers and copper foils to the outer layers, and completing all in one lamination. This simplicity makes their cost lower than conventional single-build-up boards.

Single-Build-Up 6-layer (1+4+1)

Conventional Single-Build-Up HDI Printed Boards

These boards have a structure of (1+N+1) where N ≥ 2 and N is even. This is the mainstream design for single-build-up boards in the industry, featuring buried vias in the inner multi-layer board and requiring two laminations. In addition to blind vias, they also have buried vias. If designers can convert this type of HDI into the simple single-build-up structure mentioned in Point 1, it benefits both suppliers and customers. Many of our clients have optimized their designs from this conventional single-build-up stack to a structure similar to the simple single-build-up one based on our recommendations.

Double-Build-Up 8-layer (1+1+4+1+1)

Conventional Double-Build-Up HDI Printed Boards

These boards have a structure of (1+1+N+1+1) where N ≥ 2 and N is even. This is the mainstream design for double-build-up boards in the industry, with buried vias in the inner multi-layer board and requiring three laminations. They typically do not involve stacked via designs, making manufacturing difficulty moderate. If the buried vias in layers 3-6 can be optimized to layers 2-7 (as mentioned earlier), one lamination step can be reduced, streamlining the process and lowering costs. This is exemplified below.

Double-Build-Up 8-layer (1+1+4+1+1)

Another Conventional Double-Build-Up HDI Printed Board

Though structured as double-build-up boards (1+1+N+1+1, N ≥ 2 and N even), the buried vias here are not in layers 3-6 but in layers 2-7. This design reduces laminations from three to two. However, such boards have another manufacturing challenge: blind vias in layers 1-3, which need to be fabricated by filling vias for the inner blind vias in layers 2-3 (a via-filling process for inner blind vias in double-build-up structures). Generally, HDI boards using via-filling are more costly and difficult to manufacture than those without. Thus, for conventional double-build-up boards, it is recommended to avoid stacked via designs and convert blind vias in layers 1-3 into staggered blind vias in layers 1-2 and 2-3 (buried/blind vias). Experienced designers often use this simplified approach to reduce manufacturing costs.

Unconventional 6-layer (1+1+2+1+1)

Unconventional Double-Build-Up HDI with Cross-Layer Blind Vias

Structured as double-build-up boards (1+1+N+1+1, N ≥ 2 and N even), they feature cross-layer blind vias with significantly increased depth—blind vias in layers 1-3 are twice as deep as conventional blind vias in layers 1-2. Clients with such designs have unique requirements that prohibit converting cross-layer blind vias (1-3) into stacked blind vias (1-2 and 2-3). These cross-layer blind vias pose challenges not only in laser drilling but also in subsequent processes like PTH (Plated Through Hole) and electroplating. Typically, PCB manufacturers without sufficient technical capabilities struggle to produce such boards, as their manufacturing difficulty is significantly higher than conventional double-build-up boards. This design is not recommended unless there are special requirements.

Stacked Vias 8-layer (1+1+4+1+1)

Double-Build-Up HDI with Stacked Blind Vias

Structured as (1+1+N+1+1) where N ≥ 2 and N is even, this design is used in some double-build-up boards in the industry. The inner multi-layer board has buried vias and requires two laminations. A key feature is the stacked blind vias over buried vias in layers 2-7, increasing manufacturing difficulty. However, placing buried vias in layers 2-7 reduces one lamination step, streamlining the process and lowering costs.

Cross-Layer Vias 8-layer (1+1+4+1+1)

Double-Build-Up HDI with Cross-Layer Blind Vias

Structured as (1+1+N+1+1) where N ≥ 2 and N is even, this is a double-build-up board with moderate manufacturing difficulty in the industry. It has buried vias in layers 3-6 and requires three laminations, primarily due to cross-layer blind via designs. Manufacturers without sufficient technical capabilities struggle to produce such boards. If cross-layer blind vias in layers 1-3 are optimized into staggered blind vias (1-2 and 2-3)—not the stacked splitting method mentioned in Points 4 and 6 but staggered splitting—it would significantly reduce manufacturing costs and streamline production.

Optimization Triple-build-up & more

Optimization of Other HDI Stack Structures

Triple-build-up printed boards or PCBs with more than three build-ups can also be optimized using the design principles above. A full triple-build-up HDI board typically requires four laminations in its complete production process. By applying the design logic of single or double-build-up boards, one lamination step can be reduced, improving board yield. Many of our clients have examples: initial stack designs required four laminations, but after optimization, the PCB could be produced in three laminations while meeting the functional requirements of a triple-build-up board.

Why Use HDI PCBs?

Compared to surface-mount technology (SMT) and through-hole boards, HDI PCBs offer numerous technical advantages:

What to Consider When Designing HDI PCBs?

Like any PCB technology, HDI PCBs require specific considerations: