Detailed Explanation of Common HDI PCB Stack-up Structures

Common Layering Structures for HDI PCB Boards

HDI (High-Density Interconnect) PCBs feature micro vias, fine trace widths/spacing, and stacked layers, enabling dense interconnectivity and compact design. These characteristics make HDI PCBs suitable for high-performance, space-constrained applications.

Common Layering Structures for HDI PCBs:

1. 2+N+2 Structure: 2 internal layers, N external layers (used for 6-layer boards).
2. 3+N+3 Structure: 3 internal layers, N external layers (used for 8-layer boards).
3. 4+N+4 Structure: 4 internal layers, N external layers (used for 10-layer and higher boards).
4. 5+N+5 Structure: 5 internal layers, N external layers (used for 12-layer and higher boards).

Blind Vias: Blind vias connect inner layers to outer layers, reducing PCB thickness.

Buried Vias: Buried vias create connections between inner layers, optimizing layer usage.

Typically, blind and buried vias range from 0.075 to 0.2 mm in diameter and are created using methods such as laser drilling, plasma etching, and photosensitive etching, with laser drilling being the most common.

HDI PCBs, with their high-density components and interconnections, are ideal for complex designs like 5G, IoT, and automotive applications. Their stack-up design is customized based on the specific requirements of each project.

Simple First-order Laminated Printed Circuit Board (First-order Laminated 6-layer Board, Stacking Structure is (1+4+1))

This type of HDI PCB stackup, specifically a first-order laminated PCB, is the simplest, where the inner multilayer board has no buried holes and is completed by pressing once. Although it is a first-order laminated 6-layer board, its manufacturing process is very similar to that of a conventional multilayer PCB design that is laminated once. The key difference is that it requires laser drilling of blind vias and multiple processes.

Since this HDI PCB stackup structure has no buried holes, the 2nd and 3rd layers can be made into a core PCB in the production, and the 4th and 5th layers are used as another core board. The outer layer is added with a dielectric layer and copper foil, and the middle is added with a dielectric layer and then pressed once, which is very simple and has a lower cost compared to conventional first-order laminated boards.

Conventional First-order Laminated HDI Printed Circuit Board (First-order Laminated HDI 6-layer Board, Stacked Structure is (1+4+1))

The structure of this type of HDI PCB stackup is (1+N+1), (N≥2, N is even). This structure is the mainstream design of the industry’s first-order laminated boards at present. The inner multilayer board has buried vias and needs to be pressed twice to complete. In addition to blind holes, this type of single-layer laminated board also has buried holes.

If the designer can convert this type of HDI PCB into the above-mentioned first type of simple first-order laminated PCB, it will be beneficial to both the supply and demand sides. After our suggestion, many of our customers have chosen to change the stacking structure of conventional first-order laminated HDI boards to a simple first-order laminated board similar to the first type.

Conventional Second-order Laminated HDI Printed Circuit Board (Second-order Laminated HDI 8-layer Board, Stacking Structure is (1+1+4+1+1))

The structure of this type of HDI PCB stackup is (1+1+N+1+1), (N≥2, N is even). This structure is the mainstream design of the industry’s second-order laminated HDI boards at present. The inner multilayer board has buried vias and needs to be pressed three times to complete. The main reason for this is the lack of a stacked hole design, making the manufacturing difficulty moderate.

However, as mentioned above, if the buried vias of layers (3-6) are optimized to the buried vias of layers (2-7), one pressing can be reduced. This process optimization can lead to cost reduction while maintaining the structural integrity of the HDI PCB. This type is like the example below.

Another Conventional Second-order Laminated HDI Printed Circuit Board (Second-order Laminated HDI 8-layer Board, Stacking Structure is (1+1+4+1+1))

The structure of this type of HDI PCB stackup is (1+1+N+1+1), (N≥2, N is even). Although it is a second-order laminated HDI board structure, since the position of the buried vias is not between the (3-6) layers but between the (2-7) layers, such a design can also reduce the number of presses by one, making the second-order laminated HDI PCB, which typically requires a 3-pressing process, optimized to a 2-pressing process.

However, this type of board presents another difficulty in HDI PCB manufacturing. There are blind vias in the (1-3) layers, which are split into (1-2) and (2-3) layer blind vias for manufacturing. It is necessary to use via filling to make the inner blind vias of the (2-3) layers. That is, the inner blind vias of the double-layer lamination are created through via filling. Typically, HDI PCBs with via filling are more costly and the manufacturing difficulty is also significantly greater.

Therefore, for conventional second-order laminated HDI boards, it is recommended that the stacking design should avoid the use of stacked holes and try to convert (1-3) blind vias into staggered (1-2) blind vias and (2-3) buried (blind) vias. Experienced HDI PCB designers can adopt this kind of avoidance and simplification design or optimization to reduce the manufacturing cost of their products.

Another Unconventional Second-order Laminated HDI Printed Circuit Board (Second-order Laminated HDI 6-layer Board, Stacking Structure is (1+1+2+1+1))

The structure of this type of HDI PCB stackup is (1+1+N+1+1), (N≥2, N is even). Although it is a second-order laminated board structure, there are also cross-layer blind holes, and the depth capability of blind holes is significantly increased. The depth of the (1-3) layer blind holes is doubled compared to the conventional (1-2) layer blind holes. Customers with this design have their own unique requirements and do not allow the (1-3) cross-layer blind holes to be made into stacked blind holes (1-2) (2-3) blind holes.

In addition to the difficulty of laser drilling, the subsequent copper sinking (PTH) and electroplating are also difficult. Generally, PCB manufacturers without a certain level of technical level find it difficult to manufacture such boards. The manufacturing difficulty is significantly higher than that of conventional second-order laminated boards. This design is not recommended unless there are special requirements.

Second-order Laminated HDI with Blind Hole Stacking Design, Buried Holes (2-7) Layers Above the Stacked Blind Holes. (Second-order Laminated HDI 8-layer Board, Stacking Structure is (1+1+4+1+1))

The structure of this type of HDI PCB stackup is (1+1+N+1+1), (N≥2, N is even). This structure is currently used in some second-order laminated boards in the industry. The inner multilayer board has buried holes and needs to be pressed twice to complete. The main reason is that there is a stacked hole design, which replaces the above-mentioned 5th point of the cross-layer blind hole design.

The main feature of this design is that the blind holes need to be stacked above the (2-7) buried holes, which increases the manufacturing difficulty. The buried hole design in the (2-7) layer can reduce one lamination, optimize the process, and achieve the effect of reducing cost.

Second-order Laminated HDI with Cross-layer Blind Hole Design (Second-order Laminated HDI 8-layer Board, Stacking Structure is (1+1+4+1+1))

The structure of this type of HDI PCB stackup is (1+1+N+1+1), (N≥2, N is even). This structure is currently a second-order laminated board with a certain degree of difficulty in manufacturing in the industry. With such a design, the inner multilayer board has buried holes in the (3-6) layers and needs to be pressed three times to complete. The main reason is that there is a cross-layer blind hole design, which has a high manufacturing difficulty.

It is difficult for HDI PCB manufacturers without certain technical capabilities to manufacture such second-order laminated boards. If this cross-layer blind hole (1-3) layer is optimized and split into (1-2) and (2-3) blind holes, this method of splitting blind holes is not the stacking method mentioned in the 4th and 6th points above. Instead, it is a staggered blind hole splitting method, which will greatly reduce the manufacturing cost and optimize the production process.

Optimization of Other Stacking Structures of HDI Boards

Similar optimization principles can be applied to third-order lamination and higher HDI PCB stackup structures to avoid unnecessary lamination cycles. Shifting vias to eliminate stacked vias reduces fabrication steps and improves yields in the HDI PCB manufacturing process.

It’s important to note that while HDI PCB stackups offer numerous advantages, they also require specialized HDI PCB design guidelines and manufacturing expertise. Designers must carefully consider factors like signal integrity, thermal management, and manufacturability to fully harness the benefits of HDI technology.

Additionally, working closely with experienced PCB manufacturers and reliable HDI PCB fabrication services is essential to ensure successful HDI PCB production.

The common HDI stack up as follows

Understanding different HDI PCB stackup structures provides designers with more flexibility in terms of layer allocation, routing options, and component placement. This allows for efficient utilization of available space and optimization of the PCB layout.

Why Choose Highleap Electronic for HDI PCB Production

Highleap Electronic stands out as a premier choice for HDI PCB production due to its advanced manufacturing capabilities and extensive expertise in high-density interconnect technology. Our state-of-the-art facilities are equipped with the latest machinery for precision laser drilling, plasma etching, and advanced photo-sensitive techniques, ensuring that every HDI PCB meets the highest standards of quality and performance.

We employ a team of highly skilled engineers who specialize in optimizing stack-up designs to enhance signal integrity, power delivery, and thermal management, making sure your HDI PCBs are both reliable and efficient.

Additionally, Highleap Electronic is committed to providing exceptional customer service and support throughout the entire production process. From initial design consultation to final product delivery, we work closely with our clients to understand their specific requirements and deliver customized solutions that meet their unique needs.

Our dedication to continuous improvement and innovation ensures that we stay ahead of industry trends and technological advancements, offering you cutting-edge HDI PCB solutions that drive your projects forward with confidence and success.

HDI PCB Stackup Professional FAQ

1. What are the key considerations for optimizing signal integrity in HDI PCB stackups?

To optimize signal integrity in HDI PCB stackups, designers must focus on minimizing crosstalk, managing trace impedance, and ensuring proper signal routing. Utilizing controlled impedance routing, differential pair routing, and avoiding unnecessary via transitions can significantly improve signal quality.

2. How does the choice of materials impact the performance of HDI PCBs?

The choice of materials for HDI PCBs, including the type of dielectric and copper foil, directly affects the board’s electrical performance, thermal management, and mechanical stability. High-quality, low-loss dielectric materials can enhance signal speed and reduce power loss, while high-conductivity copper ensures efficient power delivery.

3. What are the advantages of using microvias in HDI PCB designs?

Microvias in HDI PCB designs allow for higher component density and improved electrical performance by enabling shorter signal paths and reducing parasitic inductance. They also facilitate multi-layer interconnections, enhancing the board’s overall reliability and signal integrity.

4. How can designers effectively manage thermal issues in HDI PCBs?

Effective thermal management in HDI PCBs can be achieved through the use of thermal vias, heat sinks, and proper layer stack-up designs. Incorporating materials with high thermal conductivity and ensuring adequate airflow in the final product can also help dissipate heat and maintain optimal operating temperatures.

5. What are the common challenges in manufacturing HDI PCBs, and how can they be addressed?

Common challenges in manufacturing HDI PCBs include ensuring precise drilling of microvias, maintaining layer alignment, and achieving consistent lamination quality. These challenges can be addressed by utilizing advanced laser drilling techniques, stringent quality control measures, and working with experienced PCB manufacturers who have specialized expertise in HDI technology.