Detailed Explanation of Common HDI PCB Stack-up Structures

Layering Structures for HDI PCB Boards

Detailed Explanation of Common Layering Structures for HDI PCB Boards

HDI boards, which stands for High-Density Interconnect, refer to high-density interconnect boards with non-mechanical drilling, micro blind vias with diameters below 6 mils, inner and outer layer trace widths/spacing below 4 mils, and pad diameters not exceeding 0.35 mm. The manufacturing method for multilayer boards with such characteristics is called HDI (High-Density Interconnect) PCB.

The layering structures commonly used for HDI PCBs typically include the following:

  1. 2+N+2 Structure: This structure consists of 2 internal layers, N external layers, along with two signal layers and a ground layer. This structure is typically used for four-layer and six-layer boards.
  2. 3+N+3 Structure: This structure consists of 3 internal layers, N external layers, along with three signal layers and a ground layer. This structure is typically used for eight-layer boards.
  3. 4+N+4 Structure: This structure consists of 4 internal layers, N external layers, along with four signal layers and a ground layer. This structure is typically used for ten-layer boards or higher.
  4. 5+N+5 Structure: This structure consists of 5 internal layers, N external layers, along with five signal layers and a ground layer. This structure is typically used for twelve-layer boards or higher.

Blind Vias: Abbreviated as Blind vias, these enable connections between inner layers and outer layers.

Buried Vias: Abbreviated as Buried vias, these enable connections between inner layers.

Most of the buried blind vias are small holes with diameters ranging from 0.075 to 0.2 mm. The methods for creating buried blind vias include laser drilling, plasma etching, and photosensitive etching. Laser drilling is commonly used, with variants such as CO2 and YAG ultraviolet (UV) laser machines.

HDI PCBs are designed to have high-density components and interconnections, which often involve multiple layers and various structures to accommodate complex electronic devices. The specific stack-up of an HDI PCB can vary depending on the application and design requirements, now, let’s discuss the various layering structures in more detail:

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

This type of board is the simplest, that is, the inner multilayer board has no buried holes and is completed by pressing once. Although it is a first-order laminated board, its manufacturing is very similar to that of a conventional multilayer board that is laminated once. It is just that it is different from a multilayer board in that it requires laser drilling of blind holes and multiple processes. Since this stacking structure has no buried holes, the 2nd and 3rd layers can be made into a core board 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 than conventional first-order laminated boards.

stacking structure is 1+4+1

stacking structure is 1+4+1

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 board 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 holes 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 into the above-mentioned first type of simple first-order laminated board, 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.

first-order laminated HDI 6-layer board, stacked structure is 1+4+1

first-order laminated HDI 6-layer board, stacked structure is 1+4+1

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 board is (1+1+N+1+1), (N≥2, N is even). This structure is the mainstream design of the industry’s second-order lamination at present. The inner multilayer board has buried holes and needs to be pressed three times to complete. The main reason is that there is no stacked hole design, the manufacturing difficulty is normal, and if, as mentioned above, the buried holes of (3-6) layers are optimized to the buried holes of (2-7) layers, one pressing can be reduced, and the process can be optimized to achieve the effect of reducing cost. This type is like the example below.

second order laminated HDI 8-layer board, stacking structure is 1+1+4+1+1

HDI 8-layer board, stacking structure is 1+1+4+1+1

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 board is (1+1+N+1+1), (N≥2, N is even). Although it is a second-order laminated board structure, since the position of the buried hole 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 board, which requires a 3-pressing process, optimized to a 2-pressing process. However, this type of board has another difficulty in manufacturing. There are (1-3) layer blind holes, which are split into (1-2) layer and (2-3) layer blind holes for manufacturing. It is necessary to use filling holes to make the inner blind holes of (2-3) layers. That is, the inner blind holes of the double-layer lamination are made by filling holes. Usually, the cost of HDI with filling holes is higher than that without filling holes, and the difficulty is also significantly greater. Therefore, for conventional second-order laminated boards, it is recommended that the stacking design should try to avoid the use of stacked holes and try to convert (1-3) blind holes into staggered (1-2) blind holes and (2-3) buried (blind) holes. Some experienced designers can adopt this kind of avoidance and simplification design or optimization to reduce the manufacturing cost of their products.

HDI 8-layer board stacking structure is 1+1+4+1+1

HDI 8-layer board stacking structure is 1+1+4+1+1

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 board 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 board 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.

HDI 8-layer board, stacking structure is 1+1+4+1+1

HDI 8-layer board stacking structure is 1+1+4+1+1

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 board 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.

Second-order laminated HDI 8-layer board stacking structure is 1+1+4+1+1

HDI 8-layer board stacking structure is 1+1+4+1+1

Optimization of Other Stacking Structures of HDI Boards

Similar optimization principles can be applied to third-order lamination and higher HDI boards to avoid unnecessary lamination cycles. Shifting vias to eliminate stacked vias reduces fabrication steps and improves yields. It’s important to note that while HDI PCB stack-ups offer numerous advantages, they also require specialized design 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 is essential to ensure successful HDI PCB production.

The common HDI stack up as follows

Understanding different HDI PCB stack-up 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.

The common HDI stack up

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 Stack-up Professional FAQ

1. What are the key considerations for optimizing signal integrity in HDI PCB stack-ups?

To optimize signal integrity in HDI PCB stack-ups, 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.

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