hdi pcb
hdi pcb

HDI PCB

High-Density Interconnect: Technology aimed at enhancing circuit performance through high-density connections.

The field of electronics is advancing at an incredible pace, with even the tiniest electronic devices packing substantial capabilities. This trend is particularly evident in devices like computers and smartphones, where sizes are shrinking, but performance is continually improving. The development of PCB plays a critical role in this, especially with the emergence of High-Density Interconnect (HDI) technology.

High-Density Interconnect is a technology that enables the placement of denser circuitry on smaller PCB areas through the use of fine traces, spacing, blind vias, buried vias, and multilayer structures. This PCB is commonly referred to as HDI PCB, characterized by smaller through-holes and pad-to-pad spacing with higher connection pad density. Thanks to these advantages, electronic devices benefit from faster signal transmission, reduced signal loss, and lower cross-talk delays. Consequently, HDI technology is rapidly growing in the PCB field, with various manufacturers actively exploring and adopting this technology.

Top HDI PCB Manufacturer – FS Tech

As mentioned earlier regarding High-Density Interconnect technology, the HDI PCB fabrication demands precise processing capabilities. Traditional CNC machining may struggle to achieve the required level of precision, resulting in roughness and other issues. For this reason, manufacturers must equip themselves with various advanced laser processing equipment. However, not all manufacturers are willing to bear the high costs associated with obtaining such equipment. Therefore, when selecting an HDI PCB manufacturer for your project, it is crucial to inspect the manufacturer’s hardware facilities.

Clearly, FS Technology is a Chinese manufacturer equipped with Excellent PCB fabrication capabilities. Founded in 2004, our company has steadily grown in size, now operating two automated smart factories capable of handling any processing detail in PCB fabrication and assembly. In manufacturing, laser drilling and cutting machines ensure the precision of PCB processing details, while in assembly, automated SMT and AI plug-in machines enable batch production. For testing, AOI and X-ray technologies deliver precise results. It’s safe to say that FS Technology is one of the most capable HDI PCB manufacturers in China.

SpecificationManufacturing Capacity
Board layers4-35 layers
Quality gradeIPC 6012 Class 2, IPC 6012 Class 3
Available materialsHigh Tg FR4 (140 °C, 150 °C, 170 °C), Special material
Board thickness0.4-6.0mm
Minimum Line Width/SpaceInner layer: Part 2 / 2mil, overall 3 / 3mil (H/H OZ base copper)
Outer layer: Part 2.5/2.5mil, overall 3/3mil (H/H OZ base copper)
Minimum aperture0.15mm-0.3mm
Solder maskGreen, Red, Yellow, Blue, White, Black, Purple, Matte Black, Matte Green
Silk screenwhite, black, yellow, blue
Surface treatmentSurface treatment
Copper thickness0.5-13 oz
Manufacturing time5-10 days

Use HDI PCB Boards in Electronic Projects

Compared to traditional circuit boards, HDI PCB boards are typically more expensive due to factors such as process complexity, special materials, and equipment requirements. Nevertheless, they find extensive applications. In the following sections, FS Technology will list the advantages of HDI PCB and offer some recommended use cases.

Benefits of HDI Technology

Advantages of HDI PCB

High-density interconnect technology is a key measure to enhance the efficiency of printed circuit boards. Ongoing improvements in copper etching technology have made it possible to combine the characteristics of multiple different PCBs within a single HDI PCB. This enables the use of HDI PCB across various applications, offering several advantages and opportunities:

Space Utilization

HDI PCB optimizes circuit board space by leveraging advanced vias such as blind vias, buried vias, and micro vias. Specialized HDI PCB factories can achieve micro via diameters smaller than a human hair, allowing for the accommodation of more components within a smaller PCB surface area.

Signal Transmission

In projects with higher signal requirements, designers need to reduce signal transmission path lengths to minimize signal loss and avoid cross delays. HDI technology employs blind vias and fine-pitch traces to achieve high-density layouts, resulting in tighter component spacing and ultimately improving signal transmission performance.

Environmental Resilience

When facing harsh environmental conditions, the wiring and connections of PCBA boards may be compromised. HDI PCB allows the construction of multilayer circuit structures with intricate internal connections and specially designed stacked micro vias, offering better resistance to vibrations, temperature fluctuations, humidity, and other environmental factors.

Cost-Effective

While HDI PCB is costlier when considering only the PCB’s cost, they contribute to overall space optimization in devices. This reduction in materials, such as casing, during final assembly not only saves on construction costs but also aligns with the trend toward miniaturization in electronic products.

Application of HDI PCB Board

In the overall PCB application market, consumer electronics account for the largest share, while the application of HDI PCB tends to lean toward high-end electronics. The chart above illustrates the application scenarios of HDI PCB in various fields, and it’s evident that it is more frequently used in applications that demand higher reliability and signal transmission.

Communications

You may have noticed the significant changes and innovations in modern communication, reflected not only in weight and size but also in functionality. This trend is an inevitable outcome. With the continuous advancement of HDI technology, circuit functionality and external conditions are more advantageous. HDI boards make devices thinner while enhancing functionality:

  • Smartwatches: These have evolved beyond just telling time to include communication and health monitoring features.
  • VR (Virtual Reality): VR glasses and headsets allow people to interact with virtual environments in physical interactive spaces.
  • Mobile Phones: Since the introduction of any-layer HDI PCB with the iPhone 4S, major smartphone manufacturers have followed suit.

Industrial Control Boards

Industrial control systems have a greater demand for high-density circuit-structured PCB. These systems consist of different parts or modules that need to work in harmony. Any error in any part can lead to system failure. HDI PCB can address various complex application environments, and their higher circuit density allows them to carry a multitude of components like sensors, actuators, and controllers, enabling precise control. It’s important to note that industrial control PCB come at a significant cost.

Automotive Electronics

With the rise of electric and new energy vehicles, there has been a sharp increase in the demand for modern automotive electronic systems. This is especially true for high-density, high-speed, and high-frequency PCB technologies. Modern vehicles have transitioned from standalone vehicles to complete communication networks. Vehicles provide real-time driving status information through satellites and a multitude of sensors, enabling smart driving. Additionally, vehicles can automatically connect with each other to facilitate information sharing.

HDI PCB Structure Introduction

To take your HDI PCB project from concept to reality, reasonable design is a must, and for this you must understand its structural characteristics!

Symmetric Structure

Printed circuit boards typically consist of inner layers and outer layers, and HDI boards are no exception. In most cases, the internal structure of a multi-layer HDI PCB is symmetrical. The inner layers act as the symmetrical axis of the HDI PCB and often contain hidden vias. These inner layers are sandwiched between the outer layers, with each inner layer containing a set of micro vias. For instance, in a basic 4-layer HDI PCB structure, the top and bottom layers make up the outer layers of the HDI circuit board, while the second and third layers constitute the inner layers of the HDI PCB.

If the structure of an HDI PCB is asymmetrical, temperature and stress on the circuit board during the lamination process may become uneven. This could lead to circuit board warping. Even after the lamination process, imbalanced stress may persist between the circuit layers, potentially resulting in bending of the internal structure of the HDI PCB. Additionally, structural asymmetry can lead to the use of more resin in areas with more copper traces during the manufacturing process, resulting in uneven board thickness of the printed circuit board.

HDI construction refers to the number of times laser drilling is employed. HDI printed circuit boards come in various structures to meet the required performance in design specifications, such as 1+N+1, 2+N+2, 3+N+3, i+N+i.

1+N+1

The structure of a 1+N+1 HDI PCB includes a high-density interconnection layer, making it suitable for BGA with a lower input and output count. It features micro vias and registration technologies capable of handling a 0.4 mm ball pitch and incorporates copper-filled vias. Applications of this type of PCB include cell phones, memory cards, and GPS devices.

The production process of 1+N+1 HDI PCB closely resembles that of multilayer PCB. The fabrication process for an N-layer HDI PCB with a 1+2+1 structure involves the following steps:

    First, the N PCB layers that will be laminated together are manufactured, followed by the creation of the two outer layers.

  1. Mechanical drilling is used to bore holes in the N innermost layers, while laser drilling is employed to drill holes in the two outermost layers.
  2. Electroplating is performed on the blind vias located within the inner layers.
  3. The two outer layers are then laminated together with the two inner layers.

2+N+2

HDI PCB Layering Methods

The structure of a 2+N+2 HDI PCB consists of two or more build-up layers with high-density interconnections. Micro vias on different layers can be staggered or stacked, increasing routing density in complex designs while maintaining a low board thickness. This makes them suitable for BGAs with smaller ball pitches and high input/output counts. Examples of applications for this structure include PDAs and game consoles.

The conventional production method for 2+N+2 stacked through HDI PCB, considering six layers, is outlined as follows:

  1. Begin the fabrication and lamination of the N (2) innermost layers of the printed circuit board for both layers 2 and 5.
  2. Employ a mechanical drilling process to bore through the inner layers, using laser drilling on layers 2 and 5.
  3. Perform electroplating on the blind vias within the inner layers, followed by laminating the innermost layers with the second and fifth layers.
  4. Electroplate the micro vias in layers 2 and 5.
  5. Manufacture the first and sixth layers, with the HDI PCB manufacturer using laser drilling to determine the locations for the micro vias and drills.
  6. Laminate layers one and six with the finished PCB layers.

ELIC

ELIC is Each Layer Interconnection, All the layers in this ELIC HDI PCB structure are high-density interconnection layers. So, it means that the conductors on any layer of the PCB can be freely interconnected using copper-filled stacked micro via structures. This produces improved electrical qualities while providing a reliable connectivity solution for highly complicated devices. CPU and GPU processors used in portable and mobile devices, memory cards, and MP3 are some of its examples.

HDI Board Vias

There are different types of PCB vias, and in HDI PCB, we specifically emphasize blind vias and buried vias. These are two advanced types of vias that require the use of more sophisticated equipment to achieve!

HDI Board Vias

Blind Via:

A blind via, also known as an interconnector or a link between layers, is a hole that extends to a certain depth and can be found on both the top and bottom surfaces of a PCB. Its primary function is to establish connections between the PCB’s external layers and one or more of its internal layers. Additionally, blind vias can serve as pathways that facilitate connections between the top and inner layers of the PCB.

Buried Via:

A buried via is a connection hole located in the inner layers of a PCB that doesn’t extend all the way to the surface. Its primary function is to establish connections between at least two of the PCB’s inner layers. Notably, the layers being connected in this scenario remain hidden from the external layers.

Utilizing buried vias in a PCB allows for expanding the board’s functionality without requiring additional space. This is achieved through a technique known as packing density, which reduces unused space while simultaneously enhancing performance.

Use

If you’re designing a High-Density Interconnector, it’s highly recommended to incorporate blind and buried vias. Here are key considerations when implementing blind and buried vias on an HDI printed circuit board:

  • Layer Positioning: The layers that can reside beneath the outer layers are typically hidden or buried. This arrangement allows for more efficient use of PCB space.
  • Reduced Signal Stub: Blind vias play a crucial role in reducing signal stubs, especially during the drilling process. This reduction ensures that there are no unwanted alterations, as signal stubs terminate at the last connected layer.
  • Space Optimization: Utilizing blind and buried vias contributes to creating additional space within the HDI PCB. By using these concealed vias, the HDI circuit board can remain thin and lightweight while expanding its available surface area.

HDI PCB Manufacturing Process

The detailed process employed by FS Technology to manufacture a straightforward six-layer HDI PCB is depicted below. In the following sections, we will provide a comprehensive explanation of critical steps, including drilling, blind plating, inner patterns, AOI of inner layers, and stack-up arrangement.

Drilling

The initial phase involves mechanically drilling holes that are buried within the layers. This process commences with the layers closest to the surface, which, in the case of a six-layer stack, are layers three and four. They are positioned between layers two and five, and then between layers one and six, in that sequence. Consequently, mechanical drilling occurs at positions 3-4, followed by buried holes at positions 2-5, and ultimately at positions 1-6. Adhering to this order is crucial; skipping layers, such as drilling holes in layers 2-5 without first drilling holes in layers 3-4, could result in significant issues.

Subsequent to this step is the drilling of any required blind holes (from the surface to an inner layer). After completing the drilling of hidden holes, blind holes are drilled from the first surface layer (layers 1-2) and the bottom layer (layers 6-5), respectively.

Blind Plating

When producing blind holes using a laser drill, it is crucial to address the phenomenon known as the “coke button.” This is another vital aspect to consider in this context.

Laser drills operate at extremely high temperatures, and their microvias have incredibly narrow apertures. As a result, coke buttons are formed and become integrated into the substrate that lines the walls of the drill hole. Due to the small size of the laser-drilled microvias, eliminating these coke buttons using conventional methods is challenging.

To tackle the issue of coke buttons, an expensive method called blind hole plating is employed. This method ensures the high quality and integrity of the circuit.

Before depositing the copper, several pre-operation activities must be completed to prevent or minimize the risk of copper oxidation.

Inner Patterns

Once the copper has been placed and electroplated to ensure adhesion, the next step is to create and etch the inner patterns. This method is similar to the one used in manufacturing a traditional FR-4 PCB. However, there are significant differences to note: the traces are considerably finer, and the microvias are noticeably smaller than through-hole vias. The distinction is evident.

AOI of Inner Layers

Following the creation of inner patterns and etching, the inner layers undergo an automated optical inspection. This inspection ensures the precise alignment of patterns and microvias on the circuit. The use of AOI at each stage of the stacking process guarantees a high-quality final product in terms of appearance.

Next in the process is the brownification step, which serves to enhance the bonding between the copper of the inner layers within the HDI stack and the substrate of subsequent layers.

Stack up Arrangement

The construction of the HDI circuit is completed by layering two and five on top of layers three and four. If any holes, typically hidden microvias, need to be drilled, this is the stage at which it is done. These holes must penetrate all three or four layers of the stack-up. The same procedure is repeated until the top layers, currently layers 2 and 5, are finished and ready. Finally, the last and outermost layers, numbered one and six, are placed on top.

Summarize

HDI is a specialized PCB manufacturing process that requires a deep understanding of the process, available materials, and various design considerations. High density interconnect PCB is employed in high-end electronic devices that demand compactness and high performance. These applications include mobile phones, touch-screen devices, laptops, digital cameras, and avionics.

It’s a positive sign that PCBA companies of all sizes are adopting this technology to better serve their customers. Among these companies, FS Technology stands out as the premier Chinese manufacturer specializing in this technology. Regardless of the project’s size, we utilize this method to produce top-quality PCB that offer high performance in a compact form at a reasonable price. If you require this technology, please don’t hesitate to contact us, and we will provide you with the most competitive HDI PCB prices on the market.

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