We live in a constantly evolving era, and like any other technology, high-density interconnects have advanced in recent years. As a leading EMS manufacturer in China, FS Technology is confident enough to provide HDI PCB manufacturing and assembly services for advanced equipment, and will demonstrate its capabilities in this area next with a high-density interconnect PCBA sample.

Overview of High Density Interconnect PCB

If you are from the technology field, you must have heard about High frequency HDI PCBA. Even if you’re unaware of it now, there is a good chance you will know about it soon because these are now used in many electronic applications. With the development of technology, the PCBA industry is switching to more advanced materials and innovative designs, such as the HDI board mentioned earlier. 

High-density interconnect, abbreviated as HDI, is a material that has been making its way into the market as a potential choice for printed circuit boards over the last several years. This new type of technology can easily help electronics manufacturers get more functions in a smaller size. HDI PCB design allows for smaller vias and capture pads, and higher connection pad density. Since the density of HDI PCBA is higher than that of traditional PCBA boards, the upgrading speed of this technology is the fastest in the PCBA processing industry. PCBA suppliers use this technology to add more electronic components on both sides of the bare board, which means that customers’ electronic products can defeat its precise opponents with faster signal transmission, less signal loss and crossover delay. If you choose FS Technology as your Chinese supplier, you will get more advantages and convenience, please quickly get your HDI PCB price list through the contact email above.

HDI PCB&PCBA samples manufactured by FS Technology

HDI PCB Manufacturing Capabilities

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 treatmentImmersion Gold, OSP, Hard Gold, Immersion Silver
Copper thickness0.5-13 oz
Manufacturing time5-10 days

Advanced HDI PCB manufacturing process

The detailed process of FS Technology to manufacture a simple six-layer HDI PCB is shown in the figure below. Next, we will explain in detail some of the most important links including: Drilling, Blind Plating, Inner Patterns, Automated Optical Inspection of Inner Layers, Stack up Arrangement.

Six-layer HDIPCB manufacturing flow chart


The first stage is to drill holes into the ground that are buried mechanically. It begins with the layers closest to the surface, which would be layers three and four for a stack with six layers. They will come between positions two and five and then one and six in the following order. As a result, the mechanical drilling at positions 3-4 will be followed by buried holes at positions 2–5 and finally at positions 1-6. Chinese HDI Fabricators must comply with this order; skipping layers, such as drilling holes in layers 2-5 without first drilling holes in layers 3-4, might create significant problems.

Following this step is drilling any blind holes that need to be done (surface to one inner layer). Following 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 blind holes are produced using a laser drill, a “coke button” phenomenon must be taken care of. This is another crucial point to take into consideration in this context.

Laser drills work at exceedingly high temperatures, and their micro vias have incredibly narrow apertures. This results in the production of coke buttons, which are then incorporated into the substrate that makes up the walls of the drill hole. Because of the laser-drilled micro via size, it is difficult to eradicate these coke buttons using traditional procedures.

An expensive method known as blind hole plating is used to tackle the problem with the coke button. This method guarantees the high quality and integrity of the circuit.

Before depositing the copper, we must complete a few pre-operation activities to avoid or limit the risk of copper oxidation.

Inner Patterns

After the copper has been placed and electroplated to ensure it will stick, the inner patterns will be created and etched. This method is comparable to the one used in manufacturing a traditional FR-4 PCB. The traces are substantially more refined, and the micro vias are noticeably smaller than through-hole vias. The difference can be seen.

Automated Optical Inspection of Inner Layers

The inner layers are subjected to an automated optical inspection at this point. This verifies that the patterns and micro vias are precisely located where they should be on the circuit. Using AOI at each successive step of the stacking process guarantees that the final product will have a high-quality appearance. The brownification process comes next, and its purpose is to strengthen the bonds between the copper of the inner layers of the HDI stack and the substrate of subsequent layers of the HDI stack.

HDI PCB Stack up Arrangement

The construction of the HDI circuit is finished when layers two and five are layered on top of layers three and four.   If holes need to be drilled (typically hidden miro vias), they are drilled at this step. These holes need to penetrate all three or four layers of the stack-up. The same procedure is repeated until the top layers, which right now layers 2 and 5, have been finished and are ready. After that, the last and outermost layers, numbers one and six, are placed on top.

HDI PCB Structure Design Guidelines

There is an inner layer and an outer layer on every HDI PCB, whether it is a FPC HDI PCB or a rigid HDI PCB. In most cases, the interior of a multilayer HDI PCB has a symmetrical structure. The inner layers are the symmetry axis of the High Density Interconnection PCB, and hidden vias often penetrate them.  The inner layers are held in place by the outer layers. They each have their own set of blind micro vias that go through them. For instance, in the HDI PCB’s outside layers, the upper and lower levels are the HDI circuit board’s outer layers, while layers 2 and 3 are the HDI PCB’s inner layers in the basic 4-layer HDI PCB layout.

If the HDI PCB structure is not symmetrical, the temperatures and stresses on the circuit board will differ during the HDI PCB lamination process. This could cause the circuit board to bend. Even after the lamination process, there is still stress between the circuit layers, and the unbalanced stress may cause the HDI PCB structure to bend.

Also, if the circuit layer isn’t symmetrical, the HDI PCB manufacturing process can use more resin where there are more copper wires. As a direct consequence, the printed circuit board has an uneven board thickness.

HDI construction refers to the number of times micro vias are drilled using a laser. HDI Printed Circuit Boards have various structures to achieve the required performance based on the design specifications: The most straightforward HDI construction is called 1+N+1, indicating that the HDI PCB’s micro vias are single vias that only need a single laser drilling. 2+N+2 indicates that the HDI PCB requires laser drilling on both the top and bottom twice in addition to the first time. 3+N+3 denotes that the HDI PCBs must undergo laser drilling three times and that the top and bottom outer layers have three PCB layers each. And this goes on. 

HDI PCB Layering Methods

Simplest HDI PCB Design: 1+N+1

The structure of 1+N+1HDI PCB has a layer of high-density interconnection. It makes it appropriate for BGAs with lower input and output counts. It has micro vias and registration technologies that can handle a 0.4 mm ball pitch. Also, it incorporates copper-filled vias. Cell phones, memory cards, and GPS are some of their applications.

The production of 1+N+1 HDI PCBs is similar to that of multilayer PCBs. The following steps are taken while fabricating an N-layer HDI PCB with a 1+2+1 structure:

  • First, the N PCB layers that will be laminated together are made, and then the two outer layers are manufactured.
  • A mechanical drilling process is used to drill the N innermost layers. Laser drilling is used to boreholes in the two outermost layers.
  • Electroplating is performed on the blind vias found in the inner layers. The two layers on the outside are laminated together with the two layers on the inside.

Moderate Complex HDI Structure: 2+N+2

This structure of 2+N+2 HDI PCB consists of two or more build-up layers of high-density interconnections. Micro vias on different layers can be staggered or stacked. This helps increase routing density in a problematic design while keeping the completed board thickness low. Thus, they are well-suited for BGA with a smaller ball pitch and high input/output counts. PDAs, game consoles, etc., are examples of this structure.

Consider six layers of HDI PCB. The conventional method of production for 2+N+2 stacked through HDI PCBs is as outlined below:

  • Fabrication and lamination of the N (2) innermost layers of the printed circuit board manufacturing begin for both layers 2 and 5.
  • A mechanical drilling process is used to drill through the inner layers. Laser drilling is performed on layers 2 and 5.
  • Electroplating is performed on the blind vias found in the inner layers. The innermost layers are laminated with the second and fifth layers.
  • Electroplating is done in the micro vias found in layers 2 and 5.
  • The first layer and the sixth layer are both manufactured. The maker of HDI-printed circuit boards uses laser drilling to determine the spots for the micro vias and drills.
  • Layers one and six are laminated with the finished PCB layers.

Most Complex HDI PCB: ELIC

This HDI structure is known as Every Layer Interconnection (ELIC). 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.

Changes in HDI Board Vias

We can break down the role of the via into two distinct categories: first, we can use it to provide an electrical connection between layers, and second, to fix or position devices. Blind holes, buried holes, and through holes are the three primary classifications used when referring to the method of creating these holes. HDI boards employ Blind and buried vias. 

HDI Board Vias

Blind Hole: 

In addition to having a certain depth, we may find it on both the top and bottom surfaces of the printed circuit board. The role of the blind via, also known as the interconnector or link between layers, is to provide this function between the layers.

The function of the blind via is to provide a connection between one of the circuit board’s outside or external layers and one or more of the board’s internal layers.

That is not all that we can say about the blind. The second use case for the blind via is when it functions as a pathway that allows connections to be made between the top and inner layers.

Buried Hole: 

A buried hole is a connection hole in the inner layer of the printed circuit board that doesn’t go all the way to the surface. This hole is referred to as a “buried hole.” 

The function of the buried via, which is to link at least two of the inner layers of the circuit board, is the most important thing it does. The layers that are going to be connected in this scenario are not going to be visible from the layers that are on the outside.

In addition, using a hidden via on a printed circuit board (PCB) allows for the expansion of the board’s functionality without requiring additional space. This is made possible by the packing density technique, which reduces empty space while simultaneously boosting performance.

Use of Blind and Buried Vias on HDI PCB

If you’re designing a High-Density Interconnector (HDI) PCB, it’s well worth including or adding blind and buried vias. On an HDI printed circuit board, some of the actions that need to be taken to employ blind and buried vias are as follows:

  • The layers that can go underneath the layers are the ones that are hidden or buried. When that happens, the circuit board will have more space.
  • It is important to note that the blind via reduces signal stubs, particularly during drilling. This is why there won’t be any alterations since the signal stubs will end at the last connected layer.
  • Because of the overt function of the buried vias, it is claimed that using blind and buried vias aids in creating extra space on the HDI PCB. The hidden vias, in this case, keep the HDI circuit board thin and light while creating additional surface area.

Why use HDI PCB for electronic projects?

The Benefits of Multilayer HDI circuit board

Advantages of HDI PCB

China HDI-printed circuit boards’ key advantage is their increased efficiency. The constant improvement of copper-etching technology has made it feasible to combine the features of numerous PCBs into a single HDI PCB. Utilizing any layer HDI PCB in a variety of different applications presents a lot of opportunities for benefit. Here FS Technology lists some benefits of applying them:

Excellent space utilization

The use of blind vias, buried vias, and micro vias, in conjunction with one another, helps reduce the necessary board space. Micro vias have a diameter that is much smaller than a human hair. As a result, a smaller amount of space is sufficient to accommodate a more significant number of PCBA components.

In terms of mechanical assembly, higher density distribution of electronic components enables thinner and lighter electronic PCBAs. Mechanical assembly manufacturers can reduce the amount of materials used to build electronic products without affecting the function of electronic products, which not only saves the construction cost of the demand side, but also conforms to the trend of miniaturization of electronic products.

Fast and accurate signal transmission

Small components are used in printed circuit boards for HDI technology. These printed circuit boards use via pad and blind via technology. This enables components to be positioned far closer to one another than was previously possible. This helps to reduce signal losses as well as the possibility of cross delays, which ultimately results in an improvement to the signal transmission.

Unyielding to the harsh environment

The bare PCB board is actually just a carrier for electronic components, and electronic products may be in some extreme environments when applicable, which may have a negative impact on the wiring and connection of the PCBA. The multilayer high-density interconnect boards can provide reliable connection of stacked vias. This ensures their reliability even in the harshest and most challenging conditions.

More economical

Compared to standard printed circuit boards (PCBs), this PCB interconnected multi board system shave fewer required layers, smaller board widths, and fewer required boards, which results in total cost savings.

Common Applications of High Frequency HDI PCBA

With the change of human preference for electronic products, high-density interconnection technology is widely used in different high-frequency devices to obtain better signal transmission capabilities. Next, we will find these HDI high-frequency PCBAs from different products.

Industrial HDI Control Panel

Industrial control systems usually require more precise signal transmission capabilities, and the use of HDI high-frequency PCBA in these control systems has unique advantages. These control systems have different parts that work together to precisely control a machine or process—for example, sensors, actuators, and controllers. 8 layers HDI PCB are more reliable than standard PCBs. This means that maintenance and repair costs will go down over time.

High-end digital equipment

You find a significant change in digital devices and gadgets, not just in weight but also in size. In fact, more and more digital electronic devices are gradually being eliminated in the development of the electronics industry, and the use of PCB high-density technology is the promoter of this transformation. HDI-printed circuit boards have made it possible to make devices thinner and more portable and to use new technologies. Examples of digital devices that use HDI PCBs are:

  • Smartwatch with information processing capability and accurate body function detection.
  • A VR headset that enables people to interact with a virtual environment in a physical interaction space.
  • Since iPhone 4S uses Anylayer HDI, it has led the world to use HDI boards in high-end smartphones, including Huawei P30 series, Mate30 series, and Redmi K30 series.

Self-driving car high frequency HDI board

The main goal of New Energy Vehicle makers has been to make cars run better by adding more electronics. In fact, self-driving new energy vehicles will face many problems in practical application, such as: electric drive cannot provide kinetic energy for overweight vehicles, overheated fuselage damages fragile electronic components, energy overload, signal transmission speed and accuracy, etc. However the smaller high frequency HDI PCBA makes this possible.


The HDI PCB Manufacturing Process is a special kind of PCB manufacturing that requires a deeper understanding of the PCB Manufacturing Process, available materials, and many design considerations. We use high-density interconnect printed circuit boards (HDI PCBs) in high-end electronic devices that demand less space and high performance. Some applications include mobile phones, touch-screen gadgets, laptops, digital cameras, and avionics.

It’s a good sign that PCBA companies of all sizes are using this technology to better serve their customers, but FS Technology is definitely the best Chinese manufacturer with this technology. Considering the job size, We use this method to make PCBs of the highest quality that demands less space and high performance at a fair price.  If you need this technology, please contact us and we will serve you with the lowest HDI PCB price in the market.

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