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High Layer Count Circuit (HLC PCB): All you Need to Know

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Intricate circuit configurations, high-speed data transmission, and reliable operation are represented by high layer count (HLC) printed circuit boards (PCBs). Unlike the majority of the market standard 2-4 layer PCB, the HLC PCB can have 2-10 layers, enabling highly complex designs and high-density applications, especially in telecommunications, aerospace, medical, and automotive applications.
 
HLC PCB boards happen to be an important foundation under emerging technologies of 5G, Internet of Things (IoT), AI, and automobiles. So, learn about its design, material selection, manufacturing, and performance benefits with ELEPCB!

Understanding HLC Circuits

High Layer Count Circuit, is defined by the interconnections of conductive pathways within the high-layered structure of the PCB. These pathways create electrical paths that enable the interconnection of various parts across the circuit board. These copper pathways are interconnected through small plated holes called vias. The vias establish electrical routes between the different layers of the PCB, forming separate transmission paths.
 
The number of layers in a PCB and trace distribution among them define the feasibility of complex routing for signal transmission and power distribution. An HLC circuit is specially designed with dedicated planes for signal routing, power distribution, and grounding that may reduce signal interference and electromagnetic noise and optimize current flow.

Comparison of Multilayer PCB, HLC PCB and HDI PCB

The Development of Multilayer PCB, HLC PCB and HDI PCB

During the early period in developing electronic equipment, as circuit functions were becoming complicated, such electronic products like radios and television could not meet the need if internal circuitries were only accomplished with single layer PCBs. Thereby, for satisfying more complex requirements, the multilayer PCB came into being, which consists of three or more layers of circuitry and insulation stacked alternately. The complexity in circuit connection offers a premise to the subsequent establishment of PCB technology, furthering its development.
 
Then, with the advent of electronic equipment for miniaturization, highly integrated development, intelligent phones, small medical equipment, etc., large quantities of components in a very small board area must be integrated, with high-density circuit connections. It was within this context that HLC PCBs came into being. They can meet the high-density circuit connections and component integrations in limited board areas. The requirements for wiring accuracy and interlayer connections are very stringent.
 
After that, with the increasing function of an electronic device and reducing its size, the development of tablet PCs and wearable devices promoted the pursuit of high line density in HLC PCB. Since traditional technology can no longer meet the requirement of finer line layout and higher-density interconnection, such as the technologies of micro via hole and blind buried via hole, a new HDI PCB emerged. It is with the help of these technologies that the achievement of high-density distribution of the line can achieve a high-density interconnection of PCB technology, which is called HDI PCB.
Type
StructureManufacturing
HLC PCB

Line Layer: Very fine wiring with very small line widths and spacing, e.g. up to up to 0.05mm in some high-end applications.

Insulation: High performance insulation materials are used to ensure electrical isolation at high line densities.

Through-hole: Extensive use of microvias, blind holes and buried holes, with small through-hole diameters, e.g. microvias can be under 0.1mm in diameter, to achieve precise connections between multiple layers.

1. High-precision photolithography is used for line etching to ensure very small line widths and spacing.

2. Advanced lamination processes to ensure the integrity of the multilayer structure at high line densities.

3. Rigorous surface treatment processes, such as chemical nickel-gold plating, are required to meet the electrical performance requirements of high-density lines.

HDI PCB

Line layer: through the laser direct imaging and other advanced technologies to produce fine lines, line width and spacing can be achieved very small, such as part of the product line width up to 0.075mm.

Insulation layer: with special interlayer dielectric materials to adapt to the blind buried hole processing and signal transmission requirements.

Through-hole: Blind and buried holes are important features of its structure, the depth of the blind holes can be precisely controlled, for example, in some designs the depth of the blind holes can be controlled between 0.1 – 0.3mm.

1. Laser drilling technology is the key to manufacturing blind and buried holes with high drilling accuracy.

2. Thin inner layer material and fine inner layer graphic production process are adopted.

3. Special plating process to ensure the good conductivity of blind and buried holes.

Multilayer PCB

Line layer: The wiring is relatively thick and the line width may be around 0.1 – 0.3mm depending on the specific design.

Insulation layer: common insulation material used to separate the line layer.

Through-hole: Mainly through-hole, relatively large diameter, usually around 0.3 – 0.6mm.

1. The traditional etching process for making circuits requires relatively low precision.

2. Ordinary lamination process to press multiple layers together.

3. Through-hole plating process is relatively simple.

AspectHLC PCBMultilayer PCB
Layer
Typically 2-10 layers
Possibly more than 10 layers or even more
ApplicationMedical, Marine, Aerospace, Telecom, and Automotive.
Widely used in computer, communication,
medical and military fields
CostHigherLower
FEATUREHLC PCBHDI PCB
Layer Count
Typically 2-10 layers
Layers are between 4-16
FocusPower Boards, Motherboard PCB, and other complex circuits.Small PCB, with focus on vias.
ApplicationsMedical, Marine, Aerospace, Telecom, and Automotive.
Electronics, smart phones, digital watch.
CostHigherLower, but higher than Multilayer PCB
Signal Integrity
Good Performance
Perform well, but with limited function

Materials Used in HLC PCBs

Below is a general table that describes the advantages of different PCB materials and their respective areas of application.
 
If you want to know more about PCB materials in order to choose the right one for HLC PCB manufacturing, please check out this blog from ELEPCB!
PCB Materials and Features
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ELEPCB
MaterialDescriptionAdvantagesApplications
FR4Common substrate made of fiberglass-reinforced epoxy.Cost-effective, strong, reliable.General-purpose PCBs
PolyimideHigh-performance, heat-resistant substrate.Excellent for high-temperature uses.Heat-intensive devices
CeramicAdvanced material with superior thermal properties.High conductivity, reliable in extremes.Aerospace, military, automotive
Copper FoilConductive layer, thickness varies by power needs.Handles high power without overheating.Power electronics
PrepregResin-impregnated fiberglass for layer bonding.Structural integrity in multilayers.Multilayer PCBs
Core MaterialsFiberglass and copper laminate forming the PCB body.Strong, good insulation.All PCB types

Manufacturing Processes for HLC PCBs

Step 1:
Stack-up Design

Step 2:
Lamination

Step 3:
Drilling

The step starts with a stack-up design, where the engineers plan the position of layers ensuring signal integrity, power distribution, and thermal management. 

Then the lamination process follows this design step. In this phase, the stacked layers are attached together using high pressure and temperatures. 
This will be the initial stage that causes problems with PCB functionality if alignment or defects appear.
Subsequent to lamination, drilling is performed to create vias, which are plated to provide electrical connections between the layers. 

Step 4:
Etching

Step 5:
Surface Treatment

Step 6: Component Placement, Soldering & Testing

The etching process then occurs, where all excess copper is eliminated, leaving behind only the traces, which will make up the circuit paths.

Before component placement and soldering, surface treatment is applied. Options include electroless nickel/immersion gold (ENIG), hot-air solder leveling (HASL), or organic solderability preservative (OSP). 
Component placement and soldering are followed by multiple tests, such as AOI, electrical, X-ray, Burn-in and environmental, to ensure PCB quality.

Applications of HLC PCB

HLC PCBs are implemented in various industries with unique demands from their boards.
  • In the field of telecommunications, HLC PCBs are implemented in data transmission systems, networking equipment, and 5G infrastructure.
  • Aerospace utilizes HLC PCBs for satellites, avionics, and other space technologies, which must perform well under extreme conditions.
  • HLC PCBs for medical devices appear in equipment that includes MRI machines and pacemakers. This is due to the high-precision and reliable nature of their application.
  • For automotive electronics, they are employed in advanced driver assistance systems, electric vehicle control systems, etc.

Case Study:
How HLC PCB takes part in Smartphone Motherboards?

For example, high-end smartphones such as those in the Apple iPhone series and the Samsung Galaxy series…

Application of HLC PCB
in Smart Phones

The motherboard of the Apple iPhone 14 Pro uses HLC PCB technology, which is ultra-high line density with over 500 lines in every square centimeter. Integration of many high-performance components, such as an A16 processor, high-capacity memory, a high-resolution camera module, and a 5G communication module, are supported on the very small motherboard space-about 80-100 square millimeters. To achieve such high-density integration, it needs to apply HLC PCB line width to at least a minimum of 0.03mm, the diameter of the aperture less than 0.08mm. Apply advanced inter-layer connection technology to guarantee high-speed and stable signal transmission.

Application of
HDI PCB
in Smart Phones

Some models of the Samsung Galaxy S23 series use HDI PCB technology for the motherboard, whose line width can be as small as about 0.05mm to integrate more components into a motherboard area of approximately 90 square millimeters. 

The technology uses blind and buried via holes that enable shorter signal transmission paths between the processor and other major components. 

For example, it reduces signal transmission delay from the processor to a memory module by up to 30 percent compared to conventional multilayer PCBs, hence improving the overall performance of the phone. 

Application of Multilayer PCB
in Smart Phones

Multilayer PCBs have been employed even in a variety of low and middle-class smartphones. Suppose several entries using multilayer motherboards designed by Redmi have motherboard areas of around 120-150 square millimeters. It has line widths of around 0.15 mm with the diameter of the holes at around 0.4 millimeters. With a relatively low line density, it mainly integrates a low-end processor, a smaller capacity memory, and a basic communication module, etc., which can meet the basic functional requirements of an entry-level cell phone but has a certain gap in performance and functional integration with high-end mobile phones adopting HLC PCBs or HDI PCBs.

Conclusion

HLC PCB has become part of modern electronics for intricate, high-performance designs. Increasingly, demands are being placed on the PCB towards the HLC-compact yet powerful devices that are driving the need for size-related solutions, maximum performance, and affordable material.
Wanna know more about HLC PCBs? Contact ELEPCB, leave a comment below or just gert a quote!
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Wenxiao He
I am an experienced engineer specialized in the electronics field with valuable sales expertise and a determined mindset. If we can be of use to one another rest assured that there is no other more professional option on the market than myself.
About Benjamin

Benjamin is the general manager of ELE PCB, a leading PCB design and manufacturing company based in China. He has over 10 years of experience in the PCB industry, and has been involved in various projects.

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