HDI PCB

China high-end HDI pcb manufacturer QFPCB

As one of China’s leading Printed Circuit Board (PCB) manufacturers, QFPCB has significant expertise and a strong market position in the field of High-Density Interconnect (HDI) technology. QFPCB’s HDI PCB factory serves top-tier global clients, including those utilizing advanced HDI technology extensively in server solutions. QFPCB is capable of producing both HDI PCBs and HDI rigid-flex boards and possesses advanced HDI processes, including Any Layer HDI Interconnection technology. These technologies are primarily applied in mid-to-high-end products across downstream industries such as telecommunications, data centers, industrial control and medical, and automotive electronics.

What is a HDI PCB?

HDI (High-Density Interconnect) is a type of printed circuit board (PCB) manufactured using higher-density routing technology. Compared to traditional PCBs, HDI boards achieve miniaturization, lightweight design, and higher routing density through smaller via sizes, finer line width/spacing, and more layers of blind and buried vias. This enables efficient space utilization and high-performance signal transmission, meeting the demands of electronic devices, especially mobile devices.

Different Types of HDI Circuit Boards

As electronic devices continue to evolve toward miniaturization, high performance, and reliability, High-Density Interconnect (HDI) PCB technology has become essential to meeting these demands. Depending on the application scenarios and technical requirements, HDI PCBs can be categorized into Rigid HDI PCB, Flexible HDI PCB, and Rigid-Flex HDI PCB.

1. Rigid HDI PCB

Rigid HDI PCBs use rigid materials as the substrate, offering high strength, excellent stability, and superior heat dissipation. These PCBs are widely used in smartphones, tablets, data center servers, and communication equipment, addressing the need for high-density routing, high-speed signal transmission, and multi-layer blind and buried vias.

Features:

• Fine circuit lines with high wiring density
• Multi-layer blind and buried vias for optimized space utilization
• Excellent mechanical strength and stability

Applications:

• Smartphones
• Communication equipment
• Data center servers

2. Flexible HDI PCB

In contrast to typical flexible PCBs, Flexible HDI PCBs often utilize kapton as a material to meet high frequency, high speed and flexibility requirements. By combining a very small bend radius with HDI technology, Flexible HDI PCBs can better optimize the internal space layout. Flexible HDI PCBs utilize flexible substrates, allowing them to be bent, folded, and curved in three-dimensional spaces. These PCBs are commonly found in devices requiring spatial flexibility and lightweight design, such as wearable devices, camera modules, medical devices, and consumer electronics.

Features:

• Bendable and foldable for complex spatial layouts
• Lightweight and ultra-thin design
• Greater design flexibility

Applications:

• Wearable devices
• Medical equipment
• Camera modules

3. Rigid-Flex HDI PCB

HDI rigid-flex PCBs combine the dual advantages of traditional HDI boards and rigid-flex boards, offering unique characteristics of high-density interconnection and flexible structure. Compared to traditional PCB design, HDI rigid-flex PCBs are significantly more complex, particularly in handling rigid-flex transition areas, trace routing, and blind and buried via design. These aspects must strictly adhere to specific design rules to ensure stable electrical performance and mechanical strength. Among these, blind via fabrication technology is one of the core processes in HDI rigid-flex PCB manufacturing, serving as a critical method for achieving reliable interconnections between circuit layers.

Our continuous investment in developing world-class, fine-line Microvia technology has made us a leading Microvia HDI rigid-flex PCB board manufacturer. Our through experience and commitment in providing a complete solution for our customers helps to resolve early HDI Rigid-Flex Printed Circuit Board design issues, shorten the lead time, and deliver a high-quality, cost-efficient product.

Rigid-Flex HDI Printed Circuit Boards combine the advantages of rigid PCBs and flexible PCBs, featuring both the stability of rigid regions and the flexibility of flexible regions within a single PCB. This design is ideal for high-density integration and spatially constrained complex devices, including high-end smartphones, automotive electronics, military equipment, and industrial control systems.

Features:

• Seamless integration of rigid and flexible structures
• Stable electrical and mechanical performance
• Suitable for high-density interconnections and complex spatial layouts

Applications:

• Smartphones
• Automotive electronics
• Military equipment

8 Layer HDI stackup impedance(1st order, 2nd order, 3rd order, anylayer order) Case Study

1.  8-Layer 1st order HDI PCB Stack-Up Design

An 8-layer HDI (High-Density Interconnect) PCB typically adopts a (1+6+1) or (1+N+1) structure, where N≥2 and is an even number. This structure meets the requirements for high-density routing and stable electrical performance. Below is a typical example of an 8-layer HDI PCBs stack-up:

Layer	Description	Type
L1	Signal Layer	Outer Layer (Top Layer)
L2	Ground Plane	Inner Layer
L3	Signal Layer	Inner Layer
L4	Power Plane	Inner Layer
L5	Power Plane	Inner Layer
L6	Signal Layer	Inner Layer
L7	Ground Plane	Inner Layer
L8	Signal Layer	Outer Layer (Bottom Layer)

Blind and buried via structure

• Blind Vias: L1-L2, L7-L8
• Buried Vias: L2-L7
• Through-Hole Vias: L1-L8

The QFPCB HDI manufacturing process of an 8-layer HDI PCB begins with the production of the inner layer circuits for L3/L4/L5/L6. Next, the L2/L7 copper foil layers are laminated, followed by drilling the 2-7 buried vias and completing the L2/L7 layer circuits. Subsequently, the L1/L8 copper foil layers are laminated, and laser micro-vias are drilled between L1-L2 and L7-L8, along with mechanical drilling for through-holes spanning layers 1-8 to ensure reliable electrical connections across all layers. Finally, the outer layer circuits on L1 and L8 are processed according to conventional manufacturing workflows, followed by surface treatment and rigorous electrical testing and inspection. This process integrates multiple lamination steps, buried vias, laser drilling, and high-precision circuit fabrication technologies, achieving high-density interconnections and stable electrical performance

8-Layer 1st order HDI PCBs Stack-Up

2.  8-Layer 2nd order HDI PCBS Stack-Up Design

The standard second-lamination eight-layer 2nd-order HDI (secondary buildup HDI 8-layer board, with a stack-up structure of (1+1+4+1+1)) follows a structure of (1+1+N+1+1), where N≥2 and is an even number. This design is currently the mainstream approach in the industry for second-lamination processes, featuring buried vias in the inner multilayer, typically requiring three lamination cycles to complete. A key characteristic of this structure is the absence of stacked vias, making the manufacturing difficulty relatively manageable. If the buried vias originally designed between layers (3-6) are optimized to (2-7), one lamination cycle can be eliminated, simplifying the production process and effectively reducing manufacturing costs while maintaining electrical performance and structural stability. This process optimization not only enhances production efficiency but also offers a more cost-effective solution for mass-producing high-performance HDI boards.

QFPCB conventional 8-layer 2nd-order HDI Boards stackup:

The 8-layer 2nd-order HDI PCB with cross-layer blind via design (secondary lamination  8-layer HDI boards, stack-up structure (1+1+4+1+1)) follows a (1+1+N+1+1) configuration, where N≥2 and is an even number. This design is considered a challenging structure in the current PCB manufacturing industry due to its complex secondary lamination process. The inner layers typically feature buried vias between layers (3-6), requiring three lamination cycles to complete. The cross-layer blind via design significantly increases manufacturing difficulty, and only HDI PCB manufacturers with advanced technical capabilities can successfully produce such high-end stacked boards. QFPCB HDI PCB manufacturing factory has mass-produced cross-layer blind buried via circuit boards.

Additionally, another key challenge lies in the blind via design between layers (1-3). When split into blind vias for layers (1-2) and (2-3), the (2-3) layer inner blind vias must be processed using a via-filling technique, specifically employing a stacked via + via-filling process during the secondary lamination. Compared to HDI boards without via-filling requirements, this approach significantly raises both manufacturing costs and technical complexity.

Therefore, in conventional secondary lamination HDI designs, it is recommended to avoid stacked via designs whenever possible and instead convert blind vias from (1-3) layers into staggered blind vias for (1-2) layers and buried (or blind) vias for (2-3) layers. Experienced design engineers are often able to optimize designs in this way, simplifying the manufacturing process and effectively reducing production costs while enhancing overall product reliability and efficiency.

8-layer 2nd-order HDI copper-filled impedance stackup

3.  8-Layer 3rd order HDI PCBS Stack-Up Design

For other types of HDI boards (such as 8-layer, 3rd-order HDI PCBs requiring three lamination cycles, or 8-layer PCBs with more than three lamination cycles of any stage), optimization can be carried out based on the design concepts mentioned above. Taking the complete 3rd-order HDI board as an example, the entire production process usually requires four lamination cycles. However, by applying design concepts similar to those used for one-lamination or two-lamination boards, it is possible to reduce one lamination cycle in the production process, thereby improving the product yield.

Among the customers who mass produce 8-layers 3rd-order HDI circuit boards in QFPCB HDI PCBs Manufacturing, many of the initial stack-up designs required four lamination cycles. However, after optimizing the design, it became possible to meet the functionality requirements of the HDI board with just three lamination cycles.

8-layers 3rd-order HDI Printed Circuit Boards Impedance Stackups

4. 8-layers Anylayer HDI Pcbs Impedance Stackups

How to select Materials for HDI PCB?

When selecting materials for HDI PCBs, it is essential to first understand the application requirements, including functionality, operating environment, and size complexity. Different applications (such as high-frequency, power electronics, RF communication, etc.) have varying requirements for electrical performance, thermal management, and mechanical strength.

1. Understand the Application Requirements

• Functionality: What is the purpose of the HDI PCB? Is it for high-speed signal processing, power distribution, or RF communication? Different applications will have different material requirements, such as low loss for high-frequency applications or high thermal conductivity for power devices.
• Operating Conditions: Consider temperature ranges, humidity levels, and other environmental factors that the PCB will be exposed to. For example, automotive applications may need materials that withstand higher temperatures and mechanical stress.
• Size and Complexity: HDI boards often feature small form factors with fine-pitch components and multiple layers, so materials must support dense interconnections and reliable via formations.

2. Consider the Electrical Performance Requirements

• Dielectric Constant (Dk): The dielectric constant of the material affects signal speed and impedance. Low Dk materials are typically used in high-speed or RF applications to reduce signal loss.
• Dissipation Factor (Df): The dissipation factor represents how much signal is lost as heat. Low Df materials are desirable for high-frequency applications to reduce power loss and heat generation.
• Electrical Conductivity: Materials with good electrical conductivity, such as copper for traces, are essential for ensuring signal integrity and current-carrying capacity.

3. Thermal Management

• Thermal Conductivity: HDI PCBs, particularly those used in power electronics, often require materials with high thermal conductivity to dissipate heat. Materials like ceramic-filled PCBs or specific resin systems can help improve thermal management.
• Coefficient of Thermal Expansion (CTE): The CTE must match between the PCB layers and components to avoid mechanical stresses. Low CTE materials are preferable for HDI PCBs, especially when dealing with fine-pitch components and solder joint reliability.
• Glass Transition Temperature (Tg): The Tg of the material determines its ability to withstand temperature variations without degrading. HDI PCBs used in automotive or military applications may require materials with higher Tg to endure harsh conditions.

4. Mechanical Strength and Reliability

• Mechanical Strength: HDI boards are often subjected to mechanical stress during assembly and operation. Materials should have adequate strength to resist cracking, warping, or delamination. This is particularly important for multi-layer and thick copper HDI boards.
• Flexibility: For flexible or flexible-rigid HDI PCBs, materials like polyimide or flexible laminates are chosen to ensure the board can bend without damage.
• Fatigue Resistance: Materials used for HDI PCBs need to resist fatigue caused by thermal cycling, particularly in high-reliability applications (e.g., aerospace, automotive).

5. Material Types for HDI PCBs

FR-4 Materials:

1. Normal Speed and Normal Loss

The most common base material for HDI PCBs. It is cost-effective and has reasonable mechanical and electrical properties but might not be suitable for high-frequency or high-performance applications. These materials have relatively moderate dielectric constants and loss factors and are typically used for low frequency applications up to 5 GHz and dissipation factors of 0.02-0.014. Isola FR370HR, S1000-2M, IT-180A , NP-175F , KB-6167F and TUC Tu-768 is a typical representative of these materials, in addition to which we can also provide you with materials such as EM-828G, EM-370Z, Nelco N7000-2HT, Panasonic R1577 and others.

2. Medium Speed and Medium Loss

These materials are able to maintain more stable values of dielectric constant over frequency changes and have lower dielectric loss (roughly half that of normal speed materials). They are therefore suitable for use in applications with operating frequencies of 5-10 GHz and dissipation factors of 0.014-0.008. Common materials are TU-872 SLK, TU863+, Panasonic R5725(M4S), S7439G, IT958G, EM888K, FR408HR and Isola I-Speed. in addition to which we can also provide you with materials such as Nelco N4000-13 and N4800-20 and others.

3. High Speed and Low Loss

In applications that require high speed transmission and low loss, such as automotive electronics and advanced communications equipment, materials with low dielectric constants and low loss factors need to be selected. These materials excel in the 10 to 25 GHz frequency range, with loss factors typically ranging from 0.008 to 0.004, while they maintain lower electrical noise. QFPCB can provide you with MEGTRON6_R-5775, TUC TU883, EM528, Isola I-TERA MT40 S7338, Nelco MW1000,  Doosan DS-7409D, IT968.

4. Very High Speed and Very Low Loss (RF/Microwave)

If your HDI circuit board needs to be used in RF/microwave applications, then you need to choose materials with ultra-high speed and very low loss. These material can perform well in＞25 GHz applications and they have extremely low dielectric loss (Df ≤ 0.004). QFPCB can offer you a choice of High speed series materials as diverse as MEGTRON7_R-5785 and MEGTRON8_R-5795, Tu-933, Meteorwave4000（Nelco), IT-988, Tachyon 100G(Isola). High frequency series materials selection Rogers RO4350B, RO4835, TLY-3(Taconic), AD250(Arlon), F4BM220-300.

Polyimide Materials:

Used for flexible HDI PCBs, Polyimide is the most common substrate material for flexible PCBs, offering excellent thermal stability, high-temperature resistance, superior mechanical strength, and good electrical performance. It can operate at higher temperatures, making it suitable for applications that require high-temperature resistance, such as automotive, telecommunications, medical devices, and avionics. Flexible material selection SF201, SF202, Kapton HN, DuPont AP, FASB0520, FQX0520, ThinFlex W-1003RD-C, Panasonic RF-775

Prepreg Materials:

HDI (High-Density Interconnect) PCBs are advanced circuit boards with fine-pitch components and multiple layers, and they require specialized prepreg materials to support their manufacturing process. Prepreg (pre-impregnated) materials are key components in multilayer PCB construction, serving as the bonding agent between layers and providing mechanical support. For HDI PCBs, the prepreg materials need to offer high-performance properties, such as good adhesion, high thermal stability, and low shrinkage. Standard prepregs like Shenyi FR4, Panasonic FR4, and TUC FR4 are commonly used. Polyimide (PI) and PET-Based Flexible Prepregs Common examples include Kapton and ThinFlex prepregs.

Copper Clad Laminate (CCL):

Copper clad laminate materials have copper foil laminated onto one or both sides of cured (C-stage) dielectric. The rigid CCLs can be FR4, FR-5 or some PTFE. The typical application uses single-side clad laminate material where the copper clad is used as the outer layer and the c-stage is bonded to the sub-composite. Microvias are formed utilizing laser drilling methods. Materials available differ by reinforcement (woven glass, non-woven glass and expanded PTFE) and chemistries involved (epoxies, polyimide, polyester etc.).

Resin Coated Copper (RCC):

Resin coated copper materials are compromised of copper foil, coated with a resin dielectric material that can be directly bonded to the sub-composite. They differ by whether they are wet processable or not. In non-wet processable-coated copper materials, microvias are formed utilizing plasma or laser drilling methods.

Soldermask:

The soldermask process is a crucial step in HDI PCB fabrication, The soldermask process involves applying soldermask ink or epoxy resin to a PCB surface to protect the copper traces and pads during assembly. This layer prevents oxidation, contamination, and solder bridges, ensuring a smooth soldering process，It also prevents the unintended flow of solder during soldering operations.. Typically, the soldermask is green,black,greet,blue,red, though other colors are also available.

A prepared stencil is used to apply the soldermask material selectively to certain areas of the PCB, such as pads, traces, and via holes. For HDI and Multi layers PCBs, via holes are often plugged to prevent solder from flowing through during the soldering process. After applying soldermask ink across the entire PCB surface, it is exposed to UV light through artwork. Areas intended for soldering remain after chemical development, while the exposed areas are removed. The remaining soldermask is then fully cured, creating a durable finish.

QFPCB defines soldermask thickness, with specific measurements for different areas: ≥5μm on the knee of the track and 10-30μm on the surface, depending on copper thickness. This ensures improved electrical insulation and resilience against chemical or mechanical forces. The varying thicknesses provide more robust coverage, enhancing durability. With 70% soldermask fill on type VI via holes, QFPCB reduces the risk of rejection during assembly. QFPCB has also established agreed-upon brands and types of soldermasks for use.

Surface Finish:

Various surface finishes are applied to exposed copper areas of PCBs to protect the surface from oxidation, corrosion, and to ensure optimal solderability. These finishes vary in their application methods and are chosen based on the specific needs of the PCB design and the intended application. The most commonly used finishes include:

1. HASL (Hot Air Solder Leveling): HASL is a traditional and cost-effective finish where the PCB is dipped into molten solder, and excess solder is removed using hot air. It is widely used due to its good solderability, but it is less suitable for fine-pitch components or high-density designs, as it can leave a relatively thick solder layer.
2. ENIG (Electroless Nickel/Immersion Gold): ENIG is a two-layer finish consisting of an electroless nickel layer for corrosion resistance and a thin layer of immersion gold to ensure excellent solderability and prevent oxidation. ENIG provides a flat, smooth surface, making it ideal for fine-pitch components, especially in high-reliability applications such as aerospace and medical devices.
3. ENEPIG (Electroless Nickel Electroless Palladium Immersion Gold): ENEPIG is similar to ENIG but includes an additional layer of electroless palladium between the nickel and gold layers. This finish offers superior solderability, excellent corrosion resistance, and is particularly well-suited for applications requiring fine-pitch components and high-reliability solder joints.
4. Plating Hard Gold: Hard gold plating involves a thicker gold layer compared to ENIG, offering high durability and excellent wear resistance. It is often used for high-frequency, high-performance applications or for connectors, where surface durability is critical.
5. OSP (Organic Solderability Preservative): OSP is an organic coating that provides a protective layer over copper, preventing oxidation without adding a metallic finish. It is an environmentally friendly option and is commonly used for high-density interconnect (HDI) boards, but it may not provide the same long-term durability as other finishes.

After the surface finishes are applied, their thickness and solderability are rigorously tested to ensure they meet strict specifications. These tests ensure that the PCB can reliably accept solder during the assembly process, which is crucial for maintaining the integrity and performance of the solder joints throughout the PCB’s life cycle.

6. Consider Cost and Availability

• Cost vs. Performance: High-performance materials often come at a higher cost, so balancing the requirements with the budget is essential. For example, using standard FR-4 materials for basic functionality or less critical applications can reduce cost, while high-end materials like PTFE or ceramics are best for high-frequency or high-reliability applications.
• Lead Time and Supply Chain: Materials that are readily available and have shorter lead times may be necessary for projects with tight deadlines or those requiring large quantities.

7. Manufacturability and Process Compatibility

• Layer Count and Via Technology: HDI PCBs often involve finer vias (microvias or blind/buried vias) and higher layer counts. Choose materials that support these processes, such as those with good laminate bonding properties and fine-feature capability.
• Processability: Ensure that the material can be processed using the intended PCB manufacturing technology (e.g., laser drilling for microvias, high-frequency lamination).

8. Environmental and Regulatory Compliance

• RoHS Compliance: Ensure the materials selected comply with RoHS (Restriction of Hazardous Substances) regulations, which limit the use of harmful substances like lead, cadmium, and mercury.
• Halogen-Free: For environmentally conscious applications, select halogen-free materials to meet environmental standards.
• UL Rating: Depending on the application, a UL rating for flammability (such as UL 94V-0) might be required.

Choosing the right materials for HDI PCBs involves balancing electrical performance, thermal management, mechanical strength, and manufacturability, while considering the specific requirements of the end application. It’s essential to evaluate the material options based on performance characteristics (such as Dk, Df, and Tg), environmental conditions, and regulatory requirements. By doing so, you can ensure that the HDI PCB performs reliably and meets all the functional demands of your product.