Comprehensive Guide to HDI PCBs: Design, Advantages, and Applications

High Density Interconnect (HDI) printed circuit boards (PCBs) are circuit boards with a higher wiring density per unit area than traditional PCBs. HDI PCBs have denser interconnections and components, finer lines and spaces, and higher connection pad density. They also have smaller vias and traces, and higher layer counts. A single HDI board can accommodate the functionalities of multiple boards previously used in a device. HDI PCBs are a preferred choice for high-layer and expensive laminated boards. HDI PCBs require different fabrication and assembly processes than typical circuit boards. These circuit boards have Higher manufacturing costs, More challenging designs, More complex reworking and repairing, and Manufacturability issues.

Design Tips for HDI PCB:

1- Choose the type of vias to minimize process complexity

Choosing the appropriate type of vias is crucial, as it determines the necessary equipment, manufacturing steps, processing time, and additional costs. Opting for micro vias, blind vias, or buried vias can reduce the number of layers and material expenses. However, selecting between through-hole vias, dog bone vias, or via-in-pad vias will influence the overall process complexity.

2- Choose the minimum number of components for HDI applications

Component selection is always essential, but it is especially critical for HDI boards. The components chosen for HDI designs dictate the routing widths, placements, types, and sizes of drilled holes, as well as the overall stack-up. While performance remains the primary concern, packaging, traceability, and availability must also be considered. Replacing components or redesigning the layout can greatly increase manufacturing time and material costs.

3- Minimize stress and EMI when placing components

Asymmetric via placement can cause uneven stress and potential warpage of the circuit board, reducing yield. Dense component spacing and high-power signals may introduce EMI, affecting signal quality. Parasitic capacitance or inductance from nearby pins or pads can also impact signal integrity, making EMI modeling during the design phase essential.

4- Minimize signal integrity issues by optimizing routing

A key advantage of HDI is the use of narrower traces for signal propagation, allowing for reduced size. However, trace widths should be designed to ensure optimal signal integrity, which involves maintaining the shortest possible trace lengths, consistent impedance paths, adequate ground planes, and proper isolation of digital, analog, and power signals.

5- Choose stack-ups to minimize material costs

In addition to the selection of vias, the choice of PCB stack-ups also significantly impacts the manufacturing cost of HDI PCB electronic products. The material type and number of layers directly affect the required lamination and drilling cycles. Cost should be one of the determining factors when making these decisions.

The Advantages of HDI PCB:

Improved Reliability: Due to their smaller aspect ratio, microvias offer better reliability compared to typical through-hole vias. They are more robust than through-holes and employ superior materials and components, resulting in excellent performance for HDI (High-Density Interconnect) technology.

Enhanced Signal Integrity: HDI technology incorporates via-in-pad and blind-via techniques. These techniques help bring components closer to each other, reducing the length of signal paths. HDI technology eliminates stubs caused by through-holes, reducing signal reflection and improving signal quality. Therefore, shorter signal paths significantly enhance signal integrity.

Cost-effectiveness: With proper planning, HDI technology can reduce overall costs compared to standard PCBs. This is due to the requirement of fewer layers, smaller dimensions, and fewer PCBs needed.

Compact Design: The combination of blind and buried vias reduces the space requirements of the circuit board.

Overall by using HDI technology, designers now have the option to place more components on both sides of the raw PCB. Multiple via processes, including via in pad and blind via technology, allow designers more PCB real estate to place components that are smaller even closer together. Decreased component size and pitch allow for more I/O in smaller geometries. This means faster transmission of signals and a significant reduction in signal loss and crossing delays.

Different Types of Vias in HDI PCB:

A via is a small conductive hole that connects multiple layers of an HDI PCB, allowing signals to pass between them. There are four types of vias used in HDI PCBs:

● Through-hole via: These vias extend from the top to the bottom layer of the PCB.

● Blind via: These vias connect an inner layer to a surface layer of the PCB.

● Buried via: These vias connect two inner layers of the PCB.

● Microvia: These vias are smaller than through-hole vias and are often used in HDI PCBs to connect multiple layers without increasing the size of the board.

HDI PCB Materials Selection:

The choice of material and its construction is pivotal in the design and manufacturing of HDI (High-Density Interconnect) PCBs. The process of designing HDI interconnects entails a comprehension of potential challenges associated with specifying glass-reinforced dielectric materials.

● Copper Clad Laminate (CCL): Copper clad laminate materials involve the lamination of copper foil to one or both sides of a cured (C-stage) dielectric. The rigid CCLs may be categorized as FR4, FR-5, or certain PTFE types. A typical application employs single-side clad laminate material, where the copper clad serves as the outer layer, and the C-stage is bonded to the sub-composite.

● Resin Coated Copper (RCC): Resin coated copper materials consist of copper foil coated with a resin dielectric material, which can be directly bonded to the sub-composite. They are distinguished by their processability in a wet environment or lack thereof. For non-wet processable-coated copper materials, microvias are created using either plasma or laser drilling techniques.

● PP (Prepreg): Often referred to as B-stage, Bonding sheet, or simply Preg, Prepreg comprises fiberglass fabric impregnated with resin. During the Prepreg coating operation, the resin undergoes partial curing without hardening. During the heating phase of the PCB stack-up in the pressing process, the resin in PP will flow, adhere, and bond the PCB core with copper foil or other materials.

Design Considerations for HDI PCBs:

The processes involved in the production of HDI PCBs are often different from those used with other PCB types. Here’s what you need you know about HDI board production and some of the design considerations you’ll want to keep in mind to keep in mind throughout the production process:

Sequential Lamination: In the lamination process, the PCB core or cores are combined with copper, as well as pre-preg layers for multi-layer PCBs, by applying heat and pressure. The amount of heat and pressure needed varies from board to board. After the lamination phase is complete, the PCB manufacturer will drill vias. Unlike other types of PCBs, HDI boards go through this process multiple times. These sequential laminations help prevent shifting and breaking during drilling.

Via Fill Types: Via fill types should always match your specific application and PCB requirements. Via fill materials we work with on a regular basis include electrochemical plating, silver-filled, copper-filled, conductive epoxy, and non conductive epoxy. The most common via fill type is non conductive epoxy.

Via-in-Pad Process: The via-in-pad production process allows you to place vias in the surface of the flat lands on your PCB by plating the via, filling it with one of the various fill types, capping it and, finally, plating over it. Via-in-pad is typically a 10- to 12-step process that requires specialized equipment and skilled technicians.

Laser Drill Technology: Any-layer HDI designs frequently necessitate laser microvias, created using laser drills capable of generating up to 20-micron diameter lasers to precision-cut both metal and glass, yielding minuscule yet pristine holes. Utilizing materials like uniform glass with a low dielectric constant can further shrink hole dimensions.

HDI PCB Manufacturing Process:

The design of an HDI PCB is critical to its successful manufacturing. HDI PCB design must carefully consider the following factors:

● Line width and spacing: HDI PCBs typically have line widths and spacings of 1 mil or less. This requires precise design and manufacturing techniques to ensure that the traces are properly formed and that there is no electrical interference between traces.

● Via size and placement: HDI PCBs often use microvias, which are vias with a diameter of 6 mils or less. Microvias are more difficult to drill and plate than traditional vias, so they must be carefully placed on the PCB.

● Stack-up: HDI PCBs often have complex stack-ups, with multiple layers of copper and dielectric material. The stack-up must be carefully designed to ensure that the PCB is electrically and mechanically sound.

You can view the complete article on the PCB manufacturing process here.

Cost Effective HDI:

While some consumer products shrink down in size, quality remains the most important factor for the consumer second to price. Using HDI technology during design, it is possible to reduce an 8-layer through-hole PCB to a 4-layer HDI microvia technology-packed PCB. The wiring capabilities of a well-designed HDI 4 layer PCB can achieve the same or better functions as that of a standard 8 layer PCB. Although the microvia process increases the cost of the HDI PCB, the proper design and reduction in layer count reduces the cost in material square inches and layer count more significantly.

To manage your HDI PCB costs effectively, consider the following factors:

1) Vias and Holes: Smaller vias require more precision and cost more. More vias also increase costs.

2) Stackup and Layers: Complex stack-ups and additional layers raise costs; aim for the most efficient number.

3) Materials: The choice of core materials (like FR4, metal) and surface finishes (like ENIG) affects costs.

4) Laminations: More laminations improve performance but increase cost and processing time.

5) Stacked vs. Staggered: Stacked vias cost more due to additional materials and time required.

6) Pad Size: Optimize pad size early to reduce costs.

7) Turnaround Time: Rush orders cost more; plan ahead for savings.

8) Supplier: Choose a supplier offering high-quality products at competitive prices to avoid future expenses.

HDI PCB Industries and Applications:

HDI technology is essential for modern electronics, enabling smaller, cost-effective devices without compromising performance. Key industries using HDI PCBs include:

Consumer Electronics: HDI PCBs are found in smartphones, laptops, wearables, smart home devices, and IoT gadgets, allowing for miniaturized yet powerful consumer tech.

Communications: Essential in devices like routers, switches, and semiconductors, HDI boards support electronic communication, digital media, and network infrastructure.

Automotive and Aerospace: HDI PCBs enable compact, lightweight components for cars and aircraft, supporting features like WiFi, GPS, cameras, and sensors, contributing to more efficient operation.

Medical Devices: Used in advanced monitoring, imaging, and surgical equipment, HDI technology enhances performance and reduces device size.

Industrial Applications: HDI boards power IoT devices and smart sensors in manufacturing, warehousing, and industrial operations, optimizing performance and connectivity.