What is a high-density interconnection board (HDI) PCB and why is it used?

Compared with traditional PCBs, high-density interconnected PCBs or HDI PCBs can provide more components and connections in a smaller area due to their high-density and compact design.

The purpose of this article is to discuss the characteristics, advantages and applications of HDI PCB. We will also discuss the manufacturing processes and challenges involved in the production of these complex and advanced PCBs. This guide will give you a better understanding of HDI PCB and its applications in various industries.

what's the HDI PCB？

High-density interconnection or HDI PCB is a kind of PCB that uses micropores (smaller and denser than traditional through holes) to connect the layers of the PCB and increase the number of interconnections per unit area.

We can also say that it is a multi-layer PCB, using fine-pitch solder resist layers, high-density copper rails (wiring) and advanced materials to maximize performance and minimize costs.

The term ”high density" refers to the copper content of each layer of the circuit board. A typical HDI PCB has 140 to 160 layers, which means that there are more than 200,000 copper pads on each circuit board. The more layers, the stronger the connectivity between the circuit boards

Advantages and limitations of HDI PCB

advantages：

●Higher component density: smaller traces and spacing, more components can be placed in a smaller area.

●Improved signal integrity: The trace width and spacing of the HDI PCB are smaller, which can reduce noise and improve signal integrity.

● Performance enhancement: The overall performance of electronic equipment is better.

● Improved design flexibility: the use of multi-layer design in a smaller space can achieve a more complex and flexible circuit layout.

Limitations：

● Due to advanced technology, manufacturing costs are higher

● More difficult to design and manufacture

● The supply is limited because not all manufacturers produce it

How to choose HDI PCB material?

The following factors must be considered when choosing HDI PCB materials：

Performance requirements: You should consider the performance requirements of your application. This includes factors such as operating frequency, signal integrity, and power requirements.

Cost: Due to the complex design and manufacturing process of HDI PCB, manufacturing costs are usually higher. You should consider the cost of the material and how it fits your budget.

Manufacturing capabilities: These PCBs require specialized manufacturing technologies such as laser drilling and micropores, so you should consider the capabilities of PCB manufacturers to ensure that they can produce HDI PCBs to your specifications.

Thermal properties: The material used in this PCB will affect its thermal properties, such as its thermal conductivity and coefficient of thermal expansion (CTE).

Weldability: The materials used should be compatible with the welding process that connects the components to the PCB.

You also need to choose the following dielectric used in the HDI PCB：

Dielectric constant: polyimide

Suitable for: ● High temperature resistant materials ● excellent dimensional stability ● low dielectric constant

Dielectric constant: PTFE (polytetrafluoroethylene)

Suitable for: ● Low dielectric constant ● excellent electrical performance ● low coefficient of friction

Dielectric constant: LCP (liquid crystal polymer)

Suitable for: ● High performance ● excellent electrical and mechanical properties ● low dielectric constant and high temperature resistance

PCB with HDI and PCB without HDI

Structure: High-density interconnected circuit boards are characterized by high-density interconnection, made of thin and high-quality multi-layer wiring substrate materials; standard PCBs have larger circuit and space interconnection, and use a wider range of substrate materials.

Thickness: High-density interconnected circuit boards are thinner than standard PCBs, with thicknesses ranging from 0.089mm to 0.25mm; standard PCBs are relatively thick, with thicknesses ranging from 0.25mm to 1.5mm.

Electrical and thermal conductivity: High-density interconnected circuit boards have higher electrical and thermal conductivity, but depend on the material used; standard PCBs also have good thermal and electrical conductivity, but are slightly worse than HDI.

Production technology: High-density interconnected circuit boards use laser drilling and micropores; standard PCBs only have mechanical drilling.

Usage: High-density interconnected circuit boards are used for high-performance electronic devices that require high integration and miniaturization, such as smart phones, portable devices, high-end computers, or military equipment; standard PCBs are used for standard computers, household appliances, and industrial equipment.

Cost: High-density interconnected circuit boards are generally more expensive; standard PCBs are cheaper than HDI.

Where can HDI PCB be applied?

Electronic equipment: Connect components to the electrical system and provide grounding for the system.

Medical: Connect medical equipment and accessories. They also ensure that all these devices can work together normally.

Military: Military personnel around the world use it to help their equipment run at a more compatible speed.

HDI PCB Stacking types

PCB design can take advantage of multiple types of HDI (high-density interconnection) stacking：

Micropores HDI: PCB uses micropores (through holes with smaller diameters) with high efficiency and provides high-density interconnection.

Blind buried hole HDI: Use invisible vias on the PCB surface for connection to improve the appearance of the PCB and reduce the risk of interference. High degree of design flexibility.

Build HDI sequentially: The PCB is divided into multiple layers and has different interconnection modes.

HDI PCB stacking and lamination process

Lamination is the process of using heat and pressure to bond multiple layers of materials together. It can be used to create HDI PCBs. Compared with traditional PCBs, HDI PCBs have higher wiring density and smaller through-hole sizes.

● In microporous stacking, small holes are used to connect the material layer.

● The through holes in the PCB are used to connect the layers without passing through the outer layer.

● Sequential lamination HDI stacking does not glue all the layers together at once, but adds them to the PCB layer by layer.

HDI PCB design rules and techniques

There are several key considerations and design rules that need to be kept in mind when designing HDI PCB：

● Use smaller trace and spacing sizes

● Use high-quality materials

● Carefully consider stacking

● Choose the right manufacturer