Learn More About HDI PCB Knowledge

What You Need to Know About HDI PCB

Concept:

HDI: High-density interconnects technology. It is a multilayer board made by stacking method and micro-blind buried holes.

HDI PCB (High Density Interconnect PCB): is a relatively high circuit board using micro-blind and buried via technology line distribution density. This is a process that includes inner and outer layers. The holes and metallization are then used in the holes to achieve the function of joining each inner layer. With the development of high-density and high-precision electronic products, the requirements for circuit boards are also the same. The most effective way to increase PCB density is to reduce the number of through holes, and accurately set blind holes and buried holes to meet this requirement. Thus, an HDI board is generated.

• Take advantage of HDI:
• HDI technology helps reduce PCB costs;
• Increased linear density;
• Conducive to the use of advanced packaging;
• HDI technology has better electrical performance and signal effectiveness;
• Have better reliability;
• HDI technology is better in heat dissipation;
• Can improve RFI (radio frequency interference)/EMI (electromagnetic interference)/ESD (electrostatic discharge);
• HDI technology improves design efficiency;

Material

There are some new requirements for HDI PCBs materials, including better dimensional stability, anti-static mobility and non-adhesives. The typical material of HDI PCB is RCC (Resin Coated Copper). There are three types of RCC, namely polyimide metallized film, pure polyimide film, and cast polyimide film.

The advantages of RCC include: small thickness, light weight, flexibility and flammability, compatibility characteristic impedance and excellent dimensional stability.

In the process of HDI multilayer PCB, RCC replaces the traditional adhesive sheet and copper foil as the role of insulating medium and conductive layer. The RCC can be suppressed with a chip through traditional suppression techniques. Non-mechanical drilling methods such as lasers are then used to form micro-via interconnects.

RCC promotes the occurrence and development of PCB products from SMT (Surface Mount Technology) to CSP (chip-scale packaging), from mechanical drilling to laser drilling, and promotes the development and progress of PCB micro-vias. All of these have become RCC’s leading HDI PCB material.

In the actual PCB in the manufacturing process, the choice of RCC. There are usually FR-4 standard Tg 140C, FR-4 high Tg 170C and FR-4 and Rogers combination lamination. With the development of HDI technology, HDI PCBs materials must meet more requirements, so the main trends of HDI PCB materials should be:

• Development and application of flexible materials without adhesives;
• The thickness of the dielectric layer is small and the deviation is small;
• LPIC development;
• The dielectric constant is getting smaller and smaller;
• Dielectric loss is getting smaller and smaller;
• High welding stability;
• Strictly compatible with CTE (Coefficient of Thermal Expansion);

Technology

The difficulty of HDI PCB manufacturing is micro-hole manufacturing, through-hole metallization and thin wires.

1. Micro-hole manufacturing

Micro-hole manufacturing has always been a core issue in HDI PCB manufacturing. There are two main drilling methods:

• For ordinary through-hole drilling, mechanical drilling is always the best choice for its high efficiency and low cost. With the development of machining capabilities, its application in micro-vias has also continued to develop.
• There are two types of laser drilling: photothermal ablation and photochemical ablation. The former refers to the process of heating the operating material after high energy absorption laser to melt it and evaporate it through the formed through holes. The latter refers to the results of high-energy photons and lasers in the ultraviolet region exceeding 400 nm in length.

2. Through-hole metallization

The biggest difficulty in through-hole metallization is that it is difficult to achieve uniform plating. For the deep-hole electroplating technology of micro-through holes, in addition to using a plating solution with high dispersion ability, the plating solution on the electroplating device should be upgraded in time. This can be done by strong mechanical stirring or vibration, ultrasonic stirring, and horizontal spraying. In addition, the humidity of the through-hole wall must be increased before plating.

In addition to process improvements, HDI’s through-hole metallization method has also seen improvements in major technologies: chemical plating additive technology, direct plating technology, etc.

3. Thin line

The realization of thin lines includes traditional image transmission and laser direct imaging. The traditional image transfer is the same as the process of ordinary chemical etching to form lines.

For direct laser imaging, no photographic film is required, and the image is formed directly on the photosensitive film by the laser. Ultraviolet light is used for operation, so that the liquid anticorrosion solution can meet the requirements of high resolution and simple operation. Photographic film is not required to avoid the adverse effects caused by film defects. CAD/CAM can be directly connected to shorten the manufacturing cycle and make it suitable for limited production and multiple production.

Application of HDI PCB in automobile

Due to the needs of automobile modernization and digitization, and people’s requirements for automobile safety, comfort, simple operation and digitization. PCB has been widely used in the automotive industry, from ordinary single-layer PCB, double-layer PCB to complex multi-layer PCB. Multi-layer PCB or HDI PCB, may have cross-layer blind holes or double-layer construction layers.

In automotive circuit boards, traditional single-layer PCBs, double-layer PCBs and multi-layer PCBs are available. In recent years, the wide application of HDI PCB has become the first choice for automotive electronic products. There is indeed an essential difference between ordinary HDI PCB and automotive HDI PCB: the former emphasizes practicality and versatility to provide services for consumer electronic products. While the latter strives for reliability, safety and high quality.

HDI PCB can be divided into single-layer HDI PCB, double-layer PCB and three-layer PCB. Here, the layer refers to the prepreg layer.

Requirements for automotive HDI PCB manufacturers

Due to the high reliability and safety of automotive HDI PCB, automotive HDI PCB manufacturers must meet high-level requirements:

• Automotive HDI PCB manufacturers must adhere to a comprehensive management system and quality management system that play a key role in judging or supporting the management level of PCB manufacturers. Before being recognized by third-party certification, some systems cannot be owned by PCB manufacturers. For example, automotive PCB manufacturers must pass ISO9001 and ISO/TS16949 certification.
• HDI PCB manufacturers must be equipped with rugged technology and high HDI manufacturing capabilities. Specifically, a manufacturer specializing in the manufacture of automotive circuit boards must manufacture boards with a line width/pitch of at least 75 μm/75 μm and double stacking. It is generally believed that HDI PCB manufacturers must have a process capability index (CPK) of at least 1.33 and a device manufacturing capability (CMK) of at least 1.67. Unless approved and confirmed by the customer, it is not allowed to be modified in future manufacturing.
• Automotive HDI PCB manufacturers must follow the strictest rules for selecting PCB raw materials.

Many strategies and measures can be used to judge automotive HDI PCB manufacturers, including material supplier certification, in-process technical conditions and parameter determination, and the application of accessories. Finding reliable HDI PCB manufacturers can be an important factor in determining and judging their reliability.

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