The hole in the PCB (printed circuit board) is an important structural element, which plays a role in the electrical connection, fixing components and positioning in multi-layer PCB design. By creating a through-through channel between the different layers of the PCB, the current can flow smoothly between the conductive layers of different layers, achieving the integrity and functionality of the circuit.

1, the definition and function of the hole

The through hole, also known as the through hole, is a channel in the printed circuit board to achieve the vertical interconnection between different layers of conductive graphics. It is composed of two parts: drilling and metallization (copper plating) of the hole wall, which runs through any layer or multiple layers of the PCB, aiming to realize the electrical connection between the layers inside the circuit board, as well as the signal transmission between the chip, components and the external interface.

Structural composition

• Hole: The core of the hole is a hole formed by drilling or laser etching that runs through one or more layers of the PCB.

• Hole wall coating: On the inner wall of the hole, a layer of metal (such as copper) is usually deposited to form a conductive layer to achieve an electrical connection between the layers. This process is usually done by Electroless copper Plating or Electroplating.

• Pad: both ends of the through hole (top and bottom, for through hole; Or the surface and inner layer, for blind and buried holes) usually has a Pad (Pad), which is an area welded to the wires or component pins on the PCB layer and is the same shape, size, and metallization as other pads on the PCB.

• Isolation area: For power or ground holes, isolation rings or solder stoppers are sometimes placed around the holes to prevent short circuits or enhance heat dissipation.

Main functions of the hole:

• Electrical connection: The main role of the hole is to provide an electrical connection between different layers, so that the current can flow between the different conductive layers of the multi-layer board to achieve signal transmission, power distribution or the establishment of a ground loop.

• Component fixation and positioning: In addition to electrical functions, through-holes can also be used to secure surface-mounted components (such as SMT components), which are welded to component pins via through-hole pads, ensuring the precise position of components on the PCB. In addition, through holes can also be used as mechanical positioning holes for aligning and securing PCBS with other components during assembly.

• Heat dissipation and mechanical support: In some cases, especially for high current or high power applications, the through hole can serve as a heat dissipation path to help heat transfer from the component or inner layer to the PCB surface or heat dissipation structure. In addition, the hole can also enhance the mechanical strength of the PCB, especially in the place where there is a large area of copper layer distribution, the hole can prevent the separation between the layers due to the different coefficient of thermal expansion.

2, the type of hole

According to the position of the hole in the PCB, production method and functional characteristics, it is mainly divided into the following types:

Through hole: Through hole penetrates the entire PCB board, extending from the top layer to the bottom layer, connecting all the middle layers. Through hole is the most common type of through hole, the manufacturing process is simple, the cost is relatively low, suitable for a variety of interconnect needs, including the connection of component pins, cross-layer signal transmission, power and ground network connection.

Blind hole: The blind hole only penetrates part of the PCB layer number, one end opens at the top or bottom, the other end terminates at a middle layer, the depth is less than the board thickness. Blind holes are mainly used in multi-layer board designs that save space and increase density, especially in high-speed signal transmission and high-frequency circuits, which can reduce signal transmission paths, reduce crosstalk and delay.

Buried holes: The buried holes are completely hidden in the inner layer of the PCB and do not extend to the surface of the board. This kind of hole only connects the middle layer and is not visible to the outside, which is conducive to further improving the circuit density and reducing signal interference. Burials usually occur in the inner interconnections of multilayer boards and are particularly critical for complex, high-density circuit designs.

Back drilling: First make through holes, and then drill holes to remove excess parts that have been connected, mainly used in PCB for high-speed signal transmission, reduce unnecessary parasitic capacitors and inductors, and improve signal integrity.

Microhole: ultra-fine holes with a diameter of less than 0.1mm are commonly found in advanced packaging technologies such as high-density interconnect (HDI) boards and flexible circuit boards (FPC) to meet the needs of lightweight and short electronic products.

3, through the hole design principles and influencing factors

Through hole design should follow the following principles and consider its impact on PCB performance:

Size: The diameter of the hole, the thickness of the hole wall (aperture ratio), the size of the pad, etc., directly affect its electrical properties (such as parasitic inductance, capacitance), processing difficulty and cost. The small through-hole can increase the circuit density, but it may increase the difficulty and cost of processing. Large through-holes are easier to manufacture but may take up more valuable space.

Design rules: including hole size (aperture, hole spacing, pad size), quantity, distribution, etc., should be based on the number of circuit board layers, signal rate, current load, heat dissipation requirements and other factors reasonably set. Follow IPC standards and manufacturer recommendations to ensure manufacturing feasibility and cost effectiveness.

Parasitic parameter: there are parasitic capacitance and inductance through the hole, which affect the high-speed signal transmission. By optimizing the aperture, hole depth ratio and back pad design, the parasitic effect is reduced and the signal integrity is improved.

Quantity and layout: Too many holes can increase the cost of the PCB, because drilling and copper plating is one of the main cost factors in PCB manufacturing. A reasonable pass hole layout should minimize unnecessary pass holes while ensuring signal integrity, power integrity, and heat dissipation requirements.

Reliability: The hole quality directly affects the mechanical strength and electrical reliability of the PCB. Factors such as hole wall roughness, coating quality and thermal stress resistance should be paid attention to, and appropriate drilling and hole metallization processes should be adopted to ensure long-term stable operation of holes.

High-speed signal application: For high-speed signal transmission, the parasitic inductance and capacitance of the hole will affect the signal quality, and it is necessary to reduce signal loss and crosstalk by optimizing the hole design (such as using backdrilling technology to reduce the hole length) and using high-speed dedicated hole design software for simulation analysis.

4, through the hole manufacturing process and technology trends

• Drilling technology: including mechanical drilling, laser drilling, plasma drilling, etc., with the advancement of technology, the manufacturing capacity of microholes and high aspect ratio through holes continues to improve to meet the manufacturing needs of high-density and high-speed PCB.

• Hole metallization: mainly including electroless copper plating, copper plating and other processes, in recent years, environmental protection lead-free coating, high reliability impedance control coating and other advanced technologies have been widely used to improve the electrical performance and environmental adaptability of the hole.

• Advanced packaging technology: such as embedded component technology, fan-out wafer-level packaging, etc., put forward higher requirements for through-hole design and manufacturing, and promote the development of new through-hole technologies such as microholes and arbitrary layer interconnection.

• High-speed and high-frequency applications: With the improvement of PCB performance requirements in 5G communication, data center, automatic driving and other fields, the through hole design needs to take into account signal integrity and power integrity, the use of back drilling, step holes, coplanar waveguide and other technologies, effectively inhibit crosstalk and loss, to meet the needs of high-speed and high-frequency transmission.

In PCB design, through holes are widely used. Signal wiring, power wiring and escape wiring can not be separated from the support of the hole. Signal routing is the most basic routing method in PCB design, and through the hole is the key to realize the transmission of signals between different levels. By setting the hole reasonably, we can realize the fast and stable transmission of the signal and ensure the normal operation of the circuit board.