What are the different types of Printed Circuit Boards
Generally, printed circuit boards (PCB) are categorized according to the number of layers, substrate type, and frequency. PCB are divided into single-sided PCB, double-sided PCB, and multi-layer PCB according to the material. At the same time, PCB can also be divided into rigid PCB, flexible PCB, and rigid-flex PCB based on the material.
The single-sided PCB is the simplest type of printed circuit board. The figure below shows the structure of a single-sided PCB. The blue, yellow, and green layers are the substrate, the conductive copper layer, and the solder mask respectively. In the single-sided PCB, only one side of the substrate is coated with a copper layer, and this side is where the components are electrically connected. The single-sided PCB is cost-effective and easy to manufacture. But it has many restrictions on the circuit design because conductive paths cannot cross or overlap. Therefore, the current single-sided PCB is only used for simple circuits such as electronic toys, calculators, etc.
Figure 1: The Structure of the Single-Sided PCB
Unlike the single-sided PCB, the double-sided PCB has copper layers on both sides of the substrate. Meanwhile, components can be attached on both sides. Both through-hole and surface-mount technologies are widely used to make circuit connections on both sides.
Figure 2: The Structure of the Double-Sided PCB
The plated through holes(PTH) in the double-sided PCB act as a bridge. The walls of the plated through holes are generally plated with copper by an electrolysis process to electrically connect the circuits on one side to the other side. Due to the increased circuit density of the double-sided PCB, the double-sided PCB is suitable for more complex circuits. Compared with a single-sided PCB, it is flexible and compact. Various applications such as power monitoring and amplifiers are using the double-sided PCB.
Figure 3: Plated Through Holes (PTH) on the Double-Sided PCB
The multi-layer PCB is composed of more than 2 conductive layers, two of which are on the outer surfaces and the remaining layers are integrated into insulating layers. Between each 2 layers is the prepreg, which is a dielectric layer and can be made very thin. The number of layers in the PCB represents the number of independent conductive copper layers. Generally, the top and bottom layers are single-sided PCB and inner layers are double-sided PCB, all of which are laminated together under high temperature and pressure to form a single board. Compared with single-sided and double-sided PCB, the multi-layer PCB is good for high-speed circuits such as mobile phones and laptops, and is more flexible and compact. The figure below is an example of a 6-layer PCB.
Figure 4: The 6-layer PCB
Regarding the electrical connection between different layers, it is usually achieved through vias: plated through holes(PTH), blind vias, and buried vias. Plated through holes(PTH) access all layers of the multi-layer PCB from top to bottom. Blind vias connect either of the outermost layers of the PCB with the adjacent inner layers. Buried vias that are not visible from the outside simply connect between the internal circuit layers.
Figure 5: The vias
The substrate materials of the rigid PCB are solid materials such as fiberglass, which cannot be bent or folded. Rigid PCB may be any of the single-sided, double-sided, or multilayer PCB, depending on the needs. The main advantages include low electronic noise and vibration absorption. But rigid PCB cannot be modified or changed once they are made. The applications include laptops, temperature sensors, GPS equipment, etc.
Figure 6: The Rigid PCB
Unlike rigid PCB, flexible PCB is generally composed of rolled annealed (RA) copper foil and flexible plastic film. It allows the circuit board to adapt to a form in which the rigid PCB cannot be rotated or moved during use without damaging the circuit on the printed circuit board. Flexible PCB saves cost and a lot of space and greatly reduces the board weight and the size of the application product. In other words, it is an ideal choice for a variety of applications that require high signal trace density. Flexible PCB can be any of the single-sided, double-sided, or multilayer PCB, depending on the needs. The applications of flexible PCB include complex electronics products, organic light-emitting diode (OLED) fabrication, LCD fabrication, etc.
Figure 7: Flexible PCB
Rigid-flex PCB is a combination of the rigid printed circuit board and the flexible printed circuit board after pressing and other processes. In rigid-flex PCB, interconnections between rigid circuit boards are the flexible parts of the board. Therefore, this type of board can be folded or continuously bent and is usually formed into a curved shape during the manufacturing process. Rigid-flex PCB can be used for products with special requirements since it has both rigid and flexible areas, which can save the internal space and volume of the product, and improve the performance of the product such as higher connection reliability. However, rigid-flex PCB requires multiple production processes, leading to a low yield rate, relatively long production cycle, and high price. The applications of rigid-flex PCB are mainly in the medical, consumer electronics, and aerospace fields.
Figure 8: Rigid-flex PCB
As a special printed circuit board, high-frequency PCB offers a high-frequency range of 500MHz to 2GHz. It provides faster signal flow rates, which is suitable for high-speed designs. It has very high requirements for various physical properties, accuracy, and technical parameters. Firstly, the substrate material of high-frequency PCB should have the features of heat resistance, chemical resistance and good impacting resistance. Secondly, the dissipation factor (Df) of the board must be small, which mainly affects the quality of signal transmission. The smaller the dissipation factor, the smaller the signal loss. Furthermore, the dielectric constant(Dk) of the board must be small and stable because the signal transmission rate is inversely proportional to the square root of the material’s dielectric constant. In other words, the high dielectric constant is likely to cause signal transmission delays. High-frequency PCB substrate should also have a low water absorption characteristic because high water absorption will cause loss of both dissipation factor and dielectric constant when the board gets damp. High-frequency PCB is often used in collision avoidance systems (CAS), satellite systems, radio systems, mobile applications, etc.
Figure 9: High-Frequency PCB