circuit board fabrication

Which type of PCB substrate material is right for your PCB?

The basic characteristics of PCB depend on the performance of its substrate material. In order to improve the performance of the circuit board, the performance of the substrate material must be optimized first.

In recent years, the focus of the circuit board market has shifted from traditional hardware products, such as desktop computers, to wireless communications, such as servers and mobile terminals. Mobile communication devices such as smartphones have promoted the development of PCBs towards high density, lightweight and multifunctionality. The realization of circuit technology is inseparable from the support of substrate materials, and its technical requirements are closely related to the performance of PCBs.

The quality and reliability of PCBs and final products depend to a large extent on the selection of substrate materials. Different types of PCBs have different characteristics and requirements. Next, we will introduce in detail how to select suitable substrate materials based on these characteristics.

When there is a requirement for copper foil
PCB boards are moving towards higher density and finer lines, especially HDI PCBs. The traditional method of forming lines is to use thin copper foil substrates through the process of imaging and etching. However, as the requirements for line width (L) and line spacing (S) become smaller and smaller, thin copper foil substrates with a thickness of 9μm to 12μm can no longer meet the needs. Therefore, many PCB manufacturers have turned to etching to reduce copper foil, but this method not only involves too many steps, but also has difficulty controlling thickness and is more expensive. Therefore, thinner copper foil is more ideal.

In addition to the thickness of the copper foil, the current fine lines also have requirements for the surface roughness of the copper foil. In order to improve the bonding ability of the copper foil with the substrate material and ensure the peeling strength of the conductor, the copper foil surface needs to be treated to reduce the roughness. In addition, the protrusions on the surface of the copper foil increase the peeling strength, but it should be noted that the protrusions may cause over-etching problems when the line is etched. Therefore, for fine lines, low-roughness copper foil is required. Even if the roughness is reduced, the peeling strength of the conductor still needs to be maintained, so special surface treatment needs to be applied to the surface of the copper foil and the substrate material.

When there is a requirement for insulating medium
The main technical feature of HDI PCB lies in the stacking process. In order to meet the requirements of high-density fine lines, an insulating medium layer with low dielectric properties, low thermal expansion coefficient and high heat resistance is required.

At present, MSPA technology has been applied to the mass production of fine lines, while SAP is used to manufacture lines with line width and line spacing less than 10μm. High-density and thin PCBs make HDI PCBs no longer need core layers, but can be interconnected on any layer. Therefore, for HDI PCBs with the same functions, their area and thickness will be reduced by 25% compared with HDI PCBs with core layers. Therefore, it is necessary to apply thinner insulating layers on these HDI PCBs to ensure electrical performance.

When there is a requirement for high frequency and high speed
With the development of communication technology, the demand for high frequency and high speed transmission is also increasing. In order to meet these needs, substrate materials with good dielectric properties and low surface roughness are needed.

Commonly used high-frequency substrate materials include fluororesins, PPO or PPE resins, and modified epoxy resins. Different types of high-frequency substrate materials can be selected according to the frequency of the product. For example, fluorine-based dielectric substrates such as PTFE are usually used for products with frequencies above 5GHz.

It should be noted that when high-frequency signals are transmitted, the surface roughness of the copper foil will affect the signal transmission loss, so the surface roughness of the copper foil needs to be controlled and the roughness needs to be reduced as much as possible. For signals exceeding 10GHz, the surface roughness of the copper foil is required to be less than 1μm, and it is best to use an ultra-surface copper foil with a surface roughness of 0.04μm. In addition, it is necessary to select a suitable oxidation treatment and bonding resin system.

When high thermal resistance and heat dissipation are required
As the power of electronic devices increases, thermal management becomes increasingly important. Selecting a substrate material with good heat dissipation performance can effectively reduce the temperature of the device. Common heat dissipation substrate materials include metal substrates and aluminum substrates. Compared with traditional heat dissipation methods, the use of metal substrates can dissipate heat better, and is smaller and less expensive. The key lies in the bonding performance between the metal substrate and the circuit layer.

How to choose the substrate material of PCB?
In the modern electronic era, the miniaturization and thinning of electronic devices have made rigid PCB and flexible PCB more and more common. So, what type of substrate material is suitable for them?

1. Rigid PCB: Generally, FR-4 glass fiber composite material is used as the substrate material, which has high heat resistance and low thermal expansion coefficient, and can adapt to different application scenarios.

2. Flexible PCB: It is usually used in fields such as smartphones, wearable devices, medical devices and robots, mainly using polytetrafluoroethylene and advanced polyimide substrates. In order to meet the requirements of high-speed and high-frequency signal transmission, it is necessary to pay attention to the dielectric constant and dielectric loss of flexible substrate materials.

By selecting appropriate substrate materials, the needs of different types of PCBs can be met, thereby improving the performance and reliability of the product.