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PCB/PCBA knowledge
10 PCB Performance Indicators – All You Need to Know

There are some key performance indicators about PCB, including:

  • Physical properties: peel strength/coefficient of thermal expansion/peel strength
  • Chemical properties: Tg/Td/Z-CTE
  • Electrical properties: Dielectric constant/dielectric loss/flame retardancy
  • Environmental performance: water absorption/CAF resistance/CTI

10 PCB performance indicators are listed in detail below for you to test a PCB(Printed Circuit Board):

Table of Contents

Glass transition temperature Tg

Glass transition temperature Tg is an important characteristic parameter of PCB materials. It refers to the temperature at which the material changes from the glass state to the rubber state. When the temperature is lower than Tg, the PCB material is in a rigid glassy state; when the temperature is higher than Tg, the material becomes soft and flexible like rubber, with reversible deformation properties.

Glass transition temperature Tg
Glass transition temperature Tg

The glass transition temperature Tg has an important influence on the performance of PCB materials, which can affect the thermal deformation characteristics, mechanical strength and electrical properties of the material. Therefore, during the PCB design and manufacturing process, it is necessary to select the appropriate material and Tg temperature according to the application environment and requirements. For example, if the application environment temperature is high and the PCB is required to have good thermal stability and mechanical strength, it is necessary to select a PCB material with a higher Tg temperature.

Thermal expansion coefficient

The coefficient of thermal expansion (CTE) of PCB is an important parameter that measures the dimensional stability of the material when temperature changes. The thermal expansion coefficient is divided into X-axis, Y-axis, and Z-axis thermal expansion coefficients. It generally refers to the Z-axis expansion coefficient because it has the greatest impact on material reliability. CTE describes the ratio of the change in material length per unit temperature change to the original length. For PCB materials, the linear thermal expansion coefficient is usually used to measure the linear change in size when the temperature changes.

The thermal expansion coefficient of a PCB has an important impact on its performance and reliability. The lower the CTE value, the better the dimensional stability of the material, and vice versa. In the manufacturing process of electronic equipment, components that meet performance requirements need to be welded (installed) to the printed circuit board through appropriate methods to form a circuit board assembly with certain functions. Since components and substrates are made of different materials, that is, they have different coefficients of thermal expansion, the impact of thermal expansion needs to be evaluated by comprehensively considering the actual conditions in different directions of the material itself and the component material.

Thermal decomposition temperature Td

Thermal decomposition temperature Td refers to the temperature at which PCB materials begin to decompose at high temperatures. This is also one of the important parameters in formulating the PCB hot replacement process.

Thermal decomposition temperature of PCB material will affect its stability and life at operating temperature. If the thermal decomposition temperature of the PCB material is low, it will easily decompose and oxidize at high temperatures, leading to degradation and failure of material performance. Therefore, when selecting PCB materials, its thermal decomposition temperature needs to be considered to ensure its stability and lifespan at operating temperatures.

Copper foil peeling strength

Peel strength is a measure of the bond between a conductor and the base material. The thickness of the copper foil will affect the peel strength value of the test. The default is 1oz thick copper.

Copper foil peeling strength
Copper foil peeling strength

Copper foil peeling strength is one of the important indicators for evaluating PCB quality. Peel strength test generally refers to the bonding strength test between copper foil and base material or copper foil and browning film. Use a universal tensile testing machine to vertically stretch the copper foil at a certain rate to detect the force value when the copper foil peels off from the base material, thereby calculating its peel strength.

Water absorption and hygroscopicity

The material’s ability to resist water absorption when exposed to air or immersed in water. Moisture easily expands and spreads, causing delamination of the substrate. Moisture also affects the substrate’s ability to grow conductive anode filaments (CAF).

Water absorption and hygroscopicity are an important indicator to pay attention to because it will directly affect the performance and stability of the PCB. Here are some key points about PCB water absorption and hygroscopicity:
Influencing factors: The water absorption and hygroscopicity of PCB are mainly affected by its material composition and manufacturing process. For example, some PCB materials may contain hydrophilic groups or pore structures, which will increase the water absorption and hygroscopicity of the PCB.
Performance impact: When PCB absorbs moisture, its key performance parameters such as dielectric constant and thermal expansion coefficient may change. These changes can cause delays or distortions in signal transmission, affecting the performance of the entire electronic device.
Reliability issues: PCBs exposed to high humidity for a long time may absorb water and swell, leading to dimensional changes, deformation, or cracking. These problems will not only affect the installation accuracy of electronic components but may also cause circuit failures and reduce the reliability of electronic equipment.
Protective measures: To reduce the water absorption and hygroscopicity of PCB, some protective measures can be taken. For example, apply a waterproof coating on the PCB surface or use materials with low moisture absorption, etc. In addition, during the design and manufacturing process, the application environment and humidity conditions of the PCB should also be fully considered, and appropriate materials and processes should be selected.

Flame retardancy

The flame retardancy of PCB is an important performance indicator used to evaluate the combustion characteristics of the material after flame ignition. According to different flame retardant properties, PCB can be divided into three grades: V-0, V-1 and V-2.

The testing and evaluation of flame retardant properties are usually carried out using the international standard UL94 and the domestic standard GB/T 2408. During the test, professional flame retardant performance testing instruments were used to conduct ignition tests and vertical burning tests on the PCB. The ignition test is used to judge the self-ignition performance of the material, and the vertical burning test is used to evaluate the flame extension of the material. Through strict testing processes and accurate test data, reliable flame retardant performance evaluation reports can be provided.

Dielectric constant

The dielectric constant of resin is smaller than that of glass cloth. As the resin content increases, the dielectric constant becomes smaller.

Dielectric constant
Dielectric constant

The dielectric constant is an important parameter to measure the electrical properties of insulating materials. Specifically, it represents the relative permittivity of the insulating material filled between the capacitor plates. The larger the dielectric constant, the better the insulation performance.

For PCB boards, the dielectric constant is a very important parameter, because when signals are transmitted on the PCB, the speed and signal completion will be affected by the dielectric constant. Generally speaking, the larger the dielectric constant, the slower the signal transmission speed. Therefore, when selecting PCB board materials, the impact of dielectric constant needs to be considered based on actual needs.

In addition, the dielectric constant is also affected by factors such as temperature and humidity. Therefore, in practical applications, the impact of these factors on the dielectric constant needs to be fully considered to ensure the performance and reliability of electronic devices.

Loss factor

The loss factor (also known as loss tangent or loss tangent) is a parameter that describes the energy loss of a material under the action of an electric field. The larger the loss factor, the higher the energy loss of the material under the action of the electric field.

The loss factor of PCB is mainly affected by factors such as the dielectric constant, conductivity, frequency and temperature of the material. In high-frequency applications, the loss factor of PCB will increase significantly, resulting in attenuation and distortion of signal transmission. Therefore, in high-frequency circuit design, PCB materials with low loss factors need to be selected to ensure signal integrity and stability.

In addition, the PCB manufacturing process will also have an impact on the loss factor. For example, factors such as PCB surface treatment, lamination process and copper foil thickness will have a certain impact on the loss factor. Therefore, in practical applications, appropriate PCB materials and process parameters need to be selected according to specific application needs and manufacturing process requirements to reduce the loss factor and improve the performance and reliability of the circuit.

CAF resistance

The CAF resistance of PCB refers to its ability to resist ion migration, especially in humid environments. CAF, or Conductive Anodic Filament, is an electrochemical reaction that occurs in a humid environment, causing a conductive channel to form between the anode and cathode in the circuit, thereby causing a short circuit.

CAF resistance
CAF resistance

When a PCB is in a humid environment for a long time, the materials inside it may absorb moisture, resulting in increased hygroscopicity of the base material. Therefore, in order to improve the CAF resistance of PCB, materials with low moisture absorption and special surface treatment need to be selected.

Resistance to tracking index CTI

PCB tracking resistance index (CTI) refers to the highest voltage value on the surface of a solid insulating material that can withstand 50 drops of electrolyte without forming leakage traces under the combined action of electric field and electrolyte, expressed in V.

Conclusion

When evaluating PCB performance, examining a range of indicators is crucial for ensuring optimal functionality and reliability. The ten key performance indicators provide a comprehensive view of the PCB’s efficiency, functionality, and quality. By assessing factors such as signal integrity, thermal management, reliability, manufacturing yield, and more, one gains insights into the PCB’s overall performance. A holistic approach to analyzing these indicators enables the identification of strengths and areas for improvement, empowering designers and engineers to refine designs, enhance performance, and achieve higher-quality PCBs for various applications.

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