PCB Blog - Key points and principles of selecting fr4 pcb varistor

The awareness of environmental protection in the field of industrial production of fr4 pcb is becoming stronger and stronger. Domestic and foreign regulations have been formulated to clearly limit the use of toxic materials. Therefore, the development of green solder that can avoid pollution and replace traditional alloys has become one of the important issues facing the brazing industry. For example, many researchers at home and abroad have been or are working on the development of Lead - Free Solder and Cadmium - Free Solder. At the same time, the newly developed solder alloy should follow the principle of reducing cost and improving performance. For example, the soft solder alloy used for chip soldering is developing towards high strength and high reliability, and its development principle is that the alloy does not contain precious metals, and its mechanical properties are between soft solder and gold-based solder.

Sn/Pb solder has been widely used, which is inseparable from its excellent performance and low cost. The Pb-based solder is generally soft and can absorb the mechanical strain caused by the thermal expansion mismatch between the chip and the substrate. However, if the mechanical stress is repeated continuously (plus thermal cycle), strain accumulation will occur at the grain boundary of the filler metal, leading to microcracks, resulting in an increase in thermal resistance, and finally fatigue damage. Moreover, Pb and its compounds will have carcinogenic effect after accumulating to a certain extent in human body. Sn is often used as a filler metal base because it is easy to form intermetallic compounds with various metals and has low melting point. The wettability of solder based on Sn is better than that based on Pb. At the same time, Pb can improve the oxidation resistance of Sn-based solder and reduce the melting point of Sn-based solder.

Therefore, the matrix of highly reliable solder for SMT is mainly Sn and Pb alloys. At present, the main solder used for chips is Sn/Pb based materials. But at present, electronic products are developing towards miniaturization, high density and high performance, and the size of solder joint is getting smaller and smaller, while the thermal, electrical and mechanical loads it bears are getting higher and higher, which requires the solder to have excellent fatigue and creep resistance. The creep resistance of traditional Sn/Pb solder is poor, which can not meet the use requirements. In other fields, there are also constant requirements for the properties of solder alloys, such as the need for composite solder in the automotive industry, the need for amorphous solder in the brazing of ceramics and metals, and the requirement for low melting point of solder in heat-sensitive electronic components. Therefore, the development of green solder alloys with ideal performance and cost has become a research hotspot.

1. Application principle of zinc oxide varistor

Varistor is a voltage limiting protection device. Using the nonlinear characteristics of the varistor, when the overvoltage occurs between the two poles of the varistor, the varistor can clamp the voltage to a relatively fixed voltage value, thus realizing the protection of the subsequent circuit, and enabling the PCBA board to have a certain self-protection ability. The main parameters of the varistor include: varistor voltage, current capacity, junction capacitance, response time, etc. The response time of the varistor is ns, which is faster than the air discharge tube and slightly slower than the TVS tube. Generally, the response speed of the varistor used for overvoltage protection of electronic circuits can meet the requirements. The junction capacitance of the varistor is generally in the order of hundreds to thousands of pF. In many cases, it is not suitable to be directly applied to the protection of high-frequency signal lines. When it is applied to the protection of AC circuits, because its junction capacitance is relatively large and increases the leakage current, it needs to be fully considered when designing the protection circuit. The flow capacity of the varistor is larger, but smaller than that of the gas discharge tube. The varistor is used in parallel with the protected electrical equipment or components. When lightning overvoltage or transient operation overvoltage Vs occurs in the circuit, the varistor and the protected equipment and components bear Vs at the same time. Because of the fast response speed of the varistor, it quickly presents excellent nonlinear conductivity characteristics in nanosecond time. At this time, the voltage at both ends of the varistor drops rapidly, far less than Vs, so that the actual withstand voltage on the protected equipment and components is far lower than the overvoltage Vs, So as to protect the equipment and components from the impact of overvoltage.

2. Selection of varistor voltage of zinc oxide varistor

Select the voltage V1mA under the specified current of the varistor according to the protected power supply voltage. The general selection principles are:

For DC circuit: V1mA ≥ 2.0VDC

For AC circuit: V1mA ≥ 2.2V effective value

In particular, it is pointed out that the selection standard of the varistor voltage should be higher than the power supply voltage. While the device can be protected, the varistor with high voltage should be selected as far as possible, which can not only protect the device, but also improve the service life of the varistor. For example, the withstand voltage of the device to be protected is Vdc=550Vdc, and the working voltage of the device is V=300Vdc, so we should select the varistor with the voltage of 470V, the range of the voltage of the varistor is (423-517), the negative error of the voltage of the varistor is 470-47=423Vdc, which is greater than the device's supply voltage of 300Vac, and the positive error is 470+47=517Vdc, which is less than the device's withstand voltage of 550Vdc. Attention must also be paid to:

1) It must be ensured that the continuous working voltage will not exceed the allowable value when the voltage fluctuates, otherwise the service life of the varistor will be shortened;

2) When using the varistor between the power line and the ground, sometimes the voltage between the line and the ground will rise due to poor grounding, so the varistor with higher nominal voltage than that used between the lines is usually used.

3. Selection of flow rate

Generally, the flow rate given by the product is the current value that the product can withstand when conducting pulse test according to the waveform, impact times and gap time given by the product standard. The impact number that the product can withstand is a function of waveform, amplitude and gap time. When the current waveform amplitude is reduced by 50%, the impact number can be doubled. Therefore, in practical applications, the surge current absorbed by the varistor should be less than the flux of the product fr4 pcb.