PCB thermal analysis and thermal design techniques

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PCB thermal analysis and thermal design techniques

1. Source of PCB heat

In addition to the useful work, a part of the power consumed by the power adapter during operation is converted into heat. The heat generated by the power adapter causes the internal temperature to rise rapidly. If the heat is not dissipated in time, the temperature will continue to rise, and the components will fail due to overheating, and the reliability of the power adapter will decrease. SMT increases the mounting density of power adapter components, reduces the effective heat dissipation area, and the temperature rise of the power adapter seriously affects reliability. Therefore, research on the thermal design of the power adapter PCB is very important. The direct cause of the temperature rise of the power adapter PCB is due to the existence of circuit power components, the electronic components have different degrees of power consumption, and the heat intensity varies with the power consumption. The two phenomena of temperature rise in PCB are: 1 local temperature rise or large area temperature rise; 2 short time temperature rise or long time temperature rise.

There are three main sources of heat in the power adapter PCB: the heat of the electronic components, the heat of the PCB itself, and the heat from other parts. Among the three heat sources, the component generates the largest amount of heat, which is the main heat source, followed by the heat generated by the PCB. The external heat input depends on the overall thermal design of the power adapter.

The heat generation of components is determined by their power consumption. Therefore, components with low power consumption should be selected first in design to minimize heat generation. The second is the setting of the working point of the component. Generally, it should be selected within its rated working range. When working in this range, the performance is good, the power consumption is small, and the service life is long. The power device itself generates a large amount of heat, and should be designed to avoid full load operation. For high-power devices, the principle of derating design should be implemented, and the design richness should be appropriately increased, which is beneficial to increasing the stability, reliability, and heat generation of the power adapter.

The PCB is composed of a copper conductor and an insulating dielectric material, and it is generally considered that the insulating dielectric material does not generate heat. The copper conductor has a resistance due to copper itself. When the current passes, it will generate heat. When a small current of mA (milliampere) and μA (microampere) is passed, the heating problem is negligible, but when the current is high (100 mA or more) When you pass, you can't ignore it. It is worth noting that when the copper conductor temperature rises to 85 ° C, the insulating material itself begins to yellow, the current continues to pass, and finally the copper conductor is blown. In particular, the copper conductor in the inner layer of the multilayer PCB is surrounded by a resin having poor heat conductivity, and heat dissipation is difficult, so the temperature inevitably rises, so special attention should be paid to the line width design of the copper conductor. In fact, when designing the PCB layout, the trace width is mainly determined by the heat generation and heat dissipation environment. The cross-sectional area of the copper conductor determines the wire resistance (the signal loss caused by the line resistance in the digital circuit is negligible), and the thermal conductivity of the copper conductor and the insulating substrate affects the temperature rise, which in turn determines the current carrying capacity. For example, the cross-sectional area of the copper conductor is constant. When the allowable current value is 2A and the temperature rise value is lower than 10 ° C, the line width should be designed to be 2 mm for the 35 μm copper foil and 1 mm for the 70 μm copper foil. . It can be concluded that when the cross-sectional area, allowable current and temperature rise value of the copper conductor are constant, the heat dissipation requirement can be satisfied from two aspects of increasing the thickness of the copper foil or increasing the line width of the copper conductor.

2. Circuit thermal analysis

Circuit thermal analysis is divided into three steps: first estimating the heat generated in the component, then estimating the heat emitted by the PCB or heat sink, and finally estimating the ambient temperature at which the component will operate. The PCB or heat sink will dissipate the heat of the component by convection, conduction or radiation. Conductive heat dissipation is mainly through the heat conduction of the power device chip metal lead frame and the copper foil on the PCB. Once the PCB copper foil or discrete heat sink conducts heat, it provides a large enough surface area for convective heat dissipation to dissipate heat into the air.

There are also some difficulties in convection heat dissipation. At high temperatures, the thermal resistance increases. For this reason, thermal resistance is used as a thermal analysis parameter. If the thermal resistance Rja from the junction to the outside is given in the component data, the value indicates the temperature rise when the component is not connected to the heat sink or is not soldered to the PCB. The key thermal resistance in thermal design is the thermal resistance Rjb from the chip to the PCB and the thermal resistance Rjc from the chip to the package surface. Rja can be measured with two JEDEC standard PCBs, one for single-sided PCB and the other for multilayer PCB. If you have Rjb and Rjc specifications, you can estimate the true temperature rise of the component. When measuring Rja, there are no other chips on the PCB. When there are power supplies and other heat-dissipating chips around the components, and when the PCB is in a fanless plastic case with limited space, the actual temperature rise will be higher than the Rja measurement. The value is because the top surface of the plastic package of most components transmits almost no heat. The thermal conductivity of epoxy resin is 0.6 ~ 1W / (m · K) (watts per meter Kelvin), while the thermal conductivity of copper is 400W / (m · K). Therefore, the thermal conductivity of copper is 400 to 600 times higher than that of plastic.

The final step in thermal analysis is to estimate the ambient temperature, which is important. For example, the laboratory air temperature is 25 ° C and the chip on the bench is working at 50 ° C. When these chips are placed at an ambient temperature of 50 ° C, the temperature of the chip will reach 75 ° C. However, in the estimation of the ambient temperature step, it is sometimes impossible to determine the environmental conditions in which the component may work.

When analyzing PCB thermal power consumption, it is generally analyzed from the following aspects.

(1) Electrical power consumption, that is, the power consumption per unit area of the PCB and the power consumption on the PCB.

(2) The structure of the PCB, ie the size and material of the PCB.

(3) PCB mounting method (such as vertical installation, horizontal installation), sealing condition and distance from the housing.

(4) Thermal radiation, ie the emissivity of the PCB surface, the temperature difference between the PCB and the adjacent surface and their absolute temperature.

(5) Heat conduction, that is, the conduction of the radiator and other mounting structural components.

(6) Thermal convection, that is, natural convection and forced cooling convection.

The analysis of the above factors is an effective way to solve the PCB temperature rise. Often in a product and system, these factors are interrelated and dependent. Most of the factors should be analyzed according to the actual situation. Only for a specific actual situation can the parameters such as temperature rise and power consumption be calculated or estimated correctly.

3. Basic requirements for PCB thermal design

When designing a PCB, especially for surface mount PCB design, the thermal expansion coefficient matching problem of the material should first be considered. There are three types of package substrates for components: rigid organic package substrate, flexible organic package substrate and ceramic package substrate. The substrate is packaged by four methods: molding technology, molded ceramic technology, laminated ceramic technology and laminated plastic. The materials used for the substrate are mainly high-temperature epoxy resin, BT resin, polyimide, ceramic, and refractory glass. These materials have high temperature resistance and low thermal expansion coefficients in the X and Y directions. When selecting the PCB material, you should understand the package form of the component and the material of the substrate, and consider the temperature variation range of the component soldering process. Select the substrate with the thermal expansion coefficient to match the thermal stress caused by the difference in thermal expansion coefficient of the material. .

Many components use ceramic package substrate, its thermal expansion coefficient is typically (5 ~ 7) × 10-6 / ° C, the thermal expansion coefficient of leadless ceramic chip carrier LCCC is (3.5 ~ 7 ~ 8) × 10-6 / °C. Some component substrates use the same materials as some PCB substrates, such as PI, BT and heat-resistant epoxy. When selecting the substrate of the PCB, the thermal expansion coefficient of the substrate should be considered as close as possible to the thermal expansion coefficient of the material of the component substrate.

The conductor of the PCB is temperature rise due to the passing current, and the ambient temperature should not exceed 125 ° C (typical values are common, depending on the substrate selected). Since components are mounted on the PCB and also emit a portion of the heat that affects the operating temperature of the PCB, these factors should be considered when selecting the PCB material and PCB design. The hot spot temperature should not exceed 125 °C. The PCB substrate should be selected with a thicker copper foil as much as possible. In special cases, a substrate with a small thermal resistance such as an aluminum base or a ceramic base can be selected, and the multilayer structure also contributes to the thermal design of the PCB.

Currently widely used PCB substrates are copper-clad epoxy glass cloth substrates or phenolic resin glass cloth substrates, and a small amount of paper-based copper-clad substrates. Although these substrates have excellent electrical properties and processing properties, they have poor heat dissipation. As a heat-dissipating means for high-heat-generating components, it is hardly expected to conduct heat from the resin of the PCB itself, but to dissipate heat from the surface of the components to the surrounding air. However, as electronic products enter the era of miniaturization, high-density mounting, and high-heat assembly, it is not enough to dissipate heat by a very small component surface area. At the same time, due to the large number of surface mount components such as QFP and BGA, the heat generated by the components is transferred to the PCB in large quantities. Therefore, the best way to solve the heat dissipation is to improve the heat dissipation capability of the PCB itself in direct contact with the heat generating components. The PCB is conducted out or emitted.

4. PCB thermal design

There are three measures in the PCB thermal design: power reduction, heat dissipation and layout. The reduction of heat is not to generate heat; the heat dissipation is to conduct or dissipate heat, which does not affect the components; the layout is that if the heat is not dissipated, the heat sensitive components can be isolated by layout. Reducing consumption is the most fundamental solution. There are two main approaches to derating and low-power design, but they need to be analyzed in combination with specific designs. When selecting components, try to use components with small heat generation, such as chip resistors, wirewound resistors (less carbon film resistors), monolithic capacitors, tantalum capacitors (less paper capacitors), MOS, CMOS circuits (less used)锗 tube), surface mount devices, etc. In addition to selecting low-power components, temperature compensation and control of some temperature-sensitive special components is also one of the solutions.

Derating needs to consider the way of reducing consumption. Suppose a thin wire is nominally capable of passing 10A of current. The current generates more heat on it, and the wire is thickened to increase the margin. It is nominally passed through 20A. When the current is passed through 10A, the heat loss due to internal resistance is reduced and the heat is small. Moreover, because of the derating design, when the ambient temperature rises, in the case where the performance of the component is degraded, because of the margin, even if the performance is degraded, the requirement can be satisfied. Under the given conditions, when the temperature of the components in the circuit rises above the reliability guaranteed temperature, appropriate heat dissipation measures should be taken to lower the temperature to the reliability working range, which is the ultimate goal of thermal design.

Heat dissipation is the main content of PCB thermal design. For PCBs, there are three basic types of heat dissipation: thermal conduction, convection, and radiation. Thermal conduction and convection are the main means of heat dissipation. The common way of dissipating heat is to use a heat sink to conduct heat from the heat source and dissipate it by air convection. Radiation is the use of electromagnetic waves in space to dissipate heat, which has a small amount of heat dissipation, and is usually used as an auxiliary means of heat dissipation.

The purpose of PCB thermal design is to take appropriate measures and methods to reduce the temperature of components and PCB temperature, so that the system works properly at the right temperature. From the perspective of facilitating heat dissipation, the PCB is preferably mounted upright, and the distance between the PCB and the PCB is generally not less than 2 cm.