First, the importance of thermal design
Power supply products most of the electrical energy consumed by electronic devices during the working life is converted into heat emission, except for useful work.

The heat generated by electronic equipment makes the internal temperature rise rapidly. If the heat is not released in time, the equipment will continue to heat up, the device will fail due to overheating, and the reliability of electronic equipment will decline.

SMT increases the installation density of electronic equipment, reduces the effective heat dissipation area, and seriously affects the reliability of equipment temperature rise. Therefore, the research on thermal design is very important.

Analysis of temperature rise of printed circuit board
The direct cause of PCB temperature rise is the power consumption of electronic devices, which varies with the power consumption. Two phenomena of temperature rise in PCB:

(1) local temperature rise or large area temperature rise;

(2) short-term or long-term temperature rise.

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

1. Electrical power consumption

(1) analysis of power consumption per unit area;

(2) analyze the power distribution on PCB.

2. PCB structure

(1) the size of the printed board;

(2) printed board materials.

3. PCB installation mode

(1) installation mode (such as vertical installation and horizontal installation);

(2) sealing condition and distance from the housing.

4. Thermal radiation

(1) radiation coefficient of PCB surface;

(2) the temperature difference between the PCB and the adjacent surfaces and their absolute temperature;

5. Heat transfer

(1) install radiators;

(2) conduction of other installation structural parts.

6. Convection of heat

(1) natural convection;

(2) forced cooling convection.

The analysis of the above factors of PCB is an effective way to solve the temperature rise of PCB.

Most of the factors should be analyzed according to the actual situation, and only for a specific practical situation can the temperature rise and power consumption and other parameters be correctly calculated or estimated.

Third, the principle of thermal design
1, material selection

(1) the temperature rise of the PCB wires due to the current and the specified ambient temperature should not exceed 125 ℃ (the typical value commonly used). According to the selection of the plate may be different).

Since the components installed on the PCB also emit some heat, affecting the working temperature, the selection of materials and PCB design should take these factors into account, the hot temperature should not exceed 125 ℃, as far as possible to choose a thicker copper clad foil. ;

(2) aluminum base, ceramic base and other plates with small thermal resistance can be selected under special circumstances;

(3) multi-layer structure is helpful for PCB thermal design.

2. Ensure smooth heat dissipation channel

(1) make full use of components arrangement, copper skin, window opening and heat dissipation hole technology to establish a reasonable and effective low thermal resistance channel, to ensure the smooth heat export of PCB;

(2) heat dissipation through hole Settings:

Design some heat dissipation through hole and blind hole, can effectively improve the heat dissipation area and reduce the thermal resistance, improve the power density of the circuit board.

For example, a conducting hole is set up on the pads of LCCC devices. In the process of circuit production, solder fills it, so as to improve the thermal conductivity. The heat generated during circuit operation can be quickly transferred to the metal heat dissipation layer or the copper mooring set on the back through the through hole or blind hole.

In some specific cases, special design and use of the heat dissipation layer of the circuit board, heat dissipation material is generally copper/molybdenum and other materials, such as some modules on the power supply of the printed board;

(3) use of heat conducting materials:

In order to reduce the thermal resistance in the heat conduction process, the heat-conducting material is used on the contact surface between the high-power device and the substrate to improve the heat conduction efficiency.

(4) process method:

In order to improve the heat dissipation condition, a small amount of small copper material can be mixed into the solder paste, and the welding spot under the device will have a certain height after reflow welding.

The gap between the device and PCB is increased and convection heat dissipation is increased.

3. Arrangement requirements of components

(1) perform software thermal analysis on PCB, and design and control the internal maximum temperature rise;

(2) specially designed components with high heating and high radiation can be considered to be installed on a printed board;

(3) uniform distribution of heat capacity on the board, pay attention not to centralize large power consumption devices. If it is unavoidable, put the short components on the upstream of the airflow, and ensure sufficient cooling air flow through the heat consumption concentration area;

(4) make the heat transfer pathway as short as possible;

(5) make the heat transfer cross section as large as possible;

(6) the influence of thermal radiation on surrounding parts should be taken into account in component layout. Heat-sensitive parts and components (including semiconductor devices) should be kept away from heat sources or isolated;

(7) (liquid medium) capacitors should be kept away from heat sources;

(8) ensure that forced ventilation is in the same direction as natural ventilation;

(9) the air duct of additional subplates and devices is in the same direction as the ventilation;

(10) make the intake and exhaust as far as possible with sufficient distance;

(11) heating devices should be placed as far as possible above the product, and should be in the air flow path when conditions permit;

(12) the components with large heat or current should not be placed in the corners and edges of the printed board. If possible, they should be installed on the radiator, away from other devices, and ensure the heat dissipation channel is unobstructed;

(13) (small signal amplifier peripheral devices) use small temperature drift devices as far as possible;

(14) use metal chassis or chassis to dissipate heat as much as possible.

4. Requirements for wiring

(1) panel selection (reasonable design of PCB structure);

(2) wiring rules;

(3) plan the minimum channel width according to the device current density; Pay special attention to the channel wiring at the junction;

(4) the high-current lines should be as superficial as possible; If the requirements cannot be met, bus-bar can be considered.

(5) try to reduce the thermal resistance of the contact surface. Therefore, the heat conduction area should be increased. The contact plane shall be flat and smooth, and can be coated with heat-conducting silicone grease if necessary;

(6) consider the stress balance measures at thermal stress points and add thick lines;

(7) heat dissipation copper skin shall adopt the windowing method of heat dissipation stress, and use heat dissipation resistance welding to properly windowing;

(8) a large surface area of copper foil may be used;

(9) a large welding pad is adopted for the grounding mounting hole on the PCB to make full use of the mounting bolts and copper foil on the PCB surface for heat dissipation;

(10) put as many metallized through holes as possible, and make the aperture and disk surface as large as possible, relying on the through hole to help heat dissipation;

(11) supplementary means for device heat dissipation;

(12) if a large surface area of copper foil can be guaranteed, the method of additional radiator is not adopted for economic consideration;

(13) calculate appropriate surface heat dissipation area of copper foil according to device power consumption, ambient temperature and allowable maximum junction temperature (guarantee principle tj≤ (0.5 ~ 0.8) tjmax).

4. Thermal simulation (thermal analysis)
Thermal analysis assists the designer in determining the electrical properties of the components on the PCB and in determining whether the components or PCB will burn out due to high temperatures.

A simple thermal analysis simply calculates the average temperature of a PCB, while a complex one involves building transient models of electronic devices with multiple PCBS and thousands of components.

No matter how careful analysts are in building thermal models of electronic devices, PCBS, and electronic components, the accuracy of thermal analysis ultimately depends on the accuracy of component power consumption provided by PCB designers.

Weight and physical size are very important in many applications. If the actual power consumption of the components is small, the safety factor of the design may be too high, so that the PCB design adopts the power consumption value that is inconsistent with the reality or too conservative as the basis for thermal analysis.

The opposite (and more serious) is that the thermal safety factor design is too low, meaning that the components actually operate at a higher temperature than the analysts predicted. This problem is usually solved by cooling the PCB with cooling devices or fans.

These add-ons add cost, increase manufacturing time, and add a layer of instability to reliability by including fans in the design, so the PCB now USES active rather than passive cooling methods (such as natural convection, conduction, and radiative cooling) to allow components to operate at lower temperatures.

Poor thermal design will ultimately increase costs and reduce reliability, as is likely to happen in all PCB designs. Taking some effort to accurately determine component power consumption and performing thermal analysis of the PCB will help produce small, functional products.

Accurate thermal models and component power consumption should be used to avoid reducing PCB design efficiency.

1. Calculation of component power consumption

Accurately determining the power consumption of PCB components is an iterative process. PCB designers need to know the component temperature to determine the power loss, and thermal analysts need to know the power loss to input into the thermal model.

Designers first guess a working environment temperature or obtained from the initial thermal analysis estimates, and the element power input to refine heat model, calculate the PCB and related components “node” (or hot), the temperature of the second step is to use the new temperature to the power consumption of the computing element, and calculate the power input for the next step in the process of thermal analysis again.

Ideally, the process continues until the value does not change. However, PCB designers are often under pressure to complete tasks quickly, and they do not have enough time for time-consuming and repetitive determination of electrical and thermal performance of components.

A simplified method is to estimate the total power consumption of PCB as a uniform heat flux acting on the whole PCB surface. Thermal analysis can predict the average ambient temperature, allowing designers to calculate the power consumption of components and to know if additional work is needed by further repeating the calculation of component temperatures.

Manufacturers of electronic components generally provide specifications for the components, including the maximum operating temperature.

The performance of the components is usually affected by the environment temperature or the internal temperature of the components. While military products often use ceramic parts, the highest operating temperature is 125 ℃, the highest rated temperature is usually 105 ℃. PCB designers can use the “temperature/power” curve provided by the device manufacturer to determine the power consumption of components at a given temperature.

Transient thermal analysis is the most accurate method to calculate the temperature of the element, but it is very difficult to determine the instantaneous power consumption of the element.

A better tradeoff is to do the rating and worst-case analysis under steady state conditions.

PCB is affected by the various types of heat, can be applied to the typical thermal boundary conditions including: natural or forced convection from the surface before and after surface before and after thermal radiation, from the edge of PCB to equipment shell conduction, through rigid or flexible connector to the other PCB conduction, from PCB to support (bolts or adhesion fixed) conduction, interlayer between two PCB transmission of radiator.

There are many forms of thermal modeling tools available. Basic thermal modeling and analysis tools include general tools for analyzing arbitrary structures, computational fluid dynamics (CFD) tools for system flow/heat transfer analysis, and PCB applications for detailed PCB and component modeling.

2. Basic process

On the premise of not affecting and helping to improve the electrical performance of the system, accelerate PCB thermal design according to the mature experience provided.

On the basis of system and thermal analysis prediction and device-level thermal design, design defects are found through plate-level thermal simulation prediction, and system-level solutions or device-level change solutions are provided.

The effect of thermal design is tested by thermal performance measurement, and the applicability and effectiveness of the scheme are evaluated.

Revise and accumulate thermal simulation model, accelerate thermal simulation speed, improve thermal simulation accuracy, and supplement PCB thermal design experience through continuous practice process of estimating, design-measure-feedback cycle.