How important is board thermal design simulation? How to do it?

How important is board thermal design simulation? How to do it?

The Importance of Thermal Design

Power products The electrical energy consumed by electronic equipment during work, except for useful work, is mostly converted into heat and dissipated.

The heat generated by electronic equipment causes the internal temperature to rise rapidly. If the heat is not dissipated 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 the temperature rise of the equipment seriously affects the reliability. Therefore, the research on thermal design is very important.

2. Analysis of printed circuit board temperature rise factors

The direct cause of the temperature rise of the printed board is due to the existence of circuit power consumption devices, electronic devices have power consumption to varying degrees, and the heating intensity varies with the power consumption.

Two phenomena of temperature rise in printed boards:

Local temperature rise or large area temperature rise;

Short-term temperature rise or long-term temperature rise.

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

1. Electrical power consumption

(1) Analyze power consumption per unit area;

(2) Analyze the distribution of power consumption on the PCB.

2. The structure of the printed board

(1) The size of the printed board;

(2) The material of the printed board.

3. The installation method of the printed circuit board

(1) Installation method (such as vertical installation, horizontal installation);

(2) The sealing condition and the distance from the casing.

4. Heat radiation

(1) The radiation coefficient of the surface of the printed board;

(2) The temperature difference between the printed board and the adjacent surface and their temperature;

5. Heat conduction

(1) Install the radiator;

(2) Conduction of other installation structural parts.

6. Thermal convection

(1) Natural convection;

(2) Forced cooling convection.

The analysis of the above factors from the PCB is an effective way to solve the temperature rise of the printed board, and these factors are often interrelated and dependent in a product and system.

Most factors should be analyzed according to the actual situation. Only for a specific actual situation can parameters such as temperature rise and power consumption be calculated or estimated correctly.

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Thermal Design Principles

1. Material selection

(1) The temperature rise of the wires of the printed board due to the current passing plus the specified ambient temperature should not exceed 125 ℃ (commonly used typical values. It may be different according to the selected board).

Since the components installed on the printed board also emit some heat, which affects the working temperature, these factors should be taken into account when selecting materials and printed board design. The hot spot temperature should not exceed 125 ℃, and a thicker copper clad foil should be selected as much as possible.

(2) Under special circumstances, plates with low thermal resistance such as aluminum base and ceramic base can be selected.

(3) The use of multi-layer board structure is helpful for PCB thermal design.

2. Ensure that the heat dissipation channel is unblocked

(1) Make full use of technologies such as component arrangement, copper skin, window opening and heat dissipation holes to establish reasonable and effective low thermal resistance channels to ensure that heat is smoothly exported to the PCB;

(2) Setting of cooling vias:

Designing some heat dissipation through holes and blind holes can effectively increase the heat dissipation area and reduce thermal resistance, and increase the power density of the circuit board.

For example, via holes are set up on pads of LCCC devices. In the process of circuit production, it is filled with solder to improve the thermal conductivity, and the heat generated when the circuit is working can be quickly transmitted to the metal heat dissipation layer or the copper pad on the back through the through hole or blind hole to dissipate.

In some specific cases, circuit boards with heat dissipation layers are specially designed and used, and the heat dissipation materials are generally copper/molybdenum and other materials, such as printed boards used in some module power supplies;

(3) Use of thermally conductive materials:

In order to reduce the thermal resistance of the heat conduction process, heat conduction materials are used on the contact surface between the high power consumption device and the substrate to improve the heat conduction efficiency;

(4) Process method:

For some areas with devices on both sides, it is easy to cause local high temperature. In order to improve the heat dissipation conditions, a small amount of fine copper material can be mixed into the solder paste. After reflow soldering, the solder joints under the devices will have a certain height.

The gap between the device and the printed board is increased, and the convection heat dissipation is increased.

3. Arrangement requirements of components

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

(2) It may be considered to design and install components with high heat generation and large radiation on a printed board;

(3) The heat capacity of the board surface is evenly distributed. Be careful not to arrange large power consumption devices in a concentrated manner. If it is unavoidable, place short components in the upstream of the air flow and ensure sufficient cooling air flow through the concentrated heat consumption area;

(4) Make the heat transfer path as short as possible;

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

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

(7) (liquid medium) keep the capacitor away from the heat source;

(8) Pay attention to make the direction of forced ventilation consistent with natural ventilation;

(9) Additional sub-boards and device air ducts are consistent with the ventilation direction;

(10) As far as possible, there should be a sufficient distance between the intake air and the exhaust air;

(11) The heating device should be placed above the product as much as possible, and should be placed on the air flow channel when conditions permit;

(12) Components with high heat or high current should not be placed on the corners and edges of the printed board. As long as possible, they should be installed on the radiator and kept away from other components, and ensure that the heat dissipation channel is unobstructed;

(13) (Peripheral devices of small signal amplifiers) Use devices with small temperature drift as much as possible;

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

4. Requirements for wiring

(1) Plate selection (reasonable design of printed board structure);

(2) Wiring rules;

(3) Plan the channel width according to the current density of the device; 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 bars can be considered;

(5) To minimize the thermal resistance of the contact surface. For this reason, the heat conduction area should be increased; the contact plane should be flat and smooth, and heat-conducting silicone grease can be coated if necessary;

(6) Consider stress balance measures at thermal stress points and thicken lines;

(7) The heat-dissipating copper skin needs to adopt the window opening method of heat dissipation stress, and use the heat dissipation solder mask to open the window appropriately;

(8) Use large-area copper foil on the surface as possible;

(9) Use a larger pad for the ground mounting hole on the printed board to make full use of the mounting bolts and the copper foil on the surface of the printed board for heat dissipation;

(10) Place as many metallized vias as possible, and the aperture and disk surface shall be as large as possible, relying on vias to help dissipate heat;

(11) Supplementary means for device heat dissipation;

(12) In the case of using a large area of copper foil on the surface, it is not necessary to use an additional heat sink for economic considerations;

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

Thermal Simulation (Thermal Analysis)

Thermal analysis can assist designers to determine the electrical performance of components on the PCB, and help designers determine whether components or PCBs will burn out due to high temperatures.

Simple thermal analysis only calculates the average temperature of the PCB, while complex ones need to establish a transient model for electronic equipment containing multiple PCBs and thousands of components.

No matter how careful the analyst is in building a thermal model of the electronic device, PCB, and electronic components, the accuracy of the thermal analysis ultimately depends on the accuracy of the component power dissipation provided by the PCB designer.

In many applications, weight and physical size are very important. If the actual power consumption of the components is very small, the safety factor of the design may be too high, so that the design of the PCB is based on the power consumption values of the components that do not match the actual or are too conservative. thermal analysis.

On the contrary (and more serious) is that the design of the thermal safety factor is too low, that is, the temperature of the component during actual operation is higher than the analyst's prediction. Such problems generally need to be installed on the PCB by installing a cooling device or a fan. Cool to settle.

These external accessories increase the cost and prolong the manufacturing time. Adding a fan to the design will also bring a layer of instability to the reliability. Therefore, the PCB is now mainly using active rather than passive cooling methods (such as natural convection, conduction and cooling). radiation cooling) to allow the components to operate in a lower temperature range.

Poor thermal design will eventually increase the cost and reduce the reliability. This can happen in all PCB designs. It takes some effort to accurately determine the power consumption of the components, and then conduct a PCB thermal analysis. This will help produce small and functional strong product.

Accurate thermal models and component power dissipation 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 temperature of the components to determine the power loss, and thermal analysts need to know the power loss so that they can be input into the thermal model.

The designer guesses a component's operating ambient temperature or derives an estimate from a preliminary thermal analysis, and inputs the component's power dissipation into a refined thermal model that calculates the temperature of the PCB and associated component "junctions" (or hot spots) , the second step uses the new temperature to recalculate the component power dissipation, and the calculated power dissipation is used as the input for the next step of the thermal analysis process.

Ideally, this process continues until its value no longer changes. 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 approach is to estimate the total power dissipation of the PCB as a uniform heat flux across the entire PCB surface. Thermal analysis can predict the average ambient temperature, so that designers can use it to calculate the power consumption of components, and know whether other work needs to be done by further recalculating the component temperature.

General electronic component manufacturers provide component specifications, including normal operating temperature.

Component performance is usually affected by the ambient temperature or the internal temperature of the component. Consumer electronics products often use plastic-encapsulated components, and their operating temperature is 85 ℃; while military products often use ceramic components, the operating temperature is 125 ℃, and the rated temperature is usually 105 ℃ . PCB designers can use the "temperature/power" curve provided by the device manufacturer to determine the power dissipation of the component at a certain temperature.

The accurate way to calculate the component temperature is to do transient thermal analysis, but it is very difficult to determine the instantaneous power consumption of the component.

A better compromise is to perform nominal and differential analysis separately under steady-state conditions.

PCBs are subject to various types of heat, and typical thermal boundary conditions that can be applied include: natural or forced convection from front and rear surfaces, heat radiation from front and rear surfaces, conduction from PCB edge to device housing, through rigid or flexible connectors Conduction to other PCBs, conduction from PCB to standoff (bolted or glued), conduction to heat sink between 2 PCB sandwiches.

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

2. Basic process

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

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

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

Through the continuous practice process of estimation-design-measurement-feedback loop, correct and accumulate thermal simulation models, speed up thermal simulation speed, improve thermal simulation accuracy, and supplement PCB thermal design experience.