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PCB Tech

PCB Tech

PCB Tech

PCB Tech

PCB design and the technical measures of thermal design and thermal analysis
2021-09-11
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Author:Frank

1. The importance pcb of thermal design

In addition to useful work, most of the electrical energy consumed by electronic equipment during operation is converted into heat and emitted. The heat generated by the 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 the electronic equipment will decrease.
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.

pcb board

2. Analysis of temperature rise factors of printed circuit boards

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

Two phenomena of temperature rise in printed circuit boards:

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

(2) Short-term temperature rise or long-term temperature rise.
When analyzing PCB thermal power consumption, it is generally analyzed from the following aspects.

2.1 Electrical power consumption

(1) Analyze the power consumption per unit area;

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

2.2 The structure of the printed circuit board

(1) The size of the printed circuit board;

(2) The material of the printed circuit board.

2.3 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.

2.4 Thermal radiation

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

(2) The temperature difference between the printed circuit board and adjacent surfaces and their absolute temperature;

2.5 Heat conduction

(1) Install the radiator;

(2) Conduction of other installation structural parts.

2.6 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. These factors are often related and dependent on each other in a product and system. Most of the factors should be analyzed according to the actual situation, and only for a specific The actual situation can calculate or estimate the parameters such as temperature rise and power consumption more correctly. 

3. Thermal design principles

3.1 Material selection

(1) The temperature rise of the conductors of the printed circuit board due to the passing current plus the specified ambient temperature should not exceed 125°C (the commonly used typical value may be different depending on the selected board). Since the components installed on the printed circuit board also emit some heat, which affects the operating temperature, these factors should be considered when selecting materials and the design of the printed circuit board. The hot spot temperature should not exceed 125°C. Choose thicker copper clad as much as possible.

(2) In special cases, aluminum-based, ceramic-based, and other plates with low thermal resistance can be selected.

(3) The use of multilayer circuit board structure helps PCB thermal design.

3.2 Ensure that the heat dissipation channel is unblocked

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

(2) Setting of heat dissipation through holes
Designing some heat dissipation through holes and blind holes can effectively increase the heat dissipation area and reduce the thermal resistance, and improve the power density of the circuit board. For example, a via hole is set up on the pad of the LCCC device. Solder fills it in the circuit production process to increase the thermal conductivity. The heat generated during circuit operation can be quickly transferred to the metal heat dissipation layer or the copper pad on the back through the through holes or blind holes to be dissipated. In some specific cases, a circuit board with a heat dissipation layer is specially designed and used. The heat dissipation material is generally copper/molybdenum and other materials, such as printed boards used on some module power supplies.

(3) Use of thermally conductive materials
In order to reduce the thermal resistance of the heat conduction process, a thermally conductive material is 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 where the device is mounted on both sides, it is easy to cause local high temperature. In order to improve the heat dissipation conditions, a small amount of small copper can be mixed in the solder paste, and the solder joints under the device will have a certain height after reflow soldering. The gap between the device and the printed board is increased, and the convection heat dissipation is increased.

3.3 Requirements for the arrangement of components

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

(2) It can be considered to specially design and install components with high heat and radiation on a printed circuit board;

(3) The heat capacity of the board is evenly distributed. Be careful not to place high-power components in a concentrated manner. If it is unavoidable, place the short components upstream of the airflow and ensure sufficient cooling air flow through the heat-consumption concentrated 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 parts and components (including semiconductor devices) should be kept away from heat sources or isolated;

(7) (Liquid medium) It is best to keep the capacitor away from the heat source;

(8) Pay attention to the direction of forced ventilation and natural ventilation;

(9) The additional sub-boards and device air ducts are in the same direction as the ventilation;

(10) As far as possible, make the intake and exhaust have a sufficient distance;

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

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

(13) (Small signal amplifier peripheral devices) Try to use devices with small temperature drift;

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

3.4 Requirements for wiring

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

(2) Wiring rules;

(3) Plan the minimum 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 surface as possible; if the requirements cannot be met, the use of bus bars can be considered;

(5) Minimize the thermal resistance of the contact surface. For this reason, the heat conduction area should be enlarged; the contact surface should be flat and smooth, and can be painted if necessary.
Coated with thermal grease;

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

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

(8) If possible, use large-area copper foil on the surface;

(9) Use larger pads for the ground mounting holes 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 metalized vias as possible, and the aperture and disk surface should be as large as possible, relying on vias to help heat dissipation;

(11) Complementary means for device heat dissipation;

(12) In the case where a large surface area of copper foil can be used, the method of adding a heat sink may not be used due to economic considerations;

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

4. Thermal simulation (thermal analysis)

Thermal analysis can help designers 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 require the establishment of transient models for electronic devices containing multiple PCBs and thousands of components.

No matter how careful the analyst is when 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. In many applications, weight and physical size are very important. If the actual power consumption of the component is small, the safety factor of the design may be too high, so that the PCB design uses the power consumption value of the component that does not match the actual or is too conservative. Thermal analysis, on the contrary (and more serious at the same time), is that the thermal safety factor is designed to be too low, that is, the actual operating temperature of the component is higher than the analyst predicts. Such problems generally require the installation of heat sinks or fans Cool the PCB to solve it. 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 now mainly adopts active rather than passive cooling methods (such as natural convection, conduction, and Radiation heat dissipation) to make the components work 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 PCB thermal analysis, which will help produce compact and functional products. Strong product. Accurate thermal models and component power consumption should be used to avoid reducing PCB design efficiency.

4.1 Component power consumption calculation

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 in order to input it into the thermal model. The designer first guesses the working environment temperature of a component or obtains an estimated value from the preliminary thermal analysis, and inputs the component power consumption into the detailed thermal model to calculate the temperature of the "junction" (or hot spot) of the PCB and related components , The second step uses the new temperature to recalculate the power consumption of the component, and the calculated power consumption is used as the input for the next thermal analysis process. In an ideal situation, the process continues until the value no longer changes.

However, PCB designers are often under pressure to complete tasks quickly, and they do not have enough time for the time-consuming and repetitive work to determine the electrical and thermal properties of components. A simplified method is to estimate the total power consumption of the PCB as a uniform heat flux that acts on the entire PCB surface. Thermal analysis can predict the average ambient temperature, allowing designers to calculate the power consumption of components, and to know whether other work needs to be done by further recalculating the component temperature.

General electronic component manufacturers provide component specifications, including the maximum temperature for normal operation. The performance of components is usually affected by the ambient temperature or internal temperature of the components. Consumer electronic products often use plastic-encapsulated components with a maximum working temperature of 85℃; while military products often use ceramic parts with a maximum working temperature of 125℃, and the maximum rated temperature is usually It is 105°C. 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 most accurate method to calculate component temperature is to perform transient thermal analysis, but it is very difficult to determine the instantaneous power consumption of the component.

A better compromise is to perform rated and worst-case analysis separately under steady-state conditions.
PCB is affected by various types of heat. Typical thermal boundary conditions that can be applied include:

Natural or forced convection from the front and rear surfaces;

Heat radiation from the front and rear surfaces;

Conduction from the edge of the PCB to the device shell;

Conduction to other PCBs through rigid or flexible connectors;

Conduction from the PCB to the bracket (bolted or glued and fixed);

The conduction of the heat sink between 2 PCB mezzanine layers.