PCB News - Thermal design of PCB board

1. Importance of thermal PCB design

In addition to useful work, most of the electric energy consumed by electronic equipment is converted into heat emission. The heat generated by electronic equipment makes the internal temperature rise rapidly. If the heat is not dissipated in time, the equipment will continue to heat up, and the components will fail due to overheating, and the reliability of electronic equipment will decline. SMT increases the installation density of electronic equipment, reduces the effective cooling area, and seriously affects the reliability of equipment temperature rise. Therefore, it is very important to study the thermal design.

2. Printed circuit board temperature rise factor analysis

The direct cause of temperature rise of PCB is the existence of circuit power devices, electronic devices have varying degrees of power consumption, heating intensity varies with the power consumption.

Two phenomena of temperature rise in printed board:

(1) local or large area temperature rise;

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

In the analysis of PCB thermal power, it is generally analyzed from the following aspects.

2.1 Electrical power consumption

(1) Analysis of power consumption per unit area;

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

2.2 Structure of printed board

(1) Printed board size;

(2) Printed board materials.

2.3 Installation method of printed board

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

(2) the sealing condition and the distance from the shell.

2.4 thermal radiation

(1) radiation coefficient of printed board surface;

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

2.5 heat conduction

(1) install the radiator;

(2) Conduction of other installed structural parts.

2.6 heat convection

(1) natural convection;

(2) Forced cooling convection.

The analysis of the above factors from PCB is an effective way to solve the temperature rise of printed board, 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 correctly calculate or estimate the temperature rise and power consumption and other parameters.

3. Principles of thermal design

3.1 select material

(1) The temperature rise caused by the current passing through the PCB wire plus the specified ambient temperature shall not exceed 125 degree Celsius(commonly used typical value). Depending on the plate chosen). As components installed on the printed board also emit some heat, which affects the operating temperature, these factors should be taken into account in the selection of materials and the design of the printed board. The hot spot temperature should not exceed 125 degree Celsius. Choose a thicker copper-clad foil if possible.

(2) under special circumstances can choose aluminum base, ceramic base and other small thermal resistance plate.

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

3.2 Ensure that the heat dissipation channel is unblocked

(1) Make full use of components layout, copper skin, Windows and cooling holes and other technologies to establish a reasonable and effective low thermal resistance channel to ensure that heat can be exported to PCB smoothly.

(2) Setting of heat dissipation through hole

Design some heat dissipation through holes and blind holes, can effectively improve the heat dissipation area and reduce the thermal resistance, improve the power density of the circuit board. Such as in the LCCC device on the welding pad set through hole. In the process of circuit production, the solder will be filled to improve the thermal conductivity, and the heat generated when the circuit works can be quickly transmitted to the metal heat dissipation layer or the copper moor set on the back. In some specific cases, specially designed and used circuit boards with heat dissipation layer, heat dissipation materials are generally copper/molybdenum and other materials, such as the printed board used in some module power supplies.

(3) The use of thermal conductivity materials

In order to reduce the thermal resistance in the heat conduction process, the thermal conductive 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 conditions, a small amount of fine copper can be mixed into the solder paste, and the solder spot below the device will have a certain height after reflow welding. The gap between the device and the printed board increases, increasing the convection heat dissipation.

3.3 Layout requirements of components

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

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

(3) the heat capacity of the board is evenly distributed. Pay attention not to centralize the distribution of large power consumption devices. If it is unavoidable, the high components should be placed in the upstream of the air flow, and ensure sufficient cooling air flow through the heat consumption concentration 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 impact of thermal radiation on the surrounding parts. Heat-sensitive components and components (including semiconductor devices) should be kept away from heat sources or isolated;

(7)(liquid medium) capacitor is best away from heat source;

(8) Pay attention to make forced ventilation and natural ventilation in the same direction;

(9) The air duct of the attached sub-plate and device is consistent with the ventilation direction;

(10) as far as possible to make the air intake and exhaust have enough distance;

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

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

(13)(small signal amplifier peripheral devices) try to use small temperature drift devices;

(14) Use metal chassis or chassis for heat dissipation as much as possible.

3.4 Requirements for wiring

(1) Plate selection (reasonable design of printed board 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) Large current lines should be as superficial as possible; In the condition that can not meet the requirements, the use of bus bar can be considered;

(5) To minimize the thermal resistance of the contact surface. Therefore, the heat conduction area should be increased; Contact plane should be flat, smooth, if necessary can be coatedCovered with thermal silicone grease;

(6) Stress balance measures and bold lines are taken into account for thermal stress points;

(7) the window opening method of heat dissipation copper skin should be adopted, and the window opening method of heat dissipation resistance welding should be used properly;

(8) depending on the possible use of large surface copper foil;

(9) The grounding mounting hole on the printed board adopts a larger pad to make full use of the installation bolt and the copper foil on the surface of the printed board for heat dissipation;

(10) as much as possible to place the metalized hole, and the aperture, disk surface as large as possible, rely on the hole to help heat dissipation;

(11) supplementary means for heat dissipation of devices;

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

(13) The appropriate surface cooling copper foil area (tJ ≤(0.5~0.8) Tjmax) is calculated according to the device power consumption, ambient temperature and the maximum allowable junction temperature.