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What is the PCB thermal dissipation and how to optimize it?

PCB thermal dissipation so important to make it’s electrical device works properly,but how to make the PCB more dexterous? (The electrical energy consumed by electronic equipment during operation, such as radio frequency power amplifiers, FPGA chips, and power products, some of is working needed, but most of which is converted into heat and dissipated)

1. Design is important.

The electrical energy consumed by electronic equipment during operation, such as radio frequency power amplifiers, FPGA chips, and power products, some of is working needed, but most of which is converted into heat and dissipated. The electronic equipment working brings the muss heat will cause internal heat up rapidly, if not let their heat dissipate timely, the equipment heat will continue rising, which will cause the components invalid, the reliable of electronic equipment will decrease. SMT make the electronic equipment mounting density, and the thermal dissipation affective area reduce, the equipment heat rise affected the reliable severely, so to research of the PCB design is very important.

For thermal dissipation of PCB is a very important link. What is the PCB thermal dissipation technology? Let’s discuss together below

The heat emerging with electronic equipment working, it cause internal heat up rapidly, if not let their heat dissipate timely, the equipment heat will continue rising, which will cause the components invalid, the reliable of electronic equipment will decrease. So good thermal dissipation for PCB is very important.

2. Temperature rise factor analyze for PCB.

The direct cause of the temperature rise of PCB is the existence of the circuit power consumption component, all electronic device are with varying extent power consumption, heating intensity varies with power consumption.

Two phenomena of temperature rise in PCB:
(1) Partial temperature rise or large area temperature rise;
(2) Short time temperature rise or long time 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. PCB structure
(1) PCB dimension;
(2) PCB base material.

2.3. PCB install style
(1) Installation style(i.g. vertical installation,horizontal installation);
(2) The sealing condition and the distance with the case.

2.4. Heat radiation
(1) The emissivity of PCB surface;
(2) The temperature difference between PCB and its adjacent,and their absolute temperature.

2.5. Heat transmission
(1) Install heat sink;
(2)Other structural parts transmission heat.

2.6 Heat convection
(1) Natural convection;
(2) Force cooling convection.
The analysis of the above-mentioned factors from the PCB is an effective way to solve the temperature rise of PCB, these factors are often interrelated and dependent in product and system, most factors should be analyzed according to the actual situation. Only for a specific actual situation we can more accurately calculate or estimate parameters such as temperature rise and power consumption.

3. PCB design methods.

3.1. Thermal dissipation adopt PCB itself.
The PCB base material widely common use is epoxy resin glass fiber laminated sheet(FR4) or phenolic resin glass laminated sheet, and little are using CEM-1 material. These substrates have excellent electrical properties and processing properties, but their thermal dissipation are terrible. As a way to dissipate heat from high-heating components, it is almost impossible to expect heat to be conducted by the resin of the PCB itself,but to dissipate heat from the surface of the component to the surrounding air.

However, as electronic products have entered the era of miniaturization of components, high-density mounting, and PCB assembly, it is not enough to rely on the surface of the component with a very small surface area to dissipate heat. Meanwhile as the extensive use of QFP, BGA and other surface mount components, the heat generated by the components is send to PCB substrate, Therefore, the best way to solve heat dissipation is to improve the heat dissipation capacity of the PCB itself that is in direct contact with the heating element, through PCB to transfer or emit heat.

3.2. Add radiator or heat conducting plate to the high heat-generating device.

When a small number of components in the PCB generate a large amount of heat (less than 3),a radiator or heat pipe can be added to the heating device. If the temperature still high,use the radiator with with fan, to enhance heat dissipation. When amount of heating devices is large(more than 3), large heat sink(board) can be used, It is a special heat sink customized according to the position and height of the heating device on the PCB board or cut out different component height positions on a large flat heat sink. Snap the heat dissipation cover on the components surface as a whole, and contact each component for heat dissipation.However, the heat dissipation effect is not good due to the poor consistency of the height during the circuit board assembly and welding of the components.Usually add a soft heat conducting phase change material on the component surface to improve the heat dissipation effect.

3.3. For equipment that adopts free convection air cooling, it is best to arrange integrated circuits (or other devices) vertically or horizontally.

3.4. Use reasonable track design to implement heat dissipation
As bad thermal dissipation of resin in substrate, but The copper traces and plated holes are good conductors of heat. Therefore, increasing the residual rate of copper foil and increasing the thermal holes are the main means of heat dissipation
.

To evaluate the heat dissipation capacity of the PCB, it is necessary to calculate the equivalent thermal conductivity (9 eq) of the composite material composed of various materials with different thermal conductivity-the insulating substrate for the PCB.

3.5. The devices on the same board should be arranged as much as possible according to their calorific value and degree of heat dissipation.

Devices with low calorific value or poor heat resistance (such as small transistors, small-scale integrated circuits, electrolytic capacitors) should be placed in the cooling airflow. The uppermost stream (at the entrance). Devices with large heat or good heat resistance (such as power transistors, large-scale integrated circuits, etc.) are placed at the most downstream of the cooling airflow.

3.6. In the horizontal direction, high-power devices are placed as close to the edge of PCB as possible to shorten the heat transfer path; In the vertical direction, high-power devices are placed as close as possible to the top of PCB to reduce the impact of these devices on the temperature of other devices when they work.

3.7. The heat dissipation of PCB in the equipment mainly depends on air flow, so in the design, it is necessary to study the air flow path and reasonably configure the device or PCB. When air flows, it always tends to flow in places with low resistance, so when configuring devices on PCB, avoid leaving a large empty area in a certain area.The configuration of multiple PCBs in the whole machine should also pay attention to this problem.

3.8. The temperature-sensitive device is best placed in the lowest temperature area (such as the bottom of the device). Never place it directly above the heating device. It is best to stagger multiple devices on the horizontal plane.

3.9. Place the devices with the highest power consumption and the highest heat generation near the best heat dissipation position, and do not place the devices with higher heat generation on the corners and peripheral of PCB, unless a heat sink is arranged near it. When setting the power resistance, choose a larger device as much as possible, and make it have enough space for heat dissipation when adjusting the layout of the printed board.

3.10. RF power amplifier or LED PCB use metal substrates.

3.11. Avoid the concentration of hot spots on the PCB, distribute the power evenly on the PCB board as much as possible, and keep the PCB surface temperature performance uniform and consistent.

It is often difficult to achieve strict uniform distribution during the design process, but it is necessary to avoid areas where the power density is too high to prevent hot spots from affecting the normal operation of the entire circuit. If possible, it is necessary to analyze the thermal performance of PCB. For example, the thermal performance index analysis software module added in some professional PCB design software can help designers optimize the circuit design.

4. Summary.

4.1. Material selection
(1) The temperature rise of the conductors of PCB due to the passing current plus the specified ambient temperature should not exceed 125℃ (commonly used typical value. May be different depending on the selected board). As the components are installed on PCB, they also emit some heat, which affects the working temperature,material selection and design should consider these factor,hot spot should not exceed 125℃. Choose copper foil thickness as thick as possible.

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

(3) The multi-layer board structure is conducive to PCB heat dissipation.

4.2. Ensure that the heat dissipation channel is unblocked

(1) Make full use of the component layout, copper plane, solder-mask opening and thermal dissipation holes to establish a reasonable and effective low thermal resistance channel to ensure that the heat is smoothly exported from the PCB.

(2) Setting of thermal dissipation holes. Designing some heat dissipation through via holes and blind holes can effectively provide heat dissipation area and reduce thermal resistance, and improve the power density of the circuit board. Such as setting up via holes on the pads of LCCC devices.Solder fills them in PCB production process to improve the thermal conductivity. The heat generated during circuit operation can be quickly transferred to the metal heat dissipation layer or the copper plane provided on the back through the through holes or blind holes. In some specific cases, PCB with a heat dissipation layer are specially designed and used,the heat dissipation materials are 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 in the process of heat conduction, heat conduction materials are used on the contact surface between the power dissipation device and the substrate to improve the efficiency of heat conduction.

(4) Technology 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 into the solder paste, and the solder joints under the device will have a certain height after reflow soldering. The gap between the device and PCB is increased, and the convection heat dissipation is increased.

4.3. Components arrangement requirements
(1) Through the software to do thermal analysis for PCB,design control of internal maximum temperature rise;

(2) Consider to specially design and install components with high heat generation and large 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 that sufficient cooling air flows 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. Parts and components sensitive to heat (including semiconductor devices).

Contact us freely if you have any concernings to your PCB design for heat dissipation issues.

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