Progress in Heat Dissipation Technology of PCB Circuit Board and Its Electronic Components

PCB is the core of electronic equipment, including resistors, chips, transistors, etc. The chip has the highest heating power. The common CPU is 70 ~ 300 W, which is the main heat source. Due to the high integration of PCB, its heating power continues to increase. Excessively high temperature is seriously detrimental to the performance, reliability, and lifespan of electronic equipment.

Temperature-related failures of components include mechanical failures and electrical failures. Mechanical failure is when the temperature changes, the combined thermal expansion and contraction of various materials are different, resulting in material deformation, yield, fracture, etc. Electrical failure is a change in the performance of components caused by temperature changes, such as transistors, chip resistors, etc., which in turn cause thermal runaway and electrical overload. At the same time, a large number of electrons migrate and atomic vibrations are accelerated due to excessive temperature, resulting in uncontrolled ion migration and electron bombardment of atoms. This causes ion pollution and electromigration and will seriously affect the safety, stability, and life of components.

Component heat dissipation is divided into chip level, package level, and system level. Chip-level and package-level heat dissipation starts with optimizing materials and manufacturing processes to reduce thermal resistance. System-level heat dissipation is to use appropriate heat dissipation structure and cooling technology to design a heat dissipation system that meets the requirements, so as to ensure that components can work safely and for a long time. The International Organization for Semiconductor Technology Development proposes that system-level cooling is the main reason for limiting the growth of chip energy losses. This demonstrates the importance of high-performance system-level cooling techniques.

According to whether it depends on the phase change of working fluid, it can be divided into single-phase heat dissipation and multi-phase heat dissipation. Single-phase heat dissipation includes air cooling, liquid cooling, jet flow, and thermoelectric cooling. Air cooling and liquid cooling are more mature and widely used, but the heat dissipation effect is average. Multiphase cooling includes PCM, heat pipes, electrowetting, and spray. In general, multi-phase heat dissipation absorbs a large amount of latent heat due to the phase change of the working fluid, and the heat dissipation effect is better, which is the key development direction.

1. PCB and Component Heat Dissipation Methods and Characteristics

The heat transfer method of components can be summarized as heat conduction from the chip to the package shell. The bottom of the housing is connected to the PCB copper foil through leads, solder balls, etc. Copper foil conducts heat in the plane and thickness of the PCB. Heat transfer in the plane direction is by conduction and convection. However, heat conduction in the thickness direction must pass through the resin material of the substrate, and its thermal conductivity is very low. Therefore, copper-plated vias are often provided. Connect different layers of copper foil on the PCB to improve its thermal conductivity in the thickness direction.

Take Figure 1 as an example. The upper surface of the chip is connected to the heat sink, and the heat is conducted downward to the copper foil on the upper surface of the PCB through the solder balls and the substrate. Part of the heat is dissipated through convection and heat conduction in the plane direction, and the remaining heat reaches the bottom surface of the PCB through the thermal via, and is dissipated by the heat sink.

2. Advances in Single-phase Heat Dissipation Technology

Air cooling is divided into natural convection and forced air cooling. The heat flux limit is about 5 W/cm 2 . Natural convection cooling is poor but low cost. It is widely used in low heat flux devices such as TVs, etc. Forced air cooling has strong heat dissipation, simple structure, high reliability and is widely used in CPUs, data centers, etc. Its research focuses on cooling fins and flow control optimization.

Liquid cooling performs better than air cooling because the specific heat capacity of liquid is much greater than that of air. Conventional liquid cooling heat flux up to 24 W/cm². The heat flux of microchannel liquid cooling can exceed W/cm². Liquid cooling includes immersion cooling and liquid cold plates. Immersion cooling is to immerse equipment in a coolant with strong thermal conductivity and weak conductivity. It has been used for cooling data centers and base stations. The operating parameters of immersion cooling have a great influence on the cooling effect. The faster circulation of the system and the lower temperature of the liquid supply are conducive to cooling.

Liquid cold plates have lower requirements for packaging. It can directly contact components and has more application scenarios. Optimizing the channel structure can enhance heat transfer. As shown in Figure 2, the optimized model reduces flow resistance while enhancing heat dissipation. The TCP maximum temperature decreased by 0.27% and 1.08%, respectively. The temperature difference respectively decreased by 19.50% and 41.88%.

Microchannel is a new type of liquid cold plate generally embedded in metal plate. The equivalent diameter is between 10 and 1000 μm. Due to its small size, strong heat dissipation and good temperature uniformity, it is often used in the aerospace field. In addition to structural optimization, adjusting the flow distribution is more effective in reducing thermal resistance and energy consumption than simply increasing the flow rate, such as the algorithm for adjusting the microchannel inlet according to the temperature distribution. Research on new working fluids focuses on nanofluids and liquid metals. Liquid metal works better, but is more energy-intensive and corrosive. The energy consumption required by nanofluid is similar to that of water, so it is an ideal coolant.

Jet flow is an efficient cooling method. It was originally used in aerospace engines and later also in high-power chips. The heat flux exceeds 500 W/cm². The direction of the jet flow in the stagnation point area changes, and the heat transfer efficiency is high, but the cooling effect decreases rapidly away from this area. Multi-nozzle structure can solve this problem. Jet cooling research focuses on structural parameters and working fluids. Structural parameters include nozzle diameter, array, etc. In addition, the structure of the impact surface also affects the cooling effect, for example, the conical surface can increase the cooling effect by 11% compared with the flat surface. In terms of working fluids, there are many studies on nanofluids and liquid metals, which have better performance than traditional fluids.

As shown in Figure 3, thermoelectric cooling utilizes the Peltier effect, and semiconductors are commonly used as conductors. Thermoelectric refrigeration has the advantages of miniaturization and no noise. Its heat flux is up to 15 W/cm², which is very suitable for PCBs with small space. Its disadvantage is low cooling efficiency. In response to this problem, in addition to optimizing the heat exchange at the hot and cold ends, the most important thing is to improve the performance of thermoelectric materials. Key properties of thermoelectric materials include thermal conductivity κ, Seebeck coefficient α, and electrical conductivity σ, which together make up zT.

zT reflects the thermoelectric properties of the material. Generally, it is necessary to increase zT, such as increasing electrical conductivity or decreasing thermal conductivity. Doping different materials can improve the performance of thermoelectric materials, such as doping alloys into silicon crystals to form eutectic materials. Controlling the microstructure such as grain size and secondary phases can also improve the thermoelectric properties of the alloy. It is also important to choose the appropriate physical property configuration. Simply increasing α or decreasing κ can improve zT, but not necessarily get better cooling effect.

3. Advances in Multiphase Heat Dissipation Technology

A heat pipe is a highly capable heat transfer element with a heat flux of over 200 W/cm2. With its compact structure and high reliability, it is widely used in terminal electronic equipment. The heat pipe uses the working medium to evaporate at the endothermic end of the vacuum tube and liquefy at the exothermic end to transfer heat. The porous material of the tube core generates capillary force to maintain the circulation of the working fluid.

Electronic equipment generally uses ultra-thin heat pipes, which can be closely attached to the surface of components, including flat heat pipes (UFHP) and loop heat pipes (ULHP). They work the same as traditional heat pipes, with only slight changes in shape and structure. UFHP is a traditional cylindrical heat pipe punched into an ultra-thin flat plate. The ULHP, as shown in Figure 4, separates the liquid and gas in their respective channels to make the circulation more smooth. It has the advantages of long distance and anti-gravity.

Flat-plate pulsating heat pipe (FPPHP) is a special ULHP that does not need a tube core, and has the characteristics of simple structure and miniaturization. FPPHP forms a serpentine loop between the cold and heat sources. Due to the action of the heat source, the pressure instability at the evaporating end and the condensing end causes complex two-phase flow. The working fluid spontaneously oscillates in the channel to realize heat transfer.

Vapor chambers are a special type of UFHP. Compared with heat pipes with one-dimensional heat transfer, vapor chambers have higher heat transfer efficiency and better temperature uniformity on two-dimensional surfaces. As shown in Figure 5, it has advantages over conventional UFHP.

The die is the core to maintain the circulation of the working fluid, and also provides an interface for the liquid-vapor phase transition. Therefore, the start-up and performance of heat pipes mainly depend on the core structure, which can be divided into microgrooved core, sintered core and composite core structure. The optimization of the core is mainly to improve capillary force, permeability, and reduce weight to improve liquid delivery efficiency. Another key to the heat pipe is the working fluid. The thermal resistance is the smallest when the UFHP working fluid only fills the core, and too much liquid hinders the flow of steam. Nanofluid working medium has stronger phase change ability, flow velocity and flow driving force.

As a flexible component, heat pipes are often coupled with other heat dissipation technologies for better results. Heat pipe - PCM is the most common. In addition, there are vapor chambers - spray, heat pipes - thermoelectric refrigeration, etc.

PCM has the advantages of low cost, light weight, and strong heat dissipation. It uses latent heat of phase change to stabilize the temperature of components. For example, PCM melts and absorbs heat during the peak power period, and solidifies and releases heat during the low power period. PCM needs to improve thermal conductivity, such as microcapsule PCM, which increases the specific surface area of PCM to enhance thermal conductivity, and can be further improved by adding nanomaterials, metal foam or expanded graphite. PCM is also often used to fill heat sinks because the fins help conduct heat away from the PCM and the PCM also helps the fins dissipate heat.

PCM is usually coupled with other cooling methods,such as heat pipe - PCM, as shown in Figure 6. The heat pipe can improve the heat conduction of the PCM, and the PCM acts as a secondary condenser to absorb part of the heat dissipation from the heat pipe.

Electrowetting has low energy consumption and fast response, and is suitable for all kinds of chips. As shown in Figure 7, the movement and deformation of the dielectric droplet are controlled by electrodes and the phase change absorbs heat at the hot spot to eliminate the local hot spot. Its heat dissipation can reach the microchannel level. The droplet shape and phase transition mainly affect the heat transfer, which is related to the electric field strength, frequency and temperature. Evaporation can be promoted by increasing electric field strength and surface temperature.

In order to promote the formation of liquid film and reduce friction, it is necessary to optimize the structure and material of the droplet contact surface. For example, superhydrophilic nanoporous coating can promote the formation of liquid film. In addition, nanoparticles can improve parameters such as droplet contact angle and contact diameter, and increase internal disturbance of droplets to promote heat transfer.

The spray has high heat dissipation and large area cooling capacity. The heat flux limit reaches 1200 W/cm². The working medium forms tiny droplets through the nozzle, and the droplets impact the heating surface and undergo phase change to absorb heat. The disturbance of the liquid film by the impact and the phase change of the droplet greatly enhance the heat transfer. Factors affecting spray cooling are divided into operating parameters, coolant characteristics, and heating surface characteristics.

Operating parameters include flow rate, droplet diameter, spray direction, etc. Decreasing the droplet diameter promotes evaporation more than increasing the droplet velocity. In practice, multi-nozzle spray is commonly used. Nozzle placement is also a factor. The more nozzles, the greater the injection pressure, the faster the cooling rate. Applying an electric field can break up the droplets into fine droplets with a larger specific surface area to enhance heat transfer. Under the control of electric field, the electrospray heat dissipation of different forms can be increased by 2.8 times.

In addition to nanofluids, alcohol-water mixtures, surfactants can also improve heat dissipation. Alcohol-water can significantly reduce droplet surface tension and contact angle. Surfactants reduce droplet surface tension and increase droplet diameter. The liquid film can thicken faster, which is beneficial to the flow of the liquid film to take away heat. The heating surface, that is, the surface structure optimization, such as the straight groove structure can improve the heat transfer effect by 64.2%. By increasing the surface microroughness, heat transfer can be enhanced by about 116%. Heat transfer can be further enhanced by adding micro-roughness to the fin surface.

4. Development Direction of Heat Dissipation Technology for PCB and Components

Optimizing structures such as fins can enhance boundary layer turbulence for enhanced heat transfer, but also with increased flow resistance. In order to solve this multi-objective problem, improving heat transfer while reducing energy consumption is the focus of research. Orthogonal experiments, genetic algorithms, and topology are commonly used to optimize the radiator structure and operating parameters. The microstructure of the radiator surface also significantly affects the generation of bubbles in ebullient cooling and the contact angle of droplets in spray, etc.

Nanofluids have high thermal conductivity and can be used in most cooling technologies. Maintaining the stability of nanofluids is a key issue. Methods for short-term stabilization of nanofluids include sonication, changing the pH, and adding dispersants. Methods to maintain the stability of nanofluids in the long term still need to be explored. The concentration, type, size, etc. of nanoparticles will affect the heat transfer performance and flow power consumption. A high concentration of particles will enhance heat transfer and bring greater flow resistance, and a large number of experiments are needed to determine the optimal parameters. The use of nanoparticles in thermal management materials can improve heat transfer performance, which is related to particle concentration and particle shape. Currently, the incorporation of nanoparticles into PCM has been extensively studied. The use of nanoparticles for materials such as thermal interfaces, electronic packaging, etc., needs more research.

For application scenarios, multiple heat dissipation technologies are used to assist each other to achieve the optimal effect, providing new ideas for the future development of electronic cooling. Traditional such as heat pipe-PCM, heat pipe-air cooling, PCM-liquid cooling, etc., as more heat dissipation technologies are proposed, coupling new technologies is the development direction. Heat pipes and PCMs can flexibly assist other technologies and deserve further study.

Highly integrated PCBs and components tend to generate a large amount of heat in a short period of time and form local hot spots, and the cooling system needs to respond quickly. It is more economical to adjust the distribution and flow of cooling fluid according to the location of the hot spot. To this end, precise control technology is required, such as microchannels that adjust the size of the liquid inlet. Electric fields can precisely and flexibly control the flow of dielectric liquids, such as electrowetting, spraying and other scenarios. In the future, electric fields may be used to control flow in more applications.

Current electronic equipment is mainly cooled by air. Designing component layout can optimize heat dissipation, such as arranging thermal vias to improve PCB longitudinal heat conduction. According to the heating and heat resistance of the components, they are arranged along the airflow, and the high heating and heat resistant components are placed downstream, and the low heating and heat resistant components are placed upstream. Or consider the air flow backflow arrangement caused by the height of components.

In most cases, the working medium is flowing, and the vibration and noise generated by fluid pressure fluctuations, eddy current shedding, and boundary turbulent separation in the power plant are not conducive to the long-term work of electronic equipment. If the fan vibration is improved, it is necessary to adjust the wind angle according to the flow field to reduce the blade speed. The development of new power devices such as piezoelectric blades with blade bending and resonance can not only reduce vibration and noise, but also meet the needs of light weight and miniature.

5. Conclusion

Due to the high integration and high power of PCB circuit boards and their electronic components, the problem of thermal failure of electronic equipment has gradually become prominent and has become the key to restricting the development of electronic technology. Here we introduce the system-level heat dissipation of PCB circuit boards and their electronic components. technology. It is divided into single-phase heat dissipation technology and multi-phase heat dissipation technology, and discusses the research progress of air cooling, liquid cooling, jet flow, thermoelectric cooling, heat pipe, PCM, electrowetting, and spraying. Existing research mainly optimizes the heat dissipation structure, operating parameters, materials and working fluids, and heat dissipation technology coupling. Finally, several development priorities are put forward, including heat sink design, nanoparticle application, heat dissipation technology coupling, precision control technology, PCB design, vibration reduction and noise reduction, and provide suggestions for further development.