Resing electronic product specifications have led to the widespread use of large-scale integrated circuit (IC) and surface mount technology ( SMT ) assembly in modern electronic manufacturing services. In addition, circuits have been developing in the direction of miniaturization, light weight, multi-function, high performance, high speed and high reliability. The continuous expansion of the component density leads to a gradual increase in the heat flux density. In the case of semiconductor devices, excessive temperature may cause changes in electrical properties . Whenever Tj (junction temperature) rises once, thermal breakdown will occur if severe. If thermal issues are not properly addressed, component specifications will surely suffer from instability, further reducing product stability and reliability while hanging in the air. In conclusion, the thermal problems of printed circuit boards ( PCBs) are so prominent that they must be given special attention for the high performance of electronic products.
So far, the methods used by high-tech circuits for heat dissipation are difficult to meet the heat dissipation requirements set by subsequent circuits, and a new type of heat dissipation solution is needed. Based on the discussion of heat dissipation methods commonly used in electronic products, Metal Core PCB (MCPCB) was introduced as a solution to thermal problems in advanced circuits.
Traditional Heat Dissipation Methods
The heat generated by the circuit mainly comes from the heat of the components, the heat of the PCB board and the heat, which is the result of external conduction, in which the heat of the components accounts for the majority. Therefore, the heat dissipation of components has received the most attention in component layout and PCB design. Thermal resistance plays an important role in thermal design, the purpose of which is to reduce the thermal resistance on the heat conduction path while the heat is rapidly conducted to a heat sink such as a heat sink. The total thermal resistance between electornic components and heat sinks can be divided into device level, assembly level and system level. Device-level thermal resistance is also called internal resistance, while component-level resistance is called external resistance, and system-level resistance is called final resistance. The relationship between the internal and external resistance of the component and T j is equal to the relationship between the component T j and the thermal resistance, according to the following formula:
T j = P d x（R jc + R cs + R sa ）+ T 0
In this formula, Tj refers to the component junction temperature; Pd refers to the device power; Rjc, Rcs and Rsa refer to the thermal resistance from junction to case, case to heat sink and heat sink to complete the appliance, respectively. T 0 refers to the initial temperature and R jc is a fixed eigenvalue. Therefore, thermal resistance reduction can only be achieved from the perspective of R cs and R sa .
Device assembly mode plays an important role in heat dissipation and different types of assembly modes require different heat dissipation methods.
• Convex platform structure
When the component shell is in direct contact with the circuit board and assembled on the front, the cover plate boss should be used for heat dissipation. The convex platform heat dissipation refers to adding a convex heat dissipation platform to the corresponding cover plate according to the position of the heat sink in the circuit, and using a thermally conductive insulating pad in contact with the convex platform.
This heat dissipation pattern achieves a synergy of structure and PCB board design. The number of convex platforms, location, height, area and thermal pad thickness are all closely related to board performance. In addition, assembly deviations must also be considered. Therefore, this mode brings many difficulties to PCB design, PCB manufacturing and PCB assembly ( PCBA)
• Thermal tape
If the component pins are directly soldered to the PCB board without the component shell and are in direct contact with the circuit board, thermal conductive tape can be used to dissipate heat. Thermally conductive tapes are typically made of copper and come in two assembly types. One is to assemble the thermal tape on top of the component and the other end to connect with the heat sink. The other is to assemble the components to the circuit board with thermally conductive tape and connect the other end with a heat sink. The latter heat dissipation method is mainly achieved through the bottom side. Adhesive thermally conductive insulating pads can be used between components and thermally conductive tape.
This mode requires structural assembly between components and thermal tape, both of which should maintain good contact with thermal pads and components, and should not put too much pressure on the component pins. To fix the thermal tape, fixed positioning holes should be made on the circuit board so as not to affect the PCB tracking and layout. Therefore, this mode is not suitable for high density PCBs.
Also, when the thermal tape vibrates , the component pins are affected and must be carefully considered.
•Thermotube
Thermotube uses evaporative cooling, and the temperature difference between the two ends of the heat pipe is extremely large, which can quickly conduct heat. Generally, a heat pipe consists of a shell, a wick and an end cap. The inner heat pipe has a negative pressure state and some low boiling point liquid is filled. In addition, liquids of this type are prone to volatilization. There is a liquid absorbent core on the tube wall, which is made of capillary material. One end of the heat pipe is used for evaporation and the other end is used for condensation. When the evaporative part of the heat pipe is heated, the liquid in the capillary will evaporate immediately, and the vapor flows to the other part under micro- stress , generating heat and re-condensing into liquid, which flows back to the evaporating part of the heat-pipe under the capillary stress.
While the heat pipe has significant heat dissipation capabilities, it has not matured enough to be accepted by small components. Therefore, the heat pipe still needs a long way to dissipate heat from the PCB.
MCPCB has better heat dissipation performance
• Introduction to MCPCB
With the continuous development and optimization of material science and processing technology, MCPCB has been widely used in the United States and Japan. Under the same external application environment, the performance of MCPCB is better than any other type of PCB board in terms of heat dissipation, representing the high level of high-power electronic components in the world.
MCPCBs utilize thermally conductive metals, such as copper, in certain layers of a multilayer PCB.The MCPCB dissipates heat from the outside through a metal core, or by connecting to an external heat sink for fast heat dissipation. When it comes to high-density circuits, PCB boards compatible with SMT components, or circuits assembled with so many through-hole components, MCPCBs with high thermal conductivity must be picked up. First, a metal core with good heat dissipation is embedded in a multi-layer PCB, and the multi-layer PCBs can be connected by plated through holes that can conduct heat on the metal core and its surface. The MCPCB structure can be shown in the figure below.
• Advantages of MCPCB
Compared with the traditional heat dissipation mode, MCPCB has unparalleled heat dissipation advantages. MCPCB can increase the power density of the product and reduce the need to assemble heatsinks and other hardware. Also, as hardware and assembly costs fall, product volume may shrink. Finally, MCPCB plays an active role in improving product reliability and shielding electromagnetic waves, reducing electromagnetic interference.
Due to the high copper density, the quality of MCPCB is significantly better than any other type of PCB. However, the application of MCPCB does not result in higher weight due to the lesser applications of heat sinks and accessories usually equipped on other types of PCBs. When deciding to use MCPCB, you may encounter two tips about MCPCB:
Tip 1: You should choose an appropriate copper core thickness;
Tip 2: You can also choose an aluminum core PCB.