As the commonest components integrated platform, multi-layer PCBs are capable of connecting PCBs and components together. With electronic products becoming light, thin and small in size, and having high performance, IC components have become highly integrated, leading to the high integrity of PCBs. As a result, heat production has obviously increased and thermal density of PCBs has increasingly gone up especially because of the mass utilization of high-frequency IC components such as A/D or D/A type and moving up of circuit frequency. If massive thermal loss fails to be sent out, the reliability of electronic equipment will be greatly influenced. According to statistics, among the elements leading to the failure of electronic equipment, temperature accounts for as high as 55%, as the top cause. With the temperature increasing, the failure rate of electronic components will increase exponentially. As soon as the environment temperature increases by 10°C, the failure rate of some electronic components can increase to twice large. For aerospace products, this type of thermal control design can't even be ignored because the inappropriate design method for the all kinds of circuits in special environment will possibly result in the complete failure of the whole system. So, in the process of PCB design, much attention must be paid to thermal design.
The analysis should commence with the cause analysis. The direct cause of high temperature of PCBs lie in the existence of power consumption components. Each component has power consumption in different extent that arouses the change of thermal strength. There exist 2 types of temperature increase phenomena: local temperature rising or large area temperature rising and short-term temperature rising or long-term temperature rising. Heat transfer has 3 ways: heat conduction, heat convection and heat radiation. Radiation dissipates heat through electromagnetic wave motion passing through space. Since the radiation dissipation features a relatively low amount of heat, it is usually regarded as an assisted dissipation method. This passage will introduce a solution to PCB heat dissipation in the process of long-term operation in the environment with high temperature based on the heat conduction and heat sink transient heat storage technology with a type of servo PCB as an example.
On this servo PCB, there are 2 power amplifier chips with a power of 2W, 2 R/D conversion chips, 2 CPU chips, 1 EPLD chip and 1 A/D conversion chip. The overall power of this servo PCB is 9W. The servo PCB is installed in an airtight environment with limited air convection. Besides, because of the limited space, cold plate dissipation can't be installed on the servo PCB. In order to ensure the normal operation of servo PCB, only heat conduction and heat sink transient heat storage technology can be utilized to transfer the heat produced from the PCB to the body.
It is a common method to dissipate heat through metal core PCB. First, a metal board with excellent heat conduction is embedded between a multi-layer PCB. Then, heat is dissipated directly from metal board or disjunctive equipment is connected to the metal board to dissipate heat. The operating structure is shown in Figure 1.