PCB Technical - About a kind of PCB suitable for high temperature resistance

As we all know, designers are squeezing more performance out of printed circuit boards. Power density is rising, and the ensuing high temperatures can cause severe damage to conductors and dielectrics. Elevated temperature-whether due to I2R losses or environmental factors-will affect the thermal resistance and electrical impedance, resulting in unstable system performance, even if it is not a complete failure. The difference in thermal expansion rates between conductors and dielectrics (a measure of the tendency of a material to expand when heated and contract when cooled) can cause mechanical stress, which can lead to cracks and connection failures, especially when the circuit board is periodically heated and cooled . If the temperature is high enough, the dielectric may completely lose its structural integrity, leaving the first dominoes in trouble.

Heat has always been a factor that affects PCB performance. Designers are accustomed to including heat sinks in PCBs. However, today's high power density design requirements often overwhelm traditional PCB thermal management practices.

Mitigating the effects of high temperature not only has a profound impact on the performance and reliability of high-temperature PCBs, but also on the following factors:

Component (or system) weight

Application size

cost

Power requirements

A high temperature PCB is usually defined as one with a Tg (glass transition temperature) higher than 170°C.

For continuous thermal loads, at operating temperatures below Tg 25°C, high-temperature PCBs should follow a simple rule of thumb.

Therefore, if your product is in the temperature range of 130°C or higher, it is recommended to use high Tg materials.

In this article, we will discuss some design methods and techniques used in high-temperature PCB manufacturing and PCBA to help designers cope with high-temperature applications.

PCB heat dissipation technology and design considerations

Heat is dissipated through one or more mechanisms (radiation, convection, conduction), and the design team must keep these three factors in mind when deciding how to manage the temperature of the system and components.

Heavy copper PCB

radiation

Radiation is the emission of energy in the form of electromagnetic waves. We tend to think of it as something that only emits light, but the fact is that any object with a temperature above absolute zero radiates heat. Although the heat usually dissipated has the least impact on the performance of the circuit board, it may sometimes be the straw that broke the camel's back. In order to effectively remove heat, electromagnetic waves should have a relatively clear path away from the source. The reflective surface frustrates the outflow of photons and regroups a large number of photons at its source. If it is unfortunate that the reflective surfaces together form a parabolic mirror effect, they will concentrate the radiant energy of many light sources and focus it on an unfortunate part of the system, causing real trouble.

convection

Convection transfers heat to fluids (air, water, etc.). Convection is "natural": the fluid absorbs heat from the heat source, decreases in density, rises from the heat source to the radiator, cools, increases in density, and then returns to the heat source, and then repeats the process. (Recall the "rain cycle" in elementary school) Other convection is "forced" by fans or pumps. The key factors affecting convection are the temperature difference between the source and the coolant, the difficulty of the source to transfer heat, the difficulty of the coolant to absorb heat, the flow rate of the coolant, and the surface area for heat transfer. Liquid absorbs heat more easily than gas.

Conductivity

Conduction is the transfer of heat through direct contact between the heat source and the heat sink. In many ways, it is similar to electric current: the temperature difference between the source and sink is similar to voltage, the heat transferred per unit time is similar to amperage, and the ease with which heat flows through a heat conductor is similar to electric current. Conductivity. In fact, the factors that constitute a good electrical conductor often also constitute a good thermal conductor, because they all represent the form of molecular or atomic motion. For example, copper and aluminum are excellent conductors of heat and electricity. Larger conductor cross-sections can increase the conductivity of heat and electrons. Just like electrical circuits, long and tortuous flow paths can severely reduce the efficiency of conductors.

Generally, the main mechanism for removing heat from a circuit board is to conduct heat to a suitable heat sink, and convection conducts heat to the environment. The heat radiates some heat directly from the source, but most of the heat is usually taken away through specially designed channels (called "hot aisles" or "hot aisles"). The PCB heat sink is relatively large and has a high emissivity surface (usually corrugated or finned to further increase the surface area), bonding with a conductive (such as copper or aluminum) backing, which is a labor-intensive process . The PCB heat sink can also be connected to the chassis of the device to utilize its surface area. Fans are usually used to provide cooling air flow. In extreme cases, the cooling air itself can be cooled in a gas-liquid heat exchanger.