Printed Circuit Board Assemblies
Printed circuit boards (PCBs) connect electronic components together into a functional unit that can be used in a variety of applications. PCB fabrication processes, such as etching, copper deposition, and soldering, can produce significant amounts of heat that require proper thermal management to avoid overheating and possible failure. The physical structure of a circuit board also contributes to its thermal performance, as its thickness and material choices determine the ability to dissipate heat.
In addition to these physical considerations, the printed circuit board assemblies use case and operating environment dictate thermal requirements. For example, a PCB designed for use in a hot environment will need to be built with different materials than one that will be used in an office or home, as the temperature differences between these environments are significantly larger and will have an impact on heat dissipation capabilities.
When a PCB is fabricated, it is subjected to considerable heat and pressure during the lamination and automated soldering process. This thermal stress can lead to distortions in the PCB that can cause signal degradation and noise generation. Additionally, it can weaken the connection of metals to the board, leading to electrical shorts and failures.
Thermal Considerations for Printed Circuit Board Assemblies
A key consideration is the PCB’s glass transition temperature, or Tg, which refers to the temperature range in which the substrate can be flexed without failing. The lower the Tg of a PCB, the greater its flexibility and ability to dissipate heat.
Other considerations include the thermal resistance and track density of a PCB. The latter is determined by the thickness of copper on different layers in a stackup, with thicker copper having lower thermal resistance. Additionally, track density has an impact on the dielectric constant of a PCB, which is dependent on frequency and decreases with increasing frequency. This effect can cause phase distortion and noise generation, so it is important to design for a flat dielectric constant vs. frequency characteristics when designing high-frequency circuits.
Lastly, the material selection and quality of a PCB will have an impact on its thermal properties. Standard FR-4 boards can handle some amount of heat, but for high-power or high-pulse current applications, it may be necessary to consider alternative PCB materials that can better withstand thermal stresses.
Various techniques can be used to improve the thermal performance of a PCB, including using thermal pads and vias to conduct heat from critical areas to large internal copper pours on ground layers for improved dissipation. Thermal paste and fans are also useful tools to aid in cooling. Moreover, the use of a DC power integrity simulator can help identify areas in a PCB that will experience high current density and resulting higher temperatures, allowing additional thermal options to be implemented. In addition, careful placement of sensitive components on a PCB can decrease their influence on the thermal performance of other parts. These best practices may seem simple, but they can make a huge difference in the reliability of a finished product.
