Green printed circuit board (PCB)
Printed circuit board (PCB) materials are formulated to withstand a certain amount of heat. What happens when the temperatures rise beyond certain limits? Performance takes a nose dive, especially at higher frequencies. This is why cost-effective heat management is perhaps an engineer’s number-one priority.
Of course, heat-tolerant PCB materials and carefully designed circuits can tolerate a certain amount of heat. First, the circuit designer should understand the different parameters of the materials’ behavior when temperatures rise.
Heat comes from various sources
Circuit boards are assembled in increasing density to make smaller, lighter designs. A component mounted to the circuit board can produce heat – so can an external source, such as found in automotive electronic systems.
Heat causes most materials to expand
Because of the smaller wavelengths at higher-frequencies, microwave and especially millimeter-wave (30 GHz and higher) circuits have small features that can become distorted as a circuit board expands due to heat.
Adding to this problem is the demand for more compact electronic designs. It’s common to see circuits designed with materials that have higher dielectric constants with smaller circuit features for a particular frequency and wavelength.
When the temperature rises, the circuit materials expand, changing the form of transmission lines and altering the impedance of conductors from a desired value. The results? Loss of linearity, distortion and shifts in frequency because of changes in transmission-line dimensions.
Materials expand at different rates
Circuit boards are made of composite materials, including dielectric layers and conductive metal layers. These composite materials tend to expand at different rates and to different extremes.
The coefficient of thermal expansion (CTE) describes the amount of expansion a material experiences. In an ideal world, the CTE of your board’s dielectric layers are close in value to the copper or other conductive metals laminated to the dielectric materials. Then both materials expand together at high temperatures.