What Are The Effects of High Temperature on PCB Boards?

Thermal energy poses a potential threat to printed circuit board (PCB) materials. Although PCB materials are designed to withstand certain levels of thermal energy, circuit performance will be affected when the temperature exceeds a certain threshold, especially at high-frequency operation. If circuit designers have a good understanding of the various parameters, they can accurately describe the reaction of circuit materials when the temperature rises, and then through the selection of heat-resistant PCB materials and careful circuit design to withstand certain levels of thermal energy.

Thermal energy comes from many sources, and its impact on circuits varies from case to case, especially in the context of increasing board assembly density and the trend toward smaller and lighter designs. Thermal energy can be generated by components on the board or from heat sources external to the board. For example, designers of high-power radar systems are familiar with the large amounts of heat generated by vacuum tube amplification devices such as klystrons and traveling wave tubes (TWT). Recently, the use of high-density amplification semiconductors such as gallium nitride (GaN) transistors has brought hot spots to the forefront in addition to increasing the power levels of RF/microwave signals. In addition, heat sources external to the PCB, such as those in automotive electronic systems, can also increase circuit temperatures and cause reliability issues. Therefore, the key to designing circuits that are least affected by heat sources is to understand the behavior of RF/microwave circuit materials at elevated temperatures.

Thermal energy causes most materials to expand, and circuit materials are no exception. At high frequencies, the wavelengths are shorter, and microwave circuits, especially millimeter waves (30 GHz and above), have smaller feature sizes. As the board expands due to increased temperature, these feature sizes can be distorted. In addition, as the demand for smaller electronic designs grows, many circuits are designed to use materials with higher dielectric constants, resulting in smaller circuit features at a given frequency and wavelength. High temperatures cause circuit materials to expand, changing the shape of transmission lines and shifting the impedance of conductors away from the desired value (usually 50Ω). At high temperatures, circuits can experience linear losses, distortion, and even frequency shifts due to changes in transmission line dimensions.

Circuit boards are composite materials composed of dielectric and conductive metal layers, and these materials tend to expand at different rates and limits when exposed to high temperatures. This behavior of PCBs is characterized by the coefficient of thermal expansion (CTE) parameter, which describes the amount of expansion of a material with temperature changes (measured in parts per million (ppm)). Ideally, the CTE value of a PCB dielectric layer should be close to that of the copper or other conductive metal laminated on the dielectric material to ensure that the two materials expand synchronously at high temperatures and avoid stress at the interface between the different materials. Circuit designers often express concerns about the reliability of certain circuit components at high temperatures, such as plated through holes (PTHs) used to interconnect different layers of a multilayer circuit board, and the junction of dielectric materials and conductive metal materials.

When high-dielectric-constant circuit materials are used to minimize circuit function and size, such as Rogers Corp.’s low-loss RO3010™ circuit laminate, which has a dielectric constant (Dk) of 10.2 (thickness) measured in the z-axis at 10 GHz and a tolerance of ±0.30 across the board, high temperatures will affect the size and spacing of transmission lines, especially as circuit size is reduced due to the high Dk of the laminate. Similarly, circuits manufactured for millimeter-wave frequencies will also have extremely fine line widths and spacing, and the expansion of the board caused by high temperatures will change the performance of these circuits.

Line width and transmission line spacing jointly determine the degree of coupling between various components of the circuit. At the same time, the size of the circuit plays a decisive role in determining the center frequency of the resonant circuit. Taking an edge-coupled bandpass filter circuit used in a high-temperature environment as an example, the significant difference in the coefficient of thermal expansion (CTE) between the conductor and the dielectric material may cause changes in the physical space between the coupled resonators in the filter, which may, in turn, affect the performance of the filter. Passband frequency and bandwidth are adversely affected.

Shenzhen Pcbandassembly: In printed circuit board (PCB) materials, a lower coefficient of thermal expansion (CTE) means that the material expands less when the temperature rises. It is generally considered that a lower CTE value is ideal. Based on common engineering practices, the CTE value of PCB materials should be controlled below 70 ppm/°C to ensure the reliability of the circuit board. Such materials are particularly suitable for circuit applications in millimeter-wave frequency bands or where higher-precision dimming is required.

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