The thermal, mechanical, and electrical behavior of each PCB depends on the material properties of the PCB substrate, conductors, and component materials. Among these different materials, designers can maximize control over the behavior of the circuit board by selecting the correct PCB substrate material. The performance of PCB materials, especially the performance of resins and laminates, will determine the response of your circuit board to mechanical, thermal, and electrical stimuli.

The important PCB material characteristics that all designers should understand are divided into four aspects: electrical, structural, mechanical, and thermal characteristics.

Electrical performance

All important electrical characteristics that need to be considered in today‘s PCB substrate materials are reflected in the dielectric constant.

Dielectric constant

This is the main electrical characteristic to consider when designing a stack for high-speed/high-frequency PCBs. The dielectric constant is a complex number that is a function of frequency and causes dispersion in PCB substrates in the following form:

Velocity dispersion: As the dielectric constant is a function of frequency, different frequencies will experience different levels of loss and propagate at different speeds.

Loss dispersion: The attenuation experienced by a signal is also a function of frequency. A simple dispersion model indicates that the loss increases with frequency, but this is not strictly correct. There may be a complex relationship between the loss of certain laminates and their spectra.

These two effects will affect the degree of distortion experienced by the signal during propagation. For analog signals operating on very narrow bandwidths or a single frequency, dispersion is irrelevant. However, it is extremely important in digital signals and is one of the main challenges in high-speed digital signal modeling and interconnect design.

Structural characteristics

The structure of PCBs and their substrates can also affect the mechanical, thermal, and electrical performance of circuit boards. These characteristics are mainly reflected in two aspects: glass weaving and copper conductor roughness.

Glass weaving style

The glass weaving pattern will leave gaps on the PCB substrate, which is related to the resin content on the board. The average dielectric constant of the substrate can be determined by combining the volume ratio of glass and impregnating resin. In addition, the gaps in the glass weaving pattern will generate the so-called fiber weaving effect, where the substrate dielectric constant changes along the interconnection line, resulting in deflection, resonance, and loss. These effects become very prominent at frequencies of~50 GHz or higher, which will affect radar signals, gigabit Ethernet, and typical LVDS SerDes channel signals.

Copper roughness

Although this is actually a structural feature of printed copper conductors, it contributes to the impedance of interconnections. The surface roughness of a conductor effectively increases its skin effect resistance at high frequencies, leading to induced losses caused by eddy currents during signal propagation. Copper etching, copper deposition methods, and surface roughness of semi cured sheets can all affect surface roughness to a certain extent.

Thermal performance

When selecting substrate materials, it is necessary to divide the thermal performance of PCB laminates and substrates into two groups.

Thermal conductivity and specific heat

The heat required to increase the temperature of the board by 1 degree is quantified by the specific heat of the substrate, and the heat transferred through the substrate per unit time is quantified by thermal conductivity. The performance of these PCB materials collectively determines the final temperature at which the PCB reaches thermal equilibrium with the environment during operation. If you deploy a circuit board in an environment that requires rapid heat dissipation to large heat sinks or chassis, you should use a substrate with a higher thermal conductivity.

Glass transition temperature and coefficient of thermal expansion (CTE)

The characteristics of these two PCB materials are also related. All materials have a certain coefficient of thermal expansion (CTE), which is precisely the magnitude of anisotropy in PCB substrates (i.e. different expansion rates in different directions). Once the temperature of the circuit board exceeds the glass transition temperature (Tg), the CTE value will suddenly increase. Ideally, the CTE value should be as low as possible within the required temperature range, while the Tg value should be as high as possible. The cheapest FR4 substrate is available at Tg~130 ° C, but most manufacturers offer a choice of Tg~170 ° C fiber cores and laminates.

The thermal performance listed above is also related to the mechanical stability of the conductors on the PCB substrate. Especially, CTE mismatch can generate known reliability issues in high aspect ratio through holes and blind/buried through holes, where through holes are prone to rupture due to mechanical stress caused by volume expansion. Therefore, high Tg materials and other special laminates have been developed, and designers engaged in HDI design can consider using these alternative materials. When you need to choose a PCB substrate material, which PCB material characteristics are most important for your PCB? The answer depends on the application of the PCB and the environment in which the PCB will be deployed. When selecting prepreg and laminate materials for the next PCB, you should consider the following important material characteristics for your application reference.

Important PCB material characteristics

Your choice of substrate is no longer limited to FR4, but you should not easily choose PCB laminates. You should first understand how different material characteristics affect your PCB, and then choose a laminate that meets your operational requirements. Don‘t just listen to marketing speeches from laminated panel manufacturers; Take time to understand the material characteristics of each substrate and how they affect your PCB.

You can find some data about the performance of PCB materials online, but it is best to consult the manufacturer, especially for special laminated materials, because no two laminates are exactly the same, and no two laminates are exactly the same. More unique materials such as ceramics and metal core PCBs have a series of unique material characteristics.