The thermal, mechanical and electrical behavior of each PCB depends on the material characteristics of PCB substrate, conductor and component materials. Among these different materials, PCB designers can control the behavior of the board to the greatest extent by choosing the right PCB substrate material. The performance of PCB materials, especially the performance of resins and laminates, will determine how your circuit board responds to mechanical, thermal and electrical stimuli.
When you need to choose PCB substrate materials, which PCB material characteristics are most important for your circuit board? The answer depends on the application of the circuit board and the environment where the PCB will be deployed. When choosing prepreg and laminate for the next PCB, the following important material characteristics should be considered for your application reference.
Your choice of substrate is no longer limited to FR4, but you should not easily choose PCB laminate. You should first understand how different material properties affect your PCB, and then choose the laminate that can meet your operational requirements. Don’t just listen to the marketing speeches of laminate manufacturers; Take the time to understand the material characteristics of each substrate and how they affect your PCB.
You can find some data about the properties of PCB materials on the Internet, but it’s best to consult the manufacturer, especially for specialized laminates, because no two laminates are exactly the same, and no two laminates are exactly the same. And more exotic materials such as ceramic core PCB have a series of unique material characteristics.
The important PCB material properties that all designers should know are divided into four aspects: electrical, structural, mechanical and thermal properties.
Electrical property
All the important electrical properties to be considered in PCB substrate materials today are reflected in the dielectric constant.
dielectric coefficient
This is the main electrical characteristic to be considered when PCB is designed for high-speed/high-frequency PCB lamination. Dielectric constant is a complex quantity, which is a function of frequency and causes the following forms of dispersion in PCB substrate:
Velocity dispersion: Because the dielectric constant is a function of frequency, different frequencies will experience different loss levels and propagate at different speeds.
Dispersion: The attenuation experienced by a signal is also a function of frequency. The simple model of dispersion shows that the loss increases with the increase of frequency, but this is not strictly correct. There may be a complex relationship between the loss of some laminates and the frequency spectrum.
These two effects contribute to the degree of distortion experienced by the signal in the process of propagation. For analog signals operating on a very narrow bandwidth or a single frequency, dispersion is irrelevant. However, it is extremely important in digital signals, and it is one of the main challenges in high-speed digital signal modeling and interconnection design.
constitutive property
The structure of PCB and its substrate will also affect the mechanical, thermal and electrical properties of PCB. These characteristics are mainly reflected in two ways: glass weaving and roughness of copper conductor.
Glass weaving style
Glass weaving pattern will leave a gap on PCB substrate, which is related to the resin content on the board. And the volume average dielectric constant of the base material can be determined by combining the volume ratio of the base material and the glass impregnated resin. In addition, the gap in the glass weaving pattern will produce the so-called fiber weaving effect, in which the dielectric constant of the substrate changing along the interconnection line will produce deflection, resonance and loss. These influences become very prominent at 〜50GHz or higher, which will affect radar signals, Gigabit Ethernet and typical LVDSSerDes channel signals.
Copper roughness
Although this is actually the structural characteristic of printed copper conductor, it contributes to the electrical impedance of interconnection. The surface roughness of the conductor effectively increases its skin effect resistance at high frequency, resulting in the induced loss caused by the induced eddy current during signal propagation. Copper etching, copper deposition methods and the surface of prepreg will all affect the surface roughness to some extent.
hot property
When selecting substrate materials, the thermal properties of PCB laminate and substrate need to be divided into two groups.
Thermal conductivity and specific heat
The heat required to raise the temperature of the plate by one degree is quantified by the specific heat of the substrate, while the heat transmitted through the substrate per unit time is quantified by the thermal conductivity. The properties of these PCB materials jointly determine the final temperature when the circuit board reaches thermal balance with the environment during operation. If your circuit board is deployed in an environment where heat needs to be quickly dissipated into a large radiator or chassis, you should use a substrate with higher thermal conductivity.
The material characteristics of these two PCBs are also related. All materials have a certain coefficient of thermal expansion (CTE), which is just the anisotropy in PCB substrate (that is, the expansion rate is different in different directions). Once the temperature of the circuit board exceeds the glass transition temperature (Tg), the CTE value will suddenly increase. Ideally, CTE value should be as low as possible in the required temperature range, while Tg value should be as high as possible. The cheapest FR4 substrate has a TG of 130 C, but most manufacturers offer fiber cores and laminates with a TG of 170 C.
The thermal properties listed above are also related to the mechanical stability of conductors on PCB substrates. In particular, CTE mismatch creates a known reliability problem in high aspect ratio through holes and blind holes/buried holes, in which through holes are easily broken due to mechanical stress caused by volume expansion. Therefore, high Tg materials and other specialized laminates have been developed, and design engineers engaged in HDI PCB design may consider using these alternative materials.
