A PCB’s response to electrical, thermal, and mechanical stimuli depend primarily on the material of the substrate materials.
Today’s designers have a range of options for PCB substrate materials, ranging from typical FR4 substrates to specialized low-loss laminates.
Different material properties are more desirable in different applications, and designers should choose specific material properties that are important for their designs.
The thermal, mechanical, and electrical behavior of every PCB is governed by the material properties of the PCB substrate, conductors, and component materials. Among these different materials, designers have the most control over board behavior by selecting the right PCB substrate material. The PCB material properties, particularly of the resin and laminate materials, will dominate how your board reacts to mechanical, thermal, and electrical stimuli.
When you need to select a PCB substrate material, which PCB material properties are most important for your board? The answer depends on the board’s application and the environment where your PCB will be deployed. When you’re selecting prepreg and laminate materials for your next PCB, here are some important material properties that you should consider for your application.
Important PCB Material Properties
Your substrate selection is no longer limited to FR4, but you should not make the decision of PCB laminate selection lightly. You should first understand how different material properties affect your PCB and choose a laminate that satisfies your operational requirements accordingly.
Some data for PCB material properties can be found online, but it’s best to consult with manufacturers, particularly for specialized laminate materials as no two laminates are exactly the same, nor are two lots exactly the same. More exotic materials such as ceramics and metal-core PCBs give a range of unique material properties.
The important PCB material properties all designers should understand fall into four areas: electrical, structural, mechanical, and thermal properties.
1 Electrical Properties
All the important electrical properties that need to be considered in today’s PCB substrate materials are embodied in the dielectric constant.
1.1 Dielectric constant
This is the primary electrical property to consider when designing a stackup for a high speed/high frequency PCB. The dielectric constant is a complex quantity that is a function of frequency, which gives rise to the following forms of dispersion in PCB substrates:
•Velocity dispersion: Because the dielectric constant is a function of frequencies, different frequencies will experience different levels of loss and travel at different speeds.
•Loss dispersion: The attenuation a signal experience is also a function of frequency. Simple models for dispersion state that loss increases as frequency increases, but this is not strictly correct and some laminates can have a complex loss vs. frequency spectrum.
These two effects contribute to the level of distortion the signal experiences during propagation. For analog signals operating over very narrow bandwidth or at a single frequency, dispersion does not matter. However, it is incredibly important in digital signals and is one of the major challenges in modeling and interconnect design for high speed digital signals.
2 Structural Properties
The structure of a PCB and its substrate will also affect mechanical, thermal, and electrical properties in the board. These properties are largely embodied in two ways: the glass weave style and roughness of copper conductors.
2.1 Glass weave style
The glass weave style leaves gaps in the PCB substrate, and it relates to the resin content in the board. The volume proportion of glass and impregnated resin combine to determine the volume-average dielectric constant of the substrate. Furthermore, gaps in the glass weave style create what are known as fiber weave effects, where the varying substrate dielectric constant along an interconnect creates skew, resonance, and losses. These effects become quite prominent at ~50 GHz and higher, which affects radar signals, multi-gigabit Ethernet, and typical LVDS SerDes channel signals.
2.2 Copper roughness
Although this is really a structural property of printed copper conductors, it contributes to the electrical impedance of an interconnect. The surface roughness of a conductor effectively increases its skin effect resistance at high frequencies, leading to inductive losses from induced eddy currents during signal propagation. Copper etching, copper deposition method, and the surface of the prepreg all influence surface roughness to some degree.
3 Thermal Properties
There are two groups of thermal properties in PCB laminates and substrates that need to be considered together when selecting substrate materials.
3.1 Thermal Conductivity and Specific Heat
The amount of heat needed to raise the board’s temperature by a single degree is quantified in the substrate’s specific heat, while the amount of heat transported through the substrate per unit time is quantified in the thermal conductivity. Together, these PCB material properties will determine the final temperature of your board when it comes into thermal equilibrium with the environment during operation. If your board will be deployed in an environment where heat needs to be quickly dissipated into a large heatsink or enclosure, a substrate with higher thermal conductivity should be used.
3.2 Glass Transition Temperature and Coefficient of Thermal Expansion (CTE)
These two PCB material properties are also related. All materials have some coefficient of thermal expansion (CTE), which happens to be an anisotropic quantity in a PCB substrate (i.e., expansion rate is different along different directions). Once the temperature of the board exceeds the glass transition temperature (Tg), the CTE value will abruptly increase. The CTE value should ideally be as low as possible within the desired temperature range, and the Tg value should be as high as possible. The cheapest FR4 substrate will have Tg ~ 130 °C, but most manufacturers offer a core and laminate selection with Tg ~ 170 °C.
The thermal properties listed above are also related to the mechanical stability of conductors on the PCB substrate. In particular, CTE mismatch creates a known reliability problem in high aspect ratio vias and blind/buried vias, where the vias are prone to fracture due to mechanical stress from volumetric expansion. High-Tg materials and other specialized laminates have been developed for this reason, and designers working on HDI designs might consider using these alternative materials.
If you’re looking to learn more about PCB material properties, talk to us and our team of experts.