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.