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 have the most control over the behavior of the board by choosing the correct PCB substrate material. The properties of PCB materials, especially resins and laminates, will determine how your board responds to mechanical, thermal, and electrical stimuli.
When you need to choose a PCB substrate material, which PCB material properties are most important to your board? The answer depends on the application of the board and the environment in which the PCB will be deployed. When selecting prepregs and laminates for your next PCB, here are some important material properties to consider for your application.
Your choice of substrates is no longer limited to FR4, but you should not choose PCB laminates lightly. You should first understand how different material properties affect your PCB, and then choose a laminate that will meet your operational requirements. Don't just listen to laminate manufacturers' marketing presentations; take the time to understand the material properties of each substrate and how they affect your PCB.
Some data on the properties of PCB materials can be found online, but it is best to check with the manufacturer, especially for specialized laminates, as no two laminates are exactly the same, and no two are exactly the same. More exotic materials such as ceramics and metal core PCBs have a unique set of material properties.
Important PCB material properties that all designers should be aware of are divided into four areas: electrical, structural, mechanical and thermal properties.
Electrical Properties: All important electrical properties to consider in today's PCB substrate materials are embodied in the dielectric constant.
Dielectric Constant: This is the main electrical characteristic to consider when designing a stackup for high speed/high frequency PCBs. The dielectric constant is a complex quantity that is a function of frequency, causing dispersion in the PCB substrate of the form:
Velocity Dispersion: Because the permittivity 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 model of dispersion states that loss increases with frequency, but this is not strictly true, and there may be a complex relationship between loss and frequency spectrum for some laminates.
These two effects contribute to the degree of distortion that the signal experiences as it propagates. For analog signals operating over very narrow bandwidths or a single frequency, dispersion does not matter. 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 Properties: The structure of the PCB and its substrate will also affect the mechanical, thermal and electrical properties on the board. These properties are mainly manifested in two ways: the way the glass is braided and the roughness of the copper conductor.
The glass weave style can leave gaps on the PCB substrate, which is related to the resin content on the board. The volume ratios of glass and impregnating resin combine to determine the volume average dielectric constant of the substrate. In addition, gaps in glass weave patterns create the so-called fiber weave effect, where the varying substrate dielectric constant along the interconnect leads to skew, resonance, and losses. These effects become prominent at frequencies of ~50 GHz or higher, which affect radar signals, multi-gigabit Ethernet and typical LVDS SerDes channel signals.
Copper Roughness: Although this is actually a structural property of printed copper conductors, it contributes to the electrical impedance of the interconnect. The surface roughness of a conductor effectively increases its skin-effect resistance at high frequencies, resulting in inductive losses from induced eddy currents during signal propagation. Copper etching, copper deposition method, and the surface of the prepreg all affect surface roughness to some extent.
Thermal properties: When selecting substrate materials, the thermal properties of the PCB laminate and substrate need to be divided into two groups.
Thermal Conductivity and Specific Heat: The amount of heat required to raise the temperature of a plate by one degree is quantified as the specific heat of the substrate, while the amount of heat transported through the substrate per unit time is quantified as thermal conductivity. The properties of these PCB materials together determine the final temperature at which the board reaches thermal equilibrium with the environment during operation. If your board is deployed in an environment where heat needs to be quickly dissipated into a large heatsink or case, a substrate with a higher thermal conductivity should be used.
Glass transition temperature and coefficient of thermal expansion (CTE): These two PCB material properties are also related. All materials have a certain coefficient of thermal expansion (CTE), which happens to be the amount of anisotropy in a PCB substrate (ie, the rate of expansion is different along different directions). Once the temperature of the board exceeds the glass transition temperature (Tg), the CTE value suddenly increases. Ideally, the CTE value should be as low as possible within the desired temperature range, while the Tg value should be as high as possible. The cheapest FR4 substrates have Tg ~ 130°C, but most manufacturers offer core and laminate options with Tg ~ 170°C.
The thermal properties listed above are also related to the mechanical stability of the conductors on the PCB substrate. In particular, CTE mismatch creates known reliability problems in high aspect ratio vias and blind/buried vias, where vias are prone to fracture due to mechanical stress caused by volume expansion. As a result, high Tg materials and other specialized laminates have been developed, and designers working on HDI designs may consider these alternatives.
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