- PCB Laminates
- Prepreg
- Copper Foil
- Dielectric Loss
- Copper Loss
Making the right choices for materials for printed circuit boards in the US is an important issue for designers, as their choice not only decides the board’s performance, but also the price of the circuit board. The designer must select the materials before the board goes for manufacturing, as this affects its electrical and thermal properties. Circuit board prices in the US depend on the materials designers decide to use, and the selection in the early stages saves time and money.
The PCB stack-up level is the best stage for designers when selecting materials for printed circuit boards. The different materials they must choose must be for:
PCB Materials for Stack-Up
When designing a multi-layered electronic circuit board in the US, the stack-up represents all the layers in sequential order. A stack-up typically has cores, prepreg, and foil. Construction of the stack-up must remain symmetrical around the central core.
PCB Core
Laminates and copper foils make up the core of the electronic circuit board. The prepreg bonds the copper foil to the laminate. In multi-layered printed circuit boards in the USA, designers can place several cores sequentially with prepreg bonding them.
Basic Properties of PCB Laminates
As PCB laminates are made from dielectric materials, designers must regard several properties such as Electrical Properties like Dielectric Constant (Dk) and Loss Tangent (Tan δ) or Dissipation Factor (Df), Thermal Properties like Glass Transition Temperature (Tg), Decomposition Temperature (Td), Thermal Conductivity (k), and Coefficient of Thermal Expansion (CTE).
Electrical Properties of PCB Laminates
Dielectric Constant (Dk)
For signal integrity and controlled impedance, designers must consider a laminate material that has a low dielectric constant. For boards handling high-frequency and high-speed signals, the low dielectric constant is a critical factor, as a lower Dk factor offers better signal integrity, Most PCB materials feature a dielectric constant varying from 2.5 to 4.5.
Loss Tangent (Tan δ) or Dissipation Factor (Df)
The tangent of the phase angle between the resistive and reactive currents within the dielectric is the loss tangent or dissipation factor of the dielectric. Higher the value of Df, higher will be the dielectric loss, and high losses in a substrate makes it slow. A slow substrate with a high dielectric loss attenuates the signal as it travels, and this causes a loss of signal integrity.
High-frequency materials usually have very low Df, and the change in Df value with increase in frequency is also very low. The Df value for high-frequency PCB material usually varies from 0.001 to 0.030.
Thermal Properties
Glass Transition Temperature (Tg)
As temperature rises, the laminate material of the electronic circuit board transforms from the usual rigid state to a soft plastic state. As the material cools, its properties revert to the original rigid state. PCB designers must be aware of the temperature range the PCB will undergo during soldering and during application, and select a suitable substrate material with a glass transition temperature range that goes beyond.
Decomposition Temperature (Td)
Beyond a certain temperature range, the PCB material starts to decompose chemically and loses at least 5% of its original mass. The designer must opt for a PCB material with a decomposition temperature higher than what the PCB assembly will undergo during reflow soldering, and during operation.
Thermal Conductivity (k)
For good thermal management, it is important that the designer selects a material with good thermal conductivity. Thermal conductivity of the PCB material is the rate at which it conducts heat. If the material is a good conductor of heat, it will have a high figure of k. The expression for thermal conductivity of rate of heat transfer of the material is in watts per meter per degree Celsius or W/M °C).
Coefficient of Thermal Expansion (CTE)
With changes in temperature, the PCB material will expand or contract. Coefficient of thermal expansion is a very important parameter of a PCB material, as it defines the rate at which it will expand or contract with changes in temperature. If the CTE of the material is significantly different from the CTE of copper, heating or cooling of the PCB may cause the walls of vias to rupture, causing interconnection issues. Designers must choose materials with CTE value close to the CTE values of copper.
How Designers Select PCB Materials
To make it easier to work with electronic circuit board materials in the US, designers divide them into four broad categories like Normal Speed and Normal Loss, Medium Speed and Medium Loss, High Speed and Low-Loss, and Very High-Speed and Very Low-Loss.
Normal Speed and Normal Loss Materials
Being the most commonly used printed circuit board materials, these are of the glass epoxy or FR4 family. Their response of dielectric constant versus frequency is not flat, but rather their dielectric loss increases with frequency. That means, such materials are suitable for digital/analog applications only up to a few GHz. An example of this type of PCB material is the Isola 370HR.
Medium Speed and Medium Loss Materials
With a comparatively flatter dielectric constant versus frequency response as compared to the first category, these materials offer medium speed and medium losses. Their dielectric loss is typically about half that for normal speed and normal loss materials, meaning this category of PCB materials is appropriate for applications operating up to about 10 GHz. An example of this type of PCB material is the Nelco N7000-2 HT.
High-Speed and Low-Loss Materials
High-Speed and low-loss materials offer a still flatter dielectric constant versus frequency response as compared to category 1 and 2. These materials offer low losses as the operational frequency increases. Other than low losses, this category of materials also offers significantly lower electrical noise. An example of this type of material is the Isola I-Speed.
Very High-Speed and Very Low-Loss Materials
Very high-speed and very low-loss materials have the flattest dielectric loss versus frequency curve in the range, along with an extremely low dielectric loss. Such materials are suitable for applications where the frequency goes up to about 20 GHz. Tachyon 100G and Isola I-Tera MT40 are suitable examples of this type of PCB materials.
Signal Loss in PCB Materials
PCB Materials chosen by the designer affects the signal integrity in high-frequency and high-speed applications. Signal loss in PCBs has two components:
Dielectric Loss
With high dielectric loss, the substrate absorbs the signal and heats up. The heating increases with higher absorption, causing a higher signal attenuation. Moreover, the attenuation increases with a rise in frequency. The designer must balance the circuit board price in the US with the type of PCB material they select and the amount of loss functionally acceptable to the circuit board.
Copper Loss
The resistance of copper causes ohmic losses as current passes through the traces of an electronic circuit board. Higher frequencies create eddy currents within the copper traces, resulting in a skin effect that forces the current flow to occupy only outer parts of the cross-section of the trace. The net effect of skin effect is an increase in the resistance of the trace, leading to higher heat losses. For instance, for a trace with a nickel finish, much of the current starts to pass through the nickel with an increase of signal frequency.
One way designers get around the skin effect is by using wider traces, provided the increase in the board area is acceptable. Another option for designers is to use low profile, highly annealed copper foil, such as rolled copper.
Conclusion
By carefully selecting PCB materials and executing a proper stack-up for printed circuit boards in the US, designers can significantly improve signal integrity, reduce cross-talk, and reduce electromagnetic emissions. A careful choice of electronic circuit board materials has other benefits also, such as a proper impedance control over traces, possibility of a reduction in the size of the board, and improvements in the routing density. Additionally, introduction of ground and power planes in electronic circuit boards helps improve the board performance further.
