Comparison of PCB Materials for High Speed and HDI PCB Boards
Printed circuit boards, more commonly known by their abbreviation PCB, are intricate and complex structures that are made using a variety of different materials for all the various components that make up the board.
The performance and characteristics of your PCB such as the copper surface roughness, signal or power loss, thermal dissipation, interconnect impedance, and temperature rise will all depend on the materials you chose for manufacturing your PCB.
It is important to remember that there is no universal base material that will be suitable for every application. That is why you must carefully choose the optimal base material for your PCB to achieve a perfect balance between the various elements that are involved in the construction of PCBs.
You can choose the best base material for PCB by carefully comparing the properties of PCB materials. To assist you in selecting the best base material for your circuit boards, this article will compare some of the most common base materials.
Standard PCB Stackup Materials
Printed circuit boards are typically constructed by sandwiching NEMA grade FR4 nonconductive material between layers of copper. The NEMA designation denotes properties like flame retardant, glass transition temperature, loss factor, dielectric constant, z-axis expansion coefficient, shear strength, and tensile strength. FR4 is a flame-resistant PCB material making it ideal for use in a range of humid and temperature settings.
The polymer resin with or without reinforcement, fillers, and metal foil is the backbone of PCBs. Alternate layers of dielectric are sandwiched between copper layers forming, what we call, a PCB. For high-speed design, epoxy makes up the majority of the PCB material, but it can also be PPE, BT, modified acrylates, and cyanate ester.
Important Properties That All PCB Materials Must Have
Following are some of the most important properties which you need to consider when selecting PCB materials for your next circuit board:
- Relative Dielectric Constant
This sophisticated quantity is a function of the dissipation factor (Df) and relative permittivity (Dk). This quantity is a measure of a material's ability to store electrical energy in the form of an electrical field.
- Conductor Loss
Electrical conductivity of the conductors used determines the conductor losses of your material. Keep in mind that the surface roughness of copper will affect the conductor loss in copper used on a PCB. Surface roughness generally alters the impedance of the interconnect and increases the conductor loss of your PCB. Higher the conductor loss, poorer the performance of your PCB.
- Thermal Conductivity
The thermal conductivity of your PCB material is essentially a measure of how well the material can dissipate the heat generated by the board. It also influences the rise in temperature of your PCB with respect to the ambient temperature. Knowing the thermal conductivity of your material allows designers to devise a thermal management strategy for the PCB.
- Coefficient of Thermal Expansion (CTE)
This quantity helps designers understand how the material reacts to a change in temperature, i.e., how much the material expands or contracts when temperature increases or decreases respectively. Normally, PCB are anisotropic which means they expand by different amounts in different directions. Usually, designers are only concerned about the expansion along the z-axis.
- Glass Transition Temperature (Tg)
This property serves as an indication of the temperature beyond which the value of CTE abruptly rises as the temperature rises. In comparison to the CTE value below Tg, the CTE value above Tg is higher.
Dispersion is a property that represents the interaction between the material and electromagnetic field.
The search for newer and better materials is still underway and every now and then, fabricators and designers stumble across new PCB materials that are more suitable for high temperature, HDI, and high-speed circuit boards.
The properties for PCB material mentioned above influence the way high-speed signals progress through a substrate and how well they dampen mechanical shock and dissipate heat.
Some of the newly found PCB materials can be used in high temperature conditions or for microwave and millimeter Wave devices. However, their production costs are comparatively higher. The best course of action when selecting a material is to start with FR4 and see whether it fits the bill per your application's requirements.
PCB Material for HDI and High-Speed Design
Epoxy resin has served as the industry's mainstay resin for as long as one can remember. Due to its excellent adherence (to the metal foils and to itself), relatively low price point, and excellent thermal, electrical, and mechanical characteristics, epoxy has been a household name for the electronics industry.
Over time, significant modifications have been made to the fundamental chemistry of epoxy. To overcome certain drawbacks of epoxy-resin systems, resin-based PCB stackup materials are often used and perceived as superior substitutes. For instance, cyanate and polyimide ester resins are employed because of their lower Df and Dk values, whereas BT-Epoxy is frequently used for organic chip packages owing to its temperature stability.
In addition to thermosetting polymers, thermoplastic resins such as polytetrafluoroethylene (PTFE) and polyimide are also used. The thermosetting variant of polyimide, which can be found in the form of a film, is flexible in contrast to the thermoplastic version, which is comparatively inelastic and brittle.
Because of this, thermosetting polyimide resin is commonly used in the manufacturing of rigid-flex circuits as well as flexible circuits. This material is, however, more expensive than epoxy and is therefore, only employed when necessary. Many manufacturers believe that bromine-based flame retardants will be prohibited in the near future and therefore, have switched to a "Halogen-Free" requirement.
High speed and HDI PCBs benefit greatly from the characteristics outlined and explained above. Signals on PCBs can propagate faster thanks to decreased dielectric constant, which also reduces capacitive coupling between nearby signal traces. By minimizing crosstalk between neighboring signal lines, this aids in ensuring signal integrity in these circuit boards is maintained and undisturbed. This is all the more important for HDI PCBS since traces are arranged very close together.
Hemeixin is an industry-leading manufacturer of PCBs and other electronic systems. Our team of designers and engineers have served this field for many years and is therefore capable of designing and manufacturing all kinds of PCBs for all kinds of applications. To learn more about PCBs and our services, head on over to our website now!