Substrate’s Role in PCB Reliability
For any type of PCB or Printed Circuit Board, its substrate or the material the board is made of, forms its backbone for holding the traces and the components. NEMA or National Electrical Manufacturers Association has formed FR4, a standard for laminates made from glass-reinforced epoxy resins. The FR4 has many properties that make it important as a substrate for rigid PCBs. These properties include its dielectric constant and its dissipation factor or loss factor. Rush PCB Inc. discusses the importance of these properties and how they affect the reliability of a printed circuit board.
Substrate and PCB Performance
As a substrate material, manufacturers often use FR4 as a commonplace laminate for PCBs. However, most users need to be made aware of the influence the laminate material has on the electrical performance indices of a board. These include essential factors like propagation delays and signal integration. For instance, the FR4 substrate, as a dielectric material, poses serious challenges to applications using high-frequency signals.
This is because frequency is an important performance parameter that causes changes in the dielectric constant of the FR4 substrate. In fact, the dielectric constant parameter value of the FR4 substrate undergoes a drop when subjected to high-speed or high-frequency signals, as they polarize the material. Therefore, when designing PCBs for mission-critical applications, designers must understand and consider the properties of the substrate material.
Dielectric Materials for PCBs
Almost all electronic equipment today uses printed circuit boards. This is true in all fields of applications including aerospace, automotive, consumer electronics, and healthcare industries. Whether these applications handle low or high frequencies, such as millimeter waves, microwaves, or RF, PCBs are the basis for their circuits. No matter what voltage or frequency range the application requires, the designer must choose the appropriate PCB materials.
Engineers often describe the PCB material or substrate as the backbone for rigid circuits. This is primarily due to the insulating material or substrate in rigid PCBs forming the base on which the fabricator places copper tracks, electronic and mechanical components, and solder connections. Because the dielectric material affects the technical performance of the circuit, it is all the more essential for the designer to select the appropriate dielectric material for a PCB.
How to Choose Dielectric Material?
There are certain aspects the designer must look for when selecting the dielectric material for a PCB. Although this essentially depends on the requirements of the system, the designer must consider the following for the optimum dielectric material:
System Requirements
∙ The overall size of the board
∙ Number of layers in the PCB
∙ Temperature class of the circuit on the board
∙ System loss or efficiency requirements
∙ Operating power, frequency, and bandwidth
Material Properties
∙ Cost and budgetary requirements
∙ Ease of processing, compatibility with hybrid constructions
∙ Mechanical strength, chemical properties, and electromagnetic loss
∙ Stability under extreme temperatures and humid operating conditions
∙ Power handling capabilities under high-frequency operations
Electrical Properties
∙ A low dielectric constant for the signal to propagate rapidly
∙ A low dissipation factor for maximizing power delivery
∙ Consistent dielectric constant and dissipation factor over the operating temperature range
∙ Consistent dielectric constant and dissipation factor over the operating bandwidth, as these prevent signal degradation and variations in the impedance of transmission lines
The dielectric material FR4 meets most of the requirements above.
FR4 Substrate Material
The standard FR4 as defined by NEMA applies to laminates using glass-reinforced epoxy resins. The initials FR denote the property of the material to be flame or fire retardant and is compliant with the inflammability of the plastic material to the standard UL94V-0. The property of being flame or fire retardant implies that fire propagation in FR4 material has a rare chance, and in case the material does catch fire, it extinguishes quickly.
FR4 Electrical Properties
Along with the FR4 material being flame retardant and moisture resistant, it also possesses several electrical properties of great advantage. These include a fairly low dielectric constant of 5 at 1 MHz, a low dissipation factor of 0.009 at 1 MHz, an electrical field strength ranging from 800 to 1800 V/mil, and a surface resistivity ranging from 105-1010 ohms/square unit.
FR4 Dissipation Factor and Dielectric Constant
Among the dielectric properties of the FR4 material, the most important are the dissipation factor or loss factor, and the dielectric constant. As a high-frequency signal or a high-speed pulse travels along a copper track, the surrounding plastic material absorbs some part of the energy from the propagating pulse, causing its energy to reduce or attenuate. The amount of energy absorbed by the surrounding dielectric material depends on its dielectric constant and its dissipating factor and significantly influences the speed of signal propagation, crosstalk, and the characteristic impedance the material offers to the signal.
FR4 Dielectric Constant and Reliability
Variations in the dielectric constant of the substrate material can cause a few issues. The dielectric constant of the FR4 material varies with the thickness of the substrate, the roughness of the copper foil on it, its resin content, and the style of its glass weave. Moreover, the stability of the insulating property of FR4 deteriorates under the application of high power, heat, and voltage.
With the variation of temperature, the dielectric constant of the FR4 substrate becomes unstable. This leads to impedance instabilities in the board. Therefore, in controlled impedance boards made of an FR4 substrate, variations in the dielectric constant can pose serious challenges.
Moreover, the dielectric constant of the FR4 material is frequency-dependent. It has non-uniform values in circuit board applications using high-speed or high-frequency signals. This causes fluctuations in the board’s performance and the circuit’s characteristics, both not preferable in high-frequency and high-speed applications.
Additionally, the dielectric constant of the FR4 material also influences the interconnect size or transmission line. For smaller boards, the designer must choose a substrate material with a higher dielectric constant.
Conclusion
Being a sensitive parameter, one that varies with texture differences, dimensional changes, signal frequencies, and more, it is pertinent to analyze the performance of the FR4 substrate under specific operating conditions. Rush PCB Inc. suggests using signal integrity tools to help with such analysis when studying the variation in the dielectric constant of PCB materials under different operating conditions. Signal integrity tools help to diagnose the effects of dielectric constant variations on high-frequency and high-speed signals.