Tom
10-10-2002, 05:57 PM
IPC Advanced Study Guide Page Reference: Page 123, Section 2.8
Dielectric material is one that has a high resistance to the flow of direct current, and which is capable of being polarized by an electrical field. The definition for dielectric constant, sometimes known as permittivity, is the ratio of capacitance of a configuration of electrodes with a specific material as the dielectric between them to the capacitance of that same electro-configuration, with a vacuum or air as the dielectric. Air has a dielectric constant of "1" and thus is the perfect dielectric where all others are ratios that are greater than 1.
There are several metrics that are embraced under the general heading of dielectric properties. Dielectric constant, dissipation factor, dielectric withstanding voltage and surface insulation resistance are examples of such properties.
These properties become more and more important, as operating frequencies get higher and higher. Digital applications have shorter rise and fall times, resulting in higher effective operating frequencies. Radio frequency applications are pushing into higher frequencies to take advantage of less crowded frequencies.
It is useful to work in frequencies and wavelengths when doing calculations for dielectric constants and loss tangents. However, the important feature of digital pulses is the rise and fall time more so than the operating frequency. The rise and fall transitions contain many frequencies and their harmonics. The need for greater capability becomes apparent as RF designs approach 300 MHz and digital signal transition times approach 1Ns. At these frequencies a pulse, in FR4, will travel around 150 mm during the transition time. Thus impedance mismatches may raise significant signal integrity issues.
As operating frequencies increase and signal transition times decrease pressure on the designer to lower dielectric constant (Dk), and dissipation factor will continue. Keeping a lower dielectric constant provides:
- faster conductor signal speed
- shorter propagation delay times
- lower dielectric losses
- wider interconnects for the same design impedance
Dielectric material is one that has a high resistance to the flow of direct current, and which is capable of being polarized by an electrical field. The definition for dielectric constant, sometimes known as permittivity, is the ratio of capacitance of a configuration of electrodes with a specific material as the dielectric between them to the capacitance of that same electro-configuration, with a vacuum or air as the dielectric. Air has a dielectric constant of "1" and thus is the perfect dielectric where all others are ratios that are greater than 1.
There are several metrics that are embraced under the general heading of dielectric properties. Dielectric constant, dissipation factor, dielectric withstanding voltage and surface insulation resistance are examples of such properties.
These properties become more and more important, as operating frequencies get higher and higher. Digital applications have shorter rise and fall times, resulting in higher effective operating frequencies. Radio frequency applications are pushing into higher frequencies to take advantage of less crowded frequencies.
It is useful to work in frequencies and wavelengths when doing calculations for dielectric constants and loss tangents. However, the important feature of digital pulses is the rise and fall time more so than the operating frequency. The rise and fall transitions contain many frequencies and their harmonics. The need for greater capability becomes apparent as RF designs approach 300 MHz and digital signal transition times approach 1Ns. At these frequencies a pulse, in FR4, will travel around 150 mm during the transition time. Thus impedance mismatches may raise significant signal integrity issues.
As operating frequencies increase and signal transition times decrease pressure on the designer to lower dielectric constant (Dk), and dissipation factor will continue. Keeping a lower dielectric constant provides:
- faster conductor signal speed
- shorter propagation delay times
- lower dielectric losses
- wider interconnects for the same design impedance