scope:

Summary: This method is for measurement of relative permittivity (er) and dissipation factor or loss tangent (tan d) of circuit board substrates under stripline conditions. Measurements are made by measuring resonances of a length of stripline over a wide frequency range from below 1 GHz to about 14 GHz(1,2). The method permits a wide variety of specimen configurations, varying in dielectric thickness, width of center conductor, and use of clad or laid up conductor foil(3). Sensitivity to differences in tan d are enhanced by the ability to adjust the degree of coupling to the resonator by adjusting an air gap between probes and the resonator ends. Many of the principles used in IPC-TM-650, Method 2.5.5.5, are applied in this method.

Terminology: Terms used in this method include: Complex Relative Permittivity-The values for relative permittivity and dissipation factor considered as a complex number. Permittivity-Dielectric constant (see IPC-T-50) or relative permittivity. The symbol used in this document is er. K' or k' are also sometimes used. Relative Permittivity-A dimensionless ratio of absolute permittivity of a dielectric to the absolute permittivity of a vacuum. Loss Tangent-Dissipation factor (see IPC-T-50), dielectric loss tangent (see 9.2). The symbol used in this document is tan d.

Limitations: The measured effective permittivity for the resonator element can differ from that observed in an application. Where the application is in stripline and the line width to ground plane spacing is less than that of the resonator element in the test, the application will exhibit a greater component of the electric field in the X, Y plane. Heterogeneous dielectric composites are anisotropic to some degree, resulting in a higher observed er for narrower lines. Microstrip lines in an application may also differ from the test in the fraction of substrate electric field component in the X, Y plane.  Bonded stripline assemblies have air excluded between boards and thus tend to show greater er values than would be obtained with this method using specimen types A or, to lesser extent, B, as discussed in 3.0.

As with IPC-TM-650, Method 2.5.5.5, with specimen type A, or, to a lesser extent, with B (see 3.0), we expect the method to show a downward bias in measured er. This is caused by the electric field crossing clamped dielectricconductor interfaces with air included in the surface roughness.

With specimen type B, C, or D, the method shows an upward bias in measured tan d. This is caused by the surface roughness and/or surface treatment of the clad copper foil required for adequate adhesion to the dielectric.

Compared to IPC-TM-650, Method 2.5.5.5, both done with computer automated data collection, this method requires a greater degree of operator skill and more time to prepare specimens and perform measurements.

Advantages: The sensitivity of the method to differences in er of specimens should be superior to that of IPC-TM-650, Method 2.5.5.5 since the specimen comprises all of the dielectric affecting the measurement.

The method is known to be more sensitive to differences in tan d than IPC-TM-650, Method 2.5.5.5. We believe the ability to adjust the degree of probe-to-resonator coupling to a low enough value that Qloaded is close to Qunloaded (see 7.2.2) makes this possible.

The method is expected to lend itself to use of stable referee specimens of known electric properties traceable to NIST (National Institute of Standards and Technology).