IPC TM-650 2.5.5.5C

Abstract

Summary: This method is intended for the rapid measurement of the X-band (8.00 to 12.40 GHz) apparent relative stripline permittivity (see 9.1) and loss tangent of metal clad substrates. Measurements are made under stripline conditions using a resonant element pattern card, which is separated from the ground planes by sheets of the material to be tested. Further information about this method may be found in ASTM D3380-75. <br>Definitions: 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 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. The symbol used in this document is tan d (see 9.2). <br>Limitations The following limitations in the method should be noted. Users are cautioned against assuming the method yields permittivity and loss tangent values that directly correspond to applications. The value of the method is for assuring consistency of product, thus reproducibility of results in fabricated boards. <br>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 groundplane 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.&#160; Bonded stripline assemblies have air excluded between boards, thus tend to show greater er values. <br>High degrees of anisotropy of some composites can result in an increased degree of coupling of the resonant element, resulting in a falsely lower Q value. If a correction is not applied either mathematically as in 7.2.2 or by deviating from the probe gaps specified for the test pattern, an upward bias in the calculated loss tangent will result. <br>The sensitivity of the method to differences in er of specimens is impaired by the fact that the resonator pattern card remains as part of the fixture and at the same time constitutes a significant part of the dielectric involved in measurements. <br>The method does not lend itself to use of stable referee specimens of known electric properties traceable to The National Institute of Standards and Technology (NIST).