IPC TM-650 2.5.5.3C

Abstract

Purpose: This method is suitable for determining the volume permittivity, (dielectric constant) and loss tangent (dissipation factor) of insulating materials at 1 MHz. It is not dependent on either direct or indirect measurement of specimen thickness and therefore is very useful for thin films and laminates but may also be used on specimens up to approximately 6.35 mm [0.25"] thick. <br>It is useful for all ranges of permittivity and for loss tangent as low as 0.0005 providing the range and accuracy of the bridge used are adequate. <br>Description of Method: The two fluid method utilizes air as one fluid and a suitable liquid, normally Dow 200 1.0CS silicon fluid, as the second. Using an established value for the permittivity of air, the values for the permittivity of the fluid and the sample may easily be calculated. The cell spacing is fixed during all readings but does not need to be known accurately for the series of readings required. Since specimens do not require any electrodes to be applied and since many specimens can be measured at one time without changing any spacings or machine settings, the method is not only very accurate but very rapid. <br>The method has been used for measurement of PTFE and epoxy glass laminates and flexible films, e.g. polyimide. <br>Reproducibility lab to lab is excellent for permittivity provided minimal precautions are observed and bridge accuracy is appropriate. On most materials, the effects of small changes in moisture or temperature are larger than any error due to the method. Lab to lab correlation on stable material such as PTFE have shown results to be consistently within 0.005 or (0.2%).