Abstract:

A printed circuit board laminate datasheet provides dielectric constant and dissipation factor values that are used for design and specification. But studies have shown that these properties vary with changes in the moisture content of printed circuit board laminate material. A major problem can arise when material substitutions are made using laminate datasheets as a guide, especially when the data is derived from different laminate test conditions and methods. Initial impedance calculations on the basis of datasheet values may be acceptable, but actual board performance may be significantly different and may result in poorly functioning or non-functioning boards. In the past we conducted experiments to measure the dielectric properties of PCB materials as per industry adopted methods, including preconditioning of test samples. It was observed that some measurement results were not in accordance with manufacturer datasheets. This paper presents follow-up experimental studies on the dependence of dielectric constant and dissipation factor on the moisture content of laminates. Four types of PCB materials from two manufacturers, including two halogen-free and two halogenated, were tested in this study.

The paper further establishes whether preconditioning steps outlined in the IPC-TM-650 2.5.5.9 test method account for varying moisture contents in laminate samples.

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