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High-voltage electrical properties

In the application we have outlined briefly, this laminate is operating at high voltage, at high frequency, and in an adverse environment! Considering these in the order in which they appear in the main text:

• The surface resistivity is important, and will reduce with humidity, roughness and contamination. Not only must one be careful to protect the laminate, but a solder mask coating will improve its performance.
• Dielectric breakdown is certainly a possibility, though breakdown through the dielectric is relatively unlikely, assuming that the laminate stays dry. However, discharge across the surface is a real possibility, especially in the presence of RF and a humid environment. Again the superior performance of FR-4 compared to FR-2 will assist, as will controlling contamination and adding a surface coating.
• The electrical clearance recommendations should be carefully adhered to, again allowing for the environment. For this particular application, the increased clearance suggested in Table 5 for low air pressure applications will not be needed.
• The dielectric constant of the material, and its dissipation factor, will both affect performance. In particular, the dielectric constant has to be taken into account when creating any matching networks and other controlled-impedance features. In the VHF spectrum (30–300MHz) the dielectric constant of FR-4 will vary with frequency, and the losses will be higher than at low frequency. The variation of dielectric constant with frequency for this particular material may impact on a linear amplifier operating over a wide range of frequencies.

Hopefully you will have convinced the technician that your laminate is far from being ‘a bit of gash insulating material’. Be aware, however, that, in the real world, a professional application of this nature may well need specialist materials. Even so, ‘unimproved’ FR-4 is a remarkably versatile laminate.