Modern FR4 material has excellent properties up into the UHF region. If you require an input with gigaohms of input impedance, you may consider a bootstrapped guard ring around your input point.

It mostly depends on the circuitry. The phenolic boards before required ceramic standoffs for the components that required tens or hundreds of gigaohm resistance from each node to ground/node to node. But now it do not need so much, and eve

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