Bernie, We have sectioned a multilayer PCB and it is possible to see the inner layer prepreg flowing between the tracks (traces) with 14thou (0.355mm) gaps and I'm sure that the prepreg does flow down to 6thou gaps although I haven't seen it.
Our electric strength tests gave us >5kV @ 6thou gap (inner layer). This is acknowledged by IEC 950 in clause 2.9.7 "Spacings filled by insulating compound" and note 2 states "acceptable forms of construction include: - Internal insulation of multi-layer printed boards". Creepage or clearance distances don't exist, only the requirements for distance through insulation of 2.9.4 apply. Regards Richard Steele Fujitsu Telecommunications Europe Limited Jon D Curtis wrote: > > From: Bernie McEwen-LBM022 <[email protected]> > Subject: RE: Coated Printed Boards > > Any material declared as a conformal coating must meet the abrasion > resistance tests in EN60950 2.9.5. A general purpose silk-screened > solder-resist is not compliant. > > Internal layers of a multilayer PCB may have creepage distances reduced > to the clearances because they are a pollution degree one environment. > But spacings cannot be reduced further because although the laminate > layer covering the tracks would meet the abrasion test, the material > does not extend between the tracks; it sits on top, leaving a void (or a > bonding material that may have voids) between tracks. > > Provided a single layer meets the relevant proof tests, then two layers > of laminate are a compliant insulator. Tracks may cross or run together > provided at least two layers of laminate separate them. > > Regards. Bernie McEwen. > > > ----------
