Hi Jose, Er is all about the relative proportions of glass and resin.This note which I hope I have pasted in correctly explains all. The graphs that go with it have been deleted but they add nothing to this information. We all work within a range and are unable to change Er by very much. Your local laminate supplier should be able to certify as required. I won't speak for him but we take this to be our responsibility over here. Regards, Dave Heywood [log in to unmask] [[ Microsoft Word 6.0 Document : 3849 in WINMAIL.DAT ]] ---------- From: [log in to unmask] To: [log in to unmask]; Dave Heywood Subject: [TN] FAB. Dielectric constant certi