DC calculations are made for core material as such. This means the DC does not change for it in whatever MLB structure. For prepregs, DC is calculated based on expected effects after cure in the press - in the book - in the MLB. Variations are minor, within but a percent or two, for the same glass styles and resin contents used in the structures. "Good" board shops will provide pre-calculated DC numbers both for core and prepregs used. Still, the numbers vary minimally and your overall impedance will be little affected. However, for very high speed stuff, this can be critical though little FR4 is used in these designs. You can then calculate the requirements based on DC, thickness, trace widths (for differential pairs as stripline [boradside and edge rate requirements] and microstrip requirements). MoonMan --------------------------------------------------------------------------------- Technet Mail List provided as a free service by IPC using LISTSERV 1.8d To unsubscribe, send a message to [log in to unmask] with following text in the BODY (NOT the subject field): SIGNOFF Technet To temporarily halt delivery of Technet send the following message: SET Technet NOMAIL Search previous postings at: www.ipc.org > On-Line Resources & Databases > E-mail Archives Please visit IPC web site (http://www.ipc.org/html/forum.htm) for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ---------------------------------------------------------------------------------