Text item: The effective dielectric constant is: X*log(DK glass) + (1-X)*log(DK resin)= log (DK effective) where X is the fraction of glass. I think the DK of glass is ~6.2 and the DK of resin is ~3.4, but I am not positive. So the dielectric constant does change based on the resin content. It is a constant because it has no units. It is a ratio of material to air. Steve Joy Intel Corp. Stephen_Joy @ccm.hf.intel.com > Bob Vanech writes: > Avner-- > If I am wrong, I hope someone out there will make me smarter, but > the dielectric "CONSTANT" is just that--constant for that type of material. > IMOP, when calculating impedance of microstrip or stripline, the dielectric > constant of a material doesn't change, less the material's unique tolerance > range REF: IPC-D-275 Table 3-2. Therefore, unless proven wrong, the constant > DOES NOT change, but the impedance value WILL vary depending on which > formula you are using depending on you pwb construction. Bob, I only wished it was this easy. If it was I wouldn't have a weekly arguement about controlled impedence. Let me first express my opinion. The dielectric constant of material WILL change dependant on a few things, 1) resin content of the material (for FR4), 2) the frequency at which the signal is going through the material, and 3) which fab house you are using. To make my point, let me ask a question to the user's community. "What is the dielectric constant of FR4, Tg 170 degrees?" I'll gladly publish the results that come back (IF ANY). I only put #3 above because I can't seem to get a "true" value for the dielec- tric constant from our three main fab houses. No real problem because we allow them to modify our trace widths and stackup to give us back our impedence required, BUT with speeds in excess of 1.2 Ghz now, we are asking for much more precise information. Our circuits will just not work correctly with the "fidge" factor any longer. Waiting to hear any response. Mitch Morey Sr. PCB Designer TRW, Inc. Redondo Beach, CA 90278 Phone: (310)814-5765 Fax: (310)812-4949 EMAIL: [log in to unmask] ||||| ((o o)) ____________________________oOO_(_)_OOo_____________________________ PCB layout is like fine art. You have to DIE to be famous! ____________________________________________________________________ (_) (_) Text item: External Message Header The following mail header is for administrative use and may be ignored unless there are problems. ***IF THERE ARE PROBLEMS SAVE THESE HEADERS***. Resent-Sender: [log in to unmask] Precedence: list X-Loop: [log in to unmask] X-Mailing-List: <[log in to unmask]> archive/latest/1727 Resent-From: [log in to unmask] Resent-Message-ID: <"jb0WW1.0.bm6.KpTen"@ipc> Cc: [log in to unmask] Subject: FAB: Re: Controlled Impedance - Dielectric Constant- Comment To: [log in to unmask] Message-Id: <9605211516.AA07229@dodger> Date: Tue, 21 May 96 08:16:28 PDT From: "Mitch S. Morey" <[log in to unmask]> Old-Return-Path: <[log in to unmask]> Resent-Sender: [log in to unmask] Received: by ipc.org (Smail3.1.28.1 #2) id m0uLt4g-00008hC; Tue, 21 May 96 10:10 CDT Resent-Date: Tue, 21 May 1996 10:16:48 -0700 Received: from ipc.org by simon.ipc.org via SMTP (940816.SGI.8.6.9/940406.SGI) id KAA25638; Tue, 21 May 1996 10:16:48 -0700 Received: from simon.ipc.org (IPC.ORG [168.113.24.64]) by ormail.intel.com (8.7. 4/8.7.3) with SMTP id IAA05318 for <[log in to unmask]>; Tue, 21 Ma y 1996 08:24:56 -0700 (PDT) Received: from ormail.intel.com by relay.hf.intel.com with smtp (Smail3.1.28.1 #2) id m0uLtIw-000qDLC; Tue, 21 May 96 08:24 PDT