Bill, Notwithstanding my prior message, I doubt this particular board needs that much analysis. This is a "coarse" construction for a controlled impedance board: the impedance on the outerlayer of this board, if not previously specified and controlled, will vary considerably supplier to supplier, up to or more than 20% if I had to take a SWAG at it ... and as Jeff (Bush) noted, you would need traces about .022" wide to hit 50 ohms, so unless there are .022" wide traces it is unlikely to be a controlled impedance design what is telling to me is the requirement for 2 oz of foil/side on the inner core - that's a lot of copper, relatively speaking. My guess would be that this board supplies a lot of current ... The only good reason to call out 7628 in this case that I can think of is to keep the board cheap; 7628 being the lowest common denominator in glass styles, universally available (I think), and very cost effective for building up thickness (as noted by others in this mail thread). fyi: Dk varies by resin system, epoxy generically around 4.3; other resin systems are different within the epoxy family, the Dk will vary between laminators within the specific laminator's offerings, by resin and glass percentage by prepreg or core style within the resin percentage and prepreg style at frequency (sometimes) or at temperature .... laminators have spec sheets but they aren't awfully user friendly to the uninitiated; as others have said: call your favorite fabricator's application engineer for guidance on these matters Valerie "Brooks,Bill" <[log in to unmask]> Sent by: TechNet <[log in to unmask]> 07/13/2004 12:46 PM Please respond to "TechNet E-Mail Forum."; Please respond to "Brooks,Bill" To: [log in to unmask] cc: Subject: Re: [TN] 7628 prepreg? So in a stackup that requires a .031 core with 2 oz. Cu both sides and a symmetrical build up of 2 layers of 7628 on both sides (4 total) and then 1.5 oz Cu finished outside... Would the objective then be to control the spacing from the outer conductors to the inner ground plane at .014 in.? Is the DK of 7628 different than other glass epoxy materials? I would like to calculate the impedance of the traces on this board. Is there a materials sheet available on-line that describes the electrical and physical characteristics of the prepreg material? And does it change when laminated? This is an older board and I'm just trying to decipher the designer's intent. Thanks in advance... Bill Brooks PCB Design Engineer , C.I.D., C.I.I. Tel: (760)597-1500 Ext 3772 Fax: (760)597-1510 http://pcbwizards.com -----Original Message----- From: Jeffrey Bush [mailto:[log in to unmask]] Sent: Tuesday, July 13, 2004 4:10 AM To: [log in to unmask] Subject: Re: [TN] 7628 prepreg? 7628 glass has different yield thickness based on resin content and the rheology. You can purchase 7628 in epoxy from a yield per ply thickness of .0065" to where you are with yours. 7629 glass can give you up to .009" in yield as this is some what heavier glass. I think the standard has been a .007" per ply yield - this is fairly standard from laminate suppliers. Jeffrey Bush Director, Quality Assurance and Technical Support 76 Technology Drive - POB 1890 Brattleboro, Vermont 05302-1890 Tel. 802.257.4571.21 Fax. 802.257.0011 [log in to unmask] http://www.vtcircuits.com --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 ----------------------------------------------------- --------------------------------------------------- Technet Mail List provided as a service by IPC using LISTSERV 1.8e 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 or (re-start) delivery of Technet send e-mail to [log in to unmask]: SET Technet NOMAIL or (MAIL) To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest Search the archives of previous posts at: http://listserv.ipc.org/archives Please visit IPC web site http://www.ipc.org/contentpage.asp?Pageid=4.3.16 for additional information, or contact Keach Sasamori at [log in to unmask] or 847-509-9700 ext.5315 -----------------------------------------------------