I'll give it a shot... When the resin in a prepreg fills in the copper on adjacent layers, depending on the copper weight and amount of retained copper after etch (e.g. a plane might be 80% copper, and a signal layer might be 20% copper), the prepreg thickness after lamination an vary quite a bit. It may be the design in question has heavy copper signal layers such that a HRC 7628 is pressing out at only .0068. Typically, 7628 b-stage pressed out between solid foil will be closer to .0075. A laminator (who makes the prepreg) can give you whatever resin content you ask for. For example 7628 might have 40%RC, or 44%, or 47%. In such a case the 47%RC might be called a high resin content 7628. One laminators 45% RC 7628 may also press out with a different thickness than another laminators 45%RC 7628 due to differences in rheology (flow characteristics) of the resin. So the final dielectric thickness depends on the board design, the specific resin, the press cycle, and other factors, not just the RC%. It may vary by where you measure it as well; high pressure areas (where there is a lot of copper) on the board will have thinner dielectric thickness than low pressure areas. I would recommend to board designers not to specify the prepreg, but rather to specify the intended dielectric thickness, or better still, the intended impedance (along with any minimum dielectric spacing, etc.), and let the fabricator select the prepreg which will give the desired result. Tim Reeves Process Engineer Tyco Printed Circuit Group - Dallas Division (800) 783-9223 ext. 1120 -----Original Message----- From: TechNet [mailto:[log in to unmask]]On Behalf Of Brooks,Bill Sent: Tuesday, May 18, 2004 12:50 PM Subject: FW: [DC] Core Constructions Is there a materials expert out there that can answer Mike's question below? B-stage epoxy resin is somewhat of a mystery to many designers. Perhaps someone can clarify the reference to "High Resin Content" as apposed to ? what other options are available and how they affect the construction of the FR4 board. Please copy to the group too so we can all learn from this. 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: Mike Valle [mailto:[log in to unmask]] Sent: Tuesday, May 18, 2004 12:24 PM To: [log in to unmask] Subject: [DC] Core Constructions Hi, I need some help trying to find some information on FR-4 construction and thickness. I've come across a 4-layer stackup on a cross-section detail on one of our drawings and I can't seem to find it in IPC-4121. Our b-stage construction calls out for 1 ply of b-stage HRC (high resin content) 7628 .0068. Another detail on another drawing calls out for 1 ply of b-stage HRC (high resin content) 7628 .008. What I've been finding in IPC-4121 is that there is a FR-4 11 (7.087) 7628 40% RC and a HFR-4 11 (7.087) 40% RC, which is a high tg. So, my question is - is there such a thing as HRC - high resin content with varying thicknesses? Thanks, Mike Valle Project Engineer, PCB Design Shure Inc. (847)424-4616 [log in to unmask] --------------------------------------------------- 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 -----------------------------------------------------