I will be out of the office until March 11th. If you have any questions please contact Troy White at extension 4717. >>> TechNet 03/01/02 15:42 >>> Hi Genny, Jumping in late . . . to echo what others have already said there are tradeoffs when specifying cap lamination over foil lamination. Cap lamination uses a top and bottom piece of laminate to cap the board. Foil lamination uses prepreg and copper foil on the tops and bottoms. Cap lamination has higher material costs than foil lamination. Cap lam. ensures the outer (top & bottom) dielectric spacing by using the laminate thickness for that dimension. This can be crucial for an impedance controlled board and the increase in product consistency can be worth the added expense. The added material costs for cap lamination usually get passed back to the customer. Your fab house can tell you when a design is outside their foil lam. capability. FR-4 Prepreg is a woven glass fabric that's been impregnated with uncured (or partially cured) epoxy resin. The glass fabric comes in different styles ranging from thinner to thicker 106, 1080, 2113, 2116 & 7628. There are other styles but these are the most popular ones. To create a particular prepreg thickness the laminate suppliers pick one or more plies of glass fabric and combine them with the epoxy resin. Different prepreg constructions have slightly different resin contents and that may affect the prepreg's dielectric constant slightly. Laminate is fully cured prepreg (so the suppliers claim - heated debate to ensue) with an outer metal cladding - that's the 1/2, 1 or more ounce copper foil. When you construct a PCB panel one or more sheets of prepreg are placed between each layer of laminate during the Lay-up process. Several PCBs get stacked together in something called a lamination book. The Lamination process involves loading several of these lamination books into a heated vacuum assisted lamination press. During the press cycle some of the prepreg's resin is squeezed out as it flows and fills the internal features of the PCB. As long as you've got good fill the less resin squeeze out (resin loss) the better. Your fab house should know how much resin they lose for a particular lay-up configuration - it depends on the surrounding layers - % ground vs. % signal & the weight of copper. If there isn't enough resin to completely fill-in the features you'll have resin voids & scrap panels. Unlike a glue, the epoxy resin is polymerized (or cured) during the lamination process but it is the stuff that creates a bond between layers of laminate. For a prepreg analogy, consider a heavily starched shirt or . . . . a fabric tablecloth that has Mountain Dew soaked into it. The water evaporates leaving a tablecloth impregnated with Mountain Dew, it's stiff. If you fold it or hold it unsupported you get Mountain Dew powder all over and can see a crease left behind. If you are really rough you can tear the fabric. Prepreg acts the same way. The uncured epoxy resin is brittle and the lighter the glass style the more prepreg dust you'll get from general handling. Hans Integrity First - Service Before Self - Excellence in All We Do ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~ Hans M. Hinners Electronics Engineer Warner Robins - Air Logistics Center (WR-ALC/LUGE) 226 Cochran Street Robins AFB GA 31098-1622 mailto:[log in to unmask] Com: (478) 926 - 5224 Fax: (478) 926 - 4911 DSN Prefix: 468 -----Original Message----- From: Genny Gibbard [mailto:[log in to unmask]] Sent: Tuesday, February 26, 2002 12:56 PM To: [log in to unmask] Subject: Re: [TN] The board looks funny... Thanks Paul. I believe you have hit the nail on the head. This may seem like an elementary question, but is it better to spec more pre-preg, like your standard second sketch, or go with your first sketch? I guess I don't have a good concept of what prepreg is or when it should be used. What I think I know about it is that it is not fully cured FR4 and that the pressing of the layers together is what finishes the cure process. I think I believed that it was sort of the 'glue' that held a multilayer board together and thought it should be placed in the middle. We have only just started to try to be more specific in what materials we spec, and before, we trusted the manufacturer to figure out what to use. I believe we probably used to get a construction based exactly on your second sketch. But this board is built like your first sketch, which, amazingly enough, was what we spec'd in our brand new revised readme file. It is comforting to know they read my readme file...even if I don't know what I am talking about. Thanks. Any good sources of info on what pre-preg is and where it should be used would be much appreciated... -----Original Message----- From: Paul Brionez [mailto:[log in to unmask]] Sent: February 26, 2002 11:00 AM To: TechNet E-Mail Forum.; Genny Gibbard Subject: Re: [TN] The board looks funny... Genny It sounds like your PCB was built with two B stages with FR4 between the copper layers and the prepreg between layers 2-3. Similar to this Most 4-layers 1- cu 1- cu FR-4 pre-preg 2- cu 2- cu pre-preg Filler FR-4 3- cu 3 - cu FR-4 pre-preg 4- cu 4 - cu Genny Gibbard wrote: > Hi, > We just received a new rev of a PWB in house. It has not been populated > yet. It looks like it has been built correctly - the right layer order, > connections, etc - but it looks different than the rest of our 4 layer > boards. Normally the traces on the inner layer that is closest to the > surface appear dark in colour and the deeper layer appears lighter. This > board is the opposite. The layer closest to the surface is light in colour > and the deeper layer is dark. Also, the FR4 seems much easier to see > through than usual. > > Any thoughts or explanations for this? > Thanks for your time. > > Genny Gibbard (mailto:[log in to unmask]) > > ---------------------------------------------------------------------------- ----- > 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 e-mail to [log in to unmask]: SET Technet NOMAIL > To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest > 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 > ---------------------------------------------------------------------------- ----- ---------------------------------------------------------------------------- ----- 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 e-mail to [log in to unmask]: SET Technet NOMAIL To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest 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 ---------------------------------------------------------------------------- ----- --------------------------------------------------------------------------------- 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 e-mail to [log in to unmask]: SET Technet NOMAIL To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest 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 --------------------------------------------------------------------------------- --------------------------------------------------------------------------------- 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 e-mail to [log in to unmask]: SET Technet NOMAIL To receive ONE mailing per day of all the posts: send e-mail to [log in to unmask]: SET Technet Digest 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 ---------------------------------------------------------------------------------