Here's a brief primer on foil. I'm no expert. I kept waiting hoping that someone more qualified than me would provide this information. I'll be the first to admit that I plagiarized a lot this from literature from Gould and Oak-Mitsui. I looked for links to similar information on both of their web sites and found none. Perhaps someone from either or both of these companies can provide links to better information. Electrodeposited foil is produced by plating onto a cathode drum rotating in an electrolytic solution. Current density, contact time (drum speed) and solution parameters determine the foil characteristics, surface roughness, grain structure and other properties (tensile, elongation, etc.) The two different sides of the foil have very different appearance and characteristics. The "drum" side is typically very smooth. The other side of the foil is designated as "matte" though I've also heard it called the "tooth" side since it tends to have dendrites from the electrodeposition process. The foil then goes through treating. This is predominately chemical and provides such features as mechanical and chemical bond enhancement, as well as corrosion resistance. One thing to remember is that both sides of the foil aren't necessarily treated the same. Foil thickness at this point is measured by weight, thus the designation 0.5 oz and 1.0 oz which stands for the weight of a sheet of a specified size. And right now I can't find what that size is. Once the foil is incorporated into the board and etched into circuitry, it's no longer possible to measure it by weight so thickness here is measured in more familiar units (mils and microns). Historically (and let me qualify that by saying I've been in this business about 14 years), foil has been used by laminate manufacturers with most of the treatment on the matte side which was placed against the prepreg when making the cores. The drum side was left facing out for the fabricators to build the circuits on. The only treatment on the drum side was a conversion coating to keep the copper form oxidizing. Fabricators would then use chemical and mechanical surface preparation methods to improve photoresist adhesion and optimize circuit yields. The smooth copper surface on the drum side of the foil makes for a very poor bond when laying up multilayer boards with prepreg. As is so often the case, those who get their first get all the good real estate, and that's what happened here. The laminate manufacturers used up that wonderfully rough "tooth" side of the foil. To address this, fabricators use an oxide or other bond-enhancement treatment on the foil to promote adhesion between foil and prepreg. Then a few years ago, innovations started to occur. I think double-treat came first, which meant that many of the treatments that were used before only on the matte side were now also applied to the drum side. The benefits and detractions of doing this are beyond the scope of this little primer. Let's just say that double-treat has the benefit of not requiring a subsequent oxide or bond enhancement step during board fabrication IF you don't damage it in handling. This was followed by someone having the idea of flipping the foil and having the laminate manufacturers build cores with the matte or tooth side sticking out. This is called reverse-treat foil. Again, I'm going to avoid a detailed list of benefits and detractions. At it's simplest, putting the tooth to the outside is better for building thin (and I'd call thin < 0.004") cores since you needn't worry about tooth-to-tooth issues. It also tends to result in higher yields mostly from two factors: better resist adhesion to the matte surface and fewer problems associated with trying to etch the "teeth" embedded in the core dielectric. Lastly, there's reverse-double-treat, which combines both aspects. The foil is treated for bond enhancement on both sides and cores are built with matte side out. It offers the same benefits as reverse-treat, especially for thin cores where the double-treat surface avoids the need to go through oxide. This was important when 2 and 3 mil cores were first introduced. Getting them through an oxide line was nearly impossible without shredding them. As material transport improvements have been made, especially with conveyorized oxide and bond enhancement processes, the need for double-treat on very thin cores has abated. > ---------- > From: Earl Moon[SMTP:[log in to unmask]] > Reply To: TechNet E-Mail Forum. > Sent: Thursday, September 20, 2001 12:46 PM > To: [log in to unmask] > Subject: Re: [TN] External conductor starting thickness prior to > plating?? ? > > Shirley, > > For some of us less informed, no joke, could you explain the ED foil > process > and how such things as drum rotation speed affects thickness. Also, if you > can, please explaing various processes used to prepair outer layer foil > surfaces for subsequent cleaning, imaging, plating, and etching processes. > I'm especially interested in how thin is too thin under what processing > conditions. > > Thank you much, > > Earl > > -------------------------------------------------------------------------- > ------- > 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 > -------------------------------------------------------------------------- > ------- > --------------------------------------------------------------------------------- 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 ---------------------------------------------------------------------------------