Earl We don't try to adjust the cure to fine tune the dielectric properties of the finished board. I didn't mean to imply that. I apologize if I didn't make that clear. You still have to advance the cure of the prepreg enough that the board sticks together and has the necessary mechanical and thermal properties for the board to withstand assembly and perform reliably. I don't have data to back this up, but I suspect that the amount of undercuring necessary to cause a significant shift in the dielectric constant would not be satisfactory. For the finished board electrical properties, more important than the degree of cure is how you get there. The press cycle parameters (how fast do you heat up, what temperatures do you hold it at and for how long, when do you apply pressure and how much) determine the amount of resin flow and the curing kinetics. All of these effect the pressed finished thickness and thus the dielectric constant of the prepreg opening. Two different manufacturer's can build the exact same board out of the exact same material, each using a different press cycle and the boards will have different impedance. Neither is doing anything wrong, just different. How much different? Probably a few tenths of a mil per opening. On a 6 or 8 mil dielectric spacing this is probably insignificant. But if you are looking at 4 mils or less, it starts to be significant if you're shooting for a 10% or less spec. What's important is for the fabricator to know how their boards are going to turn out and do it repeatably. And to be able to predict the impedance on a design before it's going to be built. As for our plans, that's where our focus is. SPC in conjunction with our suppliers on incoming material and pressed prepreg thickness to characterize and minimize variability. SPC on line width control. And improved understanding of TDR measurement and modeling. > ---------- > From: Earl Moon[SMTP:[log in to unmask]] > Sent: Monday, December 03, 2001 12:03 PM > To: [log in to unmask]; [log in to unmask] > Subject: Re: Dielectric constant vs degree of cure , FR4 > > Michael, > > Your very eloquent answer probably is quite correct. However, how many of > us > work in an uncured or over cured world. > > I know the question was stated as the difference between cured and > uncured, > but how can we make boards with less or more cure that are acceptable as > specified. Out of the window is out of spec, right? > > Having worked with very low dielectric constant materials (styrafoam as an > example), approximating air, I know it is not practical to attempt, at > this > time, building circuitry with such materials. My answers to the initial > question were based on what is required to make a circuit practical in > terms > of manufacturability, acceptance, and performance. > > I realize you folks at Merix are on the cutting edge. If you could share a > bit of your future plans, based on your excellent answer to the original > question concerning degree of cure, we would all appreciate this. > > Respectfully, > > Earl Moon > --------------------------------------------------------------------------------- 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 ---------------------------------------------------------------------------------