I know you've had answers to the contrary, but the answer to the first question is "yes". When an electric field is applied to an insulator, components of the insulator re-arrange themselves in response to this field. The components range from the sub-atomic (electrons and nuclei) to, in polymers, macromolecular chains. The dielectric constant of the insulator is very dependent on how much of this re-arrangement can occur. In an uncured polymer, the polar chains of the monomer can move more freely under the applied electric field. When cured, the chains become crosslinked and movement is restricted. The material can no longer hold as much charge and the dielectric constant will go down. This same mechanism explains why dielectric constant is changed by temperature and frequency. A material above it's Tg will have a higher dielectric constant than it does below Tg. Above Tg the polymer is freer to re-orient itself to the electric field. Frequency looks at the same thing just with respect to time. Under DC voltage, the dipoles in the polymer have an infinite time to respond and will increase the dielectric constant as they do so. Start alternating the field though and now the most restricted parts of the polymer can no longer respond. The dielectric constant starts to drop. As you keep increasing the frequency, less and less of the dipoles within the material can respond fast enough to keep up. An analogy is the relative rate at which you can wave your arms up and down versus wiggling your fingertip. The less you have to move the faster you can do so. The answer to your second question is "yes" also, but for a different reason. An FR4 board that was undercured enough to see a significant impact on electrical properties would have enough thermal and mechanical issues from the undercuring that it would be unusable. What press parameters can influence is the amount of fill and flow of the resin in the prepreg and the finished press thickness of these dielectric. The "science" of pressing PWBs revolves around balancing the needs to have a material that flows then cures. As the prepreg is heated up the resin viscosity drops and the material starts to flow, filling in the circuits on the adjacent layers. But the crosslinking reaction is also thermally driven. As the resin cures, the viscosity drops. Balancing these competing mechanisms is done by adjustment of press temperature ramp rates as well as application of vacuum and pressure. > ---------- > From: tech[SMTP:[log in to unmask]] > Reply To: TechNet E-Mail Forum. > Sent: Thursday, November 29, 2001 7:00 AM > To: [log in to unmask] > Subject: [TN] Dielectric constant vs degree of cure , FR4 > > Does the dielectric constant ( and resultant impedance results) of FR4 > vary with degree of cure of the resin system ? I.e can multi layer press > parameters influence finished board impedances ? Any info. will be > appreciated. > Paul Greene > > -------------------------------------------------------------------------- > ------- > 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 ---------------------------------------------------------------------------------