If you only considered one of several factors, contributing to impedance control, as dielectric thickness tolerance, you would understand the variation. We all know it is typical for dielectric thicknesses of core and laminate material to be in the range of + or -10% tolerance. If you add to that etched trace width tolerances as (whatever the supplier' process manages), you can see impedance changes on the order you describe. Using TDR and X-Sectional analysis together, it is possible to build better statistical models for, at least, some materials and suppliers to get closer to your requirements. Earl Moon ([log in to unmask]) ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################ ################################################################ TechNet E-Mail Forum provided as a free service by IPC using LISTSERV 1.8c ################################################################ To subscribe/unsubscribe, send a message to [log in to unmask] with following text in the body: To subscribe: SUBSCRIBE TechNet <your full name> To unsubscribe: SIGNOFF TechNet ################################################################ Please visit IPC's web site (http://www.ipc.org) "On-Line Services" section for additional information. For technical support contact Hugo Scaramuzza at [log in to unmask] or 847-509-9700 ext.312 ################################################################