Colleagues, I wonder if you can throw any light onto a problem I have with a specific impedance controlled board where the prediction does not match the actual measured results. The board is 12 layer FR4 with controlled impedance on layers 3, 4, 6, 7, 9 and 10. Layers 2, 5, 8 and 11 being the planes. So there are two signal layers between each pair of planes, all impedances should be 48 ohms (+/-10%). The problem is that the measured impedances from a finished board do not agree with the prediction based on the actual measurements from a cross section of the measured board. I give 2 examples: All dielectric spacings are copper to copper, all figures in microns: Layer 3: track width avg 126, spacing to layer 2 = 135, to layer 5 = 260. Thickness = 16. Layer 6: track width avg 121, spacing to layer 5 = 100, to layer 8 = 270. Thickness = 16. L3 measured impedance = 57.3 avg, and calculated from above figures = 50.6 L6 measured impedance = 54.3 avg, and calculated from above figures = 46.2 Dielectric constant of 4.6 used. Of course the Er can be out but it could not explain such a large difference. The offset stripline model used works within normal tolerances for all our other work but for some reason not this particular job. Any ideas? Has anyone a model that predicts nearer to the actual results? Regards John Bownass ################################################################ 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 ################################################################