Hello all, an odd question that has come up in some evaluations:

I have a  "known good" impedance calculator formula from IPC-2141 that has worked for years on Polyclad 226 material.

I know my circuit widths, dielectrics, stackup and coupon design are good.

THE DIFFERENCE tried Polyclad 370 material      Higher Tg (170 vs 140)
                                                DK/Er (4.5 on both)

THE PROBLEM -   my formula says 48-52 ohms
                        coupons read on 2 different TDRs at 59-63 ohms
                        One has a Tr @ 30ps the other is newer so probably a quicker Tr

THE QUESTION(S) 1) Am I in error ?
                        2) Should I be using a compensation factor ?
                        3) Do I have incorrect data on the 370 material ?
                        4) Have the laws of physics forsaken me for some sin against the stray molecular particle ?

Any help would be greatly appreciated, especially on questions 1,2 & 3.

JOE SENDZ....



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