Simulations should match measurements.\u00a0 Otherwise, what good are they?\u00a0 When doing signal integrity simulations, that starts with comprehensive stackup modeling.\u00a0 I quite often hear that stackup editor calculations don’t match measurements, but when I probe deeper, I usually find that the stackup has not been modeled properly.\u00a0 More often than not, the calculated impedance ends up being much lower than the measured impedance.\u00a0 Why is that?\u00a0 The usual culprit is dielectric constant, Er.\u00a0 Most folks will put in their dielectric heights and a value of 4.0 for the dielectric constant for a FR4 board.\u00a0 However, dielectric constant varies with dielectric height, usually within a range of 3.5 to 4.5.\u00a0 The thinner dielectrics typically have a dielectric\u00a0constant closer to the 3.5 value, and since most stackups have a lot of 4-mil and 5-mil dielectric layers, that means that 4.0 is too high, and that the simulated stackup is showing too\u00a0low of an impedance.<\/p>\n
Dielectric constant is also very important in determining timing numbers.\u00a0 With many of today’s modern busses being source-synchronous, skew is more important than actual flight time so this is becoming less of a concern, but there are still plenty of common-clock interfaces that require accurate prediction of signal flight time.<\/p>\n
So, talk to your manufacturer and make sure you are including the right dielectric constant in your stackup calculations.\u00a0 It is a crucial first step to making sure your simulations are accurate.<\/p>\n