Predicting the impedance effects of interstitial copper leaves in bookbinder flex
Application Note AP538 

Predicting the effects on impedance of interstitial copper leaves in bookbinder flex

We sometimes get asked about the effects on a structure's impedance from an interstitial copper layer – either from a folded flex or from an interstitial leaf in a bookbinder flex.

To model the effects of an interstitial copper layer, you can use one of the multi-dielectric controlled impedance structures in the Si8000m or Si9000e, setting the Er of one of the dielectrics in the multi-dielectric substrate to the approximate value for air, i.e. set Er = 1.0000.

From the Si8000m's structure list choose the Offset Stripline 1B2A structure. In this example H3 serves as the separation from the interstitial copper leaf.

Set the structure parameters to their appropriate values, but set the value of Er3 to 1.0000. If necessary, goal seek on trace width to achieve the target impedance (In this example, 50 ohms.) Set the height of H3 to 20 mils.

Doing this in the Er3 section of the structure above allows you to graph how close the copper on “the next structure above” needs to be to be “out of harm’s way,” to use a Rick Hartley phrase.  It’s a quick sanity check which can be made pre design and in Si8000m prior to building the stack and used to set yourself a design rule…

Switch to the Si8000m Sensitivity Analysis tab

From the Si8000m's Impedance v Changing Parameters dialog, choose the H3 parameter and set its start value to 3 mils and the end value to 20 mils with a 0.5 mils increment. Click Calculate.

The Si8000m charts the impedance varying between 45 and 50 ohms as H3, the distance to the interstitial copper leaf, varies between 3 mils and 20 mils.

So, for this example, at anything greater than 20 mils distance the impedance is largely unaffected, at 10 mils the impedance drops by 1 ohm and at 3 mils distance the impedance has dropped by 10%.

To see this for yourself, if you don't yet have an Si8000m or Si9000e license, simply ask Polarcare for your 2 week evaluation license now...