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The Bare (Board) Truth: What you Want vs. What you Get
Wednesday, December 14, 2011 | Mark Thompson, CID+, Prototron Circuits

Seasons greetings all! In this column, I will discuss what designers may want vs. what fabricators may need.

I’ll start with material selection. Many designers lately are asking for low-resin, high-glass pre-pregs as interfaces for impedance designs. This is understandable from their standpoint, because the Dk is higher and the glass weave is wider. This works well for differential pair performance, but it is sometimes difficult from a fabrication standpoint. Let me give you an example:

Let’s say the customer is dictating the use of 7628 or 7629 type pre-preg plies in an effort to keep the pre-preg Dk above 4.0, and the dielectrics in the stackup call for a single ply of 7628 to achieve required impedances. Because this ply is predominantly glass and less resin, the potential for resin starvation at lamination (depending upon the layer interface – GND, SPLIT, SIGNAL) is high. This ply is typically avoided by the fabricator in favor of one of equal dielectric value, one composed of more plies of a more resin-rich material. But doing so generally requires the fabricator to tweak the lines/spaces to deal with the lower DK of the more resin-rich materials.

This was more of an issue when fabricators were using FR-4 flavors, because the dielectric constant swing from resin-rich material to less resin-rich was a greater percentage. As an example, even the high-temp FR-4 types went from an approximation of 3.3 Dk for 106 pre-pregs to 4.7 for thicker cores.

Some of today’s material alternatives have less deviation across the board, so-to speak.

For example, ITEQ’s IT-180 or IT-180A is approximately 4.1 Dk for the resin-rich plies like 106, and up to 4.5 for thicker cores. Many designers still use an approximation for calculating impedance (yes, this is true), and many designers assume when literature says the Dk will be 4.5 at 1 gig, they can model all impedances as such. But many times they do not notice the Dk value expressed is for a core .018” or thicker, where the core make-up DOES use more glass-rich pre-preg plies.

Of course the reality is that today’s lack of board real estate means more layers and finer lines. Finer lines typically mean less dielectric between ref planes and therefore a lower Dk preg or core. If a case like the one stated above, it is critical to meet with your fabricator to review the impedances at the earliest possible point.

If you know me at all you know I write most of my columns relative to controlled impedance. Over the last 10 years many of our customers have specified either dielectrics in a stack-up or a specific Dk value or both, but with NO information about any controlled Impedances they may have.

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