Glass weave skew problems may be solved

At data rates of 28Gbits/s per channel, even if you do everything right, there is still the time bomb of glass weave skew that can ruin your day. Earler this year, Isola introduced GigaSync laminate may be the solution to defuse this statistical problem.

Here's the problem. The unit interval for a single bit at 28Gbits/s is only 36ps. If the line-to-line skew in a differential channel, from all sources such as routing length differences, packages, connectors, and drivers is more than about 10psec (~25 percent of the UI), an eye diagram may be sufficiently collapsed to dramatically increase BER (bit error ratio).

One solution is to match the length of the two lines that make up a differential pair to within 5 mils. With a signal speed roughly 6 mils/ps, this would give less than 1ps of line-to-line skew. You think you're safe. Then along comes a bear hidden in the woods that eats your lunch.

Even if the lengths in the differential pair are perfectly matched, the local variation in the effective dielectric constant between the two lines in the differential pair will create a difference in speed between the signals traveling on each line, which will result a time-delay skew.

The problem comes down to statistics. What's the chance that the two lines in your differential pair will be aligned over the glass weave just right to have enough skew to collapse your eye diagram? How lucky do you feel?

Over the years, a number of solutions have been offered to mitigate this problem: route traces off the glass weave in a PCB (printed-circuit board) axis, route in a zig-zag, or use a flatter glass yarn in the PCB material.

A new option to mitigate the statistical time bomb of glass-weave-induced skew was introduced earlier this year by the Isola Group, a supplier of laminate materials.

"If you have a homogenous material, with the resin and glass having the same dielectric constant, you will not have a glass-weave skew," said Tarun Amla, executive VP and chief technology officer of Isola when I spoke to him by phone. This was, he explained, Isola's goal in developing a new PCB material. "We looked at all the factors that contribute to glass weave skew and decided this solution would be the most robust."

The new, modified epoxy resin system has a Dk of 4.13 and the glass yarn also has a Dk of 4.13. This is lower than the standard glass yarn which is typically on the order of 6.7. The resin acts as an index matching fluid and all signals on all lines see the same effective dielectric constant.

But, low glass weave skew is only one of the speed bumps in the path to ever higher data rates in copper channels. Loss is another dominant one. While the Df of the new GigaSync is listed as 0.006, the losses above 10Gbit/s in low loss laminate channels is dominated by copper. And, the problem with copper at these high signal frequencies is not just the copper conductivity and skin depth. Copper losses can increase more than a factor of two due to surface roughness.

The skin depth of copper at 10GHz is 0.6 microns. When the skin depth is significantly thinner than the surface roughness, the conductor losses dramatically increase over smooth copper.

For the highest data rate channels, ultra smooth copper is needed. Typical rms roughness of reverse treated foil (RTF) is 1.7 microns. Even VLP (very low profile) copper foil has an rms roughness on the order of 1.4 microns.

To take advantage of the low glass skew and the low dissipation factor of GigaSync, Isola has been working with a foil manufacturer to introduce a foil with an rms surface roughness of 0.3 microns.

Since the resin system is epoxy based, Amla reports the peel strengths with this smoother foil and no special foil treatment are typically four pounds per inch. "The resin flows and fills like regular FR4 and is robust to standard lamination processes."