Printed Circuit Design & Fab Online Magazine - Current Issue

Current Issue

Bill Hargin

At higher speeds, the micro-environment around traces can alter simulation results.

In the past few weeks, e-mails from multiple sources crossed my inbox asking about the relationship between epoxy resin and signal behavior. With a couple of SI guys on one side and a career PCB manufacturing guy on the other side hitting me the same week with different versions of the same question, I thought it would make for an interesting column topic.

FIGURE 1 shows the initial image I was provided, along with the question what surrounds a typical trace? At face value, this may sound trivial, but it’s a reasonable issue for a signal-integrity practitioner to be concerned about.

Read more: What Surrounds a Typical Trace?

Bill Hargin

The high percentage of resin coverage can cause skew issues at higher speeds.

Since June, I’ve been writing about glass-weave skew (GWS). If you haven’t read those articles, you may want to go back and bone up on the subject. We’ll review a few points here.

A serial link’s differential-skew budget shrinks as bit rates increase. For example, a 1Gbps (500MHz) signal would have roughly 250ps of skew tolerance. That’s a wide window, and why most engineers didn’t need to worry about GWS 20+ years ago. Fast forward to 10Gbps (5GHz), and the skew tolerance will decrease proportionally to around 25ps.

When working at frequencies below 1GHz, or when using whatever materials the fabricator has in stock due to schedule constraints, don’t worry about glass style. If signaling at higher speeds and there is time to plan, read on.

Read more: Why You Should Quit Using 1080 Glass

Bill Hargin When sourcing materials, consider dual-ply glass.

Au: This column is a comprehensive follow-on to the July column introduction on glass-weave skew and the discussion in August and September of various mitigation strategies. With some overlap, these may be read together or independently.

In my July article, Part One of a series on glass-weave skew, I introduced its causes and when or why a hardware designer might care. In Part Two I discussed various mitigation techniques and cost. Part Three presented a deeper dive into the impact of glass styles on precipitating or mitigating skew. Part Four will cover dual-ply and low-Dk glass.

While glass-weave skew (GWS) is a real problem, it’s hard to characterize because it is statistical in nature. What is the chance one line in a pair will see a different dielectric constant than the other? It depends on the pitch of the lines, the length of the lines, the laminate composition, and the relative chance alignment of the glass bundles under the two lines.

Read more: How to Avoid Getting Totally Skewed, Part Four