Printed Circuit Design & Fab Online Magazine - Current Issue

Material Matters

Bill Hargin

Can signal-integrity test vehicle results be accurately simulated?

Ed.: This is Part 2 of a three-part series on preparing for next-generation loss requirements.

Here in Part 2 of the series, I’ll outline the means by which insertion-loss requirements are determined. In Part 3, I’ll suggest a better method for obtaining more accurate Df numbers without having to go to the trouble of building test boards.

As I stated in last month’s column, if you want to stay on top of the parameters that contribute to loss, there are a lot of factors to juggle. Frequency, copper weight, resin system, glass characteristics, dielectric thickness, trace width, copper roughness, and fabricator processing all contribute to the discussion if you’re savvy, driving fast, with both eyes open.

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Bill Hargin

Can signal-integrity test vehicle results be accurately simulated?

Ed.: This is Part 1 of a three-part series on preparing for next-generation loss requirements.

There are a lot of factors to juggle to stay on top of the parameters that contribute to loss. Frequency, copper weight, resin system, glass characteristics, dielectric thickness, trace width, copper roughness, and fabricator processing all contribute to the discussion if you’re savvy and driving fast, with both eyes open.

If frequencies aren’t increasing, no need to worry. But if your windows are getting chopped in half year-over-year, read on.

Background. Several years ago, I marketed laminates for servers. Older generations bumped up frequencies incrementally, but then we ended up dealing with frequencies that doubled from one generation to another, with downward pressure on costs.

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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