Eric Bogatin's Blog: What I Learned This Month

I was revising my lecture on ground bounce in the Essential Principles of SI class and found a very simple rule of thumb for describing the total inductance of the return path of a conductor. Ground bounce is all about the voltage created across the return path conductor when the return current of a signal line passes through it.

Of course, if there is no other conductor sharing this same return path, then the ground bounce noise generated may be a “who cares”. However, if another signal path also uses the same conductor for its return path- shared return paths- then the ground bounce noise created by the first signal switching will be seen as noise by the innocent victim line.

The amount of ground bounce generated depends on the total inductance of the signal path and the dI/dt of the switching current. This can be on the order of 20 mA/nsec for a 1 nsec rise time signal, or even 60 mA/nsec, for a 300 psec rise time signal, such as found in DDR3 signals.

But what is a good estimate for the total inductance of the return path? Is it 0.1 nH, 1 nH, or even 10 nH? Of course, the most common answer to all signal integrity questions- and most others- is “it depends”. However, sometimes, an OK answer NOW! is better than a good answer late. If you want a rough estimate NOW!, a rule of thumb is the tool to use.

For other than a few simple geometries, inductance is really hard to calculate. The approximations available are pretty complicated. I used the built in 2D field solver, and integrated circuit simulator in Agilent’s ADS to calculate for me the total inductance for a simple geometry of two adjacent, rectangular signal lines, such as might be found in a leaded package. Each line has the same line width and there is some spacing between the two lines.

I then fixed the spacing and swept the line width. The total inductance per length of one of the lines is what is plotted in the figure. It has the features expected. As the width of the return path increases, its total inductance decreases. As the spacing between them decrease, the total inductance also decreases.

What is interesting is that for the case of the line width equal to the spacing, independent of the line width, the total inductance is pretty darn close to 10 nH/inch. This makes for a simple, easy to remember rule of thumb.

When the signal and return path conductors are of the same size and the spacing between them is on the order of their line width, the total inductance of the return path is 10 nH/inch.

If the lead lengthis 0.5 inches, this is 5 nH of total inductance. One DDR3 signal line switching through this will generate 5 nH x 60 mA/nsec = 300 mV of ground bounce. Let three signals switch throug the same common lead and you get almost 1 v of ground bounce. That’s a lot! and not so uncommon.

If you want to learn more about inductance, check out more of the content on our web site relating to inductance, cross talk, switching noise and ground bounce at www.beTheSignal.com.

I had an interesting conversation with Shankar Srinivasan, a research scientist at the Edison Welding Institute. He and his team have developed a clever way of enabling soldering with a variety of solders without flux.

The purpose of a flux is to chemically reduce the oxide layers present on all tin surfaces. Remove the oxides and the metals can wet and you get a metallurgical bond. The problem with fluxes is that they are full of corrosive chemicals. If you leave any flux residue on the surface there is the potential of corrosion which is a long term reliability problem.

What Shankar’s team has developed is an ultrasonic soldering tool that uses cavitation at the tip to disrupt and dislocate oxides at the surface of the metals being joined. The molten solder acts as the acoustic transfer medium for the ultrasonic energy. The cavitating micro bubbles burst on all surfaces, cleaning the surfaces and exposing wettable, oxide-free metal.

This technique can be applied to a variety of solders, lead free in particular, and a variety of metal surfaces. It may have immediate application in hand soldering, repair and rework and in some wave solder applications. Eliminating the use of flux means no cleaning operations are needed, which means no waste water, and that will contribute to a greener process.

Interested in more technology topics, check out the columns I’ve written that are posted on www.beTheSignal.com.

Surfing the web can be an infinite time sink and it can quickly lead to information overload syndrome. While there is a lot of stuff in cyberspace, not all of it is worth the time to open the page and close it. Periodically, I’ll share some of the sites I come across that I think are exceptionally useful for signal integrity engineers. I’ll try to keep these listed on my web site, beTheSignal.com, in the resources section. Here are a few sites I’ve visited recently I think are important.

Prentice Hall, my publisher, has created a series of books on signal integrity. These have been bundled into the Modern Semiconductor Design Series. There are now 34 titles in this series. At an average of 2 inches in thickness each, this is about five feet of signal integrity. If you are starting a library, or want to suggest some books for your company library as must haves, this is a great place to start.

A number of large semiconductor companies have an applications engineering center for their customers. Altera Corp offers a Signal Integrity Center with a bunch of app notes, webinars, columns, models, tools and case studies with design guidelines.

Agilent Technologies has a similar deal, providing a portal to all the SI applications on their extensive web site. Here you will find links to app notes, webinars, product technical reports and descriptions of tools and techniques to characterize passive interconnects and active signals.

Agilent sometimes suffers from multiple personality disorder. They excel in both the hardware side of the SI solution and the software side, with their ADS tool suite. Though they are still learning to play together, as an end user, we often have to deal with them as separate companies. For the SI portal to Agilent software product applications, again, an extensive collection of app notes, videos, and webinars, check out their web site.

While we are talking about Agilent, there is also the new SI blog that Colin Warwick, a product marketing manager with the software side of the company, has created. For a marketing guy, he is pretty sharp technically and always has an interesting tip to share.

Another blog I visit regularly is Rick Merritt’s, an editor for EE Times. As a journalist for EE Times, he has his finger on the pulse of what’s happening in our industry.

In addition, the other popular journalist with a blog who covers our field is Paul Rako, editor with EDN. He covers the whole analog field, not just signal integrity.

There are many, many more web sites with valuable info and I will mention some of them in future posts. Please send me your comments on your favorite sites and I will consider adding them to my blog and to the resources page of my web site.

As Kansas is not a hot bed of signal integrity, I have to travel to most of the classes I do. Last week, I taught  a class in Ft Collins, CO and decided to drive across the state. Everyone complains about how flat and boring Kansas is. However, on this trip, I passed some new scenery which I think will become much more common in Kansas and other states.

On the US Dept of Energy map of wind power in the US, it is clear why tornado alley, where I live, is also wind power alley. Kathleen Sebelius, the governor of Kansas, has called Kansas the Saudi Arabia of wind. The US Dept of Energy estimates that 20% of the US electricity demands can come from the wind in this corridor. As an example, one of the 3 Mega-watt turbines pictured above can generate the energy equivalent of 12,000 barrels of oil in 1 year.

This is part of the basis of the plan T.Boone Pickens revealed last week in Topeka, Kansas. You’ve seen his “I’ve got a plan” promo ads on TV. He decided to kick off his nation wide tour to describe the details of this plan with a first stop in the wind capital of the US, Topeka.  After recently seeing first hand the growth of wind farms in Kansas, Susan and I decided to attend his standing room only presentation.

He started out writing the number $700B on the board. This is the money the US spends on imported oil each year. We effectively burn this money, while our suppliers invest it. His plan to wean the US from foreign oil is to first replace the current electricity production from natural gas with wind power. Second, create an electrical power distribution grid to transmit this generated power throughout the US. The third part of his plan is to promote the use of natural gas powered cars.

While I completely agree with his vision of the problem and the use of wind and other renewable energy sources, and the installation of a transmission infrastructure, I disagree with him about the car power of the future. I think we should look to transition our society from the era of hydrocarbons to the era of renewable energy resources. As we say in technology development, I think investing in developing efficient electric cars has more head room than a similar investment in natural gas car.

New advances like MIT Chemist, Daniel Nocera’s, catalyst that cracks water into hydrogen and oxygen with solar energy could directly feed the needs of an electric vehicle fleet.

How’s all this tie into beTheSignal.com? If any of you have watched our online lectures, you’ve noticed our copyright page with the tornado theme. Given the role Kansas will play in the future, we are changing the tornado theme to a wind energy generation theme.

I’m working on a series of online lectures on power integrity, to be released in the next month. Sign up on the web site and you will be notified when they are released. Look for the windmills and think of renewable energy technologies.