The ever changing world of high speed digital design
Transistor gate lengths as small as 22 nanometers have made possible integrated circuits with performance at ultra-high speeds and incredible transistor densities.
At the turn of the 21st century the upper limit on transistor count of an integrated circuit was 200 million transistors with data speeds as high as 2.5 gigabit per second over a single data path. Currently, integrated circuits are commonly made with more than a billion transistors with data rates as high as 28 gigabits per second over a single data path.
These advances in integrated circuit technology have made possible products that could only have been imagined a few years ago. In addition to all of the products that have been in use for many years, two very diverse new product categories have developed around these advances.
These are the ultra-dense cell phones and tablets that have crept into every aspect of our lives and, driven by these same devices, the ultra high performance routers, switches and servers that provide all of the Internet services they require.
The first of these has driven component packages to lead pitches as small as 0.4 mm or 16 mils with components on both sides of the PCB substrate. This has given rise to build up PCBs and laser drilled blind vias as small as 0.1 mm or 4 mils. This technology had driven laminate manufacturers to develop ultrathin laminates and prepregs which have glass weaves that are uniformly spread out to improve the quality of the laser drilled blind vias.
The effect of these changes is not apparent when one looks at the size of the cell phones and tablets as the external packages remain relatively the same. What these changes have done is make it possible to put entire systems, such as GPS, in the same package with the phone as well as allow the user to surf the web and watch real time TV and movies on these same devices. On top of all this high quality cameras are included that allow the user to easily take movies and still photos. This is remarkable innovation made possible by improvements in IC technology.
The second of these has driven the performance of routers, switches and servers used in the cloud to unprecedented highs in a very short time period. Figure 1 is a terabit router introduced in 2002. It uses half a rack, weights 350 pounds (160 kilograms) and consumes seven kilowatts of power. Figure 2 is the same terabit router five years later in 2007. It is only 1U high, weighs 22 pounds (10 kilograms) and consumes 700 watts.
Figure 1 A Terabit Router in 2002. Photo courtesy of Arista Networks
Figure 2 A Terabit Router in 2007.
More recently, the capacity of the 1U router shown in Figure 2 has increased to 2.5 terabits per second in the same size package with 32 each 40 gigabit per second ports across the front panel. Where these new technologies takes performance seems to have no limits.
The fastest signal on a PCB in the 2002 router was 2.5 gigabits per second. The fastest signal on a PCB in the 2007 router is 10 gigabits per second. The routers developed in 2014 contain signals as fast as 32 gigabits per second.
These “digital” signals are operating in the frequency band once considered microwave. As a result, laminate considerations that once applied only to RF and microwave are integral to success with these products.
Among these are copper loss, loss in the laminates and uniformity of the glass weave, none of which were issues as recently as 2000 with digital products. In fact, demands placed on laminate suppliers by the digital world are more difficult than those placed by the RF/microwave community.
The difference between the RF and digital world has blurred as a result of the speed increases in digital electronics. RF and microwave have usually involved signals in excess of a few megahertz while digital was confined to below 100 megahertz. As described above, high performance digital products now operate in the same frequency spectrum as many microwave products, blurring the line between these two technologies.
Along with the challenge of engineering data paths that operate in the microwave spectrum, changes in power delivery has emerged as a major source of design problems. These same ICs that have billions of transistors often require several different power supply voltages.
Examples are: 1.0V for the core at as much as 100 amperes, 1.5V for the I/O at dozens of amperes, 1.1V for the phased locked loops and 1.2V at several amperes for the memory interface. It is not uncommon for a PCB design to have more than a dozen different supply rails with some recent designs having more than 25 different Vdd rails. Successful power delivery system design is emerging as the most difficult part of many new designs.
Until recently, much of the electronics industry has relied on PCB fabricators to select the materials, design the stackup and calculate the impedances needed on a PCB. This method worked reasonably well before these semiconductor advances were made.
With current and future designs, how well the power delivery system performs along with the quality of the signal paths is intimately tied to how the PCB stackup is engineered. PCB fabricators are not equipped with the technical skill to account for all the issues that need to be dealt with when designing the PCB stackup. As a result, design and signal integrity engineers must take charge of this part of the design process. This requires substantial new skills on the part of this team.
These performance advances require a whole new set of design and fabrication disciplines, as well as far more knowledge of materials available with which fabricate PCBs needed by these advances, not necessary as recently as the year 2000. Due to the rapid changes in technology, university courses and text books have not been able to keep pace, leaving students with an information gap that interferes with their success doing design work on these new products.
Where do engineers and designers turn for the information necessary for success with these new design demands? One place to turn is a seminar being offered by a training company located in Silicon Valley that specializes in this area.
This company is Speeding Edge, whose president, Lee Ritchey, has been actively participating in designs of this complexity and shares this knowledge in three day seminars offered around the world.
About the author
Lee Ritchey is president of Speeding Edge, a leader in the design and fabrication of very high performance systems employed in the Internet and elsewhere. He teaches seminars on these topics around the world and has taught more that 9000 engineers and designers how to be successful in this arena. He can be reached at www.speedingedge.com
If you enjoyed this article, you will like the following ones: don't miss them by subscribing to :
