Thursday, September 25, 2008


In the realm of high-speed digital design, signal integrity has become a critical issue, and is posing increasing challenges to the design engineers. Many signal integrity problems are electromagnetic phenomena in nature and hence related to the EMI/EMC discussions in the previous sections of this blog. We will discuss what the typical signal integrity problems are, where they come from, why it is important to understand them and how we can analyze and solve these issues. Several software tools available at present for signal integrity analysis and current trends in this area will also be introduced.

The term Signal Integrity (SI) addresses two concerns in the electrical design aspects – the timing and the quality of the signal. Does the signal reach its destination when it is supposed to? And also, when it gets there, is it in good condition? The goal of signal integrity analysis is to ensure reliable high-speed data transmission. In a digital system, a signal is transmitted from one component to another in the form of logic 1 or 0, which is actually at certain reference voltage levels. At the input gate of a receiver, voltage above the reference value Vih is considered as logic high, while voltage below the reference value Vil is considered as logic low. Picture - 1 shows the ideal voltage waveform in the perfect logic world, whereas Picture - 2 shows how signal will look like in a real system. More complex data, composed of a string of
bit 1 and 0s, are actually continuous voltage waveforms. The receiving component needs to sample the waveform in order to obtain the binary encoded information. The data sampling process is usually triggered by the rising edge or the falling edge of a clock signal as shown in the Picture 3. It is clear from the diagram that the data must arrive at the receiving gate on time and settle down to a non-ambiguous logic state when the receiving component starts to latch in. Any delay of the data or distortion of the data waveform will result in a failure of the data transmission. Imagine if the signal waveform in Picture - 2 exhibits excessive ringing into the logic gray zone while the sampling occurs, then the logic level cannot be reliably detected.

Picture 1

Picture 2

Picture 3

Typical SI Problems

“Timing” is everything in a high-speed system. Signal timing depends on the delay caused by the physical length that the signal must propagate. It also depends on the shape of the waveform when the threshold is reached. Signal waveform distortions can be caused by different mechanisms. But there are three mostly concerned noise problems:

--> Reflection Noise
Due to impedance mismatch, stubs, vias and other interconnect discontinuities.

--> Crosstalk Noise
Due to electromagnetic coupling between signal traces and vias.

--> Power/Ground Noise
Due to parasitics of the power/ground delivery system during drivers’ simultaneous switching output (SSO). It is sometimes also called Ground Bounce, Delta-I Noise or Simultaneous Switching Noise (SSN).

Besides these three kinds of SI problems, there are other Electromagnetic Compatibility or Electromagnetic Interference (EMC/EMI) problems that may contribute to the signal waveform distortions. When SI problems happen and the system noise margin requirements are not satisfied – the input to a switching receiver makes an inflection below Vih minimum or above Vil maximum; the input to a quiet receiver rises above Vil maximum or falls below Vih minimum; power/ground voltage fluctuations disturb the data in the latch, then logic error, data drop, false switching, or even system failure may occur. These types of noise faults are extremely difficult to diagnose and solve after the system is built or prototyped. Understanding and solving these problems before they occur will eliminate having to deal with them further into the project cycle, and will in turn cut down the development cycle and reduce the cost. In the later part of this chapter, we will have further investigations on the physical behavior of these noise phenomena, their causes, their electrical models for analysis and simulation, and the ways to avoid them.

Where SI Problems Happen

Since the signals travel through all kinds of interconnections inside a system, any electrical impact happening at the source end, along the path, or at the receiving end, will have great effects on the signal timing and quality. In a typical digital system environment, signals originating from the off-chip drivers on the die (the chip) go through c4 or wire-bond connections to the chip package. The chip package could be single chip carrier or multi-chip module (MCM). Through the solder bumps of the chip package, signals go to the Printed Circuit Board (PCB) level. At this level, typical packaging structures include daughter card, motherboard or backplane. Then signals continue to go to another system component, such as an ASIC (Application Specific Integrated Circuit) chip, a memory module or a termination block. The chip
packages, printed circuit boards, as well as the cables and connecters, form the so-called ifferent levels of electronic packaging systems, as illustrated in below. In each level of the packaging structure, there are typical interconnects, such as metal traces, vias, and power/ground planes, which form electrical paths to conduct the signals. It is the packaging interconnection that ultimately influences the signal integrity of a system.

SI In Electronic Packaging

Technology trends toward higher speed and higher density devices have pushed the package performance to its limits. The clock rate of present personal computers is approaching gigahertz range. As signal risetime becomes less than 200ps, the significant frequency content of digital signals extends up to at least 10 GHz. This necessitates the fabrication of interconnects and packages to be capable of supporting very fast varying and broadband signals without degrading signal integrity to unacceptable levels. While the chip design and fabrication technology have undergone a tremendous evolution: gate lengths, having scaled from 50 μm in the 1960s to 0.18 μm today, are projected to reach 0.1 μm in the next few years; on-chip clock frequency is doubling every 18 months; and the intrinsic delay of the gate is decreasing exponentially with time to a few tens of pico-seconds. However, the package design has lagged considerably. With current technology, the package interconnection delay dominates the system timing budget and becomes the bottleneck of the high-speed system design. It is generally accepted today that package performance is one of the major limiting factors of the overall system performance.

Advances in high performance sub-micron microprocessors, the arrival of gigabit networks, and the need for broadband Internet access, necessitate the development of high performance packaging structures for reliable high-speed data transmission inside every electronics system. Signal integrity is one of the most important factors to be considered when designing these packages (chip carriers and PCBs) and integrating these packages together.

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