They say you can't judge a book by its cover. But in the component world, the quality of the package, or "cover," that houses the IC is as critical as the performance of the IC itself. In high-speed systems, IC packaging can mean the difference between the success or failure of a design, a product or an entire company.

One of the long-held assumptions is that IC packages are ideal and they merely form the connection between the die and the IC. But with high-performance, high-speed systems, this assumption no longer holds true. Standard component packages-QFP, PLCC and DIP-have inductances in the Vcc and ground leads that can seriously degrade performance. As outputs switch logic states, current to charge and discharge the signal wires or transmission lines passes through these inductances.

The result is Vcc and ground bounce, which are voltage transients that cause the power rails on the die to move away from those on the printed-circuit board. The result is decreased operating voltage for the die and noise spikes on the output signals. Failures occur when outputs switch logic states. As edge speed increases or the rise time decreases, these spikes become larger.

The use of BGA packages for high-performance ICs has helped reduce these problems by providing lower-inductance paths in the power leads. However, many BGA packages have been designed with lowest cost as the first priority. This results in a BGA package with unsatisfactory power lead inductances.

Where does this leave the system designer? As new ICs are chosen, it must be determined how much Vcc and ground bounce each component will have when operating in worst-case mode. In some cases, the IC manufacturer will have measured package inductance allowing for the calculation of Vcc and ground bounce. This is not usually the case, however, so the designer will have to make the measurements independently.

Ultimately, successful component packages result when there is good communication between system designers and component manufacturers.

A full discussion of this topic is presented in Chapter 38 of Right the First Time: A Practical Handbook on High Speed PCB and System Design, Volume 1. This chapter is available as a feature story at www.EEdesign.com.

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