Design Guide for High-Speed Controlled Impedance Circuit Boards
|Publication Date:||1 March 2004|
This guide is intended to be used by circuit designers, packaging engineers, printed board fabricators, and procurement personnel so that all may have a common understanding of each other's area.
The goal in packaging is to transfer a signal from one device to one or more other devices, through a conductor. High-speed designs are defined as designs in which the interconnecting properties affect circuit performance and require unique consideration.
The term ''high-speed'' as applied to logic or digital designs needs clarification in its usage. The three most common interpretations of high-speed are as follows. (1) High-speed as a reference to the rate of change of signal amplitude with time (frequently called the edge rate of a pulse) constitutes the most important usage. The edge rate puts the greatest performance demand on interconnecting structures. (2) High-speed as a reference to the data transmission rate (bits or bytes per second) is often used to describe the ''speed'' of a system. However, high data rates can be achieved with parallel bus architectures that do not necessarily require improved performance of an interconnecting structure. (3) High-speed as a reference to the speed (distance per unit time) of a signal propagating between devices has the smallest usage and, in many cases, is not important to the application.
Controlled impedance is the maintenance of some specified tolerance in the characteristic impedance of an interconnect line (transmission line) that is used to connect different devices on a circuit. Controlled impedance is often a design consideration for high-speed digital or high-frequency analog circuits. However, the reverse is not true, that is, high-speed digital or high-frequency analog circuit designs may not need to consider controlled impedance. The purpose of this document is to help the designer understand when controlled impedance should be considered in his/her circuit design and to describe concepts important to controlled impedance design.