18 Chapter 1 Introduction 1.9 Printed-Circuit Boards printed-circuit board An IC is normally mounted on a printed-circuit board (PCB)[or printed-wiring (PCB) board(PWB)]that connects it to other ICs in a system.The multilayer PCBs of fiberglass that are laminated into a single board about 1/16 inch thick. PCB traces Individual wire connections,or PCB traces are usually quite narrow,10 to mil 25 mils in typical PCBs.(A milisone-thousandth of an inch.)PCB fine-line technology,the traces are extremely narrow,as little as 4 mils wide with 4-mil spacing between adjacent traces.Thus,up to 125 connections may be routed in a one-inch-wide band on a single layer of the PCB.If higher connection density is needed,then more layers are used. surface-mount Most of the components in modern PCBs use surface-mount technology technology (SMT) (SMT).Instead of having the long pins of DIP packages that poke through the board and are soldered to the underside,the leads of SMT IC packages are bent to make flat contact with the top surface of the PCB.Before such components are mounted on the PCB,a special"solder paste"is applied to contact pads on the PCB using a stencil whose hole pattern matches the contact pads to be soldered.Then the SMT components are placed(by hand or by machine)on the pads,where they are held in place by the solder paste (or in some cases,by glue). Finally,the entire assembly is passed through an oven to melt the solder paste. which then solidifies when cooled Surface-mount component technology,coupled with fine-line PCB tech- nology,allows extremely dense packing of integrated circuits and other mponents on a PCB.This dense packing does more than save space.For very high-speed circuits,dense packing goes a long way toward minimizing adverse analog phenomena,including transmission-line effects and speed-of-light limitations. To satisfy the most stringent requirements for speed and density, modules (MCMs)have been developed.In this technology,IC dice are not mounted in individual plastic or ceramic packages.Instead,the IC dice for a high-speed subsystem (say,a proc cessor and its cache memory)are bonded directly to a substrate that contains the required interconnections on multiple layers.The MCM is hermetically sealed and has its own external pins for power, ground,and just those signals that are required by the system that contains it. 1.10 Digital-Design Levels Digital design can be carried out at several different levels of representation and abstraction.Although you may learn and practice design at a particular level. from time to time you'll need to go up or down a level or two to get the job done. Also,the industry itself and most designers have been steadily moving to highe levels of abstraction as circuit density and functionality have increased Copyright 1999 by John F.Wakerly Copying Prohibited 18 Chapter 1 Introduction DO NOT COPY DO NOT COPY DO NOT COPY DO NOT COPY DO NOT COPY DO NOT COPY DO NOT COPY DO NOT COPY DO NOT COPY Copyright © 1999 by John F. Wakerly Copying Prohibited 1.9 Printed-Circuit Boards An IC is normally mounted on a printed-circuit board (PCB) [or printed-wiring board (PWB)] that connects it to other ICs in a system. The multilayer PCBs used in typical digital systems have copper wiring etched on multiple, thin layers of fiberglass that are laminated into a single board about 1/16 inch thick. Individual wire connections, or PCB traces are usually quite narrow, 10 to 25 mils in typical PCBs. (A mil is one-thousandth of an inch.) In fine-line PCB technology, the traces are extremely narrow, as little as 4 mils wide with 4-mil spacing between adjacent traces. Thus, up to 125 connections may be routed in a one-inch-wide band on a single layer of the PCB. If higher connection density is needed, then more layers are used. Most of the components in modern PCBs use surface-mount technology (SMT). Instead of having the long pins of DIP packages that poke through the board and are soldered to the underside, the leads of SMT IC packages are bent to make flat contact with the top surface of the PCB. Before such components are mounted on the PCB, a special “solder paste” is applied to contact pads on the PCB using a stencil whose hole pattern matches the contact pads to be soldered. Then the SMT components are placed (by hand or by machine) on the pads, where they are held in place by the solder paste (or in some cases, by glue). Finally, the entire assembly is passed through an oven to melt the solder paste, which then solidifies when cooled. Surface-mount component technology, coupled with fine-line PCB tech￾nology, allows extremely dense packing of integrated circuits and other components on a PCB. This dense packing does more than save space. For very high-speed circuits, dense packing goes a long way toward minimizing adverse analog phenomena, including transmission-line effects and speed-of-light limitations. To satisfy the most stringent requirements for speed and density, multichip modules (MCMs) have been developed. In this technology, IC dice are not mounted in individual plastic or ceramic packages. Instead, the IC dice for a high-speed subsystem (say, a processor and its cache memory) are bonded directly to a substrate that contains the required interconnections on multiple layers. The MCM is hermetically sealed and has its own external pins for power, ground, and just those signals that are required by the system that contains it. 1.10 Digital-Design Levels Digital design can be carried out at several different levels of representation and abstraction. Although you may learn and practice design at a particular level, from time to time you’ll need to go up or down a level or two to get the job done. Also, the industry itself and most designers have been steadily moving to higher levels of abstraction as circuit density and functionality have increased. printed-circuit board (PCB) printed-wiring board (PWB) PCB traces mil fine-line surface-mount technology (SMT) multichip module (MCM)