Routing (electronic design automation)

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In electronic design, wire routing, commonly called simply routing, is a step in the design of printed circuit boards (PCBs) and integrated circuits (ICs). It builds on a preceding step, called placement, which determines the location of each active element of an IC or component on a PCB. After placement, the routing step adds wires needed to properly connect the placed components while obeying all design rules for the IC. Together, the placement and routing steps of IC design are known as place and route.

Printed circuit board board to support and connect electronic components

A printed circuit board (PCB) mechanically supports and electrically connects electronic components or electrical components using conductive tracks, pads and other features etched from one or more sheet layers of copper laminated onto and/or between sheet layers of a non-conductive substrate. Components are generally soldered onto the PCB to both electrically connect and mechanically fasten them to it.

Integrated circuit electronic circuit manufactured by lithography; set of electronic circuits on one small flat piece (or "chip") of semiconductor material, normally silicon

An integrated circuit or monolithic integrated circuit is a set of electronic circuits on one small flat piece of semiconductor material that is normally silicon. The integration of large numbers of tiny transistors into a small chip results in circuits that are orders of magnitude smaller, cheaper, and faster than those constructed of discrete electronic components. The IC's mass production capability, reliability and building-block approach to circuit design has ensured the rapid adoption of standardized ICs in place of designs using discrete transistors. ICs are now used in virtually all electronic equipment and have revolutionized the world of electronics. Computers, mobile phones, and other digital home appliances are now inextricable parts of the structure of modern societies, made possible by the small size and low cost of ICs.

Placement is an essential step in electronic design automation - the portion of the physical design flow that assigns exact locations for various circuit components within the chip's core area. An inferior placement assignment will not only affect the chip's performance but might also make it non-manufacturable by producing excessive wirelength, which is beyond available routing resources. Consequently, a placer must perform the assignment while optimizing a number of objectives to ensure that a circuit meets its performance demands. Together, the placement and routing steps of IC design are known as place and route.

Contents

The task of all routers is the same. They are given some pre-existing polygons consisting of pins (also called terminals) on cells, and optionally some pre-existing wiring called preroutes. Each of these polygons are associated with a net, usually by name or number. The primary task of the router is to create geometries such that all terminals assigned to the same net are connected, no terminals assigned to different nets are connected, and all design rules are obeyed. A router can fail by not connecting terminals that should be connected (an open), by mistakenly connecting two terminals that should not be connected (a short), or by creating a design rule violation. In addition, to correctly connect the nets, routers may also be expected to make sure the design meets timing, has no crosstalk problems, meets any metal density requirements, does not suffer from antenna effects, and so on. This long list of often conflicting objectives is what makes routing extremely difficult.

In electronics, crosstalk is any phenomenon by which a signal transmitted on one circuit or channel of a transmission system creates an undesired effect in another circuit or channel. Crosstalk is usually caused by undesired capacitive, inductive, or conductive coupling from one circuit or channel to another.

Antenna effect

The antenna effect, more formally plasma induced gate oxide damage, is an effect that can potentially cause yield and reliability problems during the manufacture of MOS integrated circuits. Fabs normally supply antenna rules, which are rules that must be obeyed to avoid this problem. A violation of such rules is called an antenna violation. The word antenna is something of a misnomer in this context—the problem is really the collection of charge, not the normal meaning of antenna, which is a device for converting electromagnetic fields to/from electrical currents. Occasionally the phrase antenna effect is used in this context, but this is less common since there are many effects, and the phrase does not make clear which is meant.

Almost every problem associated with routing is known to be intractable. The simplest routing problem, called the Steiner tree problem, of finding the shortest route for one net in one layer with no obstacles and no design rules is NP-hard if all angles are allowed and NP-complete if only horizontal and vertical wires are allowed. Variants of channel routing have also been shown to be NP-complete, as well as routing which reduces crosstalk, number of vias, and so on. Routers therefore seldom attempt to find an optimum result. Instead, almost all routing is based on heuristics which try to find a solution that is good enough.

Computational complexity theory focuses on classifying computational problems according to their inherent difficulty, and relating these classes to each other. A computational problem is a task solved by a computer. A computation problem is solvable by mechanical application of mathematical steps, such as an algorithm.

Channel router

A channel router is a specific variety of router for integrated circuits. Normally using two layers of interconnect, it must connect the specified pins on the top and bottom of the channel. Specified nets must also be brought out to the left and right of the channel, but may be brought out in any order. The height of the channel is not specified - the router computes what height is needed.

A via or VIA is an electrical connection between layers in a physical electronic circuit that goes through the plane of one or more adjacent layers. To ensure via robustness, IPC sponsored a round-robin exercise that developed a time to failure calculator.

Design rules sometimes vary considerably from layer to layer. For example, the allowed width and spacing on the lower layers may be four or more times smaller than the allowed widths and spacings on the upper layers. This introduces many additional complications not faced by routers for other applications such as printed circuit board or multi-chip module design. Particular difficulties ensue if the rules are not simple multiples of each other, and when vias must traverse between layers with different rules.

Multi-chip module

A multi-chip module (MCM) is generically an electronic assembly where multiple integrated circuits, semiconductor dies and/or other discrete components are integrated, usually onto a unifying substrate, so that in use it is treated as if it were a single component . Other terms, such as "hybrid" or "hybrid integrated circuit", also refer to MCMs.

Types of routers

A PCB as a design on a computer (left) and realized as a board assembly populated with components (right). The board is double sided, with through-hole plating, green solder resist and a white legend. Both surface mount and through-hole components have been used. PCB design and realisation smt and through hole.png
A PCB as a design on a computer (left) and realized as a board assembly populated with components (right). The board is double sided, with through-hole plating, green solder resist and a white legend. Both surface mount and through-hole components have been used.

The earliest types of EDA routers were "manual routers"—the drafter clicked a mouse on the endpoint of each line segment of each net. Modern PCB design software typically provides "interactive routers"—the drafter selects a pad and clicks a few places to give the EDA tool an idea of where to go, and the EDA tool tries to place wires as close to that path as possible without violating design rule checking (DRC). Some more advanced interactive routers have "push and shove" (aka "shove-aside" or "automoving") features in an interactive router; the EDA tool pushes other nets out of the way, if possible, in order to place a new wire where the drafter wants it and still avoid violating DRC. Modern PCB design software also typically provides "autorouters" that route all remaining unrouted connections without human intervention.

In electronics engineering, a design rule is a geometric constraint imposed on circuit board, semiconductor device, and integrated circuit (IC) designers to ensure their designs function properly, reliably, and can be produced with acceptable yield. Design rules for production are developed by process engineers based on the capability of their processes to realize design intent. Electronic design automation is used extensively to ensure that designers do not violate design rules; a process called design rule checking (DRC). DRC is a major step during physical verification signoff on the design, which also involves LVS checks, XOR checks, ERC, and antenna checks. The importance of design rules and DRC is greatest for ICs, which have micro- or nano-scale geometries; for advanced processes, some fabs also insist upon the use of more restricted rules to improve yield.

The main types of autorouters are:

CADSTAR

CADSTAR is a Windows-based electronic design automation (EDA) software tool for designing and creating schematic diagrams and printed circuit boards (PCBs). It provides engineers with a tool for designing simple or complex, multilayer PCBs. CADSTAR spans schematic capture, variant management, placement, automatic and high-speed routing, signal integrity, power integrity, EMC analysis, design rule checks and production of manufacturing data.

Zuken Inc. is a Japanese multinational corporation, specializing in software and consulting services for end-to-end electrical and electronic engineering. Zuken came into existence as a pioneer in the development of CAD systems in Japan to contribute to electronics manufacturing. The literal translation of Zuken is "graphics laboratory." Established in 1976 in Yokohama, Japan, it is listed on the Tokyo Stock Exchange; net sales amounted to USD216 million for the year 2011.

Specctra is a commercial PCB auto-router originally developed by John F. Cooper and David Chyan of Cooper & Chyan Technology, Inc. (CCT) in 1989. The company and product were taken over by Cadence Design Systems in May 1997. Since its integration into Cadence's Allegro PCB Editor, the name of the router is Allegro PCB Router. The latest version is 17.2.

How routers work

Many routers execute the following overall algorithm:

For detailed routing, the most common technique is rip-up and reroute aka rip-up and retry: [1]

This process repeats until all nets are routed or the program (or user) gives up.

An alternative approach is to treat shorts, design rule violations, obstructions, etc. on a similar footing as excess wire length—that is, as finite costs to be reduced (at first) rather than as absolutes to be avoided. This multi-pass "iterative-improvement" routing method [20] is described by the following algorithm:

Most routers assign wiring layers to carry predominantly "x" or "y" directional wiring, though there have been routers which avoid or reduce the need for such assignment. [21] There are advantages and disadvantages to each approach. Restricted directions make power supply design and the control of inter-layer crosstalk easier, but allowing arbitrary routes can reduce the need for vias and decrease the number of required wiring layers.

See also

Related Research Articles

Wire wrap

Wire wrap was invented to wire telephone crossbar switches, and later adapted to construct electronic circuit boards. Electronic components mounted on an insulating board are interconnected by lengths of insulated wire run between their terminals, with the connections made by wrapping several turns of uninsulated sections of the wire around a component lead or a socket pin.

Electronic design automation (EDA), also referred to as electronic computer-aided design (ECAD), is a category of software tools for designing electronic systems such as integrated circuits and printed circuit boards. The tools work together in a design flow that chip designers use to design and analyze entire semiconductor chips. Since a modern semiconductor chip can have billions of components, EDA tools are essential for their design.

Circuit diagram graphical representation of an electrical circuit

A circuit diagram is a graphical representation of an electrical circuit. A pictorial circuit diagram uses simple images of components, while a schematic diagram shows the components and interconnections of the circuit using standardized symbolic representations. The presentation of the interconnections between circuit components in the schematic diagram does not necessarily correspond to the physical arrangements in the finished device.

Place and route is a stage in the design of printed circuit boards, integrated circuits, and field-programmable gate arrays. As implied by the name, it is composed of two steps, placement and routing. The first step, placement, involves deciding where to place all electronic components, circuitry, and logic elements in a generally limited amount of space. This is followed by routing, which decides the exact design of all the wires needed to connect the placed components. This step must implement all the desired connections while following the rules and limitations of the manufacturing process.

TARGET (CAD software)

TARGET 3001! is a CAD computer program for EDA and PCB design, developed by Ing.-Büro Friedrich in Germany. It supports the design of electronic schematics, PCBs, and device front panels. It runs under Windows and is available in English, German and French.

OrCAD electronic design automation software

OrCAD Systems Corporation was a software company that made OrCAD, a proprietary software tool suite used primarily for electronic design automation (EDA). The software is used mainly by electronic design engineers and electronic technicians to create electronic schematics and electronic prints for manufacturing printed circuit boards. OrCAD was taken over by Cadence Design Systems in 1999 and was integrated with Cadence Allegro since 2005.

Signal integrity

Signal integrity or SI is a set of measures of the quality of an electrical signal. In digital electronics, a stream of binary values is represented by a voltage waveform. However, digital signals are fundamentally analog in nature, and all signals are subject to effects such as noise, distortion, and loss. Over short distances and at low bit rates, a simple conductor can transmit this with sufficient fidelity. At high bit rates and over longer distances or through various mediums, various effects can degrade the electrical signal to the point where errors occur and the system or device fails. Signal integrity engineering is the task of analyzing and mitigating these effects. It is an important activity at all levels of electronics packaging and assembly, from internal connections of an integrated circuit (IC), through the package, the printed circuit board (PCB), the backplane, and inter-system connections. While there are some common themes at these various levels, there are also practical considerations, in particular the interconnect flight time versus the bit period, that cause substantial differences in the approach to signal integrity for on-chip connections versus chip-to-chip connections.

FreePCB

FreePCB is a printed circuit board design program for Microsoft Windows, written by Allan Wright.

In electronic design automation, maze runner is a connection routing method that represents the entire routing space as a grid. Parts of this grid are blocked by components, specialised areas, or already present wiring. The grid size corresponds to the wiring pitch of the area. The goal is to find a chain of grid cells that go from point A to point B.

KiCad free software suite for electronic design automation (EDA)

KiCad is a free software suite for electronic design automation (EDA). It facilitates the design of schematics for electronic circuits and their conversion to PCB designs. KiCad was originally developed by Jean-Pierre Charras. It features an integrated environment for schematic capture and PCB layout design. Tools exist within the package to create a bill of materials, artwork, Gerber files, and 3D views of the PCB and its components.

Altium Limited is an American, Australian-domiciled owned public software company that provides PC-based electronics design software for engineers who design printed circuit boards. Founded as Protel Systems Pty Ltd in Tasmania, Australia in 1985, Altium now has regional headquarters in the United States, Australia, China, Europe, and Japan, with resellers in all other major markets.

TopoR is an EDA program developed and maintained by the Russian company Eremex. It is dedicated to laying out a printed circuit board (PCB). The current version is 6.3.17875 as of 2017-09-20.

Comparison of electronic design automation (EDA) software

PCB (software) free and open-source software suite for electronic design automation

PCB is a free and open-source software suite for electronic design automation (EDA) - for printed circuit boards (PCB) layout. It uses GTK+ for its GUI widgets.

DipTrace

DipTrace is an EDA/CAD software for creating schematic diagrams and printed circuit boards. The developers provide a multi-lingual interface and tutorials. DipTrace has 4 modules: schematic capture editor, PCB layout editor with built-in shape-based autorouter and 3D-preview & export, component editor, and pattern editor.

Pulsonix Software für Schaltplan Erstellung und Leiterplatten Design

Pulsonix is an electronic design automation (EDA) software suite for schematic capture and PCB design. It is produced by WestDev, which is headquartered in Gloucestershire, England, with additional sales and distribution offices overseas. It was first released in 2001, and runs on Windows.

EasyEDA is a web-based EDA tool suite that enables hardware engineers to design, simulate, share - publicly and privately - and discuss schematics, simulations and printed circuit boards. Other features include the creation of a Bill of Materials, Gerber and pick and place files and documentary outputs in PDF, PNG and SVG formats.

Proteus Design Suite electronic design automation software

The Proteus Design Suite is a proprietary software tool suite used primarily for electronic design automation. The software is used mainly by electronic design engineers and technicians to create schematics and electronic prints for manufacturing printed circuit boards.

References

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Further reading