Placement (EDA)

The PCB layout is a step in the layout design (in short: lay out ) of electronic circuit boards. Here, the designed electrical circuit diagram of the manual or automatic placement (see autoplacer ) is converted to the required components on the circuit board in an interconnect network. It is performed by using a so-called autorouter almost exclusively on the computer by hand or automated today.

CAD software for PCB design includes not only the circuit design and its simulation often auto- placer and autorouter. In order for a car router can deliver meaningful results, it design rules above must be specified. If one does not encounter these automatic functions at boundaries, so that circuit traces must be routed manually, at least partially.

Approach to PCB layout

Unbundling occurs after creating a wiring diagram, which can also be in the form of a netlist and placement of components on the screen. After these steps, there is a so-called " rat's nest " (English Board nest ), in which all the electrical connections by side on the shortest paths.

With the unbundling and Others PCB technology, component geometry, position of the components, signal propagation delays, current strengths as well as location of the outer terminals (connectors) must be considered. This work is supported by commercial layout programs (CAD ) systems such as Pulsonix EDWinXP, OrCAD, Eagle, TARGET 3001!, Cadence Allegro, Siemens AG V CAD, Altium Designer (formerly Protel ), or expedition PCB. An open source KiCad and gEDA layout programs are known. In particular, by components databases and the checks of design rules layout programs facilitate the job of conductor unbundling. Be issued, the CAD data for platinum preparations, lamp and Shopping typically in Gerber format, as well as a parts list (English: Bill of Materials, BOM).

• Creating component descriptions in the layout program associated component library.
• Input of the circuit diagram with the corresponding software module.
• Input of the circuit board geometry ( manually or import the data from a mechanical CAD program).
• Input circuit board technology ( layer stackup, used Via technology, rules for trace widths, spacing rules )
• Placing the components on the printed circuit board ( first those components that have to be in certain places, such as potentiometers, which protrude through the front panel)

Further positioning is performed so that:

• The wiring connections are as short as possible
• The predetermined rules for the compounds of the components are adhered to
• There is sufficient space for the cabling of lines
• Specifications from the PCB production are taken into account

Design rules can be very large under certain circumstances. But you are allowing the layout to perform the unbundling with less costly iterations and error checks.

The placement is interactive in general. The for producing compounds are represented as " rubber bands " between the ports to allow an advantageous placement. To place non-critical components at the end of placing today also automatic Platzierhelfer are in less dense designs thoroughly ( autoplacer ).

Unbundling with autorouter

If divestiture program (auto router) is used, boundary conditions must first be entered into the layout tool, so that the router such rules be observed. For an autorouter provides useful results, the routing strategies can be specified. Whether it makes more sense to use an autorouter, or to route by hand, is highly dependent on the type of design. If several similar, digital designs are processed, so it may be useful to optimize and reuse rules and routing strategies. Partial performs the control input more time than manual PCB layout by an experienced layout artists.

Often, rules must be observed, which can not be specified in some routers, such as neutral grounding, reference points for analog voltages properties when vias soldering, thermal management and thermal. Can be disentangled with the autorouter here may only parts of the design.

Manual unbundling (manual installation)

In manual route first critical lines such as high-frequency lines or buses will be laid. Often one selects the preferred direction of the conductors on the top side perpendicular to those on the bottom. At each change of direction can now be replaced by a via the plane.

When laying the layout software provides support by displaying a seam around the circuit path, which facilitates compliance with design rules. During the unbundling, it is often necessary to move individual components again. In high-quality EDA programs so there is usually the opportunity to disentangle individual signals semi-automatic and let already push by "push aside" or "push and shove " routed traces and components from the corridor just been working hand- printed conductor.

After the layout of the software by means of automatic design rule test (English Design Rule Check DRC) check that all specified design rules are adhered to and that there are no logical errors, such as non-connected components or conductor crossings.

Limits of unbundling

In very densely populated circuit boards, not all tracks may be laid under certain circumstances, by hand, because there is no free " lanes " exist more for the connection of some component connection points. These missing links must then be formed during the assembly using wire jumpers or be laid by hand as wires. This is justifiable for reasons of cost, but also due to the technical requirements ( high frequency signal propagation times, reliability ) in only a few products. In addition, there are hardly any space at trace widths and spacings of 75 to 150 micron pads for connection of wires. As an alternative to wire bridges there are solder jumpers ( jumpers) and " zero - ohm resistors ". If necessary, you can also increase the number of layers or working in multi-layer printed circuit boards with plated-through holes that are not on all layers ( so-called blind vias and buried vias, see through connection ).

Guidelines for PCB design ( design rules )

In the PCB design guidelines are more often observed, for example:

• Restrictions on the production technology used, such as minimum trace widths, minimum drill diameter, distance from copper surfaces to the edge, etc.
• Compliance with the characteristic impedance of special cables (e.g., at high frequency, Ethernet, USB )
• Compliance with the cable lengths for signals with respect to each other ( data buses, balanced signals ) by meanderlines
• Crosstalk between lines ( eg in compact devices such as mobile phones );
• Specifications of the signal propagation time (eg line lengths for PCI, PCI Express);
• Into account the electromagnetic compatibility ( EMC);
• Compliance with isolation distances from about 50 V after standard ( eg according to VDE: distances for clearances, creepage distances ).

Result of the PCB design (output)

The result is a (hopefully complete) unbundled design that no longer contains air lines and carries the complete information in itself, able to produce a printed circuit board can. It consists of several layers of virtual copper layers and connections between them, as well as the component dimensions and placement coordinates. From these data i.d.R. Production data ( Gerber data) for the exposure and etching obtained from the manufacturer. Further, the so-called "drill file" be a file with information about the arrangement and sizes of the required holes generated with the aid of a CNC drilling tool can edit the board. Also information for the boundary-scan test and flying -probe test can be drawn from the platinum File.