PCB unbundling

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User interface of a layout software.
Circuit board with placed components, so-called "rat nest". The green "rubber bands" must be laid as conductor lines.
Finished layout from the picture above; the connections are made by means of vias on the top ( top layer , blue) and the bottom ( bottom layer , red).

The circuit board disentanglement is a work step in the layout design (short: layout) of electronic circuit boards . After the manual or automatic placement of the components on the circuit board (cf. Autoplacer ) , the designed electrical circuit diagram is converted into a conductor track network . Today it is almost always carried out on the computer by hand or automatically with the help of a so-called autorouter .

In addition to circuit diagram design and its simulation, CAD software for circuit board design often also includes auto placers and auto routers. In order for an autorouter to deliver meaningful results, design rules must be given beforehand. If you do not do this, these automatic functions reach their limits, so that at least some of the conductor tracks have to be laid manually.

Procedure for unbundling printed circuit boards

The unbundling takes place after the creation of an electrical circuit diagram , which can also be in the form of a network list , and the placement of the components on the screen. After these steps is called a "rat's nest" ( English RATSNEST before), are shown by each other in which all the electrical connections to the shortest paths.

When unbundling, u. a. PCB technology, component geometries, position of the components, signal propagation times, current strengths and the position of the external connections ( plug connectors ) are taken into account. Layout programs make the work of conductor path unbundling easier, in particular through component databases and the checking of design rules. Issued CAD data for platinum preparations, lamp and Shopping typically in Gerber format as well as BOM (English: Bill of Materials , BOM).

  • Creation of component descriptions in the component library belonging to the layout program.
  • Input of the circuit diagram with the corresponding software module.
  • Input of the circuit board geometry (manually or importing the data from a mechanical CAD program).
  • Input of the circuit board technology (layer structure, via technology used, rules for track widths , spacing rules)
  • Placing the components on the circuit board (first those components that need to be in certain places, e.g. potentiometers that protrude through the front panel)

The further placement is carried out in such a way that:

  • the line connections are as short as possible
  • the specified rules for connecting the components are observed
  • there is sufficient space for laying the cables
  • Specifications from the circuit board production are taken into account

The design rules can be very extensive. However, they enable the layout designer to perform the unbundling with less time-consuming iterations and error checks.

The placement is usually done interactively. The connections to be created are shown as "rubber bands" between the connections in order to enable an advantageous placement. For placing uncritical components at the end of the placement process, automatic placement aids ( autoplacers ) are also suitable today in less dense designs .

software

The work is supported by layout programs. B .:

see also: EDA: Programs for the design of printed circuit boards

Unbundling with autorouter

Modern multi-layer designs with BGAs are usually processed automatically with unbundling programs (autorouter); for this purpose, boundary conditions (rules) must be entered in the layout tool beforehand. Routing strategies can be specified so that an autorouter delivers useful results. Whether it makes more sense to use an autorouter or route by hand depends heavily on the type of design. If several similar, digital designs are to be edited, it can be useful to optimize rules and routing strategies and to reuse them. For small projects or special requirements, e.g. B. HF, the rule entry takes more time than the manual PCB unbundling by an experienced layouter. Iterative layout is sometimes necessary if the autorouter does not find a (complete) solution to the unbundling problem.

Often, rules must be observed that cannot be specified in some routers, e.g. B. neutral point grounding, reference points for analog voltages, properties during soldering, heat management or thermal vias . Here it is possible that only parts of the design can be disentangled with the autorouter.

Manual unbundling (manual laying)

With manual unbundling, critical lines such as high-frequency lines or buses are laid first. The preferred direction of the conductor tracks on the top is often chosen to be perpendicular to that on the bottom. With every change of direction, the level can now be changed by means of a through-hole plating .

When laying, the layout software offers support by displaying a seam around the conductor track, which makes it easier to comply with the design rules. During the unbundling, it is often necessary to move individual components again. With high-quality EDA programs, there is usually the option of disentangling individual signals semi-automatically and using “push aside” or “push and shove” to push already laid conductor tracks and components out of the corridor for the conductor track that has just been processed by hand.

After the layout of the software by means of automatic design rule test ( English Design Rule Check check that all defined design rules are followed and there is no logical error, DRC), for example. B. disconnected components or track crossings.

Limits to unbundling

In the case of very densely populated circuit boards, not all conductor paths can be laid by hand, because there are no longer any free "lanes" to connect some component connection points. These missing connections must then be formed by wire bridges during assembly or laid as wires by hand. For reasons of cost, but also due to the technical requirements ( high frequency , signal propagation times , reliability), this is only justifiable for a few products. In addition, with conductor track widths and distances of 75 to 150 µm, there is hardly any space for soldering eyes to connect wires. As an alternative to wire bridges there are solder bridges (jumpers) and "zero ohm resistors" . If necessary, you can also increase the number of layers or, in the case of multilayer circuit boards, work with vias that are not located on all layers (so-called blind vias and buried vias , see vias ).

Design rules for PCB unbundling

When unbundling printed circuit boards, additional specifications must usually be observed, e.g. B .:

  • Limitations of the manufacturing technology used such as minimum conductor track widths, minimum drilling diameters, distance from copper surfaces to the edge etc .;
  • Compliance with the wave impedance of special lines (e.g. for high frequency , Ethernet , USB );
  • Compliance with the cable lengths for signals with reference to each other (data buses, symmetrical signals) through meander lines;
  • Crosstalk from lines (e.g. in compact devices such as cell phones );
  • Specifications of the signal propagation time (e.g. cable lengths for PCI , PCI Express );
  • Consideration of electromagnetic compatibility (EMC);
  • Compliance with insulation distances from approx. 50 V according to the standard (e.g. according to VDE : distances for clearances , creepage distances ).

Result of the PCB disentangling (output)

The result is a completely disentangled design, which no longer contains air lines and which contains all the information needed to produce a printed circuit board. It consists of several layers of virtual copper layers and connections between them, as well as the component dimensions and placement coordinates. From this data i. d. R. Production data (Gerber data) obtained for exposure and etching at the manufacturer. Furthermore, the so-called "drill file", a file with information about the arrangement and sizes of the required holes, can be generated, with the aid of which a CNC drilling tool can process the board. Information for the boundary scan test and flying probe test can also be taken from the circuit board file.

See also

literature

  • J. Händschke: Circuit board design - a manual not only for practitioners . Eugen G. Leuze Verlag, Bad Saulgau 2006, ISBN 3-87480-219-1 .
  • K. Mitzner, B. Doe, A. Akulin, A. Suponin, D. Müller: Complete PCB Design Using OrCAD Capture and PCB Editor, Elsevier / Academic Press 2019, ISBN 9780128176849