|
The connections on the PCB should be identical to the circuit diagram, but while the circuit diagram is arranged to be readable, the PCB layout is arranged to be functional. There is rarely any visible correlation between the circuit diagram and the layout. Printed circuit board layout can be performed manually (using CAD) or in combination with an auto-router. The best results are usually still achieved using at least some manual routing, simply because the design engineer has a far better judgment of how to arrange circuitry. Surprisingly, many auto-routed boards are often completely illogical in their track routing. The program has optimized the connections and sacrificed any small amount of order that may have been put in place by manual routing. Generally, auto-routed boards are somewhat harder for a technician to repair or debug, for this reason. Historically, PCBs used to be laid out by drawing or using stick-on paper shapes on mylar film - that really WAS manual routing! The CAD printed circuit board layout consists of several layers. For illustration, often the layers will be colored and compressed into the one overlay image. When laying out boards, it’s usually best to try using actual size check plots at some stage during the design process. Most overlays need to be printed out enlarged to show the detail, but an actual size print, with mounting holes and possibly cutouts, is a great check tool. The print itself can be placed inside the actual enclosure; you can see how it will be positioned in relation to other parts. You can also place components up against the pad markings as a quick idiot-check of dimensions. The width of the tracks is a trade-off based on current flow, space available, size of parts, and electromagnetic interference. The track layout is a similar trade-off that also picks when to dodge from one side of the board to the other to avoid an obstacle, but overall normally aims to find the shortest regular path between the connected points. Given the impedance, susceptibility, and signal on tracks, the loop area is another trade-off that is considered as the design proceeds. These days most circuit boards are automatically assembled and tested, but people still install and repair them. Having a quality design can make all the difference between a product that is pleasant to work on and one that you will hate. Machine test points, for ATE (Automated Test Equipment) bed of nails testing, only need to be plain pads or lands. Some test points, however, are really intended to be for engineering test or modification - for these you could put in labeled, circled pads. A complete silkscreen identification overlay will only make a better product. The silkscreen contains important information that assists people to service and install the product. Aside from the printed circuit board functionality, the overlay is the chief distinguishing feature between a purpose-built product, and a general-purpose controller or ready-made PLC. For dense surface mount boards, you would need to leave off component values, and sometimes omit the component designators. First try to drop the size of the lettering - small text being better than no text. The silkscreen is the primary method for labeling connectors, replaceable parts, orientation, and even installation notes. It can also used as a backup for manufacturing notes, although manufacturers normally put these notes in the BOM. AirBorn Electronics. “The Printed Circuit Board (PCB) Layout.” 15 Sep. 2009 [http://www.airborn.com.au/method/layout.html].
Related Articles -
printed, circuit, board,
|
