The State of Autorouting in PCB Layout

PCB layout begins with a schematic. The schematic can be a simple paper drawing on simple boards or captured in a CAD program for more complex boards. The connections on the PCB should be identical to the schematic, but while the schematic is arranged to be readable, the PCB layout is arranged to be functional. As a result, there is rarely any visible correlation between the schematic and the layout.

PCB layout can be performed manually (using a CAD tool such as Altium Designer, PADs, or Allegro), or in combination with an Autorouter. The best results are usually achieved using or manual routing because the design engineer can exercise better judgment of how to arrange circuitry than an autorouter program is able to. Many autorouted boards are often completely illogical in their track routing because the program has optimized the connections, but sacrificed any small amount of order that may have been put in place by manual routing. Generally, autorouted boards are slightly harder for a technician to repair or debug for this reason. Furthermore, manufacturing yield is often decreased with autorouted boards.Although autorouters have become much stronger tools, there is still no comparison between and autorouted board and a hand routed board. 911EDA does not autoroute any board unless we receive specific instruction from our customer to do so.

The PCB layout project consists of several layers (911EDA has designed boards up to 24 layers) for illustration purposes. The holes, outline, and component identification layers can be combined into one diagram. A PCB will usually have mounting holes and may possibly have cutouts. By cutting an actual size checkplot and placing it in the enclosure, you can see how it will be positioned in relation to other parts. We can also place components up against the pad markings as a quick double check of sizing.

911EDA puts a lot of effort into the PCB design. It is somewhere where quality of design can make a difference. These days, most circuit boards are automatically assembled and tested. But even with the automated process there are still people working on most boards. Having a quality design can make all the difference between a product that is easy to work on and one that fights the process.

A complete silkscreen identification overlay is used on most boards and the quality of this is another way to make a better product. The silkscreen contains important information that assists people to service and install the product. Aside from functionality, the overlay is the primary distinguishing feature between a purpose-built product and a general-purposeproduct. On a purpose-built product, terminals might be labeled as: "+12v red" "-pwr black" "solenoid+" "solenoid-", whereas a general-purpose product might be labeled as: "A1 A2 A3 A4".

For dense surface mount boards, it is often necessary to leave off component values, and sometimes it is even necessary to omit the component designators. The silkscreen is the primary method for labeling connectors, replaceable parts, orientation, and even installation notes (for instance "Remove J3 while replacing battery - observe battery polarity")