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The Printed Circuit Board (PCB) Layout
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, so there is rarely any visible correlation between the
circuit diagram and the layout.
PCB layout can be performed manually (using CAD) or in combination with an Autorouter. The best results are usually still achieved using atleast some manual routing - simply because the design engineer has a far better judgement of how to arrange circuitry. Surprisingly, many autorouted boards are often completely illogical in their track routing - the program has optimised the connections, and sacrificed any small amount of order that may have been put in place by manual routing. Generally autorouted 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 PCB layout consists of several layers, for illustration often the layers will be coloured and compressed into the one overlay image. (Click here to see a typical overlay, and Click here to see a colour coded key diagram) When we layout boards, we usually try to use actual size checkplots 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, we 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 idiot-check of dimensions. The truth is rarely pure, and never simple. -- Oscar WildeWe put a bit of effort into the PCB design - it is somewhere where quality of design can make a difference. 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. And then, added to all that is Design-for-manufacture.
These days most circuit boards are automatically assembled and tested -
but people still install and repair them.
We put a complete silkscreen identification overlay on most of our boards, and the quality of this is another way we can make a better product. The silkscreen contains important information that assists people to service and install the product. Aside from the PCB functionality (which is rather obvious) the overlay is the chief distinguishing feature between a purpose-built product, such as we deliver, and a general-purpose controller or ready made PLC. We would label terminals as: "+12v red" "-pwr black" "solenoid+" "solenoid-" whereas competitors may often just give you: "A1 A2 A3 A4". They don't have much of a choice - they have no idea what you will attach to the terminals. For dense surface mount boards, we often need to leave off component values, and sometimes need to omit the component designators. The truth is, we first try to drop the size of the lettering - small text being better than no text. The silkscreen is the primary method for labelling connectors, replaceable parts, orientation, and even installation notes (for instance "Remove J2 while replacing battery - observe battery polarity"). It can also used as a backup for manufacturing notes (for example "Mount R2 approx 5mm off circuit board due to high temperature") although now we normally put these notes in the BOM. The PCB layout also needs to take account of
EMI and ESD compliance and we have a seperate page on some techniques
that we use to get our PCBs to comply. We have an (older)
tutorial on designing PCBs - it steps through the CAD process
from schematic to layout. We also support the Autotrax/Easytrax CAD package by providing
Updated video drivers. If you are intending to
design PCBs yourself, we would recommend this
manual on PCB layout.
One last rather sharp comment on choosing people to do your design work:
You need to rely on either
Skill and Experience, or Luck.
Those are the two ways a project can be successful. We hope you choose AirBorn for your job.
Most of our Etched PCB laminates are:
Different methods of PCB construction:
Reality is merely an illusion, albeit a very persistent one -- Albert Einstein Click the Continue arrow below for "PCB Etching," or go on to:
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Having a quality design can make all the difference between a product that is pleasant to work
on and one that they 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 we put in labelled, circled pads.
EMI & ESD
