Before a PCB design can be started, a schematic drawing should
exist. Whether it exists on white paper and pencil, scribbling on a
white board, or even just in your head, one must exist. If this
schematic is to be read by others, it must adhere to what everyone
knows as a 'schematic diagram'. They need to recognize symbols for
resistors, capacitors and anything else you have. From this point,
most people can get an understanding of what the circuit is. However,
it's only when you intend for this schematic to be creating into a
physical PCB design, you need to get even more strict on your drawing.
If you need a PCB design to be created from this schematic,
every symbol on it needs to have additional characteristics attached to
it. For example; that resistor and capacitor, are they through hole or
surface mount parts? What is the tolerance, and even the power
rating?
Finally, if you're going to have your PCB design assembled at a
professional assembly house, you'll need the actual manufacturing part
number for each and every component on your board. Can you imagine if
someone came to you and asked you to buy them a "vehicle"? It's the
same way when someone asks a manufacturing house to buy a "resistor".
While this guy might be about 6'7" tall, and work as a lumberjack,
surely you wouldn't buy him a small Yugo. They're cheap, but it won't
fit him at all.
This is the same with the assembly house. They 'could' go out
and attempt to decipher what your intended uses are, look at the
conditions that exists, and buy that resistor to the best of their
ability... but them having to do this, means the schematic is not portable or finished. In the end, that manufacturing house is going to charge a whole lot extra if they try and do it themselves. But in most cases, they'll just kick it back to you and tell you that your work isn't finished yet.
A schematic with all the attributes is not only professional,
it's 'portable'. Meaning, if I need to send this schematic to my a
colleague taking over the project, or even another company all
together, they would be able to take it and 'hit the ground running'.
Every phone call from my colleague about the schematic means I missed
something somewhere on the schematic. And, if I had just been swept
up by aliens, this wouldn't be good for my colleague, and it wouldn't be
good for me either.
An industry standard schematic tool is Cadence Orcad Capture (there are multiple names, but we'll stick with this for explanation purposes). If you have your schematic page open, and you select a component (a 10 ohm resistor for example), and right-click on it, a window will open up to allow you to assign properties. Some of the few basic properties that need to be included in this pop-up window are;
1). PCB Footprint - is it a 1210, TH___ (through hole), 0402, or an 0603? As I mentioned before, a PCB designer would look at the circuit to best identify what needs to be there (or he'll just kick it back to you and say it's not finished, crushing your schedule). . In this case, if we were to see that this 10 ohm resistor is intended to limit current to a high current LED, we would derive a wattage from a formula and probably pick something much larger than an 0603. They (0603's) are generally rated at about 1/16th of a watt. So, make sure you do your own calculations before assigning the footprint value just in case the PCB designer chooses a wrong value foootprint type.
2). Manufacturing part number - This is kind of an optional entry (but sometimes mandatory if the wattage or other important things arn't specified). There are probably over 20 different manufacturers making a 10 ohm resistor. Sometimes, leaving just the PCB Footprint value in place will allow the PCB designer to pick the most cost effective resistor to put in it's place. One 1210 might be twice the cost of the other 1210, so he's going to pick the cheapest one, and if you have 8 of these resistors on your board, and you manufacture 1000 boards, it adds up fast. It's also important to know that manufactures of a resistor are not always the same. Some vary quite a bit. So be sure to put the manufacturers part number down if you have a 'special' resistor in mind. Manufacturing part numbers are also important if you have an IC. WIth today's IC's, you have probably 5 different footprints for 1 IC. Some very small and others very large. And in addition, the 'PCB Footprint value', isn't always a universal footprint code. I've seen footprints that say they are an SSOP-8, but they are really an SOP-8 or and MMSOP-8. So, to keep confusion to a minimum, it's important to have the manufacturing part number, MFPN for short.
3). Trace width - This is where we learn about the 'unintended fuseable link'. If you have 3 amps coming out of a power supply, you surely wouldn't want that trace be a very thin trace. This trace will 'burn' (or blow open) just like a fuse, as fast as you can apply power to the circuit. A decent designer will do it right even if this value isn't there, but I've seen too many times, where a design house will simply 'go with what the customer gives them'. So, make sure you include this.
4). Main page - this is what I refer to as the bible of the schematic. Be sure to list anything that you feel is necessary for the design. These are very simply things like, "Place the decoupling capacitors as listed in the schematic, next to each power pin on the ICs". I once saw a PCB designed so that every decoupling capacitor was all the way on one side of the PCB, lined up very nicely, but every IC on the board itself wasn't decoupled. This is where you also declare that you have 'high speed' traces. If you have a USB pair, or a high speed memory bus, be sure to declare the tolerances of the trace length matching, etc. I once saw USB traces that were not at all treated as a differential pair, but just two nets snaked all over the board without any impedance characterization at all (it didn't work). Also include PCB mounting holes of "X" size, size and shape of the PCB, and any height requirements if this is going into a small enclosure. And don't forget to indicate that it needs to be either ROHS compliant (or not) as well.
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