Part Creation
Prerequisites:
- A rough design concept
- Keystone components
Tools:
- Logical design tool (Schematic Capture EDA)
- Physical design tool (PCB Layout EDA)
- ERP system (optional - Aliases & Supplier Parts Data)
Next Step:
This is less of a stage and more of a description of the process by which you will turn datasheets into components for use in your logical schematic capture, physical PCB layout, and component engineering ERP type systems.
There have been approximately 185 million electronic components ever documented. Of these a couple million are still actively produced, and even fewer held in stock. For example DigiKey has just over 2.5 million total SKUs, only about 650,000 of which are actually in stock. Said another way, it is incredibly likely that any given part number no longer exists (the odds are 99.6% against you), but at the same time most of the parts that you actually do want to use are probably pretty common. They have probably been used by someone else, they are probably in active production and they are probably in stock somewhere.
Each of these parts is described by a document called a datasheet. The datasheet usually contains everything you the engineer need to know about it. It should have sizing, pin names, physical characteristics, details like how hot or cold the part can get and still function, and tactical information like how to solder it to a PCB. Unfortunately most of these documents are not machine consumable, and you the engineer need to go through the painstaking process of parsing and converting these details into EDA CAD data, specifically the component’s logical symbols, pin names, and physical solder land pattern. And if you’re really lucky and you choose the right parts and the right library, someone has probably saved even you a ton of time and created the EDA models for you already.
And so our advice is rather than expect a perfect library of all 185 million parts, which unfortunately has never and probably should never exist, we recommend you use the library that does the best job of the 10,000 or so super common parts, and a pretty good job of the remaining, longer tail, 100,000 or so. But that means you will definitely need to create or improve at least a couple of part models eventually. If you’re lucky it’ll only be 10% of the parts you use, unlucky and it will be 25 or 50%, and in the absolute worst case you will be designing with parts no one has ever used before (a warning sign) and you’ll need to create them all.
When creating a part model in a library your goal is to as closely as possible represent the real world off-the-shelf device. For any unique part number, there should be 1 and only 1 correct model. There is some room for preferences when laying out a logical symbol, but for example pin 28 should always be named GND and the footprint should always be that of a 28 pin PDIP.
And so with that in mind, when creating a part model you absolutely need to capture the following four details.
Manufacturer & Part Number
This should allow an unambiguous link back to one and only one part. It should not link to a family or a part without the package defined. This should be the exact part number such that if you called that manufacturer and ordered it they would provide you with the exact part you specified.
Datasheet
This should come from the original manufacturer and contain every possible detail known about the part. The datasheet may exist for the family your part number belongs too, but that’s fine as long as the datasheet you choose is as specific as possible.
Schematic Symbol
This is the logical model of the part you will use with designing a schematic. This is the only subjective part of the component. Some designers prefer to lay out their symbols differently, which is fine, but they should (ideally and if possible) all exist as symbol variants under the same part. But that being said the pin names need to remain consistent across variants.
PCB Land Pattern (a.k.a. the Footprint)
This is the physical model for how the part connects to the PCB. The land pattern should include silkscreen for the reference designator, part outline and pin 1 location, and it should include a copper padstack for each terminal/pin the part has.
Whatever EDA tool or crowdsourced library you are using should include library management tools that will allow you to edit existing parts, create new parts, draw symbols, and generate footprints. You can get started finding, auditing, improving or creating your keystone components now.
Once your components are finished you can move on to using them in your schematic capture or PCB layout design tools.