Component Definition 

There are two fundamental classes of components which are differentiated by the party that maintains control of the specifications for the component.

  • COTS (commercial off-the-shelf) component specifications are maintained by their manufacturer (a 3rd party) and are published on a datasheet for use by Engineering to determine suitability for use.  Integrated Circuits, Valves, Fitting are examples of COTS components.
  • Custom component specifications are published by your engineering department.  The specifications contain all the information necessary for your supplier (PCB Assembly house, Machine Shop, etc.) to deliver the part to your design.  If you are using a contract manufacturer, it is the full set of documentation that is the “specification” to which the CM will build your product.  It is still published and controlled by your engineering department.


See Related Articles

A Word about Suppliers
A Word about 3D Printing


It is not strictly necessary that this detail be in the BOM if the BOM has links to this data via a unique identifier, typically your part number.

  1. Commercial-off-the-shelf (COTS) components. These are parts that are bought based on a 3rd party manufacturer specification. The manufacturer will sell a component based on a specification or datasheet that the manufacturer publishes. These parts are controlled by the manufacturer, and you are relying on their quality system to assure that the part you receive meets the datasheet specifications you used to choose the component. To fully define this part, you need to specify:
    1. Manufacturer
    2. Manufacturer’s Part Number

    If the part is configured or has any special requirements that are not detailed in the Part Number (e.g., special fittings, terminations, etc.), then a purchase specification document would be appropriate where you detail the Part Number and the special features. Some companies require a datasheet with the appropriate configuration marked on the datasheet to identify the version of the component. The goal is to be clear as to the specification of the part. Most of the time a Manufacturer and Part Number is sufficient, and the datasheet can be downloaded as needed. Be careful specifying a part by the supplier (instead of the manufacturer) as there may not be enough control over the component specification. See “A Word about Suppliers”.

    It is also a good idea to designate alternate components that are pre-approved by Engineering so that procurement is not impacted by component shortages. Admittedly, this is seldom done until the specified part is gone and then there is a scramble to identify a new part to maintain production progress. This problem can be complicated by regulatory requirements that may mean the new part must be qualified and approved before the substitution can be made; another good reason to identify alternates early.

    Finally, note that most COTS parts have a statement in their datasheet that the specification may change. If you are relying on the part for safety reasons, be sure the part has been approved by a 3rd party Nationally Recognized Test Lab (NRTL e.g., UL, TUV, ETL). This will give you confidence that the part will continue to meet safety requirements. If the part isn’t approved by an NRTL, then you will have to monitor the manufacturer and any released information on the part specification to be sure the part does not change. Many manufacturers/distributors will notify you of upcoming changes so that you can plan. In critical component cases, you may need a formal contract to ensure you have adequate notice.

    For most COTS parts, the manufacturing group will need the Manufacturer and Part Number to identify the part. Some COTS parts may need a Purchase Specification. The riskier the product (consider a medical device), the tighter the specifications.

  2. Custom Components. These parts are components and subassemblies where you are in control of the specification for the component. There are many types of parts that fit this category (e.g., PCB Assemblies, sheet metal parts, machined parts, molded parts, labels, etc.) but the common thread is that there is a document which details the design and tolerances of the component. The good news is that you have total control over a custom part because you can identify all the features of the part that are important to your design. The bad news is that you must identify all the features of the part that are important to your design, to have that control. If you leave off the wrong detail, eventually it will bite you and the design intent will not be reflected in the parts you receive.

    Below is a list of the types of documents that are necessary for different types of custom parts:

    1. Printed Circuit Board Assemblies – a typical PCBA consists of (1) custom part, the PCB Fabrication, to which you install multiple COTS components (e.g., connectors, integrated circuits, resistors, capacitors, fuses, etc.).
      1. To document the Assembly, you need:
        1. Bill of Material with:
          1. Part description (e.g., Capacitor, 0.1uF, 35v, 0805)
          2. Quantity
          3. Part Reference Designation (e.g., C1, C2, C9, etc.). This designation tells the assembler what locations to install the component.
          4. Part Manufacturer
          5. Manufacturer’s Part Number (and alternates, if any)
        2. Pick and Place file – this is used by automated placement equipment to position the component in the correct place and orientation.
        3. Assembly Drawing with any notes:
          1. Special components of processes (swage nut location and installation detail, heatsink installation, wires soldered to the board, etc.).
          2. Assembly Standards (e.g., IPC-A-610, Class 1,2,3)
          3. RoHS compliance process requirements for lead-free processes.
        4. To document the PCB Fabrication, you need:
          1. PCB Artwork files – this may be an electronic file (e.g., ODB++) or set of Gerber files detailing the artwork used for manufacture of the PCB. The artwork usually details:
            1. Silkscreen requirements – printing on the top and bottom of the PCB.
            2. Solder Mask – this allows solder to flow in certain areas and not in others. It is used to control the solder flow on component pads.
            3. Copper layer artwork showing the trace routings and vias for every layer. A (4) layer board will have (4) artwork drawings, one for each layer.
            4. Drill file – details the size hole for through-hole component leads, vias, connectors, etc.
          2. PCB Detail drawing – this document details the fabrication of the PCB including:
            1. Board Dimensions. If necessary, also include details on palletizing the boards.
            2. Board thickness, stack-up and tolerances. This will detail the overall thickness of the PCB (e.g., 0.062 is a typical PCB thickness), inner layer distances and core thicknesses.
            3. Copper Weight (thickness) for each layer. Typically, ½ oz to 2 oz for normal PCBs. Copper thickness is 1.4 mils / oz.
            4. Hole size, tolerance, and plating requirements.
            5. Material specifications (e.g., FR4, G10, UL 94V-0 flammability, etc.)
            6. Warp and Twist tolerances (e.g., 10 mils/inch)
            7. Solder Mask type, color, and tolerance (e.g., SMOBC, Green, 20 mils etch to mask registration)
            8. Silkscreen type and color (e.g., Non-conductive ink, white)
            9. RoHS compliance requirements should be stated if applicable.
            10. Board finish plating requirements (e.g., ENIG – electroless nickel immersion gold, Lead-Free HASL – Hot Air Solder Level, etc.)
            11. Board fabrication standards (e.g., IPC-A-600, Class 1,2,3)
    2. Sheet Metal Parts – Sheet metal parts must have a drawing which details:
      1. Material type (e.g., 5052 Aluminum, A1008 CRS)
      2. Dimensional tolerances. Sheet metal parts can have tight hole-hole and hole size tolerances (+/- 0.005”), but it is difficult to hold a tolerance tighter than 0.02 around a bend. Save the tight tolerances for where your design requires and relax others to save cost.
      3. Finish Specifications (e.g., processing notes, time-save, clear anodize, prime and paint)
      4. Paint specification and texture – paint manufacturer and part number. This can be difficult to get consistent and there may be batch-to-batch variation. We suggest making color chips and sending them to the metal shop, painter, and your QA for checking color against a standard.
      5. Deburr and Break Sharp Edges – important to prevent customer or assembler injury as well as tubing, wire, and cable damage in the assembly. In a typical manufacturing process, multiple people will handle this material, it’s important the risk of getting injured by handling it is as low as possible
      6. Added components like swage nuts, the part number, location, and installation side.
      7. Identify critical dimensions necessary for QA inspection and records
      8. Include a CAD STEP file with the drawing to allow automated programming of the part.
      9. Silkscreen/ pad printing information and its location
      10. The part and any finish operations should be manufactured using RoHS / REACH compliant Materials
    3. Machined Metal or Plastic Parts – these are the typical custom machined parts that are ubiquitous.
      1. Material type (e.g., 6061-T6 Aluminum, Acetal, 316 SS, etc.)
      2. Dimensional Tolerances – machined parts can have very tight tolerances depending on material, but your cost can increase significantly as tolerances tighten.
      3. Thread depth and type
      4. Deburr and break sharp edges
      5. Masking (e.g., mask certain threads before plating, or mask certain areas for grounding)
      6. Finish Specification (e.g., clear, or black anodize, irridite, etc.)
      7. Plating
      8. Surface finish detail on sealing surfaces if required
      9. Other details (e.g., Heat Treating, dimensions “before” or “after” plating, etc.)
      10. Drawing standard (e.g., ISO or ANSI)
      11. Include a CAD STEP file with the drawing to allow automated programming of the part.
      12. The part and any finish operations should be manufactured using RoHS / REACH compliant Materials
    4. Molded or Cast Parts – these items are fabricated by injecting thermoset plastics or metals into a mold. To use this process there are design standards to follow to yield parts that are not susceptible to heat sinks, voids, and other defects. Also consider the part must be able to be removed from the mold, so there are considerations for draft angles and pull directions. The manufacturing documentation must include:
      1. Material Type (Aluminum grade, ABS type, etc.)
      2. Include a CAD STEP file with the drawing to allow automated programming of the mold.
      3. Tolerances for critical dimensions, warp, and twist
      4. Notes for trimming of flash
      5. Notes for location of gate marks and how they are treated.
      6. Surface texture
      7. Material color. This is harder than it may seem, and the color can vary from batch to batch. We suggest using color chips that are split up between the resin supplier, molder, and your QA to keep control of the color variation.
      8. Post-molding operations – installing threaded inserts, post machining
      9. Over-molding – many times there is a compound part with varied materials/colors applied. An example is an elastomeric grip handle on an ABS part.
    5. 3D printed parts – See “A Word about 3D Printing”. The documentation or these parts is similar to Molded Parts. Some sections would not apply (e.g., gate marks) and the design standards are different (e.g., draft angles don’t apply), but there are other requirements for 3D printing:
      1. 3D printing technology to be used
      2. Material specification
      3. Support parameters and support removal
      4. Orientation in machine
      5. Cleaning / curing / heat treat
      6. Post print finishing (sanding, painting, etc.)
    6. Cable Assemblies – cable assemblies seem easy but can be very sophisticated. Wires and connectors tend to be the #1 quality problem in a product and where able, they should be built with automated and calibrated equipment. They are documented like other assemblies, calling out material and quantity, but they have additional requirements:
      1. Labeling – depending on requirements, you may need labels on each end of the cable labeled for jack location (where the cable attaches) and a label identifying the cable part number/revision.
      2. A schematic assembly which details which wires/terminals/connectors are on each end and how the wires are connected throughout the assembly. This can many times be handled in a Wire Run List for the cable.
      3. Quality Standards (e.g., IPC-A-620 Class 1,2,3)
      4. Special Tooling – crimp pins are designed to be installed with special tools. Although it looks like you may be able to crush the pin onto a wire with a pair of plyers, to make a connection that will work reliably for the product life, you must use the specific, calibrated crimp tool.
    7. Software / Firmware – There are many different pieces of software that may be necessary on a product, and software that is required for installing that software or firmware. They include:
      1. HEX or ELF files for loading into a microcontroller
      2. The programming software that loads the HEX or ELF file onto a specific microcontroller
      3. Test Software
      4. User Interface Software
    8. Documentation of software should include:
      1. The file(s)
      2. The programming software if the file is not an installer
      3. Instructions for loading the file into the microcontroller or computer including any configuration parameters (e.g., serial numbers or addresses).

I hope this gives you some idea of the breadth of information necessary for manufacturing. Not all this documentation is required for every product, but consider that the more documentation present, the clearer and more consistent the product manufacturing.