Are you new to printed circuit boards (PCBs)? Have you wondered what the markings mean on these complex boards? Are these markings intended to identify the components on the board? This post aims to help understand some aspects of the design process of a board and identifying components. If you aren’t familiar with the different letters presented on a circuit board, refer to this forum post. This is a good starting point as I will be referring to these markings.
Reference Designators
There are usually a lot of little markings on a full printed circuit board design. The most common are in a layer called the “silkscreen.” As you saw in the post above, the most common one is called a reference designator. These numbers are typically generated sequentially by a PCB designing software like Altium, KiCAD, Eagle, and many more. Not all manufacturers begin with “1” as they may want more unique IDs for each part. They may choose to start somewhere else. For example, “100” may be the start of a design file. A manufacturer may even throw some letters inside the format. It may be possible to have a system where the number keeps increasing sequentially across the board and not start over for each type of part, but this is not as common. The key point is that the beginning letter matters the most. This usually gives a clue as to what kind of part it may be.
Are Designators Tied to Part Identity?
It’s possible to confuse the board marking with the individual part marking. The designator on the board isn’t intended to define the component directly but rather inform you the order in which the parts were placed in the design. The designators are supposed to be in line with something called a bill of materials (a.k.a BOM). A bill of materials is a list of all components used in a design. Let’s look at a reference design to make this a little more clear.
Texas Instruments TIDA-010239
I wanted to see if any of our vendors had board designs that were open to the public. Texas Instruments made an “AC level 2 charger platform” that has an open design. First, take a look at the assembly drawing. This shows all the relevant components with their designations. Each component is shown in approximately the same location as it appears on the physical board. Here is a random snippet of components in the top right corner of the top side of the board:
How do we identify J15, U20, C114, and F4? Look at the bill of materials. The left column has a list of designators and the corresponding part numbers are found in the third column. You can use CTRL + F to quickly search for these labels to know what you are looking for.
J15 Identification
J15 is the 15th Jack placed in the design file. J15 is found on page 3, the manufacturer part number listed is 691101710002 which is from Wurth. The DigiKey part number is 732-2026-ND.
U20 Identification
U20 is the 20th integrated circuit placed in the design file. U20 is found on page 7. The manufacturer part number is DRV8220DSG. The DigiKey part number is 296-DRV8220DSGRCT-ND.
C114 Identification
C114 is the 114th capacitor placed in the design file. C114 is found on page 2. Note that this part is listed amongst other designators, that means all these other parts have the same part number. The manufacturer part number is CC1210KKX7R8BB226. The DigiKey part number is 13-CC1210KKX7R8BB226CT-ND.
F4 Identification
F4 is the 4th fuse placed in the design file. F4 is found on page 3. The manufacturer part number is 044001.5WR. The DigiKey part number is F1611CT-ND.
Note that some of the components show a “_P” at the end. This may have been added by the manufacturer to make the labels shorter, so they didn’t have to keep adding to the component count for that designator.
The Problem of Released Boards
An open-source board like the one above is relatively easy to identify what component is what. The issue is that most designed boards do not publicly release the bill of materials due to the proprietary nature of designs. Without the bill of materials, the designators only tell you what order they were placed and a generic type of part. It’s very rare to ever see a part number printed on the silkscreen as there usually isn’t any room for that. Some boards have such tight tolerances that identifiers are left out of the question. A single revision of a board should have the same markings, but it’s not guaranteed to remain the same if a new revision is released. Designators may change based on removed or added components. There are some general tips to help identify components that are missing these markings:
- Look for a part marking on the body of the part. This may help identify it directly.
- Look at the surrounding parts and size of surrounding parts. If there are transformers and larger capacitors nearby, then you may most likely be looking a power supply section of the whole circuit.
- Try to find a bill of materials when possible. This is usually a long shot.
- Try to test the component with a multimeter if possible.
- See if the Forum has asked about a particular board or component.
- Some boards may have outlines printed on the silkscreen. These may help figure out sizes of integrated circuits or other components.
If you have any other questions about printed circuit boards, please let us know.