In this post we will review these 2 types of markings and additional identification marks that can be incorporated into your printed circuit board.
Identification Marks
In terms of identification, almost all printed circuit boards have an assigned part number and revision. The part number is the higher-level identifier that says “what” is being made, and the revision is the more detailed identifier that describes “which version” is being made. Unless the customer specifically requests “no markings” the PCB is always identified with at least this information so that someone using or inspecting the circuit board can be sure of exactly what it is.
Sometimes customers choose to add additional identification such as their company name, logo, or other branding as additional types of identification.
Printed circuit board with part markings.
Traceability Marks
Regarding traceability, the goal of the markings is to be able to take a finished PCB and to “trace” it back to the specific manufacturing lot in which it was produced and ultimately to the specific processes it went through, operators that built it, and materials that were used.
The most basic element of traceability is the “Date Code” which typically indicates the week number and year of production for the part. Additional elements of traceability are often useful and may include panel number, board number, lot number, and serial number. The ultimate goal of any of these is to aid in containment, troubleshooting, and process improvement if defects or other unwanted variances occur in the manufacturing process.
Traceability Elements and Standard Markings
Epec’s standard markings include a UL designator (if appropriate for the materials used), the date code, and our logo.
- Date Code: The default date code format is WWYY where YY is the last two digits of the year, and WW is the week number of the year. Other common formats are YYWW, MMYY, YYMMDD, and other similar variations. Different manufacturing facilities may use different conventions for affixing the date code (at release, at plating, at imaging, etc.) but the important part is that the combination of date code and part number will identify the manufacturing batch that the part was made in.
- Logo: The Epec logo is trademarked and helps to ensure that the product came from a genuine Epec manufacturing facility and to quickly identify Epec-sourced parts for customers with multiple sources.
- UL: Epec facilities go to the extra expense of Underwriters Labs (UL) qualification. Circuit boards produced on UL-qualified materials will have their factory code and relevant qualification type included in markings unless prohibited by the customer. UL qualification is done by material type, so uncommon materials may not be specifically UL-qualified but will still be produced with the same equipment and processes as the UL materials.
Additional Traceable Elements:
- Lot: Some facilities have more detailed tracing capability than date code and can track manufacturing at the work order or job/release level. If the customer requests a lot number, this additional tracer will be used, if available.
- Panel #: Individual boards/arrays are manufactured together in a set, arranged to fit on a standard panel size that the manufacturing facility uses. The panel number identifies the specific panel that a board came from and can be useful for tracing a specific defect that was limited to a particular panel or set of panels.
- Board #: The circuit board number identifies the specific PCB from a panel. The combination of board #, panel #, and date code will uniquely identify any final circuit board at a high level of specificity.
- Serial #: A serial number is useful when a traceability application benefits from a single number that uniquely identifies a board from a set. A simple serial number will iterate from 1 to however many boards are in the manufacturing lot, and gaps in serial numbers will indicate undelivered or failed boards from the lot. A compound serial number can take different forms and is generally designed to contain a full set of traceable information in a single string of text designed to be scanned and used in a customer’s automated system. More information on barcode formats can be found below.
PCB with serial number markings.
Markings Location
If the customer specifies an area for markings, Epec will attempt to place all required markings in that area and will inform the customer if this causes legibility problems. If no location is specified, an Epec CAM specialist will place the information in an open area of the PCB that does not interfere with other required PCB legend or features – typically on the bottom side of the circuit board where fewer features and other legend are specified. Markings can also be placed on the array.
Manufacturing Process and Cost Factors
Markings may be applied in the copper, printed legend (silkscreen), or soldermask layers of a circuit board. If the markings are static – they don’t change from board to board and panel to panel – then there is no cost advantage to printing in one way vs. another. If boards are marked uniquely with serials or board/panel numbers, the fastest and lowest-cost method to apply markings is the silkscreen/legend layer. This is because almost all PCB manufacturers have now moved to a digital inkjet printing process for that step, and the process for the printed legend is fast, reliable, and – importantly – very amenable to dynamic data such as serial numbers, panel, board, and lot identifiers.
LPI soldermask and digital imaging for copper resist exposure are also often digital in modern PCB manufacturing, but the equipment and capabilities vary more, and performing serialization at this stage presents more manufacturing challenges. If possible, it is best to put markings in the silk screen layer for the lowest cost and fastest turn when serial or board numbers are needed.
Another consideration for the printed legend is the color. Although the cost difference for different colors of legend ink is often minimal, printing in a color other than white may come with a minimum quantity due to requiring a special color purchase. More importantly to the manufacturing process, most facilities and legend machines are configured to print only in white, necessitating a different process for other colors. The other process is usually LPI (Liquid Photo Imageable) soldermask or silkscreened soldermask where the special color is treated as a mask layer and handled similarly to the soldermask but in a second pass with the special legend color. This all requires additional machine setup and can lead to additional cost and lead time.
For the fastest turn and lowest cost, use green for the soldermask and white for legend & markings. Epec is happy to support other colors and is competitive on price, but customers should understand that the choice has some impact on the process.
Barcodes and Automation
A barcode is simply a means of encoding numbers and text in a form where a machine can easily and reliably read it with a barcode scanner. There are several different 1D (bars) and 2D (dots/squares) barcode formats. The formats differ primarily in how redundant they are (which makes them more resilient to being scratched or damaged), how dense they are (how much information they can encode in a given area), and what characters they support (older formats saved encoding space by more strictly limiting what characters can be encoded).
PCB with bar code markings.
If you don’t care about formats and just have a simple alpha-numeric string of less than 20 characters to encode, we’d suggest using Code 128 for the greatest compatibility and support across different manufacturing equipment. If your needs are more complex, please see the next section for some information on popular barcode formats.
Barcode Format Primer
- 1D:
- 1D barcode formats are older, simpler, and are the most widely supported since they also work with legacy scanning systems that may not support 2D formats. They are less information-dense than 2D formats and have more restrictions on their character sets and the amount of data that can practically be encoded.
- Code 39 - 1D, most limited character set (only supports upper case letters, numbers, and a few symbols), no redundancy. Can be implemented with a simple font. Most useful for limited amounts of alpha-numeric text. Suggested max length of 20 characters.
- Code 128 - 1D, larger character set, suggested max length of 25 characters, supports error correction, probably the most widely supported format since it also works with older 1D-only scanners.
- 2D:
- 2D formats can store much more information than 1D formats but are less widely supported by industrial scanners and less space efficient for small amounts of data. DataMatrix and PDF417 are most commonly used in industry and are largely interchangeable for most use cases. QR has been widely adopted in consumer applications and is supported by most modern phones, which will try to resolve it to a URL on the Web. Modern phones can also read all other barcode types, but an app download may be required.
- DataMatrix – 2D, full ANSI character set, up to 2335 characters, widely used in healthcare, manufacturing, and logistics, more space efficient than PDF417, can be easier to scan at odd angles.
- PDF417 – 2D, full ANSI character set, up to 1850 characters, depending on level of error correction. Widely used in government ID, transportation, and postage. Supported by older scanners than QR and DataMatrix since it works with a linear scan.
- QR – 2D, full ANSI character set, up to 4296 characters, seen often in consumer applications, supported by many phones, which will attempt to open them in browsers.
Additional Considerations
There isn’t a hard size limit, but there is a practical limit based on the effective resolution of the process and the amount of data that you are trying to encode. For serial numbers and small amounts of text, Code128 is usually the most supported, reliable, and space-efficient option. For items where you want to provide a link to a user/customer, we suggest QR codes. For a single scan of multiple fields for industrial operations, we suggest PDF417 or DataMatrix. For all the formats, there is a tradeoff where the more data you encode, the larger (or more dense/hard to scan) the barcode becomes. Another thing to keep in mind with barcodes is that the contrast between the legend and soldermask affects the ability to scan reliably. White legend on green soldermask works well and is a proven combination, but if you want to try a different combination that has less contrast, you should ask a sales engineer.
The normal method is to serialize the Arrays/PCBs on the panel and to serialize the panels so that the combination of date code, panel#, and board# is unique and specifically identifies the board. If you have specific additional requirements around serialization such as maintaining a particular range or something other than a sequential counter, it is best to provide a spec and discuss the specifics with an Epec engineer.
Summary
The main purpose of PCB markings is for identification and traceability. The most common required markings are Part, Revision, Date Code, and Manufacturer UL identifier. Other common markings include the board#, panel#, serial#, and logos or other branding.
Unless you have a good reason not to, it is best to use green soldermask and white ink for the fastest turns and the best price. Markings can be in silkscreen, soldermask, or copper layers, but if serial numbering is involved, it is best to use a silkscreen.
If you aren’t sure what barcode format to use, stick with Code128. If you have more data or want the barcode to be consumer-scannable, use QR. Other formats are fine also if there is a specific reason but sticking with the two most popular. Allowing board# and panel# in the legend is a good practice for better product traceability. Serial numbers across boards do not provide much additional advantage unless there is an application need to capture the information in a single barcode scan, but they can be useful for that use case.
