Heavy Copper PCB Marking: Date Code, Serial and Lot Traceability
Why marking is harder on heavy copper
On a standard board, the legend is a printing step nobody thinks about. On a heavy copper PCB, the surface is not flat. Thick traces stand well above the laminate, and the solder mask over and between them is uneven. Legend ink printed across a copper edge can break, smear or end up thin on the shoulder of a trace.
The result is a familiar complaint at incoming inspection: the date code is there but half of it is unreadable, or the serial label was placed over a copper step and peeled after reflow. When a field failure arrives months later, that marking is the only link between the board and its production lot.
So on heavy copper boards, marking is a layout and specification decision, not a default left to the shop.
What the marking has to carry
Decide which identifiers you actually need to resolve later. Most buyers pick from this list:
| Identifier | What it resolves | Who usually defines the format |
|---|---|---|
| Manufacturer mark | Which supplier built the board | Supplier (it is their mark) |
| Certification mark (for example UL) | Which listed construction applies | Supplier, within its own listing |
| Date code | Build week or period | Buyer states the format, or accepts the supplier's |
| Lot / batch code | Which production lot | Supplier's system, mapped in the shipment documents |
| Part number and revision | Which design revision | Buyer |
| Unique serial or 2D code | Which individual board | Buyer, if unit-level traceability is required |
A date code alone traces to a period, not a lot. If more than one lot can share a week, ask for a lot code too, or confirm in writing how the supplier maps date code to lot. A unit serial is only needed when you will track individual boards through assembly, test or field return.
Certification marks are the supplier's to apply. Only ask for a certification mark if the supplier has a listing that covers your construction, and ask for the listing evidence rather than assuming it.
Where to put it
Three rules cover most heavy copper layouts:
- Put marking on flat laminate or on a flat copper pad, never across a copper edge. Reserve a marking zone in the layout, the same way you reserve space for a logo.
- Keep it on the board, not on the panel rail. Marking on the breakaway border leaves with the border at depanelization.
- Keep it clear of what assembly will cover. Heatsinks, busbars, press-fit hardware and conformal coating masks all hide or damage marking.
Show the zone on the fabrication drawing or as a dedicated layer in the data package. If you leave the location open, the marking lands wherever there is room.
Which method to specify
| Method | Strength on heavy copper | Watch for |
|---|---|---|
| Legend ink (silkscreen or inkjet) | Standard; inkjet can add variable data such as serials | Breaks over copper steps; place on flat areas |
| Etched copper text | Part of the copper layer, very durable | Fixed text only, so a date code needs a data change or a reserved field; spacing rules for thick copper apply |
| Mask-defined text (opening in solder mask) | No extra print step | Low contrast on some finishes |
| Laser marking on mask or a label pad | Can carry a unique serial or 2D code | Ask whether the supplier offers it; define the code content |
| Adhesive label | Easy to add late | Must survive reflow and cleaning; avoid uneven copper areas |
Copper text on a heavy copper layer follows the same spacing limits as heavy copper traces. If you plan etched text, check it against the trace width and spacing limits by copper weight before release. Where legend sits on top of the mask, mask coverage over thick copper matters too, see solder mask coverage on heavy copper.
Who defines the format
Format ownership is where most traceability gaps start. A workable split:
- Buyer defines: which identifiers are required, the marking zone, part number and revision text, the date code format if you need a specific one (for example YYWW or WWYY), and serial or 2D code content if used.
- Supplier defines: its own manufacturer and certification marks, its lot code scheme, and the method it will use, which you then approve.
- Both agree: how lot code maps to date code and to the shipment documents, and what happens if the marking is unreadable at incoming inspection.
If the buyer says nothing, the supplier's default applies. That default may be fine, but you should know what it is before the first order, not after the first failure.
Readability and acceptance
State when the marking must be readable: after surface finish, after assembly reflow and after cleaning. Say how you will treat a board with missing or unreadable marking. Common options are reject, accept with a deviation, or accept if the lot can still be identified from packaging. Pick one and write it down. Board marking and identification are addressed in IPC-6012; reference it by name if you want a shared vocabulary, but the standard does not choose your content, location or format for you.
The marking on the board links to the documents in the box. How the certificate of conformance and lot records connect to that marking is covered in the shipment document package guide.
PO clause you can adapt
> Board marking. Each board shall carry [manufacturer mark] [date code in format ____] [lot code] [part number and revision] [unique serial / 2D code: yes / no], located in the marking zone shown on drawing [number, revision], applied by [method], and readable after surface finish, assembly reflow and cleaning. Marking shall not cross copper edges. The supplier shall state how lot code maps to date code in the shipment documents. Boards with missing or unreadable marking shall be [rejected / held for disposition].
Fill the brackets before you send the RFQ. This clause sits alongside the rest of your heavy copper purchase specification.
What to send with the RFQ
- Fabrication drawing or layer showing the marking zone.
- The list of required identifiers and any date code or serial format.
- Preferred marking method, or a request for the supplier's proposal.
- Readability conditions and the rule for unreadable boards.
Send the package through the quote page so the supplier can confirm the method before quoting.
FAQ
Can legend ink be printed over thick copper traces?
It can be printed, but it often breaks or thins at copper edges. Reserve a flat area for any text that must stay readable.
Is a date code enough for traceability?
A date code traces to a build period. If several lots share a period, also require a lot code or a written mapping from date code to lot.
When do I need a unique serial or 2D code?
Only when you track individual boards through assembly, test or returns. It adds data handling on both sides, so require it on purpose.
Can I ask for a UL mark on my heavy copper board?
Only the supplier can apply its certification mark, and only if its listing covers your construction. Ask for the listing evidence.
Who decides the marking method?
The supplier proposes, the buyer approves. Write the approved method in the PO so it stays the same on reorders.
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