Skip to content
WhatSMDSign in

SMD marking codes explained

A marking code stands in for a part number that does not fit on the body. It names the part only once you know the package, and often the circuit around it as well.

Last reviewed

What the code is

A SOT-23 body is about 2.9 by 1.3 mm. A part number such as MMBT3904 will not fit on it at a size anyone can read, so the maker prints a short code, most often two or three characters and lists it in the datasheet, usually in a table headed Marking or Ordering information.

Nobody hands these codes out. Each maker picks its own, and a code means something only beside that maker's datasheet. Another maker may print the same characters on a different part, or a different code on the same part.

Why one code fits many parts

Two characters drawn from the ten digits and the 26 capital letters allow 1,296 codes. Every maker of small parts draws from that same short list, so the same pair turns up on unrelated parts.

Second sources work the other way. A maker that copies a popular part usually copies its code too, which is why many makers print 1AM on the MMBT3904. Other makers give a well-known part a code of their own: in our tables, BAT54 in SOT-23 is printed with 15 different codes. These are the ones the most makers use:

Read the package, then decide what is code

Settle the package before the code. The same characters mean different parts in different packages, so the package shortens the list first. Count the leads and measure the body; SMD package sizes has the common ones side by side.

Then decide which characters belong to the code. Makers add marks that are not part of it:

  • A small letter straight after the code, as in 1Ap, is often a date or factory mark. WhatSMD's search reads a small last letter that way.
  • A second line under the code is usually a lot or date code.
  • Some datasheets write the code with a star, as in A7*: the star stands for a letter that names the factory.

Code lists then record the result as if it were a code of its own. In our tables the BAV99 in SOT-23 is printed A7s, A7W and A7p as well as A7, and every one of them is the same part.

A third character can still be part of the code. A76 is a code of its own: in our tables it is printed on 7 parts, among them BSS123, OPA374AI, OPA374AIDBV and PJA3476, while A7 is printed on 264 parts.

If two identical parts on one board differ only in their last character, that character is most likely a date or lot mark. If you cannot tell, look up both readings.

Worn characters are easy to misread, 0 for O and 5 for S among them; How to identify an SMD component lists the usual pairs.

Worked examples: 6H, 1AM and A6

Each figure here is counted from our tables, so it changes as they grow.

  • 6Hin SOT-23

    11 parts in SOT-23 from 14 makers. In every package: 82 parts in 16 packages.

    Kinds of part in SOT-23

    • 2 ICs
    • 2 transistors
    • 1 diode
    • 1 MOSFET
    • 5 with no kind in our tables

    Printed by the most makers

    • MMBF5486, 4 makers
    • BC818, 2 makers
    • FMMV2104, 2 makers

    Its parts here are of 4 different kinds, so the code alone cannot settle it. The circuit decides first: what pin 1 is wired to, and whether the part sits on a supply rail or on a signal line. A meter reading separates what is left.

    All 11 parts printed 6H in SOT-236H in every package

  • 1AMin SOT-23

    36 parts in SOT-23 from 23 makers. In every package: 51 parts in 8 packages.

    Kinds of part in SOT-23

    • 36 transistors

    Printed by the most makers

    • MMBT3904, 14 makers
    • 2N3904S, 2 makers
    • MMBT3904LT1, 2 makers

    Every part here whose kind our tables give is a transistor, and MMBT3904 is the one most makers print with this code. Another maker's MMBT3904 will usually replace it; check the pinout in that maker's datasheet first.

    All 36 parts printed 1AM in SOT-231AM in every package

  • A6in SOT-23

    34 parts in SOT-23 from 36 makers. In every package: 261 parts in 28 packages.

    Kinds of part in SOT-23

    • 25 diodes
    • 4 ICs
    • 2 MOSFETs
    • 3 with no kind in our tables

    Printed by the most makers

    • BAS16, 15 makers
    • MC2838, 3 makers
    • 1S2838, 2 makers

    Most of its parts here are diodes, led by BAS16. The other parts on the list are the reason to check the circuit before you order, because a code that is usually one part is not always that part.

    All 34 parts printed A6 in SOT-23A6 in every package

Where to look a code up

The maker's datasheet has the final word on its own parts: search the part number with the word marking and read the table. When you do not know the part number yet, you need a list that runs the other way, from the code to the parts.

WhatSMD's search runs that way from our tables: the makers' own marking tables first, then published code lists, each part labelled with where it came from. To browse instead, the online SMD code book lists every code by first character, package and part type.

Try it

Search 6H and narrow it to SOT-23, then compare the kinds of part it lists with the example above.