Date Code
Reading and Decoding the YYWW Convention
The dominant date-code format in the semiconductor and passive-component industries is the four-character YYWW string, where the first two digits are the final two digits of the calendar year and the last two are the ISO 8601 week number. A marking of 2615 therefore means the part was assembled in week 15 of 2026, which falls in mid-April. Because the year field is only two digits, the code rolls over every century; a part stamped 0512 could be week 12 of 2005 or 1905, a distinction that almost never causes confusion in practice but does require care when handling legacy or NOS inventory. ISO weeks run from 01 to 52 in most years and reach week 53 in years whose calendar carries an extra week, such as 2020 and 2026.
No single standard mandates one ordering, so manufacturers diverge. Many follow YYWW, some reverse it to WWYY, and others prepend a plant, fab, or assembly-site letter such as a leading A or P before the four digits. A handful of vendors still use a single-digit year plus a three-digit Julian day, and ceramic capacitor makers sometimes encode the year as a letter (A for 2010, B for 2011) followed by a month letter. The only reliable way to decode an ambiguous marking is to cross-reference the manufacturer datasheet or marking specification rather than assuming YYWW. For RF assemblies that combine die from multiple suppliers, the assembly date code on the package may differ from the wafer date code recorded internally.
A date code is not the same as a lot code. The date code answers when, normally to the week; the lot code answers which batch, tying back to a specific wafer run or assembly travelers. One date code can span several lots, and a single lot may be marked across two adjacent weeks. High-reliability and space-grade parts carry both, with the lot identifier captured on the certificate of conformance and barcode label for full genealogy, while the date code on the body serves as the fast visual screen during incoming inspection.
Shelf Life and Solderability Triggers
For RF components the date code is most consequential as the clock that governs solderability and storage limits. As parts age, tin and tin-lead lead finishes oxidize and copper-tin intermetallic layers thicken, which degrades wetting during reflow. Industry practice, codified in JEDEC J-STD-002, is to re-test solderability once a part passes roughly 24 months from its date code, and to bake or reball if it fails. Moisture-sensitive surface-mount devices add a second age-related limit through their floor life and bake requirements. Procurement specifications frequently cap the allowable age, with a common rule being a date code no older than 24 months at receipt for commercial work and tighter windows for flight hardware.
Counterfeit Screening Using Date Codes
Date-code analysis is a frontline counterfeit-detection tool. Genuine production reels show a natural spread of date codes or a tight cluster consistent with a single run, whereas remarked or relabeled counterfeits often display implausible codes: a week number above 53, a future date, identical codes across parts that should differ, or codes that predate the part's known introduction. SAE AS6171 and AS5553 incoming-inspection flows treat anomalous date codes as a trigger for deeper testing such as XRF, decap, and electrical screening. The example below shows the arithmetic used to convert an ISO week to an approximate calendar date during inspection.
Year = 2000 + YY (for codes after 1999)
Approx. date ≈ Jan 1 of Year + (WW − 1) × 7 days
Age in months from date code:
Agemo ≈ (Yearnow − Yearcode) × 12 + (Monthnow − Monthcode)
Solderability re-test trigger (J-STD-002):
Re-test required when Agemo > 24
Example: code 2418 → Year = 2024, WW = 18 → approx. May 1, 2024. Evaluated June 2026, Agemo ≈ 25 → solderability re-test required.
Common Date-Code Formats Compared
| Format | Example | Encodes | Resolution | Where Seen | Ambiguity Risk |
|---|---|---|---|---|---|
| YYWW | 2418 | Year + ISO week | 1 week | ICs, MMICs, connectors | Low |
| WWYY | 1824 | ISO week + year | 1 week | Some passives, EU vendors | Medium (vs YYWW) |
| YYMM | 2405 | Year + month | 1 month | Modules, cables | Medium |
| Y-Julian | 4123 | Year digit + day 001 to 366 | 1 day | Older mil-spec parts | High |
| Letter year | P15 | Coded year letter + week | 1 week | Ceramic capacitors | High (needs key) |
Frequently Asked Questions
How do I read a YYWW date code?
Split the four digits into two pairs. The first pair is the last two digits of the year and the second pair is the ISO week, 01 to 52 or 53. So 2418 is week 18 of 2024, around the first week of May. Watch for vendors that reverse the order to WWYY or add a plant letter, and always confirm against the datasheet. Week 53 appears only in calendar years that carry a 53rd ISO week, such as 2020 and 2026.
What is the difference between a date code and a lot code?
The date code says when a part was made, usually to the week; the lot code says which production batch or wafer run it came from. One date code can cover several lots, and a lot can span two marking weeks. Under MIL-PRF-19500 high-reliability parts carry both: the date code on the body for fast visual screening and the lot ID on the certificate of conformance and barcode for full genealogy during failure analysis and recalls.
Why do RF distributors enforce a date-code window when buying parts?
Buyers commonly require a date code no older than 24 to 36 months. Older finishes oxidize and intermetallics grow, so JEDEC J-STD-002 solderability re-testing kicks in past roughly 24 months, and moisture-sensitive parts have age-linked floor-life limits. A tight window also flags counterfeits, since implausible or uniform date codes across a reel are a classic remarking signature caught at incoming inspection.