Start with the designator
The silkscreen beside a component tells you what class of part belongs there. That is the first identification step, and it is independent of the package.
| Prefix | Part | Prefix | Part |
|---|---|---|---|
| R | Resistor | U | Integrated circuit |
| C | Capacitor | J / P | Connector / header |
| L | Inductor | Y / X | Crystal / oscillator |
| D | Diode | F | Fuse |
| Q | Transistor / MOSFET | SW | Switch |
| TP | Test point | FB | Ferrite bead |
Two boards can carry the same designator and completely different parts, so the designator identifies the role, not the value. The value comes from the BOM revision and the marking on the package.
Then the package
Package codes are a language of their own, and the same function appears in several of them:
- Passives: 0402, 0603, 0805, 1206 (imperial sizes), with 01005 and 0201 at the small end of what this plant places.
- Small transistors and diodes: SOT-23, SOT-323, SOD-123.
- Integrated circuits: SOIC, TSSOP, QFN, QFP, BGA and CSP. The letter pattern tells you whether the pins are on the sides (QFP), underneath the body (QFN) or in a grid of balls (BGA).
- Electrolytics: a cylindrical can with a stripe, often the tallest part on the board.
A package that is physically smaller than the footprint on the board is a defect waiting for first article; a package that is larger will not sit flat.
Case markings and what they do not tell you
Small passives often carry no marking at all. Larger parts carry a manufacturer code, a lot code and sometimes a date code, but not the full part number — and manufacturers change the marking between revisions.
That is why identification in production is a three-way match: the BOM says what should be there, the marking says what arrived, and the footprint says what the board was designed for. Two out of three is not enough.
What you cannot see
Under a BGA or a chip-on-board glob top, there is nothing to read. Identification there is by position (the designator in the CAD data), by X-ray for the ball pattern, and by electrical test for the connection. This is one reason area-array assemblies depend on first-article inspection and X-ray rather than on visual checks.
A practical check routine
- Read the designator, find the BOM line, confirm the value and package.
- Read the case marking and confirm manufacturer and part family.
- Check polarity marks against the assembly drawing.
- Confirm the footprint matches the package.
- For area-array parts, confirm the ball pattern and placement under X-ray.
Workmanship is judged to IPC-A-610, and the acceptance criteria for the bare board itself are separate — the board is accepted to IPC-6012B before anything is placed. The assembly side is described on the PCBA manufacturing page, and incoming checks in the quality gates note.
