KEMI Docs · SMT Practice
Preparing for SMT Assembly — BOM and CPL
Last updated: 2026-08-30
A bare-board order needs only Gerbers; assembly needs two more files — a BOM saying what to mount and a CPL (pick & place) saying where and at what rotation. The quality of these two files decides both how fast you get a quote and how low the defect rate is.
1
BOM — six required columns
| Column | Example | Why |
|---|---|---|
| Designator | R1, R2, C3, U1 | The key linking to the CPL; grouped lists must match quantity |
| Value | 10kΩ 1%, 100nF 50V | Basis for substitution decisions |
| Package | 0603 (1608 metric), SOT-23 | Determines nozzles and feeders |
| MPN | exact manufacturer part number | Without it, parts get substituted at will |
| Quantity | 12 | Must equal the designator count |
| Fit | SMT / DNP | Unmarked DNPs trigger 'missing part?' queries |
Mark DNP, don't delete the row
Deleting unfitted parts from the BOM makes empty footprints look like missing material. Keep the row and mark it DNP — that is the convention assemblers expect.
2
CPL — coordinates and rotation
Five columns: designator, X, Y, rotation, side. Export it from your CAD — never write it by hand. The catch: origin location, zero-rotation orientation and bottom-side mirroring conventions all differ between CAD families. Assemblers calibrate against your silkscreen, so always send the Gerbers (silk and paste layers included) together with the CPL.
⚠ Rotation values are never fully trusted
Polarised parts — tantalums, diodes, IC pin 1 — routinely arrive 90 or 180 degrees off due to CAD convention differences. Clear polarity marks on silk are what lets the assembler cross-check. On tiny boards without silk, provide a one-page assembly drawing.
3
Stencil and paste layers
The paste stencil is cut from your Gerber paste layers (GTP/GBP) — do not omit them at export when ordering assembly. Large thermal pads (QFN) usually get reduced apertures to avoid floating parts; mention any special requirements up front.
4
Final checks
- BOM designator total = CPL row count = parts on the board.
- No part present in the BOM but missing from the CPL, or vice versa (usually a THT handling question).
- Every polarised part has a visible silk mark.
- No end-of-life or long-lead parts — checking stock by MPN speeds up the quote.
- State whether THT parts are included and whether hand-soldering is requested.
FAQ
Is there a standard BOM template?
No — any spreadsheet with the six columns works. Fastest route: take your CAD's BOM export and add MPN and DNP columns.
Who sources the parts?
Turnkey (assembler buys), consigned (you supply), or mixed. Prototypes usually go turnkey, with critical ICs consigned. Mark the mode as a BOM column.
Can small quantities be assembled?
Yes — fixed costs (programming, line setup) just dominate the per-board price at low volume. For a handful of boards, compare against a hand-assembly quote.
Can I get an instant quote from a BOM?
Upload the BOM to KEMI's instant quote and assembly labour is priced immediately; parts cost follows in the formal quote after BOM review.
See also
This article summarises the working rules KEMI uses in real design and fabrication work. For production, part datasheets and fab specifications take precedence.