SMT Assembly Data Package: Gerber, BOM and Placement

A surface-mount build starts long before the first panel is printed, and it starts with data. When the assembly package is incomplete, the factory stops, asks questions and waits, and the lost time lands directly on the delivery date. A complete SMT assembly data package removes that risk. It also protects the customer, because a well-defined package answers most questions up front and keeps design intent out of the hands of anyone who does not need to see it. The list below covers what a contract assembler actually needs, and why each item matters.

Why the data package decides the schedule

An assembler has to build a stencil, program the placement machine, verify the material set and write a test plan before production can start. Each of those tasks depends on a different file. If one is missing, the work stops at that step rather than proceeding in parallel, and a two-day clarification can push a whole batch. Supplying the complete set at the request-for-quote stage is the single most effective way to shorten the front end of a project. It also makes quoting more accurate, because the assembler can price the actual material and the actual placement count instead of guessing. The same package defines the first-article inspection plan, so the customer and the assembler agree in advance on what a correct build looks like. A package that is complete on day one therefore shortens the schedule twice: once at quotation and once at first article.

Gerber files and the layer set

The Gerber set is the geometric description of the board, and the assembly package should include all of it rather than only the copper. The pad layer defines where solder paste and components go, the solder mask layer defines the openings that keep the mask off the pads, the silkscreen layer carries the reference designators, and the stencil layer defines the paste apertures. The assembly house needs the pad, mask, silkscreen and stencil layers to cut the stencil and to write the placement program, so those four cannot be omitted. Include the board outline layer as well, since panel dimensions and fiducial placement depend on it. Keep the files in a standard format with a documented unit and datum, and check them against the manufacturable PCB design guidelines before release, because a Gerber set that cannot be fabricated cannot be assembled either.

Bill of materials

The bill of materials must map one line to one physical part. Every row should carry a reference designator list, a manufacturer part number, a description and a package or footprint identifier, and the assembly package is only as reliable as that mapping. Where a part is available from more than one source, list the approved alternatives explicitly instead of leaving the substitution to the buyer. Where a part is not to be substituted, say so. Two errors cause most of the delays here: a reference designator that appears in the bill of materials but not on the board, and a footprint in the CAD library that does not match the part ordered. Both are cheap to catch at the data review stage and expensive to catch on the line.

Pick-and-place file and coordinate conventions

The pick-and-place file tells the machine where each component sits and how it is rotated. Supply it as a text or spreadsheet file, in metric units with millimetre coordinates, and state the origin explicitly. The usual convention places the origin at the lower-left corner of the board so that every coordinate is positive, which avoids sign errors in the machine program. Rotation conventions also need to be documented: define what a rotation of zero degrees means for each package type and whether the file is mirrored for bottom-side placement. When a board is assembled in a panel, confirm whether the coordinates refer to the single board or to the panel, because the assembler needs the correct set to program the machine. Ambiguity in these three conventions, units, origin and rotation, is the most common source of mis-placed components.

Placement drawing

A placement drawing is essentially the pad and silkscreen layers of the board printed as a reference view, and it does two jobs. It lets the programmer confirm that the machine coordinates match the intended positions before the first article is built, and it gives the line a visual check during setup and first-article inspection. Mark the top and bottom views clearly, include the origin and the fiducials, and add a table of reference designators with their values. On dense boards, annotate the drawing with the orientation of polarized parts, because a reversed diode or electrolytic capacitor found at first article is far cheaper than one found after a full batch has been reflowed.

Assembly and test instructions

Not every special requirement fits in a Gerber file. A short assembly instruction document captures them: a component that must not be washed, a connector that must be masked before conformal coating, a part that is fitted after the second soldering step, or a specific torque for a screw terminal. Add the target solder paste type and any restriction on flux chemistry. The test instruction belongs in the same package, since the assembler needs to know which parameters to verify, what limits apply and what equipment is expected. Where functional test is not required, say so explicitly rather than leaving it blank; an empty field is read as unknown, and unknown becomes a question. Documenting the assembly sequence as part of the PCBA development process keeps the instructions current instead of accumulating on a separate sheet.

Confidentiality and partial release

Design data is valuable, and a full release is not always necessary. Where the product is still in development, the customer can extract only the layers the assembler needs, which is usually the pad, mask and stencil set rather than the complete inner-layer artwork. A signed confidentiality agreement supports the same goal from the other direction. Either approach lets the factory do its job without holding the complete design, and both are normal practice. The one thing to avoid is withholding a file the assembler genuinely needs in order to protect it; that trade only moves the risk from confidentiality to schedule. gopcb reviews the data package with the customer before quotation so that both sides agree on what has been supplied and what is still outstanding.

Finally, version control the package. State which revision of each file applies to the order, and keep the design and fabrication records together so that a change to the board and a change to the bill of materials never travel separately.

FAQ

Do I need to send the full Gerber set for assembly? The assembler needs the pad, solder mask, silkscreen, stencil and outline layers to cut the stencil and program the machine. Inner-layer artwork is usually not required for assembly and can be withheld if confidentiality is a concern.

What units should the pick-and-place file use? Metric units with millimetre coordinates, and an origin stated explicitly, normally the lower-left corner of the board. Document the rotation convention and whether the coordinates refer to a single board or to the panel.

What happens if the bill of materials and the board do not match? The assembler must stop and resolve the discrepancy, which costs production time. Run a comparison between the reference designators in the CAD file and the bill of materials before releasing the package.