Gerber files describe the copper layers, solder mask and silkscreen of a circuit board, but they say very little about the components that will sit on it. Many engineers who order custom PCB assemblies for the first time assume these files are enough, only to receive a list of questions that delays the build. Assemblers need a fuller picture of what goes where, how it should be handled and what the finished board must achieve. Preparing this information in advance helps quotes arrive faster and reduces the risk of errors on the production floor.
A Complete Bill of Materials
The bill of materials (BOM) tells the assembler exactly which parts to buy and fit. A clear BOM usually includes the following details:
- Reference designators that match those on the board layout, such as R1, C5 or U3.
- Manufacturer names and part numbers, not just generic descriptions like “10k resistor”.
- Quantities per board and the package or footprint for each part.
- Parts marked as “do not populate” (DNP), so the assembler knows to leave them off.
- Approved alternative parts in case the preferred component is out of stock.
Pick-and-Place Data
Automated assembly machines rely on precise coordinates to place each component. Most CAD tools can export this data, which is often called a centroid or XY file and generally contains the following:
- The X and Y position of each component relative to a defined origin point.
- The rotation angle, which ensures that polarised parts are placed in the correct orientation.
- The side of the board on which each component sits.
Assembly Drawings and Special Instructions
Some requirements cannot be captured in data files alone. Assembly drawings and written notes help clarify details such as these:
- Polarity markings for diodes, capacitors and LEDs, along with pin 1 indicators for chips.
- Mechanical parts, such as heat sinks, standoffs, screws or connectors fitted by hand.
- Processes such as conformal coating, adhesive application or potting, including areas to keep clear.
Build Quantity, Timing and Parts Sourcing
Commercial details shape how the assembler plans and prices the job. Prototype PCB assembly services in particular often handle small batches with tight deadlines, so clear information on these points avoids back-and-forth before work begins:
- The number of boards required and whether future production volumes are expected.
- The target delivery date and whether an expedited service is needed.
- Whether the assembler should source all parts (turnkey) or whether some will be supplied by the customer (consigned).
- Whether a partial build is acceptable if certain components are delayed.
Testing and Quality Requirements
The assembler also needs to know how the finished boards will be judged. Sharing quality expectations upfront usually involves the following:
- The acceptance standard, such as IPC-A-610 Class 2 or Class 3.
- Firmware files and instructions if the boards need to be programmed before shipping.
- Functional test procedures or test fixtures, where applicable.
- Inspection requirements, such as automated optical inspection or X-ray for hidden joints.
- Whether flux residue must be cleaned from the boards after soldering.
Packaging and Compliance Details
How the boards are delivered matters as much as how they are built, especially for products heading into regulated markets. Customers can specify points such as these:
- Compliance requirements, such as RoHS or other material restrictions.
- Antistatic packaging and labelling preferences.
- Serial number or batch labels for traceability.
- Moisture barrier bags for boards that will be stored before use.
- Documentation to be included, such as inspection reports or certificates of conformance.
Sending a Complete Package From the Start
Gerber files are only one part of what an assembler needs to build a board correctly. Supplying a detailed BOM, placement data, drawings, commercial details, quality requirements and packaging instructions together reduces delays and helps the first build match the design intent.
Reach out to MPN Tech and let us assist you in bringing your custom PCB design to life.
