Generating Printed Circuit Board Documentation

 In PCB, PCB Design

Designers generate ideas that fabricators and assemblers subsequently convert into reality. While it is an important responsibility for fabricators and assemblers to faithfully follow the instructions provided by the designer, it is obligatory on the latter’s part to document their ideas in a manner suitable for others to follow.

For getting PCBs or Printed Circuit Boards fabricated and assembled, designers document their requirement through CAD files, Gerber files, Drill files, BOMs, and similar. However, for conveying special instructions, designers use notes, which must be clear and unambiguous, to let the manufacturer envision the designer’s requirements.

PCB manufacturing passes through many stages, and each stage requires specific information. Designers typically prepare the complete package of information before approaching their contract manufacturer. For a smooth operation, the designer must know beforehand the features and limitations of the machines they expect the manufacturer to use. This may require the designer to consult their contract manufacturer before generating the documentation for their project. Prior consultations lead to clear and unambiguous documentation that not only reduces the possibilities of errors and misunderstandings, but also results in accurate quotes, and reduces the turn-over time.

Gerber Files

The primary requirement for PCB manufacturers is the Gerber files for the board. This information allows them to fabricate a board that is very close to the intended design.

Gerber files are 2-D binary vector images describing every single layer of the PCB design. While one Gerber file describes the silk screen, another will describe the top copper layer, and so on. The international standard Gerber files typically include two types—RS-274-D and RS-274-X. Most manufacturers follow the RS-274-X, as this is the extension Gerber format containing all information. Designers use CAD software capable of generating both formats. They use suitable file extensions to allow easy identification of the layers the file represents.

PCB LayerGerber Extension
Top Layer (Copper)NameOfPCB.GTL
Top Silkscreen/OverlayNameOfPCB.GTO
Top Solder maskNameOfPCB.GTS
Top SMD PasteNameOfPCB.GTP
Bottom Layer (Copper)NameOfPCB.GBL
Bottom Silkscreen/OverlayNameOfPCB.GBO
Bottom Solder maskNameOfPCB.GBS
Bottom SMD PasteNameOfPCB.GBP
Board OutlineNameOfPCB.BOR
Inner Signal Layers (Positive Polarity)
(Only for multi-layered PCBs)
NameOfPCB.G2L, NameOfPCB.G3L, NameOfPCB.G4L, NameOfPCB.G5L, NameOfPCB.G6L, NameOfPCB.G7L, NameOfPCB.G8L, NameOfPCB.G9L, NameOfPCB.G10L, etc.
Inner Plane Layers (Negative Polarity) (Only for multi-layered PCBs)NameOfPCB.G2P, NameOfPCB.G3P, NameOfPCB.G4P, NameOfPCB.G5P, NameOfPCB.G6P, NameOfPCB.G7P, NameOfPCB.G8P, NameOfPCB.G9P, NameOfPCB.G10P, etc.
NC Drill File (Excellon)NameOfPCB.XLN

Note: The above file naming format is only a suggestion and not a requirement. The advantage of following a naming format is easy identification of the layer to which the file belongs. The board outline Gerber file must contain all the cutouts and mills. Usually, designers do not use the drill report or rack definition as the NC Drill file. Rather, they include the information within the NC Drill File.

Read About: PROBLEMS WITH THE GERBER FILE FORMAT AND SOLUTIONS

Unless specifically instructed, the manufacturer will make several assumptions—type of laminate and surface finish chiefly. Other information that Gerber files do not convey are impedance control, testing, copper weight, board thickness, and board contour and dimensions. The designer must include separate documentation for conveying this information. Rather than having the manufacturer deduce much of the information from the Gerber files, the designer will do better to add a simple order form to inform the manufacturer on the number of layers, board dimensions, minimum trace width and spacing, total quantity, and requested turn-around time for the PCB. Including a read-me file with the same information could act as a cross-check for errors and omissions.

pcb gerber files
Source: https://en.wikipedia.org/wiki/File:AutoTRAXDEXPCB.png

Instruction Notes and Read-Me Files

Through instruction notes and/or read-me files the designer can convey important information to both the manufacturer and the assembler. Typically, these are a simple list of features the designer requests them to consider. In some cases, adding a description of the final product also helps to convey a better idea. Rather than a lengthy note, a clear and concise description is more helpful.

It is possible for designers to include textual information on one of the layers of the Gerber files. Although this method does ensure the information reaches the intended, the information thus transmitted has severe limitations. The advantage with the standard read-me file or the instruction note is the designer can update them with each revision. A better idea is to update the read-me file and include its updated revision number in a layer in the Gerber files. That way, the manufacturer will be aware of the existence of a read-me file/instruction note even if it is missing.

Bill of Materials

Assembling the board requires part of the information from the Gerber files, such as the copper layers, SMD paste, and silkscreen/overlay files. However, these are necessary only for reference, and to show the placement of SMD paste.

In addition to the above, crucial information assemblers require is the BOM or Bill of Materials and the positional or XYRS data. The BOM must contain information about all components on board, including their reference designator, value, description, package type, part number (retailer or manufacturer), and if possible, a link to the webpage of the distributor. To allow the assembler to decide whether to use a pick-and-place machine or manual assembly, it is preferable to include the packaging information for the component, such as whether it is available on tape, reel, cut tape, or in bags.

Some assembly houses also accept .XYRS files format, a combination of BOM and XYRS data. The file is usually tab delimited and contains the fields mentioned below.

ColumnDescriptionExample
PartPart Name (Designator)U1, R10, C15
XX-coordinate of part (mils, orientation from bottom-left)1210.00
YY-coordinate of part (mils, orientation from bottom-left)910.00
RotationRotation of part (degrees)180
SideLayer on which the part will be present1/T/top for Top
2/B/bott for Bottom
TypeSMD or THC1/SMT/SMD for SMD
2 for THC
X-SizeX-Dimension of the part (mils)Package size, Pad footprint
Y-SizeY-Dimension of the part (mils)Package size, Pad footprint
ValueValue of the part12K, 0.22µF, ATMEGA328P-AU
FootprintFootprint of partR0603, C0805, TQFP-100
PopulateOptional (Whether to populate this part or skip)1 to populate, 0 to skip
MPNManufacturers Part NumberMF-CAP-0402-27pF

It is necessary to check the BOM or .XYRS file thoroughly for accuracy and errors. Having a third party within the organization review the documentation is an easy way to weed out errors and spelling mistakes.

Conclusion

It is easy to marginalize documentation related to a PCB as this is a time-consuming and laborious process. However, spending some time up-front for documentation results in manufacturing and assembly processes proceeding smooth and fast. One can infer whether the documentation is clear, concise, and unambiguous, from the number of queries the contract manufacturer makes—ideally, there should be none.