Essential Checklist for PCB Assembly files" A guide to standard formats"

PCB assembly cannot be started without a set of specific files called PCB assembly files. With the help of these files, an idea regarding the information, position, and orientation of all layers of the components is given to the manufacturer. They might also be known as manufacturing files, fabrication data, assembly files, and an array of other names.

Any fabricator or assembler is unable to work without these PCB assembly files. To avoid any discrepancy and to ensure a smooth flow of work, PCB assembly files need to be sent in a proper format.

The requirements may vary between PCB assembly manufacturers, but below is a list of widely used and accepted file formats for PCB assembly.

List of PCB assembly files

When the PCB design is set, some files must be exported to ensure accurate PCB assembly. The most important of PCB assembly files include

  • BOM (bill of materials)
  • Assembly files
  • Centroid data
  • Gerber file
  • NC Drill file

BOM

A Bill of Materials (BOM) serves as a comprehensive inventory of components utilized in a circuit board, and its format can be conveniently represented in an Excel chart. Whether for partial turnkey or full turnkey services, the BOM incorporates essential details to streamline assembly and inspection processes.

The BOM includes key elements such as

  1. Item
  2. Reference Designator
  3. Description
  4. Notes
  5. Package information
  6. Manufacturer’s Part Number
  7. Quantity required per board
  8. Manufacturer Details

To enhance clarity and facilitate assembly inspection, it is prudent to denote unassembled positions with “Do Not Include” (DNI) in the BOM. This strategic addition ensures a seamless and efficient evaluation of the assembly at different stages.

The precision and integrity of the components sourcing and assembly program hinge significantly on the accuracy of the Bill of Materials (BOM). Careful attention and accuracy are paramount when inputting data into the Excel file.

Gerber files :

PCB assembly

Another PCB assembly file is a Gerber file. Gerber files are a standard file format used to convey information about printed circuit board (PCB) designs in manufacturing processes. Gerber files contain shape and location data for each element in a printed circuit board (PCB) layout.

Gerber files are commonly compressed into .zip/.rar.

These files store information in an ASCII text format and have 4 key elements listed as Configuration parameters, Aperture definitions, XY coordinate locations for draw and flash commands and Draw and flash command codes. These gerber files contain vital details about each PCB layer for production. At a minimum, the PCB assembler requires three essential files: Silkscreen, Copper (Track), and Solder Paste layers. Alongside Gerber files, it is crucial to provide drill files indicating hole locations, a board outline file, and relevant mechanical layers for special processes like cut-outs or castellation. Alternatively, ODB++ files, comprehensive archives with all fabrication data, can serve as a substitute for traditional Gerber files.

3 main Gerber file formats are RS-274-D , RS-274-X and Gerber X2 format.

PCB Assembly NC drill file

The NC drill file, commonly known as an Excellon file, functions as an additional document essential to be submitted alongside the Gerber file when providing instructions to the PCB manufacturer. Specifically outlining the position and dimensions of drill holes on the board, this file holds particular significance for routing and drilling machines.

PCB Assembly Centroid files

Centroid files function as the digital plans for your circuit board, guiding machines on component placement.
certain CAD tools automatically generate the Centroid file, others may require manual modification before its creation.

This file details the position and orientation of all surface mount components, encompassing information like reference designator, X and Y coordinates, rotation, and board side (Top or Bottom). It specifically lists surface mount parts, providing crucial details for accurate assembly. precise files is essential to guarantee the accurate positioning of components.

Various software tools such as Eagle, Altium, and Cadence can generate these files, emphasizing the need for compatibility with the assembly machine’s software and adherence to the appropriate format.

PCB Assembly Files

Similar to the PCB fabrication drawing, the assembly drawing is typically auto-generated from PCB design CAD tools. Its purpose is to equip assembly machine operators with sufficient information to verify the correct rotation and polarity of each assembled component, particularly when there is no silkscreen printed on the circuit boards. Manufacturers accept the .pdf format for these drawings, which should distinctly highlight the pin 1 position of each IC and the reference designator of every component. Additionally, common polarity markings, such as positive terminal markings for tantalum capacitors and cathode markings for diodes, are crucial elements to include.

Other Instructions:
When creating the PCB Assembly files, it is crucial to include informative notes for the assembly provider. In addition to fundamental instructions, take into account the following considerations:

  1. Identify vital components, such as connectors or heat sinks, necessitating specific mounting instructions, and incorporate these details in the assembly notes on the drawing.
  2. Clearly illustrate any additional hardware, such as stiffener bars, handles, or ejectors, along with precise mounting instructions included in the drawing notes.
  3. For critical assembly areas, provide an enlarged and isolated “detail” or explored view to offer a more detailed perspective.
  4. Clearly indicate the placement of assembly stickers and labels directly on the drawing to enhance clarity throughout the assembly process.
Base MaterialFR-4
Layers2
Product TypeIndustrial
Different Design1
Delivery Format Single PCBSingle PCB
PCB Thickness1.6
PCB ColorBlue
SilkscreenWhite
Surface FinishHASL (With LED)
Outer Copper Weight1 oz
Via CoveringTented
Board Outline Tolerance+-mm (Regular)
Confirm Production FileNo
Remove Order NumberNo
Gold FingersNo
Castellated HolesNo
Edge PlatingNo
Appearance QualityIPC Class Standard
Silkscreen TechnologyInk-jet/Screen Printing Silkscreen

Why PCB assembly files are important

The user can handle these PCB assembly files for the manufacturing of PCBs by using documentation and multiple design tools. With full assembly documentation, the user can be sure of an insight of inspection techniques and methods necessary to deliver the final product.

If the user fails to communicate the vital information to the manufacturer using these files, user may end up overcoming several errors.

The PCB assembly files are important due to several reasons listed below:

  • The absence of PCB assembly files may cause positioning and orientation-related errors which might be costly during repairs.
  • Some data cannot be obtained from silkscreen which can be easily obtained from the use of these files.
  • If vital information like through-hole components’ position or polarity markings when soldering manually can cause an illogical lack of compatibility.

Apart from these errors, there is a component quantity mismatch, absence of polarity marking and pin, missing footprints, and more.

In conclusion, PCB assembly files are indispensable in ensuring accurate and efficient manufacturing processes, providing essential information for component positioning, orientation, and assembly. Failure to communicate this vital data may result in errors, underscoring the critical role of comprehensive PCB assembly files in facilitating seamless production and minimizing potential issues.

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