How to Avoid File Errors When Designing Printed Circuit Boards
Contract electronics manufacturers understand that with printed circuit boards (PCBs) becoming more sophisticated, their importance, as the building blocks of technological devices also grow stronger. With different types of PCBs now being used in everything from children’s toys to home automation systems and cars to smartphones, design engineers are finding themselves increasingly challenged to stay ahead of the ever-changing option of materials, design and layout software, and manufacturing capabilities. With technology changing daily, it also creates new ways of making errors while designing and developing PCBs.
Issues Leading to PCB Failure
While there can be external issues leading to PCB failure such as environmental issues, absence of test planning, no review process, not using correct tools, being unfamiliar with the manufacturing requirements, there can be some internal issues that are of a more fundamental nature. These include schematic errors and missing files.
Avoiding Schematic Errors
PCB designers must learn to use the available Computer Aided Design (CAD) and PCB layout tools. With proper use of sophisticated design tools, designers can avoid unintentional errors in layout and manufacturing specifications. A thorough training in the use of these tools helps tremendously, and this can have a positive impact on the efficiency and productivity of the organization.
Gerber Files and Related Errors
Typically, the result of the design phase of any PCB design project is a set of Gerber files. The manufacturer of the board utilizes these files to actually fabricate the PCB. Therefore, the Gerber files must be accurate and complete to allow boards to be manufactured that perform to the designer’s intention, without failure or design flaws.
In the PCB industry, Gerber files are the standard for passing details of manufacturing information to the fabricator of the board. Even for a single PCB, there are multiple Gerber files, with separate Gerber files describing silkscreen details, individual board layers, and so on.
The completion of PCB design leads to the Gerber files being generated for the manufacturer. However, the process can have a number of inherent potential problems:
Files Missing— Not All CAD programs are equal, and some generate individual files for each layer of the board with all types of data. The designer must know the files the manufacturer needs for board fabrication, and make sure the CAD package generates all of them. Negligence from the designer’s side can lead to the manufacturer not receiving the full complement of files necessary to produce the boards successfully.
Zero Length Files—While creating Gerber files, incomplete information can lead the CAD software generating files of zero length, not containing any information for the fabrication process. The designer needs to validate the file sizes in the directory to ensure there are no files with zero length.
File Labels—ailure to follow standard procedures for labeling files can create confusion for the manufacturer. While this can lead to mistakes in the boards produced, there can also be production delays, which can be both expensive and time-consuming to correct. Such confusion and delays are avoided with clear and detailed labeling.
Gerber Formats—Manufacturers accept Gerber files generated in 274D and 274X formats. Of the two, the 274X is the preferred format, as it has the information about embedded apertures within the file contents. On the other hand, the 274D format requires the designer to supply separate aperture list information for the manufacturer to understand the specifications of the designer. Therefore, the manufacturer is forced to follow a manual step when processing the 274D format files, leading to an opportunity for errors in manufacturing. There is also a possibility of the designer not generating the aperture list information or not including it with the other files.
Missing Drill File or Tool List—The fabricator needs this information to know where the holes should be placed on the PCB, their sizes, and the tool to use for drilling them. Apart from including the tool information within the drill fie, providing this information separately helps to avoid delays in manufacturing caused by manual entry and guesswork.
Insufficient Inner Clearances—The designer must ensure sufficient clearance between a drill hole and the adjacent inner layer of copper. Insufficient clearance can lead to shorts between the inner copper layers of the board.
DFM Not Followed—PCB design must conform to Design For Manufacturing (DFM). A discussion with the manufacturer is necessary for the designer to understand the different fabrication and manufacturing techniques along with the capabilities available with the manufacturer.
Designers can combine the use of best practices along with self-checks of Gerber files before dispatching them to PCB manufacturers. With these checks, designers can improve the speed of the process, and avoid delays.
PCB Design Creation Process Checklist
|Drilling||Have you included the drill files and tool lists?||Y|
|Are the hole sizes specified smaller than required?||Y|
|Silkscreen||Have you included the populated top and bottom silkscreen files?||Y|
|Outline||Have you included the mechanical/outline files?||Y|
|Copper/Silkscreen Combos||Is the company name and part number on the copper layer alone?||Y|
|Is the company name and part number clearly visible through the silkscreen layer?||Y|
|Revision Control||Have you placed the proper updated revision control levels on each layer?||Y|
|Documentation||Have you included a readme text file containing information critical to your design?||Y|
How Manufacturers Help Designers
When designing, it is an advantage for designers to know and understand the technical capabilities of the manufacturer who will be fabricating their PCBs. For instance, modern PCB manufacturing techniques have progressed to a stage where it is no longer necessary to make films—they use Direct Imaging Systems such as the MLI5800 from Maskless Lithography Inc.
Such systems allow LDI digital direct imaging of innerlayer and outerlayer circuitry, which eliminate phototools and the associated registration errors. The system allows imaging of very thin lines and spaces, as it has high accuracy of feature positioning and front-to-back registration accuracy.
This improves the layer-to-layer registration, allowing the manufacturer to build reliable HDI designs with small annular rings