A substantial amount of effort is required to optimize any PCB design. In preparation for the manufacturing phase, the proper PCB checks are essential. As such, the aim of all designers ultimately centers around enhancing functionality, ease of manufacturing, and ease of testing. Optimizing a PCB for manufacturing involves thoroughly analyzing it for potential improvements and testing for possible issues that could affect manufacturing.
Contract manufacturers (CMs) follow design for manufacturing (DFM) principles to assess the likelihood of potential issues that could emerge during manufacturing. Assessing the viability of a PCB in terms of DFM involves the following preliminary steps.
The Design for Manufacturing (DFM) Assessment
- Design review: Upon your CM initially receiving your design data, your design gets an assessment or design review. The format you utilize for the data transfer determines the manner of processing by your CM.
- Format assessment: If your design uses a CAD database format, then it will go directly to engineering for assessment. However, if it utilizes a different format, importing will generally be required, which would increase the processing time. It is important to know, in advance, which formats are considered reliable or acceptable by your CM.
- Components assessment: Your CM will assess your bill of materials (BOM) to determine which components are nearing their end of life (EOL) or require longer lead times. The aim of the component assessment is to eliminate components that do not meet the recommended standard for manufacturing.
- Component availability assessment: Your CM will match your listed components to a master component database, which will provide information on real-time component availability. This information is continuously updated and affords the CM the ability to make accurate recommendations for any necessary component replacements. If there are no exact matches to your component(s) due to discontinuation (EOL), your CM will use this database to recommend an appropriate functional replacement.
Alongside these preliminary checks, your CM will evaluate the following factors to assess the overall manufacturability of your design.
Factors for Assessing PCB Manufacturability
- Component spacing: When placing components too close, it creates problems with automated pick-and-place and adversely affects testing (fixtures).
- Component placement (location and rotation): Component placement affects every aspect of PCB design. Therefore, where and how you place a component also affects a board's solderability. If a larger component precedes a smaller component through the wave, it will shadow that component and possibly cause solder joint issues.
- Missing solder mask between pads: This causes shorts (electrical) between the pins due to solder bridging during the assembly process.
- Incorrect surface mount pad sizes: This will result in tombstoning. This is when a component, such as a capacitor or a resistor, partially lifts from the pad on one end.
- Acid traps: Routing your traces at an acute angle can lead to problems when removing etching chemicals from your PCB, thus promoting PCB fabrication issues.
- Drill optimization: This incorporates the unifying drill sizes, thus reducing the number of drill sizes in use. Optimizing drills can reduce fabrication costs.
Besides checking for manufacturability and functionality, your CM will also assess the provided data for testability based on the principles of design for testability (DFT). This involves evaluating the design based on DFT guidelines.
Design for Testability (DFT) Guidelines for PCBs
Testability affects numerous areas of PCB design. Overall, this includes building or modifying a test fixture (if required), test point pads (in-circuit tests), as well as component spacing in relation to board edges (spacing). In the process of assessing your design for testability, your CM will consult the test specifications you provide. The information gleaned from this data allows CMs to develop a test method that addresses board specifications. You can optimize the DFT process and assist your CM by completing the following:
- Provide markups on the schematic that includes connection and test points per their test review, in addition to assessing the layout.
- Include specific test points in your schema to assist your CM with this preparation. Overall, the design assessment and testing methodology will assist in determining the type of fixture required. This helps to determine if modifications are necessary for the design to increase the successful fault coverage of the test procedures.
- Work with your CM to incorporate all relevant testing information into a test instruction (TI) document, which should include illustrations and descriptions for use during testing.
- Understand and assess the types of tests available to make informed design decisions and optimize the testing process. Tests have specific advantages and disadvantages, and it is important to be familiar with testing specifications.
- For instance, the flying probe test is best suited for prototyping or low-volume production runs since it is easy to set up for testing. However, the Flying probe test is slower than the ICT. Also, the in-circuit test (ICT) fixtures and fixed programming are best suited for large production runs of proven PCB designs. Choosing the right type of test can make a significant difference in the overall outcome.
- Testability: Testability affects numerous areas of PCB design. Overall, this includes building or modifying a test fixture (if required), test point pads (in-circuit tests), as well as component spacing in relation to board edges (spacing).
Your CM can technically develop testing specifications; however, doing so for your design requires you to work closely with them and provide as much information as needed about circuit operation and functionality. Diligence at this stage is crucial to ensure that your PCB is thoroughly tested and meets your expectations. It is also important to follow PCB layout guidelines to enhance testability.
PCB Layout Guidelines to Optimize Testability
The following are crucial considerations in terms of PCB layouts for design testability:
- In-circuit test points should be on every net of the design to ensure complete test coverage.
- All test points should have a 50 mil clearance to pads and components.
- All test points should have a 100 mil clearance from the board's edge.
- It should be possible to conduct in-circuit tests on both sides of the board at once, and I recommend setting this up in advance with your CM.
Optimizing PCBs for Cost-Effectiveness
Optimizing your design for testability can reduce overall costs. Expenses incurred during PCB production broadly fall into the following four categories:
- Component costs
- Blank PCB manufacturing costs
- Testing costs
- Assembly costs
Although the four main categories of expenses can greatly vary depending on the exact design specifications, the cost of testing can equate to 25% to as much as 30% of the total cost for a finished PCB. Therefore, it is imperative that your designs adequately incorporate DFT and DFM. The overarching goal is to design a PCB that is highly testable while affording the ability to rapidly isolate issues related to component failures and manufacturing errors. The optimization of DFM and DFT is only truly possible when you work in collaboration with your CM.
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And to help you get started on the best path, we furnish information for your DFM checks and enable you to easily view and download DRC files. If you’re an Altium Designer or Cadence Allegro user, you can simply add these files to your PCB design software. For Mentor Pads or other design packages, we furnish DRC information in other CAD formats and Excel.
If you are ready to have your design manufactured, try our quote tool to upload your CAD and BOM files. If you want more information on CAD files or how to incorporate your design into a CAD format, contact us.