When manufacturing printed circuit boards, designers want the best quality without any unpleasant surprises in cost. However, some unintentional design errors could drive those costs up. When gauging PCB assembly cost, consider the various contributions of design and documentation that can prepare a design for manufacturing (DFM) or lead to excessive revision/rework.
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| Decreases Cost When.. | Increases Cost When.. | |
| BOM |
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| Footprints |
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| DFM |
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| Rework |
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| Documentation |
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A Clean BOM, the First PCB Assembly Cost Driver
Starting with a clean PCB bill of materials is essential to controlling the overall cost of the board. Board manufacturing can overrun budgets and timelines if component selections aren’t cost-efficient or have extended lead times. Incorrect parts cost you more time and money in debugging, while parts entering end-of-life (EOL) production status could force a board redesign. BOM scrubbing may seem unimportant, especially for design engineers working with familiar parts, yet these parts can change over time. CAD libraries may not always be up-to-date, and there may be newer and better options that the design team is unaware of.
Inaccurate Footprints Lead to Rework and Board Failures
The incorrect PCB CAD footprint models may result in several different manufacturing failures. Wrong pad sizes can cause SMT parts to float off the pad during soldering, while inadequate pad spacing can cause faulty solder joints or problems inserting through-hole pins into their holes. Component outlines drawn too large may not allow for a compact board placement, while outlines too small may result in an inability to assemble the board. Therefore, using the correct footprint pattern for the components soldered there is essential. When possible, industry-standard footprints are preferred. Manually created footprints require internal verification to avoid redesigning the board later, and all necessary data, including attributes and STEP files, are required to translate the layout into a physical board.
Follow DFM Rules for a More Manufacturable PCB
In the rush to design and build a prototype board, glossing over the manufacturing requirements is not unusual. Prototypes often get more attention than a production board, i.e., manual rework. However, these boards frequently enter production without any corrections, and manufacturing problems will start. You may encounter poor solder joints, tombstoning parts, connectors, and other hardware that are difficult to reach and work with.
It is best to use good DFM practices in the initial layout. You will also want to review the design for DFM rule violations to ensure your board is ready for full production from the start. This check will save you the time and expense of redesigning the board and re-validating that design after implementing corrections.
Rework Happens; Is Your Design Ready for It?
At some point, an in-development circuit board will require some rework. Rework can arise because of an assembly problem, debugging, or a design change. Plan for extra space in your design whenever possible to avoid additional clean-up work and repair any collateral damage due to the rework. Give smaller parts enough clearance to their associate processors and other large components and moving parts obscured by taller components.
At the same time, don’t forget that your PCB needs a design-for-test approach to streamline testing, especially for mid-to-high-volume productions. Include test points for ICT or flying probes and/or probe points for manual tests. Boards not set up for test will require additional manufacturing steps for validation, which will add to their overall expense.
Instill Strong Documentation Practices
Lastly, take the time to prepare good documentation. Unclear or unreadable documentation will force your contract manufacturer to slow down their process to get your answers. To dodge these problems, take time creating documents and use all the resources in your PCB design CAD system. These could include templates and automated features to simplify drawing creation, BOM, and other manufacturing file generators.
Your Contract Manufacturer for High-Quality Productions
Another helpful resource in all of these areas is to enlist the help of your PCB contract manufacturer to reduce PCB assembly costs. Here at VSE, we have over forty years of industry experience helping our customers find and avoid these problems, and we can help you do the same.
