Some people say that absolutism is a viable concept and that some absolutes are attainable. This may be true in the abstract realm of philosophy; however, tangible absolutes such as absolute zero temperature or a zero-defects production rate are difficult, if not impossible, to reach. In fact, insistence on perfection can be paralyzing. This is not to say that an explicit target is not useful; on the contrary, it is absolutely necessary, as it serves as the target from which an acceptable range of tolerance can be defined.
The implications for exceeding acceptable PCB tolerances
Well-defined PCB tolerances are essential for the building of high-quality circuit boards. Virtually all PCB manufacturing steps are influenced by tolerances, many of which are defined by you during design. If these are chosen in conjunction with your contract manufacturer (CM) to fall within the range of tolerance for their equipment and processes your board can approach the best possible result and an optimal yield rate is attainable. Before defining how to select the best PCB tolerances we should clearly define defects and which types affect our target of a 100% production yield rate.
How Do Tolerances Impact the Production Yield Rate?
The yield rate for fully assembled PCBs or PCBAs is most often associated with building a large number of boards, which in many cases requires a transition from prototype to high volume manufacturing. In other cases; particularly for specialized designs of critical systems for aerospace, medical devices, and industrial applications, low volume production is the final stage of manufacturing. Whether low or high volume the objective of the final stage of PCBA production is a perfect yield rate or zero boards that are defective to the extent that they cannot be used as intended.
PCB defects whose root cause may be in fabrication can range from the mechanical; such as delamination, bends or breaks to the less obvious that can distort electrical operation; such as contamination or moisture on or within the board. Assembled boards can also be affected by moisture and contamination; therefore, it is best to employ PCB moisture protection methods during and after manufacturing. In addition to defects that may not show up until the board has been installed and is in operation, there are the conspicuous ones that render the board unusable.
The number of boards produced divided by the number of boards that are usable is the yield rate. The difference is the number of defective boards that require rework (additional manual actions to correct minor defects and bring the board to a usable state) or must be reproduced. For PCBAs where rework will not correct the defect, redesign is likely required. This can mean additional man-hours, as well as increased manufacturing and testing costs.
How PCB Tolerances Can Improve the Yield Rate
The importance of your choice for fabrication and assembly services cannot be overstated. Making the right choice can be the difference between receiving boards that are built to meet or exceed regulatory standards based upon their IPC classification or not. Likewise, the benefits of DFM for your PCBA development cannot be overstated. Tailoring your design decisions to fall within the range of PCB tolerances for your CM’s equipment and processes ensures that your boards can, in fact, be built. The constraints defined by regulation establish the acceptable boundaries for your CM’s range of DFM tolerances. Your selection for PCB tolerances must be within these ranges.
The absolute range of capability of your CM’s equipment for a specific manufacturing step defines its process window. As an example, the absolute minimum diameter that a drill hole can be bored defines the smallest width of the process window for creating vias. Likewise, the maximum bore width defines the largest process window width for creating vias. Provided that each of these physical dimensions is regulatory compliant you are free to select any size in the range. However, selecting the extremes is the worst choice as it places more pressure on the drilling process to be precise and the probability for error is greatest. By contrast, choosing the middle of the process window is the best selection and the probability for error is the lowest. Consequently, the possibility of a defect significant enough to render your board unusable is minimized.
By selecting PCB tolerances that are at or near the center of the process window for your board’s manufacturing steps you can reduce the probability of board defects to virtually zero and eliminate the negative effect of correctable process defects on your yield rate.
|Tempo's Custom PCB Manufacturing Service
At Tempo Automation, not only do we lead the industry in fast, high-quality PCB manufacturing for prototypes and low production, but we also employ a white box approach to ensure that your design is optimized with our processes and capabilities.
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 PCB tolerances or how they can be used to improve your board’s yield rate, contact us.