BoardSurfers: Managing Minor Spacing DRCs Using Manufacturing Tolerances

While translating boards from different PCB design applications or changing design units in the later phase of a design cycle, a wide variety of mathematical round-offs may occur for geometric computations. As DRCs are logically computed, the DRCs created with minimal differences (for example, 0.01 mils) fall within manufacturing tolerances. Even when two traces running in parallel take a turn, they cause exceedingly minute spacing violations between their arced path. Such spacing violations are often waived off while reviewing the DRCs as they are considered insignificant. However, waiving off these spacing violations repeatedly is a time-consuming and frustrating task.

Well, to resolve these violations in one go, you can enable Design for Fabrication (DFF) checks integrated into Allegro^® PCB Editor. You can define tolerance values for DFF copper spacing and mask spacing that apply to certain spacing checks to allow for manufacturing tolerance limits. Setting the manufacturing tolerance minimizes the need to review spacing violations that fall under manufacturing specifications; and eliminates the need to waive the DRCs.

Enabling DFF Tolerances Checks in Allegro PCB Editor

To set up the manufacturing tolerance, open the Analysis Modes dialog In PCB Editor:

1. Choose Setup ─ Constraints ─ Modes.
2. In the Analysis Modes dialog, expand Design for Fabrication checks and double-click the Manufacturing Tolerance Allowance option to open the DFM Global Tolerance window.
   
   

   

3. Specify the allowable tolerance value for DFF Copper Spacing rules as suggested by the manufacturer and click OK.
   
   

   

   The tolerance values are always positive. If the value of spacing constraint between traces is 5 mils and the manufacturing tolerance is 0.01 mils, the DFF DRCs will be reported only if the spacing is less than 4.99 mils.

   
   

   Similarly, you can set the manufacturing tolerance for DFF Mask rules, which will be valid for spacing between two mask objects.

In the End

Incorporating manufacturing tolerances into the DRC generation process saves hours of design revisions and reduces turn-around time. In this post, we saw that setting up these tolerances is quite simple. If there are any changes in manufacturing specifications, you can modify the tolerance values and re-run online DRCs. That's the icing on the cake! The next time you review spacing DRCs, consider specifying manufacturing tolerance checks to save time and effort.

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