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Teardrops (also called fillets) are the blending area of a cline entry into a pad, while tapers are the gradual transition from one line-width to another along a path. These two core concepts appear in nearly any PCB or IC package substrate today. They act to smoothen the intersection of the two objects, eliminating acute angle acid traps while also contributing to better signal integrity along the path.
However, any time you modify your design, these items need to be updated. If you swap a via to a padstack of a different definition or size, remove a trace into a pad (potentially causing the pad to be removed completely if there are no other connections), change the entry angle, update a line width, or cross through a constraint region; there are countless ways to impact the teardrops and tapers. To get an accurate, complete design ready for analysis or manufacturing, teardrop and taper elements MUST be present. They impact surrounding plane shape voiding and push neighboring segments.
This is why the Cadence® Allegro® tool suite offers full dynamic filleting and tapering abilities for designers. By freeing you of the responsibility for managing these objects, more time can be focused on other design complexities, such as routing your high-speed nets.
What capabilities do you have in the 17.2 tools and how do you take advantage of them through all phases of the design cycle?
In the early stages of your design, having teardrops enabled ensures that routing is valid. With dynamic mode enabled, choose the design-and-correct (allow DRCs) flow; this mode will create the teardrops, even if they are in DRC conflict with a nearby object. Doing so, you get real-time feedback where more spacing is needed to get an ideal route. Eliminate the hassles of a correct-by-design flow where you can’t add the route because a fillet can’t be placed, leading you to question where the problem is or how to correct it.
To work in this mode, turn on dynamic teardrops and tapers in the settings. Make sure that fillets are enabled in the fillet configuration form and set up which pad shapes need teardrops and the shape they should take. Add the NO_FILLET attribute to any nets which do not need them, and in your cross-section, turn on filleting for the layers requiring the process. If you only require fillets in certain areas of the drawing, you can add gloss keepout areas. From now on, as you make or break any connections in the database, the teardrops and tapers will automatically adjust to the changes.
Once your design has gone to manufacturing, any engineering change order which comes in should result in the minimum of changes to the design. Therefore, you want the confidence to know that nothing about your teardrops and tapers will change in areas of your design other than where you are working, even if you move your design to a new version of PCB Editor which may have enhanced capabilities.
For this reason, when you get to this stage, the recommendation is often to lock the teardrops and tapers down outside of your change area. Quick tools, specific to fillets, are available to fix and unfix all teardrops or individuals. With the click of a button, you can lock these down from any changes, safely modify your design in the impacted area, and rely on the dynamic procedures to update things ONLY where you have unlocked the teardrops. None will be regenerated or modified in any way elsewhere in the design.
Why not turn off the dynamic mode and use manual mode instead? PCB Editor supports a full manual mode, but this shifts the effort back to you to ensure that nothing is missed or forgotten. Allowing the tool to track your changes and make the implied updates doesn’t just reduce the chance for errors, it speeds your time to market.
With every release, whether it is a major, minor, or quarterly update, improvements are made to the teardrop and taper flows.
In the recent 17.2-2016 quarterly update, pad filleting has been improved to understand stacked vias and vias in pins. Teardrops will always be based on the largest pad, not simply the pad that your cline connects to. This comes with improvements to teardrops on complex pad shapes like rounded and chamfered rectangles.
Teardrop based on pad
Teardrop based on largest pad
Allegro is always improving. So, keep an eye on the layout editor Release Notes for future developments.