The Allegro PCB Editor 16.6-2015 release now provides support for differential-pair return path vias, a method to add return path vias to a differential pair easily during the Add Connect command. Return path vias are also referred to as ground reference vias or stitching vias. They provide a current return path to minimize signal degradation in PCB and packaging interconnects by adding additional via(s), which are normally connected to ground, next to the diff pair via transitions.
The various capabilities include:
- Diff pair return path via setup
- Adding return path vias during Add Connect
- Working with diff pair and return path vias during Slide
- Defining via structures for use as return path vias
Diff Pair Return Path Via Settings
The return path vias functionality is available in the RMB context menu when a diff pair is selected for routing. After defining the return path net and via, you then will be able to select one of the six return path via styles to use when the diff pair is transitioning between layers.
Setup Tab
- Return Path Net: Select Net for the return path via. The menu contains a list of previously selected net names and the browse button will allow you to select a net name from the design. The dialog can be filtered by net name or by nets that have been assigned a voltage property (DC nets).
- Return Path Via (Padstacks): Select the padstack to be used as a return path via. The menu will contain a list of previously selected padstacks and the browse button will allow you to select a padstack from the design or library. The dialog can be filtered by padstack name or by the padstacks specified in the Physical Constraints Via list for the selected return path net (PCS net via list).
- Return Path Via (Via Structures): Select a pre-defined via structure to be used as a return path via. The menu will contain a list of previously selected via structures and the browse button will allow you to select via structure from the design. A stacked via structure can be created using Route –> Via Structure –> Define:
Spacing Tab—Define the Parameters for Each of the Six Return Path via Styles
- 1 Via: This style adds one return path via next to one of the diff pair vias. User has option to select Left or Right to specify which side of the diff pair to add to the return path via as well as the via spacing and angle. The default value for angle is set to 45 degrees for right, and 135 degrees for left.
- In line: This style adds two return path vias aligned in a straight line with the diff pair vias. User can specify return path via to diff pair via spacing.
- Equidistant: This style adds one return path via placed equidistant between the diff pair vias. User can specify return path via to diff pair via spacing. Enabling the Mirror option places one return path via equidistant between the diff pair vias on same side as diff pair pad entry routing.
- Offset: This style adds two return path vias offset with respect to the diff pair vias. User can specify the return path via to diff pair via spacing and angle. The default value for angle is set to 45 degrees.
- Diamond: This style adds two return path vias placed equidistant on each side of the diff pair vias. User can specify return path via to diff pair via spacing.
- Rectangular: This style adds four return path vias in a rectangular/square style centered on the diff pair vias. Users will need to specify x and y values in order to select this style for use.
Default value for spacing is set to Minimum DRC except for the rectangular style where the spacing defaults to 0. All entered user-defined values will be in database units and specifying different units is not supported:
Adding Return Path Vias During Add Connect
After the return path via setup is complete, you can enable one of the return path via styles from the RMB – Return Path Vias during diff pair routing. Enabled (check) return path via style to be used when routing diff pairs but changing the setting will not affect any previously added return path vias, which works similar to RMB – Via Pattern enabled settings:
Working with Diff Pair and Return Path Vias During Slide
You have the ability to slide the return path vias as a group, along with the diff pair transition vias, by selecting any one of the vias during the Slide command. It is sometime necessary to adjust the location of the either the return path vias or the diff pair transition vias, which can be accomplished by unchecking RMB – Return Path Vias Group. Once this RMB option is unchecked you will then see the RMB – Single trace mode when sliding a diff pair transition vias individually as normal:
Defining Via Structure for Return Path Vias (Allegro)
A via structure in Allegro is a series of blind/buried and/or micro vias stacked directly on top of each other that span multiple layers in a PCB. These via structures have traditionally been used in conjunction with the Create Fanout command (Route –> Create Fanout), which allows the attaching of the via structure (clines and vias) to the SMD pins of a device to complete the fanout (pin escape). Currently, defining a via structure in Allegro requires that the vias in the structure are physically connected to a pin, which in most cases means a cline is included when the via structure when created. For obvious reasons, these types of via structures should not be used as return path vias.
There is a simple solution: create a via structure that contains via objects only and no clines by placing the stacked vias inside the boundary of a SMD pad, then create the via structure using Route –> Via Structure –> Define:
A stacked via inside of SMD pin# 5 can be used to create a via structure for return path vias.
NOTE: These via structures will be exploded to their individual objects when they are used as a return path via but will remain as members of the return path vias group.
Limitations
- Return path vias
- Return path vias option is only available during Add Connect command
- Support is available for differential pairs only. Single-ended nets are not supported
- Multiple differential pair routing is not supported (i.e., Add connect of two or more differential pairs at a time)
- Using diamond and equidistant return path via styles will require a single click to set the gather point of the diff pair after/before it comes in contact with the return path vias
- Existing via structures that were created for use with the Create Fanout command may not be able to be used as return path vias as they normally contain cline objects
- Return path vias group
- Return path vias group is only available during the Slide command
- All vias that are part of the return path vias group will retain the associated net name when they were added, which is required to ensure that the return path net remains associated to the return path vias
- This results in a behavior change, compared to diff pair routing vias without return path vias. The diff pair transition vias will always retain the associated net name when they were added, even if the cline path back to a pin is deleted
- Even though the return path vias group is recognized by the Slide command, it is not possible to use other commands (i.e., Move, Copy, Delete, etc.) to perform actions on the group
- You have the ability to copy the individual objects that make up the return path via group but the copy will will not be recognized as a return path via group
- Expect the same bubble/smooth results during the slide of diff pair/return path vias compared to just diff pair transition vias only. If it is required to slide the group for long distances and in multiple directions, then it is recommended to delete and add them at the new location instead of using the Slide command
Please share your experiences using these new capabilities.
Jerry “GenPart” Grzenia