Boardsurfers: An Introduction to Allegro DesignTrue DFM Rule Aggregator

Design companies often work with multiple PCB fabricators and each fabricator may have a different set of DFM rules. It is a customary practice followed by design companies to create a common denominator rule set that can be applied to all fabricators. This process is time-consuming, inefficient, and often subject to human errors. Moreover, the whole process has to be repeated with the introduction of new technology or new fabricators.

Cadence® Allegro® platform has introduced an automated solution to this problem in its Release 17.4-2019 HotFix 013.  A new utility Allegro DesignTrue DFM Rule Aggregator is now available with your Layout Editor licenses that combine DFM rules and produces an aggregated set of rules with a common denominator, which is based on the most conservative values. This solution is primarily focused on the technology files obtained by fabricators who partnered with Cadence through the DesignTrue DFM Rules web portal but can also work on the technology files exported from Allegro Layout Editors which have DFM rules.

When working with rules (technology files) obtained from the DesignTrue DFM Rules web portal, the DFM Rule Aggregator groups multiple constraint sets (CSets) that represent similar layer types, copper thickness, manufacturing classes and merges them to produce a common denominator group of CSets in a single technology file. For the technology files generated outside the web-portal, some manual intervention is required.

To start Allegro DesignTrueDFM Rule Aggregator, enter dfmagr at the command prompt.

Now to import technology files, choose File ─ Import Technology Files and select the files you want to import. Multiple files can be imported in a single step as you see in the following image.

In this example, I am importing technology files from three different vendors. After the files are imported, CSets from individual technology files become visible with their respective constraint values.

Technology files that are generated from the DesignTrue DFM Web portal can be aggregated using the Group ─ Auto Group All menu option.

The CSets that are not automatically aggregated (generated from Allegro Layout Editors) can be aggregated by selecting them manually by using the right-click option Create Group.

The most conservative value for each CSet within an aggregated rule set group is being used to create an aggregated CSet. The conservative value is based on the type of value a rule requires. To support my statement, I am showing a few examples:

• Set to the lowest value for ratios. In this case, the Aspect Ratio in the aggregated CSet will be the lowest of all values, which is 6.

• Set to the maximum value for all dimensional values and angles. In this case, the Acid Trap Minimum Angle in the aggregated CSet will be the maximum of all values, which is 90 degrees.

• Set to the greatest value of physical size or spacing. In this case, Trace to Trace Copper Spacing in the aggregated CSet will be the greatest of all values, which is 25.00

Managing Conservative Values

Allegro DesignTrue DFM Rule aggregator uses DFM Aggregation Policy file (dfm_aggreg.xml) to manage the conservative values. The policy file is located at <install_directory>/share/pcb/consmgr and can be customized at SITE level (<CDS_SITE>/pcb/consmgr) to override the default behavior.

The XML element AGGREGATE_CRITERIA of the policy file determines the rule value. The key data types are RATIO and ANGLE. The legal values assigned to these data types are either MIN or MAX, which represent the smallest and the greatest numerical values, respectively. For all dimensional data types, the MIN/MAX value can only be overridden by adding an entry into the XML element OVERRIDEN_CONSTR_IDS.

When adding a new DFF rule as an exception, the id field specifies the constraint name found in the technology file. In the following example, the DFF Mask category rule, Mask to Trace constraint name is DFF_MASK_TO_TRACE.

The current conservative value is set to MAX. To make an exception to the conservative value and to use the MIN value the XML can be edited as follows:

<OVERRIDEN_CONSTR_ID id="DFF_MASK_TO_TRACE" set="MIN"></

OVERRIDEN_CONSTR_ID>

You can also override the value from any other CSet irrespective of the DFM Aggregation Policy by selecting the Override option from the right-click menu. In the following snapshot, the Mask To Trace value is picked from Vendor2. The overridden value in aggregated CSet is indicated with the yellow background color.

Exporting Aggregated CSets

To export aggregated CSets, select File ─ Export ─ Aggregated Cset. This command generates a technology file with aggregated CSets. To set up DFM rules in the Layout Editors, you simply need to import this file in layout editors using the File ─Import ─ Tech File option.

Over to You

I hope this post motivates you to experiment with DesinTrue DFM Rule Aggregator. You have all the functionality to combine, modify, and get production-ready technology files that can be used universally across designs. Try Allegro DesinTrue DFM Rule Aggregator and stay ahead of the DFM curve!