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Best Practices:

Working with Real-Time DFA AnalysisSeries XL and GXL

Product Version 16.0

June 2007

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19912007 Cadence Design Systems, Inc. All rights reserved.

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Best Practices: Working with Real-Time DFA Analysis

June 2007 3 Product Version 16.0

1

Using Real-Time DFA Analysis . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Licensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Adoption Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Uprev of Package Symbol Library . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 6

Sampling Auto-Generated DFA Placebounds . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Package Side and End . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10

DFA Constraint Spreadsheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Class-Based Considerations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 13Overriding Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 15

Table Compression . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Table Access from DesignPath Variable . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Read-Only Version . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Loading Spreadsheet into Allegro PCB Editor . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

DFA-Driven Interactive Placement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 17

Design Rule Check . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Legacy Placebound Shape . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Constraint Manager DRC Worksheet . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

User Preference Controls . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 19

Contents

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1

Using Real-Time DFA Analysis

New technology inside Allegro PCB Editor offers real-time package to package clearancechecking during interactive component placement. Driven from a two-dimensionalspreadsheet array of classes and package instances, real-time feedback in the form of aspacing circle provides minimum clearance requirements based on the package's side-to-

side, side-to-end, or end-to-end profiles. With thousands of components on todays boards,real-time DFA analysis feedback increases the designer's productivity and efficiency byplacing components to corporate or EMS guidelines and helps reduce the dependency onreferencing specifications or interfacing with third-party, computer-aided manufacturing(CAM) systems.

Licensing

DFA analysis is unavailable in Allegro PCB Editor XL.

Adoption Considerations

The DFA application is not considered "use out of the box" technology. Adoption, especiallywith larger companies, requires some planning between CAD and manufacturing groups.When considering the DFA application, a review of your package symbol placebound shapesagainst the clearance parameters is necessary. Questions to consider:

Are your current placebound shapes drawn tightly around the package extents notaccounting for clearance requirements to other packages? If so, you may not requiremigration to the new DFA placebound shape.

Are your current placebound shapes drawn outside the extents of the pins or assemblyoutline to account for clearance requirements? If so, Cadence recommendsincorporating the new DFA placebound shape that can be automatically generated withthe utility,dfa_update.

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Best Practices: Working with Real-Time DFA AnalysisUsing Real-Time DFA Analysis


Figure 1-1 Compact placebound geometry (l) and extended placebound (r)

Legacy placebound shapes may be drawn to include the space required for rudimentaryclearance checking and may not integrate well with DFA spreadsheet-driven rules. Althoughthe DFA checker reverts to the legacy placebound shape if a dfa_placebound shape is notfound, it is estimated this model is inaccurate for many companies as it lacks precision dueto the static nature of a single placebound shape for multiple combinations of clearancerequirements. Customers are cautioned to sample the output ofdfa_updateto ensureconformance with their package spacing guidelines.

Uprev of Package Symbol Library

Thedfa_update utility automates theuprev of all or a family of package symbols requiringthe addition of the DFA-placebound shape and the DFA_DEVICE_CLASS property. Theauto-generated rectangular placebound shape is drawn from the extents of the symbol pinsand assembly outline. The property allows package symbols to be classified into class orgroup-based objects. The utility can be run from Start Run on a Windows platform. Thebenefits of applying the DFA_DEVICE_CLASS property to library-based symbols allows theDFA Constraint Spreadsheet to be viewed hierarchically with the option of reducing thenumber of instances in the spreadsheet. Purge Grouped Symbols, located within thespreadsheet, removes symbol instances belonging to classes. Candidates for class-basedentries include symbols with common spacing requirements. The class comprises a family of

packages, such as all BGAs or heterogeneous combinations, such as SOICs and TSOPs.

When using this utility, Cadence recommends reading symbols from your current library butwriting to a new library. Symbols in Release 15.5 cannot be used with downrev software. Inthe example below, thedfa_update utility updates all.dra files in a 15.2 library. It adds theDFA placebound shape and writes the output to a 15.5 library. Properties are notrecommended when performing a global update.

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1. From Start Run, enterdfa_update.

2. In the Enter Symbol Namefield, browse to the location of any.dra file in your current

library.3. Edit that symbol name, reducing it to the suffix *.dra as shown in Figure 1-2 on page 7.

4. In the Enter Destination Directory field, browse to the location of the newly created 15.5library name.

5. Leave the DFA_DEV_CLASS property field blank.

6. Click Update.

7. Review the log file.

8. Sample a variety of particular symbols for company compliance.

Figure 1-2 Global update of all package symbols

In Figure 1-3 on page 8, all BGA are updated and assigned a class property of BGAbecause their clearance requirements are identical.

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Figure 1-3 All BGAs updated and assigned class property of BGA

Sampling Auto-Generated DFA Placebounds

It is critical that you sample several but unique packages before releasing the library to

production. The results from thedfa_update utility, in some cases, may not conform to yourcompany guidelines. Figure 1-4 on page 9exemplifies auto-generated DFA placebounds,which display in yellow. The assembly outline displays in blue. The DFA_BOUND_TOP andDFA_BOUND_BOTTOM subclasses are located in the PACKAGE GEOMETRY class.
http://-/?-http://-/?-

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Figure 1-4 Auto-generated DFA Placebound Shapes

Figure 1-5shows auto-generated DFA placebound shapes for a package with fiducials.

Figure 1-5 Auto-generated DFA Placebound Shapes for a Package with Fiducials

Figure 1-7shows an example of auto-generated DFA placebound shapes for a complexassembly outline.
http://-/?-http://-/?-

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Figure 1-6 Auto-generated DFA Placebound Shapes for a Complex Assembly Outline

Package Side and End

The software determines the package side and end based on the longer and shorterdimensions of the package symbol. If Allegro PCB Editor cannot determine the side or

end, it uses the most conservative value of the string entered in the spreadsheet for thatparticular package to package requirement.

For example, if a cell contains a string of 25:50:100, the value of 100 is considered themost conservative. Package symbols built to a square or with more than four sideswould be examples where the software cannot determine between a side or an end.

Caution

Do not assume the leaded edge of a package is the side. In Figure 1-7, an8-pin SOIC shows the side where you normally might think the end is

located.

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Figure 1-7 Package Side and End

DFA Constraint Spreadsheet

Package symbol spacing rules for Side-Side,End-End, andSide-End profiles are entered intocells in the spreadsheet, which supports top and bottom side views. The spreadsheet has a.dfa extension and can be opened by choosing Setup - DFA Constraint Spreadsheet or as astand-alone application. DFA files can be stored on disk and be controlled by the design pathvariable DFACNSPATH.

Purge controls help reduce the number of unused symbols or symbol instances belonging toclass objects. The Show Symbol Classifications utility provides an alternative, yet moreintuitive method for adding the DFA_DEV_CLASS property to package symbols. It alsoprovides a viewing mechanism for the class structure.

A stand-alone, unlicensed version of the DFA Constraint Spreadsheet, shown in the followingfigure, is available if an Allegro PCB Editor environment is installed. The stand-alone versionis recommended for development and offers the ability to write class information to the library.These controls, in the Show Symbol Classification user interface, are disabled in Allegro PCBEditor XL. The stand-alone DFA Spreadsheet can be launched from the Start menu or withthe commanddfa_dlg.exe.

Upon opening a blank spreadsheet, a two dimensional array of package symbols can bequickly created using the following steps.

1. Do one of the following:

a. Choose Start Programs Cadence SPB PCB Editor Utilities DFA SpreadsheetEditor.

b. Click Start Run. In the Open field of the Run dialog box that appears, enterdfa_dlgto launch the DFA Spreadsheet Editor.

Note: These taskscanbe accomplishedbyusing thespreadsheet with Allegro PCB

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Editor, except for writing properties to the library.

2. Click Browse for Symbols.

3. Click Package/Mechanical symbolsin the DFA Symbol Browser dialog box.

4. ClickDisplay Symbols from Library. At this time, all symbols in your PSMPATHpopulate the DFA Symbol Browser dialog box.

5. Click OKto quickly generate the table.

6. To apply a global spacing value, enter a value in theDefaultcell.

7. Click the grey cell in the upper left corner of the grid to highlight all cells. Alternatively,you can select cells as you do in Excel by clicking your left mouse button.

8. ClickApply to Selected Cellsto apply the spacing value to all cells.

Figure 1-8 DFA Constraint Spreadsheet

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Class-Based Considerations

Table sizes based on sourcing libraries are expected have hundreds or thousands ofentries. Classifying package symbols into hierarchical structures can help reduce thesize of the table significantly. As mentioned earlier, the dfa_update utility can be usedto add this property. Alternatively, the property can be assigned using the Show SymbolClassifications utility found in the DFA Spreadsheet as shown in the following figure.

1. Click on Show Symbol Classification.

2. If your library symbols already have DFA_DEVICE_CLASS properties assigned,those values are represented in the UI along with the list of instance-basedpackage symbols under thePackage Symbolcategory.

3. Select the class entry; click Updateto transfer them to the table as shown inFigure 1-9 on page 14.

4. The class based entr ies occur at the beginning of the table. A blue box surroundsthe values. Note the values inherit the setting of theDefaultcell.

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Figure 1-10 Cutting symbols and pasting into new class

Overriding Cells

The method for editing cells is similar to that used in Microsoft Excel. Drag cells with the leftmouse button or use the Ctrl or Shift keys to extend the selection. In Figure 1-11 on page 16,the cell entry of 25:20:25 represents the syntax for S-S:E-E:S-E.

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Figure 1-11 Cell Editing

Table Compression

Before releasing the table to production, the owner may elect to reduce the number of entries,(assuming class hierarchy exists), by removing symbol instances using Purge ClassifiedSymbols. This step can be applied in the Allegro PCB Editor on a design instance basis.

Table Access from DesignPath Variable

DFA Spreadsheet files should be centrally archived for thedesign team to access. The designpath variable DFACNSPATH set sthe default location of these files. The PCB designer canuse the file browser to navigate for .dfa files or the data browser that defaults to the locationset by this variable.

Read-Only Version

The stand-alone version of the DFA Spreadsheet Editor offers a setting to save the file in aread-only mode. When PCB designers open the file, its contents cannot be modified. A read-only file can be opened by the development version of the spreadsheet editor and modified.

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Loading Spreadsheet into Allegro PCB Editor

The PCB designer can load only one spreadsheet into Allegro PCB Editor. If a change occursas to how the board is assembled or soldered, additional spreadsheets can be applied tooverwrite the database contents. ChooseSetup Constraints DFA ConstraintSpreadsheet.To open a new spreadsheet, select one of the following:

File Open:standard file browser searching for .dfafiles.

File Browse DFA Library: search path aligned with DFACNSPATH variable.

The Allegro PCB Editor database stores the spreadsheet, assuming a save has beenperformed.

DFA-Driven Interactive Placement

DFA feedback requires the use of Place Manual. A new component placement iconperforms the same action as Place Manual Hide.

Designers currently using themove command for placement can get the benefits of the rightmouse buttonmirrorcommand, in addition to single-click execution; no find filteradjustmentsarenecessary. When placing a component to a DFA rule, a circle that representsthe clearance appears when the component being placed is at the minimum distance toadjacent components. A slight pause may be noticed when the circle appears. This allowsyou to instantiate the component at the minimum spacing. Pause levels can be controlled inthe User Preference Editor.

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Constraint Manager DRC Worksheet

DFA errors are itemized in the Design Worksheet under the heading of Package to PackageDFA Spacing. The worksheet provides information on the type of error in terms of side or end,subclass, required DRC value, actual clearance, and violating objects listed as referencedesignators

Figure 1-13 DFA Spacing Worksheet in Constraint Manager

User Preference Controls

The User Preference Editor features the category Dfa_drc, which includes:

dfa_pause_level Sets the pause level inplace manualcommand. If DFA DRC check is enabled

when the component reaches the minimalDFA spacing of another component, itpauses briefly, so it can be placed as closeas possible to the other component without aDFA DRC violation.

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display_nodfa_drc_marks If set, DRC markers for DFA violations do notdisplay dynamically during interactiveplacement.

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