Backdrilling in SPB15_7

Backdrilling in SPB15_7Ron Dallas, Teradyne Inc.Abstract – Backdrilling is used to remove stubscaused by plated through holes in circuit boards. Thepresence of the stub affects high speed signal integrity.Communicating the requirements for backdrilling toa fabrication vendor with prior versions of Allegro wasa tedious and error prone process.This paper presents a new backdrilling capabilityavailable in the Allegro 15.7 release and how itimproves the generation of backdrilling data forfabrication.The Cadence Best Practices Backdrilling documentwhich describes in detail how to use the new capabilityis attached to this paper for reference.I. CAVEATSThis presentation was written based on pre-releasesoftware due to conflicts between the deadline forpaper and the release schedule for 15. There may beslight differences between what is presented and thereleased software.ANDThis paper will NOT discuss signal analysis. It isassumed that the user knows whether backdrilling isneeded or not. It does dis cuss methods of specifyingbackdrilling for fabrication.II. WHAT IS "BACKDRILLING"?A. Why do we Backdrill?Backdrilling is the removal of the plated holebarrel, to a specified depth, by drilling the plated holea second time with an oversize drill.In Fig. 1, the hole was backdrilled from the bottomthrough signal layer S3 into the dielectric d2.Fig. 1 Example of BackdrillingA signal traversing the original via from layer S1to S2 would also travel down the rest of the via to thebottom of the board and reflect, creating a waveformdisturbance.B. Communicating backdrilling informationOur current methods of communicatingbackdrilling information are rather primitivecompared to normal drill information.We need to specify the location, size, depth andside to drill from for the backdrills that we want.However Allegro only creates one drill instance perhole. We need two, one for all the original drills anda second for the few (hopefully) that we want tobackdrill.To commu nicate this, we can change the drill Fig.for the backdrill holes and/or change the drill size toa fractional value, like 13.1 mils to indicate drill firstat 13 mils, then backdrill from the top. Similarly 13.2would indicate backdrilling from the bottom.C. Changing the design for backdrillingThe engineer tells the PCB designer which holes ornets need to be backdrilled. In either case, a lot ofmanual poking around on the board is necessary tolocate the desired vias and replace them with newvias that have modified drill figures and/or drill sizes.Manually poking around the design is tedious andcan introduce errors if vias are missed. And, ofcourse, the designer has to create the new padstacksbefore he can do the replacement.The major problem with changing just the drill Fig.is that no NC drill data is generated to specify theholes–their location has to be derived by listingpadstack locations with particular drill figures.If the drill size is changed, we get NC drill datawhich is a significant improvement.The depth is either determined by the vendor–stopat first layer with a trace–or by a note on thefabrication drawing. In the first case, a dangling linesegment would stop the drill too early. If the depth isspecified in a fabrication note the drill could sever aconnection accidentally placed on a layer too close tothe outside of the board. The vendor should identifythis as an issue but the problem is often overlooked.III. ALLEGRO 15.7 SOLUTIONIn Allegro 15.2 capability for creating drill data forblind and buried vias was added. In Allegro 15.7, thisCDNLive 2006 Session #5.10

Integrating Thermal Analysis with Front to Back Process 2is extended for backdrilling. Additional legends,diagrams, and NC drill files are generatedspecifically for the backdrill data. A check box hasbeen added to the NC drill parameters form to enablethe generation of NC data for backdrills.The backdrilling analysis tool assists the engineerand designer when specifying backdrills. This isaccessed by the command "backdrill setup" or fromthe menu under "Manufacturing."To specify and constrain backdrilling some newproperties have been added.A. Drill Legend and DiagramThe drill legend capability has been ext ended toinclude some backdrill specific items in the drilltemplate file. These are described in detail in the onlinedocumentation.Additional legends and diagrams are created forbackdrilling, one per depth and side. The user willprobably want to make additional fabrication filmsfor these, a small amount of additional work thatgreatly clarifies the intent for the fabricator.The drill diagrams, which look like other drilldiagrams, and the legend are created on additionalsub-classes in MANUFACTURING class. A sampledrill legend is shown below. Note that the header lineincludes the word backdrilling.Fig. 3 Head of Drill Tape FileC. AnalyzerThe analyzer assists the user in defining andchecking backdrilling. A table defines the alloweddepths, based on layers, for backdrilling and thereport shows whether the intended result wasachieved.The default rows in the table give the analyzer freerein to define as many different drill depths asneeded. Since it always gives the minimum stubpossible within the enabled rows, the default givesthe best signal performance, and is the most costlysince many drill setups are needed.Fig. 4 Default Analysis TableFig. 2 NC Dill LegendB. Drill FileAn NC drill file is created for each drill depth andside. They are named similar to the other drill files,with a "-bd-from-to" string following the board name,like:board-bd-top-11.drlThe default is a good starting point to guide userswhile setting up a more restrictive set of depths,using the report to see where a longer stub can betolerated to minimize the number of different depths.Then the user can add rows restricting backdrilling tospecific layer sets.The header of the drill file shown in Fig 3. clearlyindicates that it is for backdrilling.Fig. 1 Analysis Table with specified depthsThe results are reported in both a text file and aCDNLive 2006 Session #5.10

Integrating Thermal Analysis with Front to Back Process 3drill-diagram like display on theMANUFACTURING/ BACKDRILL-FLAGsubclasses.If html reports are enabled, the analysis report,backdrill_analysis.log, has clickable links to thepadstacks listed.Fig. 5 shows two sections of the analysis report.Notice in the upper section that one pin/via has been"excluded" from backdrilling because it was atestpoint. The lower section identifies the specific testpoint with a hyperlink for the coordinates.Fig. 2 Backdrill report - 2 sectionsFig. 6 shows both the drill diagram and thebackdrill analysis for that section of the design.Notice the square with a "T" in it that indicates thatthis pin could not be backdrilled because it was a testpoint on the start side of backdrill drill.BACKDRILL_EXCLUDE (symbol, pin, via)BACKDRILL_MIN_PIN_PTH (symbol, pin)BACKDRILL_OVERRIDE (symbol, pin, via)BACKDRILL_PRESSFIT_CONNECTOR (symbol)The analysis tool uses the properties to determinewhether backdrilling can be done to achieve thedesired stub. They are described in detail in theCadence documentation.IV. UNSOLVED PROBLEMThe new backdrilling capability very nicely createsthe drill data needed for backdrilling. However thereis one major problem left–the need for additionalclearance between the drill and adjacent copper.The drill used for backdrilling has to be largeenough to completely drill out the plated hole barrel.The original drill typically tears the edges of the holeand the plating gets applied to this rough edge. Thismeans we need a drill large enough to remove thetorn piece as well so several mils of additional sizeare needed.In addition, there is a manufacturing tolerance onthe location of the drill, which may not be exactlycentered on the original drill. This adds some moremils to the size.Then there is the tearing of the backdrill hole. Thisis bare substrate, like the board edge or an unplatedhole, and tearing it allows contaminates to wickcopper into the substrate. In some cases copper wouldbe exposed which would not be good at all. Evenmore mils are needed to keep copper out of the tornarea.Altogether, an additional 5 mils of clearance mightbe needed. In other words the pads and anti-padsneed to be 10 mils bigger in diameter than theoriginals.Fig. 3 Analysis and Drill diagramD. PropertiesThe main property used for backdrilling isBACKDRILL_MAX_PTH_STUB. This is attached to nets ,generally using the Constraint Manager, to indicatethat the padstacks on the net should be backdrilledand the maximum stub length allowed.Other backdrill properties can exclude specificpins or vias and specify the depth to drill instead ofdetermining it from where traces connect to thepadstack. These are:Fig. 4 Example of ClearancesTo save space on non-backdrilled layers, thisadditional clearance can be added only to the sectionof the padstack that is backdrilled. In any case, Thepad stack needs to be changed, unless designer wasclever enough to anticipate backdrilling when the viawas added. The BACKDRILL_MAX_PTH_STUBproperty can be used to highlight nets while routingto identify the nets requiring backdrilled vias to guideCDNLive 2006 Session #5.10

Integrating Thermal Analysis with Front to Back Process 4the designer while creating vias.V. CONCLUSIONSThe old method of backdrilling in Allegro, usingthe following steps, was tedious, time consuming anderror prone. It involved a lot of time on the part of theengineer and designer to simply identify which viasneed to be backdrilled and how far.OLD METHODCreate custom padstacksHighlight netsIdentify vias/pins and sideReplace padstacksGenerate drill dataAdd notes for backdrillingTakes daysError proneVI. ACKNOWLEDGMENTSThanks to Ed Hickey of Cadence, who encouragedme to write this paper. His paper Best Practices–Working with Backdrilling published in the Cadence15.7 on-line documentation was a great help in usingthe new capability. A copy is included at the end ofthis paper.VII. AUTHOR CONTACT INFORMATIONRon DallasTeradyne, Inc.600 River Park DriveNorth Reading, MAron.dallas@teradyne.comThe new backdrilling capability in Allegro version15.7 is quicker–hours vs. days–and is less errorprone. The ability to change the padstacks to get theneeded additional clearance would reduce the timefrom hours to minutes.NEW METHODAdd property in constraint managerReplace padstacksGenerate drill dataTakes hoursLess ErrorsCDNLive 2006 Session #5.10

Best Practices: Working with BackdrillingProduct Version 15.7July 2006

© 2006 Cadence Design Systems, Inc. All rights reserved.Printed in the United States of America.Cadence Design Systems, Inc., 555 River Oaks Parkway, San Jose, CA 95134, USATrademarks: Trademarks and service marks of Cadence Design Systems, Inc. (Cadence) contained inthis document are attributed to Cadence with the appropriate symbol. For queries regarding Cadence’strademarks, contact the corporate legal department at the address shown above or call 800.862.4522.Open SystemC, Open SystemC Initiative, OSCI, SystemC, and SystemC Initiative are trademarks orregistered trademarks of Open SystemC Initiative, Inc. in the United States and other countries and areused with permission.All other trademarks are the property of their respective holders.Restricted Print Permission: This publication is protected by copyright and any unauthorized use of thispublication may violate copyright, trademark, and other laws. Except as specified in this permissionstatement, this publication may not be copied, reproduced, modified, published, uploaded, posted,transmitted, or distributed in any way, without prior written permission from Cadence. This statement grantsyou permission to print one (1) hard copy of this publication subject to the following conditions:1. The publication may be used solely for personal, informational, and noncommercial purposes;2. The publication may not be modified in any way;3. Any copy of the publication or portion thereof must include all original copyright, trademark, and otherproprietary notices and this permission statement; and4. Cadence reserves the right to revoke this authorization at any time, and any such use shall bediscontinued immediately upon written notice from Cadence.Disclaimer: Information in this publication is subject to change without notice and does not represent acommitment on the part of Cadence. The information contained herein is the proprietary and confidentialinformation of Cadence or its licensors, and is supplied subject to, and may be used only by Cadence’scustomer in accordance with, a written agreement between Cadence and its customer. Except as may beexplicitly set forth in such agreement, Cadence does not make, and expressly disclaims, anyrepresentations or warranties as to the completeness, accuracy or usefulness of the information containedin this document. Cadence does not warrant that use of such information will not infringe any third partyrights, nor does Cadence assume any liability for damages or costs of any kind that may result from use ofsuch information.Restricted Rights: Use, duplication, or disclosure by the Government is subject to restrictions as set forthin FAR52.227-14 and DFAR252.227-7013 et seq. or its successor.

Best Practices: Working with BackdrillingContentsBackdrilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Licensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7Net Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8Applying the BACKDRILL_MAX_PTH_STUB Property in Constraint Manager . . . . . . 8Stub Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9Exclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Pin Plating Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11Applying the BACKDRILL_MIN_PIN_PTH Property . . . . . . . . . . . . . . . . . . . . . . . . . 11Pressfit Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12User Overrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Setup and Analysis User Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14Initially Launching the Backdrill Setup and Analysis Dialog Box . . . . . . . . . . . . . . . . 15Why use the Cadence Default Passes? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16Set up of Custom Pass Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18Analysis and Reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Board Testability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25NC Drill and Legend Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Enabling Backdrill Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26Legend Customization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28July 2006 3 Product Version 15.7

Best Practices: Working with BackdrillingJuly 2006 4 Product Version 15.7

Best Practices: Working with Backdrilling1BackdrillingToday's high-speed serial I/O technology handling 5 Gbps or higher presents new challengesfor hardware engineers. Passing high frequency signals over a backplane requires minimizingthe effect of plated through-hole (PTH) stubs. This can be controlled in the design space byusing the full length of the barrel for signal layer transitions thus keeping stubs to a minimumor with the use of buried or blind vias. If these options are not available, stubs can be removedthrough a board-fabrication process called backdrilling, sometimes referred to as controlleddepth counter-boring. The procedural steps and best practices for a successfulimplementation of backdrill setup and processing in Allegro PCB Editor are discussed indetail.IntroductionBackdrilling is a board fabrication process that removes the unused section of plated throughholes; typically connector pins and signal vias. A secondary, controlled depth drilling pass(s)removes all electro-deposited plating material in the PTH ensuring signal stubs areminimized. Stubs are the source of impedance discontinuities and signal reflections whichbecome more critical as data rates increase. Backdrilling can be performed from either sideof the PCB and to multiple depths. Drill sizes used for backdrilling are typically 6 to 10 milslarger than the original tooling. Fabricators must be careful not to over drill beyond thecalculated depths but also not to under drill, leaving unacceptable stubs. Tradeoffs betweensignal quality and manufacturing costs must be considered.LicensingBackdrilling is available in Allegro PCB Editor XL and above.PrerequisitesCross section must contain material thickness for both conductor and dielectric layers.July 2006 5 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingMenu LocationCommandRelevant PropertiesRelevant ParametersAssociative SubclassesBackdrill Application InformationManufacturing - NC - Backdrill Setup and Analysisbackdrill setupBACKDRILL_MAX_PTH_STUB (net)BACKDRILL_EXCLUDE (symbol, pin, via)BACKDRILL_MIN_PIN_PTH (symbol, pin)BACKDRILL_OVERRIDE (symbol, pin, via)BACKDRILL_PRESSFIT_CONNECTOR (symbol)Drill Legend (Include Backdrill)NC Drill (Include Backdrill)BACKDRILL-FLAG-TOP (violations-top)BACKDRILL-FLAG-BOT (violations-bot)NCBACKDRILL_LYR_LYR (legend)July 2006 6 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingFigure 1-1 Backdrill Flow in Allegro PCB EditorIdentify NetsAssign the BACKDRILL_MAX_PTH_STUB propertyto nets targeted for backdrilling [value = length]Exclusions and restrictionsApply the BACKDRILL_EXCLUDE property to symbols, pins orvias to exclude these elements from backdrill processingAssign the BACKDRILL_MIN_PIN_PTH property to symbolsor pins to ensure backdrill depth does not violateminimum plating rulesOverridesApply the BACKDRILL_OVERRIDE property to pins or viasfor explicit From:To Layer assignments (used on test couponsAssign the the BACKDRILL_MIN_PIN_PTH BACKDRILL_PRESSFIT_CONNECTOR property to symbols property toconnectors if backdrilling is required from both sides of theminimum board. Contact plating rules range must be knownConfigureEstablish the number of passes for the From side(top, bottom, or both)Determine the objects to drill (pins, vias, or both)Analyze and ReportGraphical evaluation of pass setup, add passes or increasemax stub allowance to resolve remaining violationsReview log file for violations, exclusions, overrides,and general backdrilling statisticsOutputNC Legend: Enable the backdrill option. Each pass isrepresented by a unique legend similar to the blind/buried via modelNC Drill: Enable the backdrill option. Each pass isrepresented as a unique drill file. Drill size is nominal; thevendor determines drill overageTerminologyThe following terms are associated with backdrilling.July 2006 7 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingBackdrillCounter-boreGbpsPTHPress fitAllegro PCB Editor XLRMBStubSecondary drilling process that removes the unused section ofplated through holesAlternative term for backdrillingGiga Bits per SecondPlated through holeConnector whose pins are forced (spring loaded) in PCB, notsolderedFormally known as Expert or 610 seriesRight mouse buttonUnused section of a plated through hole resulting in signalquality issues (reflections, impedance discontinuity)Net IdentificationThe first step in the backdrill application is the identification of nets targeted for potentialbackdrilling. The word potential is used, as ultimately backdrilling only affects pins and viason nets that violate the maximum stub rule. Although a net is identified, it may be omitted frombackdrilling if stub lengths are within margin. The net level propertyBACKDRILL_MAX_PTH_STUB can be applied at the schematic level, Cadence or thirdparty, or within Allegro PCB Editor/Constraint Manager. The value of this property is themaximum allowable PTH stub and is restricted to length in database units.Applying the BACKDRILL_MAX_PTH_STUB Property in ConstraintManagerThe BACKDRILL_MAX_PTH_STUB property can be easily assigned to nets in ConstraintManager or by using Edit – Property in Allegro PCB Editor. The Constraint ManagerProcess steps are as follows:1. Open the General Properties worksheet located in the Net Workbook.2. Scroll across the worksheet to the Backdrill Column.3. Select relevant cells where backdrill property is to be applied.4. RMB – Change.5. Enter the maximum stub value, or enter 0 if no stub is allowed.July 2006 8 Product Version 15.7

Best Practices: Working with BackdrillingBackdrilling6. Click OK.Figure 1-2 Applying BACKDRILL_MAX_PTH_STUB in Constraint ManagerPropertyPurposeUsageValueObjectsBackdrill analysis flagBACKDRILL_MAX_PTH_STUBIdentification of nets targeted for backdrillingRequiredMax allowable stub in length (database units)NetsSTipAll backdrill application properties begin with a 'backdrill' prefix.Stub CalculationA stub is calculated from either the top or bottom side of the board to the adjacent dielectriclayer of where the trace enters the PTH. It is a prerequisite that all information pertaining toconductor and dielectric thickness be entered in the Cross Section form for stub analysis tobe performed. The following example illustrates how a stub is calculated in Allegro PCBEditor.July 2006 9 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingFor example, a trace enters a pin on Layer_5, resulting in a stub from both the top and bottomside. The top side stub includes the thickness of conductor layers Top, Layer_2P, Layer_3,and Layer_4P in addition to the four dielectric layers, which includes the dielectric layeradjacent to Layer_5. The bottom side stub in calculated in a similar manner, utilizing layersBottom, Layer_9_P, Layer_8, Layer_7_P, Layer_6 and the five dielectric layers. Each sidebased stub, not the accumulated total, is compared individually against theBACKDRILL_MAX_PTH_STUB property value.Figure 1-3 Example: Stub calculationExclusionsIt may be necessary to exclude certain objects from backdrilling even though stub violationsare present. The BACKDRILL_EXCLUDE property can be applied to symbols, pins or vias atboth the library and design level using Edit – Property. Common examples might includesolder tail connectors or a dense pin escape pattern in BGA areas. The PCB designer shouldconsult with a manufacturing engineer before assuming all pin/via objects are suitable forbackdrilling.PropertyPurposeUsageValueBACKDRILL_EXCLUDEExcludes objects from backdrilling even though stub may bepresentOptionalBoolean (true/false)July 2006 10 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingObjectsBackdrill analysis flagSymbols, pins, viasXPin Plating RestrictionsIf soldered connectors are to be backdrilled, the manufacturer may specify a minimum platingdepth to ensure its pins are fastened to the PCB properly. If pins on a connector with thisrestriction are to be backdrilled, the minimum pin plating depth into the PCB cannot becompromised.For example, consider the extreme case where backdrill passes are set up to remove theentire stub (BACKDRILL_MAX_PTH_STUB = 0). A trace connects two pins on layer TOP.When backdrilling is performed from the bottom side of the PCB, it will remove the entire stubby drilling beyond Layer 2, stopping in the dielectric material before Layer Top. The remainingplating thickness after backdrilling is the conductor thickness of layer TOP (approximately 2mils) plus a portion of the adjacent dielectric layer. The sum of these thicknesses is unlikelyto meet minimum plating requirements for the connector.Applying the BACKDRILL_MIN_PIN_PTH PropertyThe BACKDRILL_MIN_PIN_PTH property can be applied to symbols or pins using Edit –Property. Backdrill passes are prevented from removing a stub on objects within this depthrange. If a pass falls within the range of the minimum pin plating depth, the backdrill analysiscode flags that location with a "P" symbol, indicating a stub does exist, but the pin will not bebackdrilled due to the plating restriction. The backdrill code will not automatically 'back-off' toa depth where the minimum plating requirement is satisfied but results in a stub violation. ThePCB designer, when assessing P code violations, is advised to:■Consider rerouting the signals on other layers to reduce the stub length while maintainingminimum pin plating requirements.July 2006 11 Product Version 15.7

Best Practices: Working with BackdrillingBackdrilling■■■Consult with engineering and discuss the possibility of increasing the value of theBACKDRILL_MAX_PTH_STUB property on the affected nets.Create a new backdrill pass set to accommodate the targeted layer to backdrill to.Apply the BACKDRILL_OVERRIDE property to a layer that satisfies the min pin platingrequirement; treat stub violations as acceptable.PropertyPurposeUsageValueObjectsBackdrill analysis flagBACKDRILL_MIN_PIN_PTHEnsures minimum pin plating depth is not compromisedOptionalLengthSymbols, pinsPPressfit ConnectorsThe backdrill code under normal circumstances will not backdrill pins from the same side aconnector is placed on. The exception to this rule may apply to your pressfit connectors.Pressfit connector pins are spring loaded or forced into the PCB making contact with thebarrel of the hole in a specific range. A stub can result from both sides of the board up to butnot including either the trace and/or the contact range.The illustration below shows an actual pressfit pin and how a trace entering just above thecontact range results in stubs from both the top and bottom side of the PCB.July 2006 12 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingIf backdrilling is required from both sides, the propertyBACKDRILL_PRESSFIT_CONNECTOR must be applied to the symbol. Backdrilling will notbe permitted into the defined contact range.After some investigation, the contact ranges may not be specified on the actual connectorspecifications. Inquiries to the manufacturer may be required to obtain this information.PropertyPurposeUsageValueObjectsBackdrill analysis flagBACKDRILL_PRESSFIT_CONNECTORUse on pressfit connectors to permit backdrilling from bothsides of the board. Backdrill depth will not protrude into contactrange as defined by the property string.Optional where values = pin contact range; this valuemust be obtained from the manufacturerSymbolsn/aJuly 2006 13 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingTipDevelop a plan to incorporate the following properties at the library level:BACKDRILL_MIN_PIN_PTH, BACKDRILL_EXCLUDE, andBACKDRILL_PRESSFIT_CONNECTOR.User OverridesIn most cases, the backdrill layer ranges defined in the Backdrill Setup and Analysis userinterface satisfy your requirements to manage stubs. The BACKDRILL_OVERRIDE propertyallows user-specified control of layer ranges for any pin or via and from ether side of the boardor both. This is done regardless of violations that may result, such as testpoint conflicts orbackdrilling through a connection. The override property was considered for the design of testcoupons where the OEM wishes to evaluate the performance of the board fabricator withrespect to the adherence of depth ranges. The designer may override the "to" layer toeffectively drill out the layer where the trace connects the pin/via. The expected usage of thisproperty is limited; please exercise caution to prevent accidental over-drilling.PropertyPurposeUsageValueObjectsBackdrill analysis flagBACKDRILL_OVERRIDEUser-specified backdrill rangeOptional: special circumstancesTOP::BOTTOM:Pins, ViasO or W (warning)Setup and Analysis User InterfaceThe heart of the backdrill application is the Backdrill Setup and Analysis user interface locatedin the Manufacturing – NC menu of Allegro PCB Editor. This controls the side of the boardto be backdrilled from, object types, and number of passes per side. Analysis in the form ofgraphical violations and a detail log file is provided when Analyze is performed. The UI isfacilitates experimentation by editing pass sets. The actual pass sets to effectively removestubs into their margin range may require some trial runs before a final assessment is made.July 2006 14 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingInitially Launching the Backdrill Setup and Analysis Dialog BoxUpon invocation, you see two pre-seeded rows enabling backdrilling to be performed on allobjects, from either side of the board, and to all layers necessary to completely remove theentire stub regardless of the value assigned to the BACKDRILL_MAX_PTH_STUB property.Essentially if using the Cadence default passes, you allow the software unlimited bandwidthto completely remove the entire stub of nets assigned the BACKDRILL_MAX_PTH_STUBproperty. In Figure 1-4, note the number 21 in the Passes column. This number is derivedfrom the total layer count of the PCB, which 22. Backdrilling in this case would be allowed todrill any of 21 combinations of layer ranges from each side of the board but never from Top toBottom or Bottom to Top.Figure 1-4 Backdrill Setup and Analysis Dialog BoxJuly 2006 15 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingWhy use the Cadence Default Passes?These passes can provide quick and valuable feedback as to the actual layer ranges requiredto backdrill out all stubs. This of course can be a very expensive approach to backdrilling andfor most is probably not within the realms of possibilities at this time. This setting, however,could be the standard in years to come as signal rates continue to increase. After runningAnalyze, view the log file and note information in the General Analysis Section. This may giveyou an idea how to plan your strategy for your pass setup and alsoBACKDRILL_MAX_PTH_STUB values.July 2006 16 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingTipThe Backdrill Setup and Analysis dialog box can be used to experiment with differentpass set strategies. The Cadence-supplied pass sets provide quick means toevaluate the steps required to remove all stubs on nets identified with theBACKDRILL_MAX_PTH_PROPERTY.July 2006 17 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingSet up of Custom Pass SetsIn all likelihood, the hardware or SI Engineer has provided some tolerance with respect tostub lengths. The use of the Cadence default pass sets would be too aggressive andexpensive of an approach to backdrilling if margins or error are permitted. The following arethe steps required to add user defined passes.1. Disable default rows: After opening the dialog box, disable the two default rows as shownbelow.2. Insert new Pass Set: Hover over an existing row, then click Right Mouse Button (RMB) –New Pass Set to add the third row.July 2006 18 Product Version 15.7

Best Practices: Working with BackdrillingBackdrilling3. Enter side and object Information: The drop-downs provide Boolean options for side ofboard (top or bottom) and objects (pin, vias, pins&vias).4. Enter number of Passes: Estimate the number of pass sets you need to meet stubrequirements. Enter an integer in the Passes cell.5. The value entered automatically expands the pass set to that number of rows. The nextstep is to map the bottom side to user-selected layers.6. Select To layers from the dropdowns: The To Layer represents a signal or plane layer.The depth calculated includes the dielectric layer adjacent to it.July 2006 19 Product Version 15.7

Best Practices: Working with BackdrillingBackdrilling7. The log file provides the layout cross section and depths from either side. The depthincludes the adjacent dielectric layer. For example:Layer Top to Top = .700 + 6.000 = 6.7000.July 2006 20 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingJuly 2006 21 Product Version 15.7

Best Practices: Working with BackdrillingBackdrilling8. Enable one or more of the pass sets: To enable all, use RMB – Enable Pass Set.9. Adding new row to pass set: Hover over any existing row in the pass set, then RMB -Insert. The new row is added below the row the cursor is on.July 2006 22 Product Version 15.7

Best Practices: Working with BackdrillingBackdrilling10. Is Backdrilling required from the Top Side? If so, hover over any row, then RMB – NewPass Set.11. Enter Top in the From Side cell, then enter object type and number of passes similar tothe method used for Bottom side entry.July 2006 23 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingAnalysis and ReportingThe complete removal of all stubs to within their specified margins is expected to be arecursive process. It may be necessary to adjust your pass sets or max stub values to resolvethe violations that are generated after Analyze is executed. When the Analyze button isselected, violations and exclusions are graphically represented on subclasses BACKDRILL-FLAG-TOP or BOTTOM as well as listed in the log file. The violations are not DRCs, but rathercharacter-based codes that provide information as to remaining stubs or objects tagged asexclusions.The following table outlines the violation codes and suggests actions to take whenconsidering resolution:July 2006 24 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingCode Description User ActionS Maximum Stub Length Violation Add new pass, increase max stub value onnet, reroute trace to reduce stub lengthPMinimum Pin PTH LengthViolationIncrease max stub value on net, re-assesspass-setC Pin is on component side Create new pass set from other side ofboardTXROWPin/Via is a testpoint on the sidebeing backdrilledPin/Via excluded frombackdrillingPin/Via has no routedconnectionPin/Via has user overrideproperty applied defining from:tolayerPin/Via has user overrideproperty applied, warningdetected as depth exceeds etchlayer making contact to pin/viaDiscuss with ATE, move testprobe to anotherobject. A tradeoff may be necessary. Knowyour strategy before you get to this point.Review stub violations with EE/SI. Reroute ifpossible to reduce stub length.Remove backdrill_max_pth_stub propertyfrom net assuming net may be a 1 pin net.Check log file for warning codes; over-drillinginto or beyond etch layer possible.Check each location carefully to ensure thisis your intentBoard TestabilityThe decision to backdrill may impact your in-circuit test strategy. Any pin or via tagged as atestprobe will not be backdrilled from that side. This could cause a significant reduction in testcoverage as most testprobes are placed on pin and via objects. Take advantage of addingtestprobes directly on outer layer traces where feasible to do so.July 2006 25 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingNC Drill and Legend OutputThe output of backdrill data can be compared to the way buried/blind via data is output.Separate Legends and NC Drill files are created for each backdrill pass. While dielectriclayers are used in analysis calculations, the NC Drill output will be based on conductor layersonly. It is assumed fabricators will control the stop depth somewhere in the dielectric layeradjacent to the conductor layer listed in the NC output. Drill sizes listed are finished sizes.Allegro PCB Editor makes no attempt to list actual drill bits used for backdrilling.Enabling Backdrill OutputThe output of backdrill data is controlled by the parameter Include backdrill located in boththe NC Drill and NC Legend dialog boxes.July 2006 26 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingNC LegendSimilar to the standard NC Legend, the backdrill legend provides a symbolic representationof the sites to be backdrilled. The legend table lists finished size, not oversized tooling, alongwith Cadence supplied notes:1. Drill sizes listed in Legend are considered finished.2. Vendor is required to select tooling for overdrilling.3. Legend does not specify depth into adjacent dielectric layer.In Figure1-5, the NC Drill Legend accounts for all sites to be backdrilled from the Bottom sidethrough Layer_6 into the dielectric layer between Layer_6 and Layer_5. The actual depthrange into the dielectric material may vary from vendor to vendor, hence Cadence makes noeffort to supply controlled depth data.TipBackdrill notes are supplied in the default .dlt file and can be edited toaccommodate your process requirements.July 2006 27 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingFigure 1-5 Backdrill LegendLegend CustomizationThe figure above shows the total drill count as well as separate unit columns for Inches andMetric. Total Quantity is enabled by editing the ?ShowTotalQuantity variable.July 2006 28 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingThe additional Metric columns were added in two parts.1. AlternateUnits variable is set to millimeters2. ColumnDefinitions holesize2 and tolerance2 definedJuly 2006 29 Product Version 15.7

Best Practices: Working with BackdrillingBackdrillingJuly 2006 30 Product Version 15.7