al gro back drill

8/12/2019 Al Gro Back Drill

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Best Practices:

Working with BackdrillingSeries XL and GXL

Product Version 16.0

June 2007


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Best Practices: Working with Backdrilling

June 2007 3 Product Version 16.0

Backdrilling . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Licensing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Prerequisites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 5

Terminology . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 7

Net Identification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 8

Applying the BACKDRILL_MAX_PTH_STUB Property in Constraint Manager . . . . . . 8

Stub Calculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 9

Exclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 10Pin Plating Restrictions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 11

Applying the BACKDRILL_MIN_PIN_PTH Property . . . . . . . . . . . . . . . . . . . . . . . . . 11

Pressfit Connectors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 12

User Overrides . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Setup and Analysis User Interface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 14

Initially Launching the Backdrill Setup and Analysis Dialog Box . . . . . . . . . . . . . . . . 15

Why use the Cadence Default Passes? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 16

Set up of Custom Pass Sets . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 18

Analysis and Reporting . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 24Board Testability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 25

NC Drill and Legend Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Enabling Backdrill Output . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 26

Legend Customization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 28

Contents


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1

Backdrilling

Today's high-speed serial I/O technology handling 5 Gbps or higher presents new challengesfor hardware engineers.Passing high frequency signals over a backplane requiresminimizingthe 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 minimum

or 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.

Introduction

Backdrilling 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.

Licensing

Backdrilling is available in Allegro PCB Editor XL and above.

Prerequisites

Cross section must contain material thickness for both conductor and dielectric layers.


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Best Practices: Working with BackdrillingBackdrilling


Figure 1-1 Backdrill Flow in Allegro PCB Editor

Terminology

The following terms are associated with backdrilling.

Exclusions and restrictions

Analyze and Report

Output

Assign the BACKDRILL_MAX_PTH_STUB property

Assign the BACKDRILL_MIN_PIN_PTH property to symbols

Configure

Overrides

Apply the BACKDRILL_EXCLUDE property to symbols, pins orvias to exclude these elements from backdrill processing

minimum plating rules

to nets targeted for backdrilling [value = length]

Identify Nets

Apply the BACKDRILL_OVERRIDE property to pins or viasfor explicit From:To Layer assignments (used on test coupons

or pins to ensure backdrill depth does not violateAssign the BACKDRILL_MIN_PIN_PTH property to symbols

minimum plating rules

connectors if backdrilling is required from both sides of theAssign the BACKDRILL_PRESSFIT_CONNECTOR property to

board. Contact range must be known

Establish the number of passes for the From side(top, bottom, or both)

Determine the objects to drill (pins, vias, or both)

Graphical evaluation of pass setup, add passes or increasemax stub allowance to resolve remaining violations

Review log file for violations, exclusions, overrides,and general backdrilling statistics

NC Legend: Enable the backdrill option. Each pass isrepresented by a unique legend similar to the blind/buried via model

NC Drill: Enable the backdrill option. Each pass isrepresented as a unique drill file. Drill size is nominal; thevendor determines drill overage


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Net Identification

The 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 Constraint

Manager

The BACKDRILL_MAX_PTH_STUB property can be easily assigned to nets in ConstraintManager or by using Edit Propertyin 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.

Backdrill Secondary drilling process that removes the unused section ofplated through holes

Counter-bore Alternative term for backdrilling

Gbps Giga Bits per Second

PTH Plated through hole

Press fit Connector whose pins are forced (spring loaded) in PCB, notsoldered

Allegro PCB Editor XL Formally known as Expert or 610 series

RMB Right mouse button

Stub Unused section of a plated through hole resulting in signalquality issues (reflections, impedance discontinuity)


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6. Click OK.

Figure 1-2 Applying BACKDRILL_MAX_PTH_STUB in Constraint Manager

Tip

All backdrill application properties begin with a 'backdrill' prefix.

Stub Calculation

A 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.

Property BACKDRILL_MAX_PTH_STUB

Purpose Identification of nets targeted for backdrilling

Usage Required

Value Max allowable stub in length (database units)

Objects Nets

Backdrill analysis flag S


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For 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 layer

adjacent 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 calculation

Exclusions

It 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.

Property BACKDRILL_EXCLUDE

Purpose Excludes objects from backdrilling even though stub may bepresent

Usage Optional

Value Boolean (true/false)


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Pin Plating Restrictions

If 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 Property

The 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 depth

range. 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 thesignals on other layers to reduce the stub length whilemaintainingminimum pin plating requirements.

Objects Symbols, pins, vias

Backdrill analysis flag X


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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 plating

requirement; treat stub violations as acceptable.

Pressfit Connectors

The 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 but

not 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.

Property BACKDRILL_MIN_PIN_PTH

Purpose Ensures minimum pin plating depth is not compromised

Usage Optional

Value Length

Objects Symbols, pins

Backdrill analysis flag P


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If 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.

Property BACKDRILL_PRESSFIT_CONNECTOR

Purpose Use on pressfit connectors to permit backdrilling from bothsides of the board. Backdrill depth will not protrude into contactrange as defined by the property string.

Usage Optional

Value where values = pin contact range; this valuemust be obtained from the manufacturer

Objects Symbols

Backdrill analysis flag n/a


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Tip

Develop a plan to incorporate the following properties at the library level:BACKDRILL_MIN_PIN_PTH, BACKDRILL_EXCLUDE, andBACKDRILL_PRESSFIT_CONNECTOR.

User Overrides

In 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 designof 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.

Setup and Analysis User Interface

The heart of thebackdrill application is theBackdrill 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 of

graphical 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.

Property BACKDRILL_OVERRIDE

Purpose User-specified backdrill range

Usage Optional: special circumstances

Value TOP::BOTTOM:

Objects Pins, Vias

Backdrill analysis flag O or W (warning)


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Initially Launching the Backdrill Setup and Analysis Dialog Box

Upon invocation, you see two pre-seeded rows enabling backdrilling to be performed on all

objects, 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 Passescolumn. 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 Box


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Why use the Cadence Default Passes?

These passes can provide quick and valuable feedback as to the actual layer ranges required

to 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.


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Tip

The Backdrill Setupand Analysisdialogbox canbe 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.


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Set up of Custom Pass Sets

In all likelihood, the hardware or SI Engineer has provided some tolerance with respect to

stub 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.


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3. 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 Passescell.

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 layersfrom the dropdowns: The To Layerrepresents a signal or plane layer.The depth calculated includes the dielectric layer adjacent to it.


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7. 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.


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8. 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.


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10. Is Backdrilling required from the Top Side? If so, hover over any row, then RMB NewPass Set.

11. Enter Topin the From Sidecell, then enter object type and number of passes similar tothe method used for Bottom side entry.


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Analysis and Reporting

The 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 Analyzeis executed. When the Analyzebutton 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:


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Board Testability

The 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.

Code Description User Action

S Maximum Stub Length Violation Add new pass, increase max stub value onnet, reroute trace to reduce stub length

P Minimum Pin PTH LengthViolation

Increase max stub value on net, re-assesspass-set

C Pin is on component side Create new pass set from other side ofboard

T Pin/Via is a testpoint on the sidebeing backdrilled

Discuss with ATE, move testprobe to anotherobject. A tradeoff may be necessary. Knowyour strategy before you get to this point.

X Pin/Via excluded frombackdrilling

Review stub violations with EE/SI. Reroute ifpossible to reduce stub length.

R Pin/Via has no routedconnection

Remove backdrill_max_pth_stub propertyfrom net assuming net may be a 1 pin net.

O Pin/Via has user overrideproperty applied defining from:tolayer

Check log file for warning codes; over-drillinginto or beyond etch layer possible.

W Pin/Via has user overrideproperty applied, warning

detected as depth exceeds etchlayer making contact to pin/via

Check each location carefully to ensure thisis your intent


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NC Drill and Legend Output

The 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 layer

adjacent 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 Output

The output of backdrill data is controlled by the parameter Include backdrill located in boththe NC Drill and NC Legend dialog boxes.


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NC Legend

Similar 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 side

through 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.

Tip

Backdrill notes are supplied in the default .dltfile and can be edited toaccommodate your process requirements.


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Figure 1-5 Backdrill Legend

Legend Customization

The figure above shows the total drill count as well as separate unit columns for Inches andMetric. Total Quantity is enabled by editing the ?ShowTotalQuantityvariable.


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The additional Metric columns were added in two parts.

1. AlternateUnits variable is set to millimeters

2. ColumnDefinitions holesize2 and tolerance2 defined


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