Defining, Scoping & Managing PCB Design Rules in Altium NEXUS

Parent page: Pre-layout Stage

The PCB Editor uses the concept of Design Rules to define the requirements of a design. These rules collectively form an 'instruction set' for the PCB editor to follow. They cover every aspect of the design – from routing widths, clearances, plane connection styles, routing via styles, etc., and many of the rules can be monitored in real-time by the online Design Rule Checker (DRC).

Design rules target specific objects and are applied in a hierarchical fashion; for example, there is a clearance rule for the entire board, then perhaps a clearance rule for a class of nets, then perhaps another for one of the pads in a class. Using the rule priority and the scope, the PCB editor can determine which rule applies to each object in the design.

With a well-defined set of design rules, you can successfully complete board designs with varying and often stringent design requirements. Additionally, since the PCB Editor is rules-driven, taking the time to set up the rules at the beginning of the design will enable you to effectively get on with the job of designing, safe in the knowledge that the rules system is working hard to ensure that success.

Fundamentals of the PCB Rules System

The rules system built into the software's PCB editor has several fundamental features that set it apart from most other design rule systems employed by other PCB editing environments:

• Rules are separate from the objects – a rule is not added as an attribute of an object, but is rather added to the overall rule set and then scoped to apply to that object. This allows rules to be applied to multiple objects and modified or applied to different objects, which would otherwise be painful to do if having to change rule attributes at the individual object level.
• Rules are targeted (scoped) by writing a query – instead of using a set of fixed, predefined rule scopes, a flexible query system is used to define the objects to which a rule is applied. This gives precise control over the target of each and every design rule.
• Rules for any design situation – multiple rules of the same type can be defined and targeted to different sets of objects, allowing complete control over the definition of board constraints. For example, different width rules can be defined to route nets at different widths on different layers.
• Each rule has a priority – any design object can be targeted by multiple rules of the same type, catering for general and more specific situations. To resolve any rule contention, the rule priority is used. The system simply goes through the rules from highest to lowest priority and picks the first one whose scope expression(s) match the object(s) being checked.
• There are two types of rules – unary rules (rules that define the required behavior of an object) and binary rules (rules that define the interaction between two objects).

Design rules in Altium NEXUS are defined and managed from within the PCB Rules and Constraints Editor dialog as described on this page. As an alternative approach for managing the design constraint rules, the document-based  Constraints Editor ([Constraints] document tab) can be used.

Defining and Managing Design Rules

The dialog has two sections:

• The tree on the left lists the different rule categories. Expand a category to display the individual rule types available. Expand a rule type to display all rules of that type that are currently defined.
• The right side of the dialog presents information in relation to what is currently selected in the tree – either a summary of defined rules of the selected rule type or category, all defined rules for the entire system, or, if an actual rule is selected, the constraints for that rule.

The PCB Rules and Constraints Editor dialog – command central for managing design rules.

Default Design Rules

When a new PCB document is created, it includes a number of default rules, which must exist for the correct functioning of the Design Rule Check system. If a default rule is deleted, it is automatically re-created when the PCB Rules and Constraints Editor is closed. If there are design rules that you do not wish to use, the correct approach to managing these rules is to disable them. 

To disable a rule, toggle the corresponding Enable option for that rule in one of the summary lists on the right-hand side of the PCB Rules and Constraints Editor dialog. A disabled rule appears 'grayed-out'. Learn more about disabling design rules.

Use the Enable option to disable a rule that you do not need. A disabled rule appears 'grayed-out'.

Creating a New Rule

To add a new design rule from within the PCB Rules and Constraints Editor dialog, navigate to and select the required rule type within the left-hand tree then click the New Rule button below the rule summary list, or right-click over the required rule type then select New Rule from the context menu.

The new rule will be added to the folder tree and will also appear in the summary list for that rule type.

Example creation of a new PCB design rule.

To access the scope and constraint attributes for the rule, either click on the entry for the rule in the folder-tree pane or double-click on its entry in a summary list. The main editing window of the dialog will change to give access to the controls for defining the scope and constraint attributes for that rule.

Accessing the detailed controls for the rule, including scope and constraints.

To fully define the new rule:

• Give the rule a meaningful name to make it identifiable.
• Define the scope of the rule by selecting scoping options from the drop-down(s), or by entering a query (or queries for a binary rule).
• Set the constraints of the rule.
• Set the priority of the rule.

Using the Rule Wizard to Create a New Rule

A new rule can also be created using the Design Rule Wizard. Access is made directly using the Design » Rule Wizard command or by clicking the Rule Wizard button at the bottom of the PCB Rules and Constraints Editor dialog.

Use the pages of the Wizard to create a new design rule. The steps are:

1. Choose the required rule type and giving it a meaningful name (and comment if required).
2. Define the scope of the rule. You will have the opportunity to further enhance scoping through an incarnation of the Query Builder that is built into the Wizard.
3. Set the rule priority.

Use the Design Rule Wizard to streamline rule creation.

Duplicating an Existing Rule

To quickly create an identical copy of an existing rule, use the duplicate feature. This feature can be accessed in two ways:

• Use the tree on the left to navigate to the required existing rule, right-click then choose Duplicate Rule from the context menu.
• Navigate to the specific rule type in the tree on the left, select the rule to be duplicated in the summary list on the right, then click the Duplicate Rule button below the list.

Streamline creation of similar rules using the rule duplication feature.

The duplicate rule will be named the same as the original, with the addition of a suffix (e.g., _1) to distinguish it. Its definition (scope, constraints, etc.,) will be identical to that of the original. In terms of priority, it will be given the next priority below that of the original rule. So, for example, if the original rule has priority 1, the duplicate will be given priority 2.

Creating a Rule from the PCB Filter Panel

Since a design rule is scoped to apply to a target set of design objects, it can be easier (and faster) to first identify those objects then create a design rule that targets them. The PCB Filter panel provides the facility for creating a design rule, the scope of which uses the currently defined query expression in the panel. So rather than struggling to visualize what your query expression might target, use the PCB Filter panel to test and tweak a query expression until only those required objects are filtered. By then creating a rule that uses that query expression, you are guaranteed to be targeting the correct set of objects.

To add a new design rule:

1. Click on the Create Rule button. The Choose Design Rule Type dialog will appear. This dialog lists each of the rule categories and rule types that are available in the PCB document.
2. Choose the type of rule to be created then click OK (or double-click directly on the entry).
3. The PCB Rules and Constraints Editor dialog will appear. A rule of the chosen type is created and the main editing window for the rule is displayed, ready to define specific constraints for the rule. The scoping option for the rule is set to Custom Query, and the query expression from the PCB Filter panel is entered into the query region accordingly.
4. Make changes to the rule's name and constraints as necessary. Also, change its priority if needed (it will be given the highest priority by default).

Create a new design rule directly from the PCB Filter panel – the filter query expression is used as the scope for the rule.

Reusing Past Query Expressions

Over time, a number of useful query expressions will be constructed in the course of laying out various boards. Typically, you will want to apply and re-apply the same queries, not only in the same design but across different designs. To allow this, the PCB Filter panel supports the notion of Historical and Favorite queries.

As a query is entered and applied from the panel, it will be added to a query 'history list'. In addition, that query can be added to a query 'favorites list' by clicking the Add To Favorites button. Use the History and Favorites buttons in the panel to access the corresponding tabs of the same name within the Expression Manager dialog to see these lists.

Access a history of applied query expressions and build a listing of favorite expressions to be reused time and again.

To use an expression in either list, double-click on its entry or select its entry and click the Apply Expression button. The Expression Manager dialog will close and the expression will be loaded into the Filter region of the PCB Filter panel.

This functionality streamlines (and expedites) the creation of rules with requisite scoping – retrieve a historical or favorite query expression, check that it still targets the required object set (apply the filter) then create a rule that utilizes that expression in its scoping, as detailed in the previous section.

Scoping a Design Rule

Altium NEXUS's PCB editor is a rules-driven environment. Design requirements are enforced through a well-defined set of design rules that collectively define the constraints for the board. Design rules target specific objects within a design. For the PCB rules system to know for which objects a given rule applies, it needs to know the scope of that rule, i.e. the extent of its application. Scoping, or targeting the rule, is performed in the PCB Rules and Constraints Editor dialog.

The default design rules, or a new rule that is added, will have the default rule scope of All, meaning it will be applied to all objects on the board. Using the drop-down, a simple rule scope can be quickly configured.

Set the scope of a design rule to define its application to the design objects.

Rather than being restricted to a predefined list of possible target options, each design rule can be more tightly scoped by writing what is called a Query.

To enter a query, set the first Where the Object Matches drop-down to Custom Query. It will display the query currently being used by the rules engine for this rule based on the current drop-down settings.

A query is essentially an instruction to the software that defines the set of design objects to be targeted. Queries are written using query keywords. In the same way that a query can be written in a Filter panel to find a specific set of objects, a query also can be written to define the objects that each rule targets. An example might be:

InNet('VBAT') And OnLayer('Bottom Layer')

If this query were to be used as the scope for a Width rule, whenever you route the VBAT net and switch to the bottom layer, the track width would automatically change to the width specified as part of that rule's constraints. Also, upon running a design rule check, any VBAT net routing on the bottom layer would have to have the specified width or it would be flagged as a violation.

Scoping the rules is based upon the query system. Use the Custom Query option to see the current query and change it using the query keywords if needed.

Scoping Options

Simple scoping options are provided that allow you to quickly generate scope queries. Select one of the options from the first drop-down field and, if required, use the subsequent drop-down list(s) to select the appropriate target, such as a Net, Layer, Footprint, Package, and so on. The scoping options presented are focused according to rule type.

Quickly create a query using the simple scoping options.

The Custom Query option enables you to write your own, perhaps more complex but also more specific query. You can type your own specific query for the rule scope directly into the query region to the right. Alternatively, two features are available to help in the creation of logical query expressions. They are the Query Builder and the Query Helper. These can be useful when you are unsure of the syntax of a query or the keywords that are available.

Use the Custom Query option to craft a more complex query expression.

Scoping a Rule with the Query Builder

Click the button to open the Building Query from Board dialog, which enables you to create a query for targeting specific objects in the design document by the simple construction of a string of conditional statements.

The left-hand section of the dialog is used to specify the condition(s) required to target the set of objects needed. Each condition is made up of a Condition Type and a Condition Value. Drop-down fields for these populate with entries that are relevant to building a scope for the current rule type and values for the chosen condition type, respectively.

As a condition is defined, a preview of the currently built query is shown in the right-hand section of the dialog. Conditions can be ANDed and/or ORed together, and also precedence incorporated (through the use of brackets/indenting) to refine the target set of objects. When the expression for the query has been defined as required, clicking OK will load the expression into the query region of the PCB Rules and Constraints Editor dialog.

Use the Query Builder to build a logical query expression through a series of conditional statements.

Scoping a Rule with the Query Helper

With the Custom Query option enabled, click the button to access the Query Helper dialog. The underlying Query Engine analyzes the PCB design and lists all available objects, along with generic keywords for use in queries.

Working with the Query Helper is fairly intuitive, even more so when you break it down into three distinct areas as shown in the image below:

Example complex query defined using the Query Helper.

These areas are:

1. Query Region – use this region of the dialog to compose a query expression. The expression currently in effect for the rule scope will be made available in this region, by default, when the dialog is accessed. You can type directly within the region; a context-sensitive 'prompt list' of possible keywords or objects will appear as an aid.
2. Operators – this region of the dialog provides a range of mathematical and logical operators for use when constructing an expression. Click a button to add that operator at the current cursor location within the query expression in the Query region above.
3. Categories, Keywords and Objects – this region of the dialog provides access to available PCB Functions, PCB Object Lists and System Functions, which can be used to create the query expression. Clicking on a sub-category within each of these three areas will display a list of corresponding keywords or objects in the region to the right. Locate the keyword or object to be used in the query string then double-click on its entry; the entry will be inserted at the current cursor location within the query expression in the Query region above.

Use the button (bottom-left of the dialog) to verify that an expression is syntactically correct. When the expression for the query has been defined as required, clicking OK will load the expression into the query region of the PCB Rules and Constraints Editor dialog.

Query Expression Operator Precedence

Before leaving this area, it is worth taking a look at the order of precedence in place for the operators used in logical Query expressions. After all, without such knowledge, an expression may not target the objects required.

Brackets have the highest precedence within an order of precedence that has been defined for the various operators, and which determines how queries are interpreted by the software (whenever the user has not provided brackets). The sequence of this order from highest to lowest is as follows:

1. Brackets
2. Not
3. ^, *, /, Div, Mod, And
4. +, -, Or, Xor
5. =, <>, <, >, <=, >=
6. &&, ||

Rule Prioritization

To simplify the process of defining and managing rules, the idea is to define general rules that cover broad requirements and then override these with specific rules in specific situations. For this to be possible, you need to be able to prioritize the rules in order to indicate which one to use when an object is targeted by multiple rules of the same type.

For example, to specify the most commonly used routing width on the board, define a single rule that applies to every net on the board. This rule can then be overridden for a specific net (or a class of nets for that matter) by adding another rule of the same type, but with a higher priority.

Another example could be the solder mask requirements. Here you would define one mask rule that targets every pad and via on the board, which could then be overridden for the pads in a specific footprint-kind. This footprint-specific rule could further be overridden for a specific pad in that footprint if required.

An important aspect of managing the rules is ensuring that all the priorities are set appropriately. When a new rule is created, it defaults to the highest priority. Use the Priorities button at the bottom of the PCB Rules and Constraints Editor dialog to configure the priorities in the Edit Rule Priorities dialog.

Change rule priorities in the Edit Rule Priorities dialog.

Initially, the dialog will list all rule instances for the rule type that is currently selected in the PCB Rules and Constraints Editor dialog. Use the Rule Type field to change the rule type and list the specific rules defined for that type. The defined rules are listed in order of current priority – from 1 (highest priority) downwards. Select a rule entry and use the Increase Priority and Decrease Priority buttons to move it up or down in the priority order respectively.

Modifying Existing Rules

Rules, of course, can be modified at any time. Indeed, to arrive at the final working set of rules often involves a few key refinements here and there. Typically this involves scoping to ensure the target design objects are being 'picked up' by the respective rule(s) as required. Select an existing rule in the PCB Rules and Constraints Editor dialog and make changes as necessary to its scoping and constraint attributes.

Changes made to existing rule definitions are highlighted in both the folder-tree pane and the applicable summary lists. Such entries are distinguished by the rule name becoming bold and an asterisk displayed to the right of the name. The asterisk is used to reflect that the rule is an existing rule that has been modified, rather than a newly-created rule (which is displayed in bold without an asterisk).

Example of an existing rule that has been modified. An asterisk is used to flag the modification, along with the rule name being made bold.

Flagging Invalid Rules

If a rule is detected as being invalid by the system – for example, it has an issue with its scoping query expression or a value for a constraint that is not allowed – it will be flagged as being invalid. Such a rule will be highlighted in red within the PCB Rules and Constraints Editor dialog both in the left-hand rule tree, and any summary view (rule category or rule type) in which the rule appears. In addition, the text for the rule type and rule category is also presented in red within the left-hand rule tree. So if you have collapsed an area of the rule tree that contains an invalid rule, you will still be alerted to it at a higher level in the hierarchy. A warning message will also appear if you attempt to close the dialog.

An example of the system flagging an invalid design rule, in this case, an erroneous scope query expression.

Disabling Rules

In the rules-driven environment of the software's PCB editor, it is not uncommon to build up a rather impressive and comprehensive array of rules with which to successfully constrain your boards. For whatever reason along the way, you may wish to disable some rules; perhaps they are not applicable to the board in question or they need to be temporarily disabled to ease the load on the Design Rule Checker (and speed up its performance as a result!). Disabling is a good way of keeping such rules just in case they are needed again in the future.

To disable a rule, toggle the corresponding Enable option for that rule in one of the relevant summary lists on the right-hand side of the PCB Rules and Constraints Editor dialog. A disabled rule also will appear 'grayed-out'.

Example disabled rules, appearing in gray font within a summary list.

Deleting Rules

To delete a single design rule from within the PCB Rules and Constraints Editor dialog:

• Use the tree on the left to navigate to the required existing rule, right-click then choose Delete Rule from the context menu.
• Navigate to the specific rule type in the tree on the left, then select the rule to be deleted in the summary list on the right. Then click the Delete Rule(s) button below the list.

The rule name will appear bold with strike-through highlighting to distinguish it as being a deletion that is yet to be 'applied'.

Deleting a single rule.

Many rule types have default rules created when a new PCB document is created. In a similar fashion, if all specific rules for one of those rule types are deleted, the default rule will be re-added automatically the next time the PCB Rules and Constraints Editor dialog is accessed. Alternatively, default rules can be created again by clicking the Create Default Rules button at the bottom of the dialog.

Exporting and Importing Rules

Design rules can be exported from and imported to the PCB Rules and Constraints Editor dialog. This allows you to save and load favorite rule definitions between different designs.

• To export – right-click anywhere within the tree on the left of the dialog and select Export Rules. The Choose Design Rule Type dialog opens in which you can choose the design rule(s) to export. In the Export Rules to File dialog that opens, you can name the .rul file and choose the location. Exported rules are stored in a PCB Rule file (*.rul).
• To import – right-click anywhere within the tree on the left of the dialog and select Import Rules. The Choose Design Rule Type dialog opens in which you can choose the design rule(s) to import. In the Import File dialog that opens, browse to the .rul file to import. 

Design Rule Reports

A report of currently defined design rules can be generated from within the PCB Rules and Constraints Editor dialog. The report can cater to all rule categories, a specific rule category, or a specific rule type. A report can be generated by:

• Accessing the required summary list, right-clicking then choosing the Report command from the context menu, or by clicking the Report button below the list.
• Right-clicking over the respective entry in the folder-tree then choosing the Report command from the context menu.

The Report Preview dialog will open with the appropriate report already loaded. Use this dialog to inspect the report using various page/zoom controls before ultimately exporting it to a file or printing it.

Generate a PCB Rules Report.

Defining Rules on the Schematic

Design constraints (rules) can be defined prior to PCB layout by adding Parameter Set directives to the schematic source document(s) with configured rule(s). The scope of the corresponding PCB design rule, created when the design is transferred to the PCB document, is determined by the nature of the object to which the parameter is assigned. The following table summarizes the schematic parameter-to-PCB rule scope options that are supported.

In each case, the method of adding a rule-based parameter is the same:

1. Add a parameter as a rule.
2. Select which rule type to use.
3. Configure the constraints for the chosen rule type.

Checking Rule Application

Depending on the board design, a fair number of design rules may need to be defined with scopes that range from the very simple to the very complex. It is a good idea to check that the rules defined do indeed target their intended objects. Care at the rule definition stage can save wasted time and effort tracing violations caused through incorrect rule scoping.

There are essentially two methods for verifying rule scopes – either by selecting design objects and interrogating the rules that currently apply to them or by taking a rule (in different locations or the software) and observing which objects fall under its scope.

From an Object's Perspective

For any placed object in the current design, you can quickly access information about which unary design rules apply to that object. Position the cursor over the object, right-click then select Applicable Unary Rules. All defined design rules that could be applied to the selected object are analyzed and listed in the Applicable Rules dialog.

Design rules applicable to a single example design object – unary rules.

Each rule listed in the dialog will have either a check () or a cross () next to it. A check indicates the rule with the highest priority out of all applicable rules of the same type; this is the rule currently applied. Lower priority rules of the same type are listed with a cross next to them, indicating that they are applicable but, as they are not the highest priority rule, they are not currently applied.

In a similar fashion, you also can access information about the binary design rules that applies between two placed objects in a design. Position the cursor over any object, right-click then select Applicable Binary Rules. Follow the prompts to select two objects in the design. The Applicable Rules dialog will then open and display all binary design rules that apply between those objects.

Design rules applicable between two example design objects – binary rules.

From a Rule's Perspective

Checking via the PCB Rules And Violations Panel

You can also quickly see what objects a particular rule applies to from the PCB Rules And Violations panel. The panel lists all currently defined rules for the design. All rules can be viewed or you can browse specifically by rule type – provided at least one rule of any given type has been defined for the active design. As you click on a specific rule in the Rules region of the panel, filtering will be applied using the rule as the scope of the filter. Only those design objects that fall under the scope of the rule will be filtered. By employing the Mask (or Dim) highlighting feature, you can quickly see the resulting objects targeted by the rule.

An example of using the PCB Rules And Violations panel to check which objects a selected rule applies to.

Using the PCB Rules And Violations panel to interrogate which objects a rule applies to is particularly useful when creating a query for a rule’s scope(s) since a rule can be edited directly from the panel and therefore, the query can be 'tweaked' until the desired objects are captured by the scope(s).

To edit the scope(s) for a rule as well as its constraints, either double-click on the rule's entry in the panel or right-click and choose Properties from the context menu. The relevant Edit PCB Rule dialog will appear from where the changes can be made.

Rules can be edited directly from the PCB Rules And Violations panel, allowing you to further refine rule scoping and/or constraints on the fly.

Checking via the PCB Rules and Constraints Editor Dialog

The PCB Rules and Constraints Editor dialog includes a query testing facility, allowing you to quickly see what objects a particular rule applies to. Click the button at the top-right of the main editing window for the rule you want to test. The Test Queries Result dialog will appear. This dialog reflects the number of objects falling under the scope of the expression, how many are applicable to the current rule, and how many are applicable based on the rule's current priority (where multiple applicable rules exist). By clicking on a link to the right, you can quickly filter to see the applicable objects in each case directly in the workspace.

Quickly view the objects captured by a rule's scoping expression using the Test Queries feature.