Component Symbols
Introduction
The symbols used in Allegro PCB Editor are a collection of data that represents a physical entity that
describes a part, or drawing, that will be used as a part of a larger drawing which will represent a physical
board. There are five types of drawings that can be created with the Allegro PCB Symbol Editor:
• Package symbol (.psm) - This type is used for footprints that contain connectivity such as a BGA,
PLCC or DIP
• Mechanical symbol (.bsm) - This type is used for mechanical portions that may be used in a design.
This could be a mounting hole, board stiffener, or board outline.
• Format symbol (.osm) - This type represents a drawing sheet format (A through D size), a company
logo, assembly or fabrication notes, cross section diagram.
• Shape symbol (.ssm) - This type is a filled polygon used in the creation of custom pad shapes and
they will be used in the padstack editor.
• Flash symbols (.fsm) - This type represents a thermal connection used in a negative artwork film.
Procedures
Creating the Drawing
Start Allegro PCB Symbol editor and setup the design parameter to the correct type, units, accuracy and
extents that will allow you to place the needed elements in the drawing. You can set the origin of the
drawing using the Setup > Change Drawing Origin (chg origin command) in 16.3 or Move Origin
section within the Design Parameters dialog.
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The standards you use for placing the origin of the symbol is site specific however usually through hole
parts have the origin at pin 1 and smd parts have an origin in the middle of the component as is the case
for fsm, ssm, and osm symbols.
Do not set the accuracy to a greater accuracy than your board designs will be. When placing
symbols at a higher accuracy than the board designs rounding down may occur.
You can reuse customized parameter settings from one design by exporting them to a database
parameter file (.prm) with the File > Export > Parameters (param out command). Then when
you initially begin a design, import the .prm file with the File > Import > Parameters (param in
command). The techfile batch command can also be used to import or export database parameters.
Adding Pins
Selecting Layout > Pins will allow you to add both connect and mechanical pins (padstack), specified in
your padpath variable, to the drawing Connect pins are those that will have logical electrical connections
to them whereas mechanical pins will not have any logical connectivity. In the Options tab of Allegro
PCB Layout editor you can browse for the padstack you would want to use, set the connect type, set the
quantity of padstacks to use along with the spacing, rotation and pin number to start with.If you elect
to use a quantity of more than 1 Allegro PCB Layout Editor will place the padstack in the direction and
spacing specified with the pin number specified.
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As you add each pin, Allegro PCB Symbol Editor finds the library padstack, copies the padstack definition
into the symbol drawing, and displays the padstack graphics
Adding Outlines
You create the outlines (Assembly, Silkscreen, or other manufacturing data) of your symbol using
the Add > Line command. Set the Class/Subclass in the Options form to Package Geometry and
Assembly top (or whichever subclass you need to add data to) and define the outline using lines and
arcs.
You create the silkscreen for the component in the same fashion, by adding lines and arcs to the Package
Geometry/Silkscreen top subclass. When adding lines and arcs used for the outlines, Allegro, by default,
uses a zero width line. You can either override this in the Options form or at the end of the design. Set
the Undefined line with to a value in the Artwork Control Form.
Adding Labels
Labels are placeholders for component data such as assembly, silkscreen reference designators and
device types. The location of the label determines where the data is displayed in the board drawing. This
information can be moved later in the design process if needed.
Important
You must define one label in order to successfully create a footprint(.psm) type drawing.
Use the Options form to specify the drawing layer (class/subclass) for your text label. Depending on
which command you select, the Options form will default to an appropriate class/subclass setting. If you
are creating silkscreen text labels, you will need to toggle the Subclass field to Silkscreen_top.
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Adding Area Constraints
Allegro PCB Layout Editor allows you to create areas that will help you achieve your design goals. These
area are:
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•
•
•
Route Keepout - a user defined polygon that prohibits etch.
Wire Keepout - A user defined polygon that allows vias but prohibits etch.
Via Keepout - A user defined polygon that prohibits vias but allows etch.
Shape Keepout - A user defined polygon that adds shape keepout. As with all keepouts, the active
subclass provides control for individual layers or for creation on the ALL subclass.
• Package Keepout - Adds filled package/part keepout areas to your layout.
• Package Height - lets you attach properties defining a height restriction to a package/part keepout
or place bound rectangle. Keepouts allow package/part symbols whose height is below a minimum
or above a maximum to be placed in that area; place bound rectangles define package/part
boundaries in terms of height and package/ part geometry.
• Package Boundary - defines package/part boundaries indicating different height constraints under
component bodies.
Adding Constraint Regions
As designs get more dense you will find that you may need to work with constraint regions which allow
you to have a different spacing or physical line width within these regions. By creating regions within
the symbol editor it can facilitate the design process of the board by having uniform regions among like
symbols - for instance 4 BGAs of the same type in the design. To add a constraint region select Shape >
Polygon/Rectangle/Circle. Set the active class to Constraint Region and the subclass to the desired
layer. You should use care when setting the subclass as the setting All Layers could over constraining your
design. In the Options tab enter a name for the region before digitizing it. The name chosen will be added
to the layout drawing when the component is placed in the layout.
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If a part is placed on the opposite (mirrored) side of the design, the system will automatically move
all definitions from the top to the bottom layer
References
For more information, refer to Defining and Developing Libraries in the Allegro User Guide.
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