This will apply everywhere if no other rules are added. Given all these possibilities, how do we know which rule applies between two objects? Different EDA tools work in different ways of course, but the priority levels of rules are pretty standard across the industry.Īt the top level we have the board rule or default rule. This would let you define specific clearances between two tracks on the same net class such as a differential pair. For example, you can set up a rule which applies to a net class only with respect to other objects on the same net class. It is even possible to create more granular electrical rules. Once the net class is created, the user simply adds a design rule that applies to the named net class and specifies the required clearances. These properties include things like track width, via style used and of course clearances. A net class is just a user named set of signals that will have common properties on the layout. This is where net classes and electrical scope becomes useful. For example, more space may be required around power tracks than low current signal tracks. Sometimes the requirement is to apply clearances to types of signal rather than to physical areas. Then a rule is created that applies to that named area and the necessary clearances are defined. In Proteus, the room tool is used to draw out the area, specify which layers it applies to and give it a name. This type of rule is called a region rule or a room rule and may be handled differently by different EDA tools. To do this a layer rule would be created with different clearances which would override the board rule on the specified layer.Īnother common requirement is for tight clearances on several layers in the area inside and around a fine pitch SMD or BGA to enable all of the signals to fanout and escape onto the board.
For example, we may allow the space between tracks on inner layers to be smaller because they are better protected from things like dust and condensation. It’s very common however to want different clearances between objects in different parts of the PCB. The EDA tool will then monitor the distances between objects as the PCB is placed and routed and will warn us if objects violate the specified clearances. At its simplest level we define a distance between pads, between a track and a pad and between two tracks. This rule applies everywhere on the board that is not overridden by another rule. There is always a board rule, sometimes called the default rule. Physical scope starts of course with the entire PCB. Fortunately, the EDA industry has a well established and fairly standardized system for specifying electrical design rules, allowing us to limit a rule to a physical area of the PCB and/or to a certain number of signals. Modern electronic products are more complex than ever before, operating at higher speeds and needing to fit into smaller enclosures this means we need more control over electrical clearances during board layout.
Making sure that there are safe clearances between objects on the PCB is a top priority.