In PCB design, a library is also a file or a database for organizing elements that can be re-used many times during physical design. PCB libraries include symbol libraries that hold schematic design symbols; footprint libraries that contain device footprints (land patterns), and model libraries with models for the simulation programs.
While these three aspects of a library element are combined in a single technology library in IC chip design, they are typically stored in separate (technology) libraries in PCB design.
The libraries can typically be searched with a library management system. When the PCB design is being drawn, copies of the required elements are placed in the circuit schematic or layout. Here too, this procedure is referred to as instantiation, as with chip design.
Within the PCB library, different library elements can reference one another. PCB libraries are often tied to specific tools and tool providers; as a result, symbol, footprint and model libraries for PCB design are often not interchangeable between different design tools. These libraries are stored in either ASCII or binary formats, depending on the tool provider.
Table of Contents
Symbol Libraries
Symbol libraries contain the symbols needed to create the circuit diagram shown below. They can be searched, viewed, selected and instantiated in the library management system. The symbol in a symbol library typically references to one or more suit- able footprints for the associated physical device. Other information, such as the manufacturer, the vendor or the link to electrical models can generally be stored as well.
If the libraries being used do not contain a required symbol, design tools generally allow the designer to create a new symbol with a symbol editor and to store it in the symbol library. The new symbol should have all the necessary information for the design process, such as the link to the footprint, the pin assignment and, if necessary, the link to the electrical model.
Footprint Libraries
While the term “footprint” (also known as “land pattern”) is often used to describe the arrangement of pads or through-holes used to physically attach and electrically connect a component to a PCB, it is also applied in the context of libraries. Here, the term is used more broadly and encompasses all physical information for a device required for PCB design. Aside from the geometrical description of the individual layers (routing layers, solder-resist layer, solder-paste layer, silk screen layer, mechanical layer, drill holes, cut-outs, etc.), the footprint may contain three-dimensional (3D) physical device models, as well.
Footprints are managed in footprint libraries, an example of which is depicted in Fig. below.
Every device symbol on the schematic refers to one (or more) associated foot- print(s) in the footprint library. This relationship is used during the transition from the circuit diagram to the PCB layout design to derive the associated footprints from the footprint library, which are then available for placement on the PCB.
New footprints can be designed in the footprint editor and stored in a library; however, care should be taken that the footprint is suitable for the prospective fabrication technology. If the PCB is needed as a 3D model in other fabrication steps, the matching 3D physical model must be included for the device.
Model Libraries
Electrical models for simulation programs, such as PSpiceTM, LTSpiceTM or SIMetrixTM, are arranged and stored in model libraries. They contain the written description of the device’s electrical response as shown in the following figure.
The model describes the electrical behavior of basic elements, such as resistors, capacitors, and inductances. Models of semiconductor diodes and transistors are adapted to the device in question by means of a number of characteristic parameters. More complex devices are modeled by connecting a number of basic elements.
In order to protect their intellectual property, some vendor models are only avail- able as encrypted entities, which do not allow any insight into the inner workings of the simulation models.