GeomGen is integrated into Mentor Graphics LIBRARIAN. To start it you simply select the GeomGen item on the GeomGen menu. GeomGen creates geometries from the devices physical parameters, such as body width and length, lead pitch, toe to toe and heel to heel distances, etc. Since different device types have different physical characteristics (an 0805 is different from a QFP) you first need to tell GeomGen what type of device you want to build.
The physical parameters of the device can be read off of its datasheet. However, many common device types are specified in IPC-SM-782. In addition IPC-SM-782 also recommends land patterns for each device listed in the spec. It refers to these land patterns as Registered Land Patterns or RLPs. Utilitek has incorporated the physical parameters and RLPs from IPC-SM-782 into a database that GeomGen can read. Using this database to read in the physical parameters will save you a great deal of time and will avoid errors.
When you start GeomGen, you get a form that allows you to specify the mode that you wish to use. Selecting "No IPC Lookup", a device Style and then OK will bring up the form that allows you to enter the physical parameters for the device type you selected. Selecting "Use IPC"and then entering an IPC RLP number for your device and a geometry name will build the geometry to the IPC RLP. Selecting "Use IPC for Source" and entering the RLP number and geometry name will bring up the physical parameter entry form with all of the numbers filled in already. This is convenient if you have a device that is close to an IPC RLP and can use the physical parameter data as a starting point.
The list of device styles is completely controllable by the user. You can define new types of devices as you see fit by editing the configuration file for GeomGen. The rules for creating a geometry for a device type are also controlled by the configuration file.
The form for entering the physical parameters for a device varies by device type. A form for a QFP is shown below:
If you chose "Use IPC as Source" all of the numbers that have to be entered in this form would have been automatically filled in for you from the IPC database. Each form consists of 2 sections, the upper section has the physical parameters of the device and the lower section specifies the land pattern for the device. The land pattern can be read in from the IPC database, or calculated using your custom method or the IPC recommended method. If you are calculating the land pattern, you can preview the results using the "Preview Calculations" button.
The terminology for defining the physical parameters of a device and the land pattern are the same as those used in IPC-SM-782. The example used in the form above was taken from the spec and appears below:
Once you are satisfied that the device parameters are correct, hitting OK builds the geometry according to the rules for that device type. GeomGen's very powerful Geometry Engine technology builds a geometry that EXACTLY meets your company's geometry spec. If you do not have a spec, GeomGen builds very complete geometries using its own set of defaults.
What happens if you change your spec after building many geometries with GeomGen? You have not wasted your time and effort. GeomGen can retarget the geometries you already created to your new library spec with ease. Every time you create a geometry, all of the physical data is saved to a database. Once the new spec is implemented in GeomGen by changing its rules, you can use the saved data to rebuild any geometry you have previously created. This feature also allows you to retarget your geometries to several different assembly lines.
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