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Circuit Schematic Capture and Design Process
PCB Layout Design Process
 CIRCUIT SCHEMATIC CAPTURE & DESIGN PROCESS
Schematic Capture package captures the circuit schematics. From the schematics, it will generate a Netlist. This Netlist will be used as input to the PCB Layout CAD package. The use of a schematic capture package makes it possible to link the circuit schematic with the final layout; and this makes it possible to run a Design Rule Check to verify that the final PCB matches the original schematics.
The process involves taking of appropriate components from the component libraries, placing them on the schematics, and editing them if required to ensure that the component package details & silkscreen legend to be printed on the PCB are as per requirements. If a component is not in the library, the component is defined and its definition and details are added to the component library.
Existing library components may need to be further edited to reflect the actual circuit diagram, and this may require that the pins of a particular component have to be moved around as per desired circuit schematics.
Once all the parts have been placed on the schematics, then all the required connections between pins of various components are drawn. This is done in a manner so as to ensure good readability; many times two pins that need to be connected together are not physically connected by a connecting line on the schematic, rather they are given the same Net Label. A Net Label always means a connection to all Net Labels of the same name.
After all the connections have been made on the schematics, a NETLIST is prepared automatically by the Schematic capture package.
PCB LAYOUT DESIGN PROCESS
Check NETLISTS, Other Information, prepare proper set:
Review all customer supplied information including Schematics, Parts List, Net-list- and design constraints. Check their consistency and any missing Information.
We can now start the PCB Layout Design process. For this we will be using a PCB Layout CAD package.
Set Design Rules
Check for manufacturability considerations- assembly considerations - testing considerations & support considerations. All or any or some of these considerations will generally impose certain restrictions on the PCB Layout design as a "set of Custom Design Criteria".
Choose all or some of the basic DESIGN RULES that we would follow for the layout. Some of the choices concern Track Width, Track Spacing, Pad sizes, Via sizes, Routing Types, Color Settings, Net-List related etc.
Component Placement
Components that form the schematic are taken from the parts library or libraries and placed on the PCB layer. All component placement constraints are taken into account.
o Some Parts like connectors & indicators may need to be placed at fixed prespecified locations.
. Heat dissipation considerations may dictate that some parts should be a certain minimum distance away from others.
. Signals that need to be limited in length may dictate close placement of certain parts that involve those signals.
. Certain parts like crystals etc may be oriented in a specific way.
Generally most of the critical criteria are taken into account at the time of parts placement
Routing
Desired connections between the pins are displayed using appropriate display commands, and these act as a visual aid for actual routing. Critical signals are routed first - for example those that involve Impedance control, Differential pairs, timing constrained signals etc. We manually Route whatever needs to be manually routed because of such special considerations. Bulk of the non-critical nets can be auto-routed; one may decide to auto-route complete or sections of the circuit. Finally, the entire routing is completed by some manual routing and cleanup.
During the Routing process, some components may need to be moved around for better routing. Some tracks geometry may need to be calculated on the basis of impedance or current carrying capacity constraints or requirements.
If any subsections of a component have been interchanged for Routing, this would require back annotating the schematics and the net-lists.
DRC - Design Rule Check
On completion of the Layout, a Design Rule Check operation tests the PCB layout against the Net-list and the set of previously specified design rules. Any violations reported are corrected.
After DRC, a final layout cleanup is done.
Documentation & Design Files:
Now we generate all the design documentation, generally comprising of these files:
. Fabrication files - These comprise all the files that are necessary to fabricate the PCB - all the Gerber files (all layers, solder masks, silk-screens), drill data files, fabrication drawing(s), and any other necessary data.
. Assembly files- These comprise all the files necessary to assemble the PCB - The parts list, the assembly drawing, solder paste masks, SMD X/Y position files, and any other necessary data.
. Database files - These comprise all the other files created by the PCB layout CAD system for the specific design.
Final Design Rules Check for Manufacturability
. Minimum Trace Width and Trace Spacing Checks
. Minimum annular ring widths Check
. Electrical shorts between planes Check
. Un-terminated traces Check
. Coupons
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PCB Design Express
1068 West Evelyn Avenue
Sunnyvale, CA 94086
Send files at files@pcbdesignexpress.com
Contact us at 408-731-2550; Fax: 408-731-2551
Email at sales@pcbdesignexpress.com
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