[attachment=12091]
PRINTED CIRCUIT BOARDS
PCB contains printed circuit of electronic equipment on it. As per circuit, the designer designs or assembles the components and parts for electronic equipment. This helps the designing easy for the designer.
Design of PCB is considered as the last step in electronic circuit design as well as the first major step in production of PCB’s.
STEPS TO DESIGN PCB S:-
1. Design and test the prototype circuit— by hand.
2. Capture the circuit’s schematic— using OrCAD Capture or similar software.
3. Perform the physical layout of the circuit— using OrCAD Layout or similar software.
4. Fabricate, populate and test the PCB— done by ECE shop personnel or similar personnel
1. PROTOTYPING:-
With a basic idea in mind, a circuit schematic is developed and analyzed to ensure the desired functionality and performance. When creating a circuit for PCB production, a designer would also select specific components at this time. Commonly, the next step is to prototype and to test the circuit. It is also possible to use the schematic capture software along with relate software to simulate the circuit without building it on a prototyping board.
2. Schematic Capture
Schematic capture software comes in several forms. It allows the PCB designer to create an electronic schematic. This electronic schematic contains more information than its paper relatives. For example, every part
symbol in Capture contains information telling what footprint the symbol is associated with.
(Footprints are the symbols used in layout software to define the physical design of each component.)
Capture parts symbols are used in a symbolic manner. Thus, the part symbol on Capture’s screen does not show what the actual physical component looks like. It does allow the designer to connect all the components in a circuit and to test the workings of the circuit by exporting files to other software. For our purpose, Capture provides the starting point for creating a physical layout in layout software.
4. Physical Layout
A blueprint of a house tells the size of lumber to use, as well as the dimensions of the living room wall and the dimensions of the window cut into it. It gives all the physical information necessary to build the house. Physical layout software can be thought of as a “blueprint” for a PCB.
The basic building blocks used in Layout are footprints. A footprint contains all the physical dimensions related to a particular part. For example, a 14-pin dip footprint defines where each of the 14 drill holes are to be located, as well as associated information, such as text defining the part number of the component. In Layout the footprints of the various parts are placed and then routed. Routing refers to defining where the copper interconnects in the circuit will be located. Interconnects are copper paths on the surface of the PCB that connect one pin to another. Interconnects are also known as “routes” or “traces”.
Layout uses different layers to contain all of the information about the PCB. Some of the layers that will be used in doing two sided PCBs are TOP, BOT, and SST. The TOP layer contains information pertaining to the top of your PCB, such as traces (routes) that are drawn on the top. The BOT layer contains information pertaining to the bottom of your PCB, such as traces that are drawn on the bottom. The SST layer is the ‘Silk Screen Top” layer, and contains the text that will appear on the top of the PCB. There are many other layers as well, some of which will never be used in two sided PCBs.
Once the routing is complete, the board layout is finished. All that remains is to double check that no text overlaps routes and make sure there are no loose ends anywhere on the board. Once the board has been reviewed and is completed, the final step is to create the Gerber files. A Gerber file is a file that contains the information from Layout that is necessary for the prototyping machine to drill, mill, and cut the PCB. A Gerber file is created for every layer of interest.
5. Physical Creation of PCB:
LAYOUT PLANNING:-
Layout of PCB should provide all the information about the circuit which has to be drawn on the PCB.
Layout considerations are:
 Layout scale :
Generally, the layout scales used are 1:1, 2:1 or 4:1
We commonly use 2:1 because it offers a reasonable compromise between accuracy gained and handling convenience.
2:1 artwork has 4 x actual PCB area while 4:1 artwork has 16 x actual PCB area.
 Grid system:
The use of grid system is generally used in the designing of PCB for assembling and fixing of components.
The use of grid makes the layout designing more convenient and easy to place the components.
 Production facilities:
Limitations can be given in artwork table size, minimum or maximum board processing size, drilling accuracy, fine line etching performance, etc.
 PCB Sizes and standards:
Board size of PCB should not be either very big or very small. It should be optimum.
Small board advantages – Easy to detect and repair. Big circuit split into smaller units so that the same unit can be applicable to other circuits also.
BASIC APPROACHES OF ARTWORK:-
1. Ink drawing: Here, materials used are white cardboard paper, good quality Indian ink and an ink pen set. This method is followed where the conductors requires special shapes. The circumferences of solder pads and centre holes are drawn with a drawing compass and the space in between is filled with ink.
2. Black taping on Transparent Base foils: This method relieves from patience and good drawing skills. This method is fast and more precise. The availability of self adhesive pads and transfer pads made this method very fast and precise.
Self adhesive pads: These are available sticking on backing paper. They are fixed on the artwork base sheet by peeling of the backing sheet and positioning it properly and firmly pressed on base sheet. Modifications can be done by lifting the pad and pressing it in the new position.
Transfer pads: Pads are printed on a thin adhesive film which is mounted on a carrier strip. Pad is transferred on artwork base by rubbing with wooden strip and carrier strip.
3. Red/Blue taping on one transparent Base foil: The component pads are generated by using black pads. The conductor pattern is done with red transparent tape for one conductor side with blue transparent tape for other conductor side. While producing the two film masters, color separation by using special filters is applied. Respective filters are used to filter out the respective color tape conductors.
4. Black taping on Diazo films: It is basically a polyster based reprographic film. It has high dimensional stability. An artwork pattern common to both PCB sides is made in black on polyster foil. But two copies of this common layer are made by a contact copying of dimensionally highly stable Diazo film and the respective conductor patterns are there after taped on these copies.
5. Cut and strip artwork: This method uses a special foil consisting of a clear, stabilized polyster film with a red coating which is transparent to eye but opaque to U.V light. The red coating has only little adhesion to the base and is cut through by a means of a knife blade. This red coating is pealed off to leave the pattern on the transparent foil.
* After the artwork is completed, better to have an inspection and check to find out the mistakes and rectifications if any. This check and inspection has to verify that all the accepted artwork rules and standards have been followed. The inspection and check verifies whether the name of PCB circuit number, PCB specifications minimum spacing requirements, etc are correct or not.