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insrusingh
Joined: 08 Jul 2008
Posts: 16
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 20 Aug 2008 5:13 pcb layer stack up |
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HI can somebody tell me what is the difference between negative plane and positive plane?
2) What is impedence control?
3) Wha is fudicial an why it is required?
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senilicus
Joined: 26 Feb 2007
Posts: 64
Helped: 7
Location: The Netherlands
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| 22 Aug 2008 15:39 Re: pcb layer stack up |
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hi,
positive plane, a positive image of the plane, what you see is what you get. With a positve image, you see the actual plane.
Negative plane, a negative image of the plane, what you see is the clearances between the plane and other features on that layer. A negative image is of a much smaller size than a positive image.
In order to control high speed signals the impedance of the pcb trace is matched againt the component is it connected to.
a fiducial is a small copper item on the board( Mostly a ring or a solid circle). It is used to position the board on the assembly machine. (by means of an optical system). These are called "global" or board fiducial's.
Compoments with a very small pitch and BGA's often have there own fiducials. these are called "local" fiducials. These are needed because small pitch component need a very precise placement.
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House_Cat
Joined: 21 Feb 2002
Posts: 1349
Helped: 274
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| 22 Aug 2008 19:14 Re: pcb layer stack up |
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To add to the explanation already provided -
PCB Editors have two ways of representing solid copper planes - positive and negative. A postive plane is one in which you place the copper you want with pads, fills, and poured polygons. A negative plane is one in which you place marks where you want to remove copper.
The advantage of the negative plane is much smaller Gerber files for board fabrication. For a positive plane, there has to be a Gerber entry for each region of copper, and Gerber photoplotters draw that area with tens of thousands of line segments. For a negative plane, the photoplotter only has to draw the smaller areas where copper is to be removed.
Every trace on a PCB is a transmission line. For maximum energy transfer and best signal integrity, the trace should have the same impedance as the source and the load. The impedance of a PCB trace depends on the width of the trace, the distance between the trace and the signal return path, and the relative permitivity of the dielectric between the trace and the return path. The signal travels down the trace you place on the PCB, but there always has to be a return path for the current. In a controlled impedance board, we carefully control the distance between the signal trace and the return path - usually by placing the trace on a layer of the board adjacent to a continuous solid copper plane.
Automatic component placement equipment for fabrication of circuit boards uses small cameras to align the equipment with the circuit board. A fiducial is a target spot that you place on your PCB so those cameras have a well defined object on which to focus. The global fiducials let you line up with the board, and the component fiducial lets the equipment more precisely line up with small component footprints.
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