Electricity: conductors and insulators – Conduits – cables or conductors – Preformed panel circuit arrangement
Reexamination Certificate
2001-07-13
2004-07-13
Vigushin, John B. (Department: 2827)
Electricity: conductors and insulators
Conduits, cables or conductors
Preformed panel circuit arrangement
C174S255000, C361S794000
Reexamination Certificate
active
06762368
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the field of decreasing the inductance of a capacitor. More specifically, the present invention relates to decreasing the inductance of a capacitor, such as a bypass capacitor, mounted on the surface of an integrated circuit board.
2. Description of the Related Art
Computer systems are information handling electronic systems which can be designed to give independent computing power to one user or a plurality of users. Computer systems may be found in many forms including, for example, mainframes, minicomputers, workstations, servers, personal computers, internet terminals, and notebooks. Computer systems include desk top, floor standing, rack mounted, or portable versions. A typical computer system includes at least one system processor, associated memory and control logic, and a number of peripheral devices that provide input and output for the system. Such peripheral devices may include display monitors, keyboards, mouse-type input devices, floppy and hard disk drives, CD-ROM drives, printers, network capability cards, terminal devices, modems, televisions, sound devices, voice recognition devices, electronic pen devices, and mass storage devices such as tape drives, CD-R drives, or DVDs.
Components of a computer system or other electronic system may be operably coupled to each other by computer busses or other types of signal lines. Computer busses typically include a plurality of signal lines which are current pathways for allowing electronic signals to propagate along the bus. The signal lines of a computer system can be implemented as signal traces either on a circuit board or embedded in a circuit board.
Computer systems are typically assembled using circuit boards on which integrated circuits have been mounted. It is also known to include a voltage plane and a reference voltage plane (sometimes referred to as a “ground plane”) in an integrated circuit board. It is also known to incorporate a via, or conductive path, which can be used to electrically couple surface mounted structures, such as capacitors, with a voltage plane or ground plane.
As computer bus speeds have increased, the impedance of the signal paths of the busses has become more significant in the operation of the bus and accordingly the electronic system. Transmission rates between a memory controller and memory models may reach as high 800 MHz. This high data transfer rate is achieved in part by requiring that the physical characteristics of the bus between the memory controller and memory modules be within certain parameters. For example, under a specification for a memory module licensed by RAMBUS INC™ the impedance of the signal paths between the memory controller and a memory module is specified at 28 ohms.
Memory busses may utilize electrically conductive vias to route the signals among embedded signal layers in a circuit board and trace signal layers on the planar surface of the circuit board. A typical via extending through a 62 mil thick circuit board exhibits an impedance of about 40 to 60 ohms, depending on the via diameter, pad sizes, and the position of the signaling layers that the via electrically couples. The impedance of the via and the internal impedance of a surface mounted structure, such as a capacitor, are cumulative. Higher impedance may lower the quality of signal transmission of the memory bus due to signal
oise margin degradation.
It is known that impedance is related capacitance and inductance. The relationship is understood to be given in Equation 1. From Equation 1 it can be seen that increasing the capacitance decreases impedance. Increasing capacitance can be accomplished by adding capacitors in parallel. However increasing the number of capacitors requires additional area on the surface of an integrated circuit board and can increase cost to the manufacturer and consumer.
Z
=
1
j
wC
+
j
wL
Equation
1
Where: Z represents impedance
C represents capacitance
L represents inductance
w represents frequency; and
j represents the imaginary part of the complex number
It is known that increasing the ground pad size increases capacitance represented by C in Equation 1. As shown in Equation 1 above, increasing C decreases impedance. Therefore increasing the ground pad size decreases impedance. It is also known that inductance is proportional to the length of the trace. The length of the trace can be represented by L in Equation 1 above. Therefore, from Equation 1 it can be seen that decreasing the trace length also decreases the total impedance.
U.S. patent application Ser. No. 09/395,788 filed on Sep. 14, 1999 titled “Capacitive Structure for Via Impedance Tuning” (referred to as the “788 patent”) naming inventors David A. Baranauskas and Douglas E. Wallace, Jr. teaches tuning the impedance of a via. However methods may be available which reduce the impedance more than the method taught by the '788 patent. Also, methods of reducing the impedance of a capacitor may be developed which are less expensive and may be implemented in conjunction with the method taught in the '788 patent.
Similarly, U.S. patent application Ser. No. 09/605,905 (the “905 application”) filed on Jun. 28, 2000 titled “Printed Circuit Assembly having Conductive Pad Array with In-Line Via Placement” naming Doreen S. Fisher and Thad McMillian as inventors, inventors teaches a conductive pad array with an in-line via placement. However this disclosure teaches a placement of vias which provides additional space on the surface of the integrated circuit board. The '905 application does not teach a means to reduce inductance of capacitors.
The problem of impedance is noticed in determining the number of capacitors necessary to couple a voltage plane and a ground plane. Typically, the voltage plane and ground plane are adjacent and parallel, separated only by the core plane. This configuration of the voltage plane to the ground plane provides a low impedance path for a returning signal. This low impedance path minimizes the noise at high frequency. When a high frequency current (sometimes called a displacement current) is induced from the voltage plane to the ground plane through a capacitor it is said the signal “jumps” or “bypasses” the conductive path. The capacitor through which the displacement current flows is called a “bypass” capacitor.
FIG. 1
shows a cross-sectional view of a capacitor in an operational configuration on integrated circuit board
100
. In the configuration shown in
FIG. 1
, the integrated circuit board includes the following; dielectric layer
170
, voltage plane
140
, core layer
160
and ground plane
150
. Voltage plane
140
and ground plane
150
are parallel and are typically separated by core layer
160
. In many computers, voltage plane
140
is maintained at a pre-determined voltage over ground plane
150
. For example, a Dell© Dimension™ computer manufactured and sold by Dell Computer Corporation of Austin, Tex. has a voltage plane normally operating at 3.3 volts above the ground plane.
Still referring to
FIG. 1
, capacitor
110
is electrically coupled to voltage pad
130
as shown. Voltage pad
130
is electrically coupled to voltage plane
140
by some means (not shown). Similarly, ground pad
120
is electrically coupled to trace
190
. Trace
190
is coupled by some means (not shown) to ground plane
150
.
In the prior art the Institute of Electrical and Electronic Engineers (IEEE) has adopted a standard configuration of a surface mounted capacitor. The standard configuration provides information to allow manufacturers and designers to provide uniform connections from a surface mounted capacitor to a voltage plane. For a single capacitor, it is known to have an equal number of connections from the voltage plane to the voltage pad and the ground plane to the ground pad.
Referring to
FIG. 2
, capacitor
210
is shown with voltage pad
220
. Voltage pad
220
is shown electrically coupled to trace
230
. Trace
230
is shown cou
Saputro Stephanus D.
Zhang Lan
Baker & Botts L.L.P.
Dell Products L.P.
Vigushin John B.
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