Presently used to make integrated circuits with printed circuit boards, ball grid arrays (BGA's) packages are leadless, surface-mounted packages in which solder balls interconnects cover the bottom surface of the package in a checkerboard fashion. Typically, a mass reflow process is used to attach BGA's to printed circuit boards (PCB's), a term generally used for printed circuit configurations such as rigid or flexible, single, double, or multilayered boards that are completely processed. Integrated circuit (IC) is the term generally used for a microelectronic semiconductor device consisting of many interconnected transistors and other components. Typically, IC's are fabricated on a small rectangle called a die that is cut from a silicon wafer known as a substrate. Different areas of the substrate are “doped” with other elements to make them either “p-type” or “n-type.” Polysilicon or aluminum tracks are etched in one to three (or more) layers deposited over the substrate's surface(s). The die is then connected into a package using gold wires, which are welded to “pads,” usually found near the edge of the die.
Ball grid arrays formed on multilayer substrates typically incorporate within the BGA substrate pattern drilled holes in laminate called vias, which connect different layers of circuitry. Typically, at least one via is positioned between two diagonal balls on the substrate, or on the printed circuit board (PCB).
Inductance is the ability of a conductor to produce an induced voltage when cut by a magnetic flux; A conductor is a material capable of conveying an electric current. Virtually all conductors have inductance, but the amount of inductance associated with each conductor varies according to a number of factors such as type of conductive material, shape of the conductor, length of the conductor, and so forth. For example, a shorter wire has less inductance than a long wire because less conductor length cut by a magnetic flux produces less voltage. Similarly, a straight wire has less inductance than a coiled wire because the conductor concentrates more conductor length in a given area of magnetic flux.
Induction (the production of an induced current within a conductor) occurs whenever magnetic flux cuts across a conductor, such as when a wire is moved within a stationary magnetic field, or when a magnetic field fluctuates about a fixed wire. One characteristic of inductors is that the faster the speed at which the flux changes, the more voltage is induced. The flux induces change in current. For example, Alternating current (AC) circuits continually produce an induced voltage because the current is continuously changing. The faster the current changes, the higher the induced voltage, which always opposes the change in voltage. If current increased, the polarity of the induced voltage opposes the increase in current, and vice versa. However, it is not necessary for the current to alternate directions. Inductance affects DC circuits transient responses whenever the value of the DC current changes, such as when a DC circuit is turned on and off. The switch induces a transient which is a change. The transient will settle to a new value according to the response of the network. Digital signaling is a sequence of transients. Further details concerning about inductance and simultaneous switching noise can be found in the book entitled Digital Signal Integrity: Modeling and Simulation with Interconnects and Packages, by Brian Young, published by Prentice Hall PTR.
Mutual inductance typically occurs whenever two conductors are positioned closely together such that a varying fluxes resulting from a change in current in Conductor A cuts across and induces voltage in Conductor B. This induced voltage, in turn, generates a magnetic flux that cuts across and induces a voltage in conductor A. Because a current in one conductor can induce voltage in the adjacent conductor, the conductors are said to have mutual inductance. To offset this appreciable effect, traces, leads, and current return path are usually kept as short as possible.
Each of these inductance discussed above seriously affects, and in some cases limits, the input/output (I/O) processing speeds of integrated circuits. For example, in the case where all the bus outputs of a circuit simultaneously switch the same way, there will be a current surge flowing in the circuit. This current surge generates an appreciable induced voltage in the circuit's conductors. The induced voltage generates a current flowing opposite to the wave of current, reduces the amount of current flowing through the circuit, thereby slowing the settling time current flow. It is clear that faster processing times will result if system inductance can be minimized. Thus it apparent to one with ordinary skill in the art that a better design is needed.