Semiconductor device manufacturing: process – Coating with electrically or thermally conductive material – To form ohmic contact to semiconductive material
Reexamination Certificate
2000-08-25
2003-08-26
Chaudhuri, Olik (Department: 2823)
Semiconductor device manufacturing: process
Coating with electrically or thermally conductive material
To form ohmic contact to semiconductive material
C438S614000
Reexamination Certificate
active
06610591
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates generally to a ball grid array. More specifically, the present invention relates to a ball grid array used in a flip-chip type on board semiconductor assembly.
2. State of the Art
Advances in semiconductor technology have facilitated the development of smaller integrated circuits having higher operating speeds. Presently, industry possesses technology to fabricate computers, telephones, scanners and video cameras and other electronic devices which can fit within a shirt pocket or similar space, at decreasing manufacturing costs and sales prices. Much of these size reductions and higher operating speeds have been facilitated by design of smaller and smaller semiconductor devices having a larger number of electrical connections for each semiconductor device.
With the miniaturization and increased operating speeds of semiconductor devices, packaging techniques are also being revised. Of the packaging techniques, much effort is being placed to keep up with the size reductions and operating speed increases of semiconductor devices utilizing chip on board (COB) assembly techniques. Among the COB techniques for attaching semiconductor devices to a printed circuit board are wire bonding, tap automated bonding (TAB), and flip-chip attachment.
In wire bonding, numerous wires are connected to contact pads on the semiconductor device and extend outwardly over the edges of the semiconductor device to correspond with contact pads on a substrate or printed circuit board. The process requires individual connections of each contact on the semiconductor device and requires an area of the substrate substantially larger that the semiconductor device itself. Also, due to the long lengths of the wire used to connect the chip to the printed circuit board and the resistance thereof, the speed at which the semiconductor device interacts with other circuitry is affected and, generally, is slower with longer lengths of wire.
In tape automated bonding or TAB, metallic leads are disposed on a polymeric tape. The leads may be connected individually or in mass to the contact pads of the semiconductor device and to the contact pads of the substrate. Unfortunately, like the wire bonding method, this method also requires utilizing a substantial area of the surface of the substrate. Also, depending upon the length of the metallic leads on the tape connected to the semiconductor device, the speed at which the semiconductor device interacts with other circuitry is affected and, generally, is slower as the length of the metallic leads increases.
On the other hand, flip-chip attachment techniques utilize the least amount of space and offer shorter interconnections with other circuitry for potentially increased interaction response, that is the space utilized on the substrate is substantially equal to the semiconductor device itself. In a flip-chip attachment technique, bond pads on the active surface of a semiconductor device may include an array of solder balls for mounting directly to a substrate, such as a printed circuit board, a carrier, and/or another semiconductor device. The array of solder balls on the semiconductor device is commonly referred to as a ball grid array (BGA). The BGA must be a mirror image of the connecting pads on the printed circuit board so that precise connection is made. With the solder balls arranged between the semiconductor device and substrate, electrical and mechanical connection is made thereto by reflowing the solder balls.
Of the three discussed COB techniques, a flip-chip type attachment technique is believed by some to be best suited to comply with the trend of the miniaturization and increased operating speeds of semiconductor devices. However, in comparison to the interconnect bumps in a BGA utilized elsewhere in semiconductor packaging (i.e., approximately 0.8 mm-1.3 mm diameter), the prior art interconnect bumps utilized in flip-chip assemblies are of a minute size (i.e., approximately 0.3 mm-0.8 mm diameter). Because of the minute size necessary for flip-chip packaging, the choice of metalization for the flip-chip assembly interconnect bumps is solder material or variations thereof, wherein the conventional interconnect bumps employed are bumps made entirely of solder material.
Although flip-chip attachment techniques utilize less space and are more responsive than other COB techniques, there are several problems associated with flip-chip packaging and the solder balls employed therein. Among the problems include the planarity of the substrate and the semiconductor die, which planarity of both effect the solder bumps and solder balls therebetween in providing sufficient electrical and mechanical connection. Compounding this difficulty of planarity is the difficulty of providing solder balls with a consistent solder ball diameter yielding a ball height. As a result of the planarity and solder bump and solder ball height problems, the solder bumps and solder balls often become deformed and marked in test sockets during reflow and burn-in testing due to the softness of the solder material. Also, the solder bumps and solder balls may become deformed during handling of the semiconductor device. Further, the solder bumps or solder balls may the knocked off or removed from one or more of the bond pads of the semiconductor device during handling. Consequently, the solder balls are often too deformed, too marked, or missing to provide sufficient electrical and mechanical connection in the final mounting of the semiconductor device to the substrate.
Furthermore, the solder material used for the solder bumps or solder balls may be selected for mechanical properties for attaching the semiconductor device to the substrate or printed circuit board, rather than being selected for having good electrical conductivity properties during service. As the operational speed of semiconductor devices is constantly increasing, it is more important that the material for the attachment of the semiconductor device to the substrate or printed circuit board be selected for electrical properties while having the ability to form mechanical connections having the desired characteristics.
Also, the use of solder balls and solder material on the bond pads of semiconductor devices and substrates requires the use of solder flux which can be difficult to apply and control in solder reflow operations to prevent damage to the semiconductor device and substrate. Therefore, as it is desirable to minimize the use of solder and solder flux for the formation of the connections between the semiconductor device and the substrate or printed circuit board to which it is attached.
In U.S. Pat. No. 2,934,685 illustrated is a ball or sphere of having a diameter on the order 0.005 inches is made from an inert material, such as tungsten or molybdenum, coated with a layer of gold containing antimony, is attached to a semiconductor wafer, having a layer of aluminum subsequently applied thereto and the semiconductor body with the heating of the ball or sphere to cause the aluminum to penetrate the upper layer of the semiconductor body causing it to be converted to p-type conductivity.
In U.S. Pat. No. 3,496,428 illustrated is a metal contact
6
comprising a preliminary metallizing layer of gold nickel on the surface of the p-type region of a semiconductor wafer substrate and a silver dot in the shape of a somewhat hemispherical ball alloyed to the metal layer. A nickel layer is subsequently deposited on the silver dot.
In U.S. Pat. No. 5,841,198 illustrated is a ball grid array package utilizing solder balls having central cores of a material with a higher melting point than solder material surrounding the core. When the ball grid package and motherboard assembly are heated to the melting point of the solder material, the cores of the solder balls remain solid and function as spacers in preventing direct contact of the package surface and the motherboard surface. The core of the solder ball can comprise a lead tin alloy having a higher melting
Akram Salman
Jiang Tongbi
Brewster William M.
Chaudhuri Olik
Micro)n Technology, Inc.
TraskBritt
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