Metal fusion bonding – Process – With condition responsive – program – or timing control
Reexamination Certificate
1999-11-08
2001-05-22
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
With condition responsive, program, or timing control
C228S004500, C228S180500, C228S234100, C219S220000
Reexamination Certificate
active
06234374
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to a system for rapidly heating selected portions of an integrated circuit package assembly and, more specifically, to such heating using a heat source capable of selectively heating a predetermined portion of an area while the entire area is being subjected to heat-generating radiations from the heat source, the heat source preferably being a tungsten halogen light source.
2. Brief Description of the Prior Art
In the fabrication of semiconductor devices and particularly integrated circuits, it is desirable that the semiconductor chip be heated to temperatures in the range of about 200 to 300° C. prior to wire bonding in order to improve the bonding process. It is also desirable that the lead frame to which a bond is to be made be retained at a much lower temperature than the chip, this lower temperature being sufficiently low to minimize oxidation of the lead frame material, generally copper. Lead frame oxidation generally diminishes the wet solderability thereof, leading to the obvious problems resulting therefrom. This problem has existed in the prior art because the mechanism used for heating of the semiconductor chip, generally a heater block or hot plate, also supports the lead frame and is non-heat selective. Accordingly, both the chip and the lead frame are heated together as a single unit to about the same high temperature, this being the temperature required for bonding to the chip. It is therefore apparent that a system whereby the semiconductor chip can be selectively heated relative to the lead frame while the chip and lead frame are in contact with each other is highly desirable.
A proposed lid sealing apparatus and method by Bokil would replace a metal heater filament with a beam of focused infrared light to reduce heat transfer into the ceramic package. See U.S. Pat. No. 4,481,708 issued Nov. 13, 1984 and U.S. Pat. No. 4,685,200 issued Aug. 11, 1987 OF D. R. Bokil and D. R. Bokil et al., and, the article by D. E. Erickson “Hybrid Circuit Sealing-Problem Prevention Clinic”, Electronic Packaging and Production, 22(11): pp. 133-137, November 1982. The Bokil system is relatively large and complicated due to the spacing design required to focus the infrared beams towards the glass for the lid seal and the required number of infrared beams (one on each side of the package).
SUMMARY OF THE INVENTION
In accordance with the present invention, the above described problems of the prior art are minimized and there is provided an apparatus and procedure for sending defocused or unfocused heat-inducing radiations over a general area wherein certain components within the area are selectively heated at a much greater rate than the remainder of the area. The use of unfocused heat-inducing radiations eliminates the need for the focussing equipment and permits the radiations to traverse a general area without requirement for accurate positioning since the nature of the materials receiving the radiations will determine the degree of heating thereof. For example, a semiconductor chip will heat up much more rapidly than will a copper-based or aluminum-based lead frame prior to wire bonding, thereby permitting wire bonding between the chip and the lead frame without excessively heating up the lead frame. The same type of operation is available when the lead frame is replaced by a tape with conductors thereon for standard tape automated bonding (TAB) wherein it is best to heat up the metal lead portions on the tape adjacent the chip and the chip to the exclusion of the tape itself since the tape degrades at the normal wire bonding temperatures and affects the electrical properties of the tape. The tape can be between the chip and a circuit board. Reflow soldering for a solder grid array in a ceramic package to secure a semiconductor chip to a circuit board is provided in the same manner with the ceramic heating the solder to its melting temperature before the semiconductor device is heated to an unsafe temperature. Rework of a printed circuit board is provided wherein a single component must be replaced without heating the entire board and components thereon by directing the radiations only over the general area containing the component to be removed to the exclusion of the remainder of the board and heating until the component can be removed by heating of the epoxy, polyimid or the like securing the component to the board or by melting solder holding the component to the board. The component can be a package containing a chip, the chip itself which is secured directly to the board or other types of components. In the case of molded ceramic or plastic packages, it is often difficult to place symbols on the package surface due to the presence of mold release agents. These mold release agents are removed from the package surface pyrolitically by heating the mold release agent on the surface of the package to the exclusion of the remainder of the package.
It has been determined that certain materials, such as, for example, the chip will heat up at a much greater rate than will other materials, such as, for example, the lead frame material when the heat source is providing unfocused energy radiations in the range of from about 0.5&mgr; to about 2.0&mgr;. This is because the lead frame material, generally copper, heats up rapidly when the heat energy radiations are above 2&mgr; but heats slowly when the heat energy is in the range of from about 0.5&mgr; to about 2&mgr; whereas the chip material, generally silicon, germanium or a group III-V or group II-VI compound, heats up rapidly when the heat energy is in the range of from about 0.5&mgr; to about 2&mgr;. It follows that a heat source which supplies at least a substantial part of its heat energy in the range of from about 0.5&mgr; to about 2&mgr; will selectively heat the semiconductor material relative to the lead frame. The amount of the energy in the range of from about 0.5&mgr; to about 2&mgr; required will be dependent upon the heat temperature differential required as well as the rate at which this temperature differential is achieved. Accordingly, a heat source which provides a substantial portion of its heat energy in the range of from about 0.5&mgr; to about 2&mgr; is required in accordance with the present invention. The standard tungsten halogen light source generally provides a substantial portion of its heat energy in the range of from about 0.5&mgr; to about 2&mgr;.
Briefly, the above is accomplished with regard to a lead frame and chip by providing a base and disposing both a lead frame and a chip thereon in standard relation to each other. A heat source of the type described above, preferably with a reflector to direct most of the radiations to a desired area, then directs the heat to the lead frame and chip. The chip will heat up to the required bonding temperature and bonding thereto then takes place while the lead frame meanwhile remains at a temperature lower than that of the chip and sufficiently low to avoid excessive oxidation thereof. The heat source is then turned off after bonding takes place.
With reference to reflow soldering for a solder grid array (SGA) ceramic package, the ceramic material, such as aluminum nitride, will rise in temperature quickly and transfer heat to the solder balls before the temperature of the semiconductor chip has risen sufficiently to affect its properties. Even when no chip is present, the soldering takes place more rapidly since the heating is directed to the locations where the heat is required to the exclusion of other locations.
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patent: 5626280 (
Amador Gonzalo
Hwang Ming
Wang Lobo
Brady III Wade James
Ryan Patrick
Stoner Kiley
Telecky , Jr. Frederick J.
Texas Instruments Incorporated
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