PACKAGING A SEMICONDUCTOR CHIP DEVICE

Abstract: Heat generated by a semiconductor chip device (10) mounted on a substrate (14) is dissipated through thermally conductive wires (36) which are welded to the device and to a thermally conductive cover (20) disposed over the device. The wires can be of annealed aluminium and ultrasonically welded to heat generating locations on the device and to a part (54) of the cover which is electrically isolated from the remainder of the cover.

Description: The present invention relates to a packaged semiconductor chip device and to a method of making it. Techniques currently in use for dissipating heat from electronic modules include the use of radial finger cooling, pistons, and thermal grease. The option to place pistons, grease, or radial fingers for optimal cooling in the component package does not exist without extensive modification of the package structure. The same is true for other known methods including copper, wool, interlocking metal, filaments, dendrites. U.S. -A- 4,023,198 discloses a semiconductor packaging configuration in which a heat sink is bonded to a semiconductor chip or die and a conductor is bonded to the die in a metallised ceramic insulating body for achieving optimum heat transfer. No weld or fusion is shown at the chip or heat sink and there is no lead connection to the heat sink. IBM Technical Disclosure Bulletin, Vol. 28, No. 8, January 1986, page 3512, describes an improved heat sink for incorporation in semiconductor modules comprising a substrate on which are mounted semiconductor chips with a cover member seal thereover. Disposed within the cover and attached by welding to the inside of the top portion of the cover are a plurality of conductive brushlike members each of which is formed of a heat conducting resilient spring tempered wire. Multiple wires are used at each chip location. The wires are arranged such that they overlie one chip and are of sufficient length to resiliently engage the underlying chip. They are not bonded to the underlying chip however, and rely upon pressure force at the chip surface to make contact. The only control for wire contact location on the chip is through preforming the wires prior to assembly of the package. U.S.-A-4,333,102 discloses a heat dissipating disk connected by brushlike bundles of wire soldered to the silicon disk at one end and heat dissipation disk at their other ends. Similarly, DE-A-3031912 discloses wires bundled together and soldered to both the substrate and heat sink for heat conduction and DE-A-2936816 discloses wires soldered to silicon devices for cooling. The present invention seeks to provide improved thermal management in semiconductor chip packaging. According to one aspect of the invention, there is provided a packaged semiconductor device comprising a semiconductor chip device mounted on a substrate, a thermally conductive chip cover overlying the chip device and a plurality of thermally conductive wires connecting the cover to the chip device, one end of each wire being welded to the cover, the packaged semiconductor device being characterized in that the other end of each wire is welded to a predetermined heat generating location on the side of the chip device facing the cover. According to another aspect of the invention, there is provided a method of packaging a semiconductor chip device mounted on a substrate, in order to dissipate heat generated by the device, comprising the steps of: providing a thermally conductive cover suitable for mounting over the device; connecting the cover to the chip device using a plurality of thermally conductive wires; welding one end of each wire to a predetermined heat generating location on the side of the device facing the cover, and welding the other end of each wire to the cover. How the invention can be carried out will now be described by way of example, with reference to the accompanying drawings, in which:- Fig. 1 represents a cross-section of a packaged semiconductor chip device; Fig. 2 represents a cross-section of another packaged semiconductor chip device; Fig. 3 is a graphic representation of the calculated effect of various wire sizes on the internal resistance of a chip package; and Fig. 4 is a graphic representation of actual internal resistance values in a chip package. Fig. 1 is a schematic cross-section of a semiconductor chip package embodying the invention. Semiconductor chip 10 is attached to substrate 14 through bonding material 18. The package has an aluminium cap 20 which has an opening 24 formed therethrough. Closure cap 28 is provided to fill opening 24. Cap seal 30 ensures a complete closure. One end of each of a plurality of fully annealed (i.e. soft and flexible) aluminium wires 36 is bonded by ultrasonic welding to a predetermined location 38 on chip 10. The other end of each wire is bonded by ultrasonic welding to the substrate cap 20. Each weld is accomplished ultrasonically to provide a true metallurgical interconnection between the silicon chip, the aluminium wire and the cap. The ultrasonic welding provides reliable intimate contact for maximum heat dissipation. There is thus provided a direct thermal path along each wire 36 from a particular source of heat, at location 38, on chip 10 to an exterior surface. As shown the wires are welded to the cap 20 of the package. The aluminium wires have a low mass. Advantageously, the locations 38 on each chip may be chosen in an automated fashion, thereby providing the capability of welding a plurality of wires in concentrations coinciding precisely with hot spots on the chip. Thermal analysis techniques may be utilised to determine the exact locations of the hot spots. Automatic wire bonders and thermal pattern recognition tools such as infrared thermal scanners may be coupled to provide thermal analysis feedback information. The thermal information needs to be determined only once. Subsequently wire bonders may be controlled to utilise the same data. The location of hot spots could also be defined by semiconductor engineers aware of where power is being dissipated or by predetermined device design ground rules. This information could be fed into an automated welding machine for assembly of the package. Fig. 2 is a cross-section of a package similar to that of Fig. 1 with the addition of means for providing electrical isolation. Thermally conductive adhesive 50 is used to attach a ceramic washer 54 with a bondable top surface metallisation to aluminium cap 20. Wires 36 are welded to the metallisation on the ceramic washer 54. The complexity of providing this feature is less than that associated with the prior art techniques of radial finger cooling and other methods, and is much less complex than pistons. The technique described in connection with Figs. 1 and 2 is applicable for various thicknesses and shapes of chips and caps. It provides total forgiveness for variations in dimensional properties such as chip tilt, chip to cap distance and other tolerances. Aluminium wire bonded chips for signal input/output (I/O) are routine in the industry in hermetic packages, but are limited most often to smaller wire diameters due to the size of semiconductor pads which are desirably maintained as small as possible in order to reduce the amount of silicon required. Plastic dual inline packages frequently use gold wires for I/O contact, but these wires are not thermal paths in the same sense as the aluminium wires in the packages of Figs. 1 and 2. Fig. 3 shows the calculated effect of using the present invention on internal resistance of the package. In the figure, D represents wire diameter in mils (1 mil ≅ 25 µm), N the number of wires in a bundle, that is, how many wires would be attached to chip 10. In Fig. 3 the mathematical analysis assumed a uniform heat surface on the chip and did not distinguish hot spots. The internal resistance Rint at the package as measured from the chip surface to the cap surface is expressed in degrees centigrade per watt. The thermal characteristic of grease is taken as 5°C/W based on a 7.9 mm chip. The physical limit assumes a base weld of twice the diameter of the wire with welds covering 50% of the total chip surface area. The chip to cap distance is assumed to be 15 mils (375 µm). Fig. 4 illustrates the results of actual measurements superimposed on the calculated value chart shown in Fig.3. A CFET thermal chip provided measured performance of welded wires with their associated physical characteristics. For two specimens evaluated the chip to cap distance was 0.25 mm. Specimen 1 consisted of 75 wires of 125 µm diameter. Specimen 2 consisted of 65 wires each having a diameter of 175 µm. The measured internal resistance for specimens fell well within the calculated range shown in Fig. 3. Specimen 1 Rint was 6.1°C/W and specimen 2 Rint was 4.9°C/W. Experimental data further confirmed that the technique of simple program wire bonding using an appropriate cap design and wire quantity, diameter and location, is equivalent to or better than known alternatives. The implementation is simpler than grease or radial finger cooling at much reduced cost since it does not need a radical change in cap design nor is it sensitive to chip tilt and chip height. No stress is placed on the interconnections of the chip to the ceramic substrate. In addition, the disclosed technique does not have the end of life problems associated with thermal grease. Radial finger cooling on the other hand is sensitive to chip tilt and chip height and places a large amount of stress on chips causing breakage of the flip chip solder columns. Unlike prior art techniques using either grease or radial finger springs, the soft aluminium wires 36 of Figs. 1 and 2 have no residual compressive force and require only 0.3 grams to deflect 500 µm with complete relaxation thereafter. Fig. 3 provides a comparison of the thermal efficiency of the cooling technique of the present invention with the conventional technique of thermal grease cooling. Ultrasonic acoustic energy imparted to the chip and bonding material is not detrimental thereto. Samples made by this technique were cross-sectioned after wire attachment and found to have no structural damage. These samples were also evaluated electrically both before and after the wire attachment step and found to experience no degradation in operational characteristics. 500 sample wires, 25 chips with 20 wires/chip, were assembled by a method embodying the present invention and subjected to 340°C thermal cycling with no loss in bond strength and having pull strengths not varying more than ± 1 gram over all samples. Electrical tests on welded wire functional bi-polar static RAM chips have been successful. Thus, the present technique, like all flip chip cooling enhancements, complements greatly the heat extracted by the flip chip solder balls themselves. However, flip chip balls are not positioned for cooling but rather are positioned for reliability and functional convenience as illustrated at 18 in Figs. 1 and 2. Wires 36, however, are positioned for optimum cooling as above described. Thus, it can be seen that application of the present invention possesses many advantages resulting from a true metallurgically interconnected thermal path formed by means of ultrasonic welding. Heat dissipation is more efficient because the aluminium wires are placed with their ends at hot spots on the chip as determined by sophisticated thermal analysis techniques and because the wires are fixedly attached to the chip and to other hardware in the package. This technique is not subject to degradation as a result of chip tilt, height thickness, or the distance between the chip and the cap. As shown in Fig. 2 it is possible to use this cooling technique to achieve an electrically isolated environment by using commercially available electrically insulating, thermally conductive adhesive between the top surface metallized ceramic washer to which the aluminium wires are bonded and the substrate cap. There is no stress induced in the chip from the ultrasonic welding as there may be when prior art cooling techniques are used. The bonded wires do not need to exert a pressure on the chip in order to make contact. In addition, the flexible wires used in embodiments of the invention do not induce thermal stresses caused by temperature excursions on materials with different coefficients of thermal expansion. Chip power capabilities are extended as a function of the number and diameter of wires used as well as the location of the wires on the chip. The internal resistance of the module package is adjustable as a function of the number of wires used. The entire process may be easily automated using conventional automated wire bonding equipment with commercially available large diameter aluminium ultrasonic weld pattern recognition bonder equipment. The nature of the method of providing this cooling function is such that yield is not decreased as a result of the technique because there are fewer opportunities directly related to the cooling technique for damage to the chip from vibration, shock, or high temperatures.

Claims: A packaged semiconductor device comprising a semiconductor chip device (10) mounted on a substrate (14), a thermally conductive chip cover (20) overlying the chip device and a plurality of thermally conductive wires (36) connecting the cover to the chip device, one end of each wire being welded to the cover, the packaged semiconductor device being characterized in that the other end of each wire is welded to a predetermined heat generating location (38) on the side of the chip device facing the cover. A packaged semiconductor as claimed in claim 1, in which the cover (20) includes an opening (24) through which the chip device can be accessed, and in which a closure cap (28), suitable for filling the opening, is provided. A packaged semiconductor device as claimed in claim 1 or claim 2, in which the wires are of annealed aluminium and in which ultrasonic welds are used to weld the wires to the cover and to the chip device. A packaged semiconductor device as claimed in any preceding claim, in which the part (54) of the cover to which the wires are welded is electrically isolated from the remainder of the cover. A packaged semiconductor device as claimed in any preceding claim, in which the mounting of the chip device on the substrate is carried out by the flip chip interconnection technique. A method of packaging a semiconductor chip device (10) mounted on a substrate (14), in order to dissipate heat generated by the device, comprising the steps of: providing a thermally conductive cover (20) suitable for mounting over the device; connecting the cover to the chip device using a plurality of thermally conductive wires (36); welding one end of each wire to a predetermined heat generating location (38) on the side of the device facing the cover, and welding the other end of each wire to the cover. A method as claimed in claim 6, in which the cover (20) includes an opening (24) through which the chip device can be accessed and a closure cap (28) provided to fill the opening. A method as claimed in claim 6 or claim 7, in which the wires are of annealed aluminium and are welded ultrasonically. A method as claimed in any of claims 6 to 8, in which the wires are welded to a part (54) of the cover which is electrically insulated from the remainder of the cover. A method as claimed in any of claims 6 to 9, in which the chip device is mounted on the substrate by the flip chip interconnection technique.

Inventors: BULLER, MARVIN LAWRENCE, LUMBRA, DOUGLAS LEON, PHELPS, DOUGLAS WALLACE, SAMUELSEN, SIGVART JOHN, WARD, WILLIAM CARROLL
Applicants: INTERNATIONAL BUSINESS MACHINES CORPORATION
Application year: 1987
Main classification: H01L23
IPC codes: H01L23