FI86604C - Powder metallurgical injection molding process - Google Patents

Description

1 86604

Powder metallurgical injection molding process. - Pulver-metallurgiskt sprutpressingsförfarande.

The present invention relates to processes for the preparation of metal mixtures. More specifically, the present invention relates to a process for injection molding mixtures of copper and tungsten containing about 5 to 50% by weight of copper.

In the past, there have been considerable difficulties in preparing satisfactory forms from copper and tungsten alloys. It has generally not been possible to prepare alloys of copper and tungsten as a liquid mixture. Copper and tungsten alloys have previously been fabricated using compression and sintering technology and powdered metal. The density and thermal conductivity of products made by such methods are generally low due to the residual porosity. Thermal conductivity can be increased somewhat and porosity reduced somewhat by mechanical compression of these products. However, such methods are not entirely satisfactory and are limited to uniaxially compressed parts.

In high-performance electronics, and especially in military and space applications, it is common to place microcircuit chips in hermetically sealed containers known as "packages." These packages must have electrically insulated wires extending through the walls of the package. These cables must be hermetically sealed and electrically insulated. These electronic packages are generally intended to contain heat generating components, so it is highly desirable that the package be formed of materials with high thermal conductivity.

Packages must be hermetically sealed to protect the electrical components they contain. Since these packages travel in minutes from sea level to the vacuum of space, no gas leakage can be allowed.

The materials used to close the openings through which electrical wires pass are generally inelastic and their coefficients of thermal expansion differ significantly from those of most metal materials. Thermal fluctuations thus cause stress in the seal and contribute to its rapid failure.

These and other prior art difficulties can be avoided in accordance with the present invention by using the method of the preamble of claim 1, which is characterized by the features set forth in the characterizing part of claim 1. The invention provides a material composition consisting of a high density mixture of copper and tungsten with high thermal conductivity and thermal expansion that can be combined with the thermal expansion of many inelastic insulating-sealing materials such as glass and ceramics. The high-density mixing of copper and tungsten is highly impermeable to gas.

According to the present invention, a mixture of copper and tungsten is produced by a powder metallurgical process in which it is injection molded to form complex shapes and in which a sintering step using a liquid phase is used to densify a part. Such casting methods have already been widely proposed:

See, e.g., Wiech, U.S. Patents 4,374,457, 4,305,756, and 4,445,956. Descriptions of such methods can be found in these prior wiech patents.

Powder metallurgical methods using injection molding and sintering in the liquid phase have the ability to produce net, highly complex parts with very precise tolerances. Net-shaped parts are those parts or products which, in order to be useful for their intended purposes, do not require any further processing, shaping or shaping beyond the 3 86604 liquid sintering phases. The achievable tolerances are less than +/- 0.003 cm. The shapes of the parts can be very complex because the product is injection molded.

The leakage rate of helium gas in a hermetically sealed electronic package is generally defined as a maximum of 1 x 101-9 cm3 of helium per second. The leakage rates of the copper and tungsten alloy products made in accordance with this invention are generally as low as about 2 x 10-10 cm 3 of helium per second. The "hermeticity" of electronic packages made of this material is thus substantially better than what is required.

The thermal conductivity of the copper and tungsten blend prepared in accordance with the present invention is generally better than about 0.40 and preferably at least 0.42 cal-cm / cm2 sec. "C measured at about 390 ° C. This thermal conductivity is measured for a material containing about 5% by weight of copper. When copper is present in less than about 5% by weight, the advantages of the present invention are generally not fully realized. When the amount of copper is 25% by weight, the thermal conductivity is generally greater than about 0.60, and preferably at least about 0.65 cal-cm / cm2 sec. 0.80 cal-cm / cm2 sec. ° C measured at a temperature of about 390 ° C. At copper concentrations above about 50% by weight, the advantages of the present invention are generally not fully realized.

The coefficient of linear thermal expansion is usually directly proportional to the volume percentage of copper in tungsten. About 7.0 parts per million / ° C corresponds to 11 weight percent copper and 9.4 parts per million / ° C corresponds to about 25 weight percent copper.

The coefficient of linear thermal expansion of the copper-tungsten material made according to the present invention can be conventional. 86604 4 to the value of the insulating-sealing material of the electronic package by adjusting the percentage of copper in the alloy.

The thermal behavior of the copper-tungsten material products made in accordance with the present invention, especially when viewed in light of the airtightness and the net appearance of these materials, takes the field very significantly forward. Due to the availability of another net product, many previous machining and assembly needs of electronic packaging can be omitted. Because the assembly of electronic packages according to the prior art has often involved brazing and soldering steps that have allowed gas leaks, the omission of most such steps in accordance with the present invention greatly improves the reliability of electronic packages. By using the present invention, it is possible to increase the power density of the package while maintaining the same or better reliability.

It is possible to make electric wires which conduct electric current to and from the electronic package from the same copper-tungsten material made according to the invention. The thermal behavior of these wires can thus be made in accordance with the housing. Because the high-density copper-tungsten material is a good electrical conductor, the electrical efficiency of the package is also excellent.

The copper and tungsten raw materials to be used in accordance with the present invention are obtained in a very finely divided form and in a very pure state. The copper material generally has a particle size of less than about 20 microns, and the average particle size of the tungsten powder is less than about 40 microns. The average particle size of these materials is generally less than about 10 microns. The amount of surface oxygen on the particles has a significant effect on the quality of the finished product. With surface oxygen concentrations above about 5,000 ppm based on 5,866,604 copper, the results during the sintering step are very irregular and unpredictable. The concentration of surface oxygen on the butter fram particles should also be less than about 1500 ppm. The particles are substantially coaxial in shape. Impurities in raw materials should be kept to an absolute minimum. For example, as little nickel as 2% reduces thermal conductivity by up to 30-40%. As little as 0.3% of various impurities such as oxides and trace amounts of lead and tin can reduce the thermal conductivity by up to 15-20%.

EXAMPLES

The following individual examples are intended to illustrate the invention without, however, limiting it.

In a preferred embodiment of the present invention, a high purity copper-tungsten material was prepared containing 35% by weight copper and 65% by weight butter frame. The average particle size of the butter frame powder was 1 to 2 microns, with a surface oxygen content of less than about 1,400 ppm and other impurities of about 300 ppm. The copper powder used had an average particle size of 8 to 10 microns, a surface oxygen content of less than 800 ppm by weight of hydrogen, and other impurities of less than 500 ppm. Both tungsten and copper powder particles were substantially coaxial. The binder prepared contained 39.47% by weight of polypropylene, 9.74% by weight of carnauba wax, 48.73% by weight of paraffin wax and 2.06% by weight of stearic acid. The binder was mixed with 4.3% by weight of the above-mentioned copper-butter frame powders. Stirring was performed in vacuo to better wet the surface of the particles and to prevent air entrainment. This reduced the porosity of the finished product and improved the thermal properties.

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A product of the desired shape was prepared by injection molding a mixture of binder and metal powders. The product, the so-called crude part, was heated in air to about 207 ° C for 2 days to remove the wax. The resulting intermediate was then heated in an atmosphere containing 25% by volume of hydrogen and 75% by volume of nitrogen at temperatures up to about 800 ° C until the polypropylene was removed. The temperature was then raised to about 1235 ° C and held here for about 3 hours in a atmosphere containing 75% by volume of hydrogen and 25% by volume of nitrogen. The resulting sintered net product was allowed to cool to room temperature for about 6 hours. The following values were defined as physical properties of interest:

Linear thermal- Thermal conductivity 1 aa elongation coefficient Density (Cal-cm / cm2 sec. (Ppm / ° C) (g / cm3) ° C) 0.864 at 397 ° C 10.4 at 41-263 ° C 12.9 (97% of full density) 0.689 at 268 ° C 0.537 at 89 ° C ppm = parts per million The leak rate of these molded products is about 2 x 10-10 cm 3 of helium per second.

When this first example is repeated at a copper content of 5 and 50% by weight, products having the following properties 86604 are obtained, respectively: (a) At a copper content of 5% by weight, the thermal conductivity is 0.45 cal-cm / cm2 sec. ° C measured at about 39 ° C, and the coefficient of linear thermal expansion is 5.6 ppm per ° C at 41-263 ° C; and (b) at a copper content of 50% by weight, the thermal conductivity is 0.87 cal-cm / cm2 sec. ° C measured at about 390 ° C, and the coefficient of linear thermal expansion is 11.7 parts per million at ° C at 41-263 ° C.

In another example of the preferred embodiment, the copper content was reduced to 15% by weight and the butter frame content was increased to 85% by weight. Again, the same type of powders as described in the first example were used. The methods of mixing, injection molding, and binder removal were again the same. However, the sintering temperature was raised to 1450 ° C. The physical properties of interest were determined. The coefficient of linear thermal expansion was 7.56 ppm per ° C and the density was 15.3 g / cm 3. The density is 94% of the full theoretical density.

Repeating this second example gives a product with a thermal conductivity of about 0.57 cal-cm / cm sec. ° C measured at about 390 ° C.

In the third example, a highly pure copper-copper frame material containing 25% by weight of copper and 75% by weight of tungsten was prepared. The buttermilk powder used had an average particle size of 1-2 microns, a surface oxygen content of less than about 1,400 ppm and other impurities of about 300 ppm. The copper powder used had an average particle size of about 8-10 microns, a purity of about 99.95% copper, a surface oxygen content of less than 800 ppm as determined by weight loss of hydrogen, and other impurities of less than 500 ppm. Both tungsten and copper particles were essentially coaxial. The composition of the binder prepared was 39.47% by weight of polypropylene, 9.74% by weight of carnauba wax, 48.73% by weight of paraffin and 2.06% by weight of stearic acid. Tungsten and copper powders were proportioned so that 25% by weight of copper and 75% by weight of tungsten were used. The metal powder was mixed with the binder in such an amount that 4.3% by weight of the resulting mixture was binder. This mixing was performed in vacuo to better wet the surface of the particles and to leave less air. In this way, the porosity of the finished product was reduced and the thermal properties were improved. The desired shaped product was prepared by injection molding the resulting mixture of binder and metal powders. The product, the so-called crude part, was heated in air to about 207 ° C for two days to remove the wax. The resulting intermediate was then heated in an atmosphere containing 25% by volume of hydrogen and 75% by volume of nitrogen at temperatures up to about 500 ° C until the polypropylene was removed. The temperature was then raised to about 1235 ° C and maintained at that temperature for about 3 hours.

The resulting sintered net product was allowed to cool for about 6 hours. The thermal conductivity was determined to be 0.496 cal-cm / cm 2 ° C at about 390 ° C.

When this experiment was repeated using a copper powder with a purity of 99.7% and containing about 0.24% by weight of insoluble oxides and trace amounts of lead, silicon, calcium, magnesium and tin, a product with a thermal conductivity of 0.401 cal-cm / cm2 sec ° C at about 390 ° C.

When these experiments are repeated using density-maximizing particle size distributions, the thermal conductivity is better than about 0.42 cal-cm / cm2 sec. ° C.

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When this experiment was repeated using the same copper material with a purity of 99.7% and containing 2% by weight of nickel, a product with a thermal conductivity of only 0.293 cal-cm / cm 2 sec was obtained. ° C.

The foregoing are preferred embodiments that can be modified and improved without departing from the spirit and scope of the appended claims.

Claims (5)

1. Powder metallurgical injection molding method, in which liquid phase sintering is used to form net formed products, characterized in that the process comprises: a copper powder having an average particle size of less than about 20 microns and containing less than about 5000 million parts of surface acid and less than about 500 million parts of acidic acid other contaminants; vai of a tungsten powder whose average particle size is below about 40 microns and contains less than about 1500 million parts of surface acid and less than about 300 million parts of other contaminants; mixing said tungsten and copper powder in vacuo with a binder so that a mixture is formed; injection molding the mixture into a given raw form; removing the binder from said shrimp form; and sintering the shrimp mold to form a net shaped product. Powder metallurgical injection molding method according to claim 1, characterized in that said net-shaped product is of a copper-tungsten material containing about 5 - 50% by weight copper and the remainder essentially tungsten, said material having a hermetic density with a leakage rate below 1 x 10- 1 cm1 of helium per second; a thermal conductivity between about 0.4 cal-cm / cm 2 at a copper content of about 5 weight percent and about 0.68 cal-cm / cm 2 at a copper content of about 50 weight percent; and a thermal expansion between about 5.5 million parts / ° C at a copper content of 5% by weight and about 11.7 million parts / ° C at a copper content of 50% by weight. Powder metallurgical injection molding method according to claim 1, characterized in that the net-shaped product is in its form an electronic package, whose thermal conductivity lies between the values above about 0.42 cal-cm / cm2 at a copper content of 5% by weight, above about 0.60 cal. cm / cm 2 at a copper content of about 25% by weight and above about 0.70 cal cm / cm2 at a copper content of about 50% by weight. Powder metallurgical injection molding method according to claim 3, characterized in that said net-shaped product contains 5-50 percent copper and the rest tungsten. Powder metallurgical injection molding method according to claim 1, characterized in that in the process a tungsten product with such a particle size distribution is chosen to maximize the density of the mixtures.