Title of Invention

A NOVEL METALLIC PASTE COMPOSITION USEFUL AS AN INTERLAYER IN DIFFUSION SOLDERING.

Abstract

This invention relates to a novel metallic paste composition useful as an interlayer in diffusion soldering. The novel metallic paste composition consists of a first metallic element which is in liquid state at room temperature, a second metallic element in powder form which can form an intermetallic compound with atleast one of the metals to be joined and a third metallic element in powder form which can form an intermetallic compound with the metal with which the second metallic element powder does not form an intermetallic compound. This enables fabrication of metal joints for a wide spectrum of metals.

Full Text

The present invention relates to a novel metallic paste composition useful as an interlayer in diffusion soldering. The novel metallic paste composition of the present invention will be useful in and related to diffusion soldering for joining two metals/alloys in an easier way. Its intended application is in consumer, medical, telecommunication, automobiles, microelectronics, high temperature and high power applications, high reliability applications and the like. The known process of diffusion soldering for joining metals/ alloys, metallic/ alloy components (parts) involves using an interlayer of low melting joining element or alloy sandwiched between them at appropriate temperature and pressure for appropriate time. The joining element or alloy reacts with the parts to form intermetallic phases, by the principle of isothermal solidification, which are stable at relatively higher temperature than the processing temperature of the joint. Suitable selection of the combination of the metals/ alloys/ parts to be joined, the joining element/ alloy, their relative thickness and the processing condition governs the formation of intermetallics and thereby the properties of the resultant joint. Reference may be made to D.M. Jacobson and G. Humpston, Soldering and Surface Mount Technology, vol. 10, pp.27-32, 1992; P.K. Khanna, S.K. Bhatnagar, G. Dalke, D. Brunner and W. Gust, Mater, Sci. and Engg. 'B' vol. 33, pp. L6-9, 1995; S. Bader, W. Gust, H. Hieber, Acta. Metall. Mater., vol. 43, pp.329- 337, 1995; P.K. Khanna, S.K. Bhatnagar, L.S. Chang and W. Gust, Zeitschrift fur Metallkunde, vol. 90, pp.470-74, 1999, wherein the known methods of providing the interlayer in the diffusion soldering process are such as by using performs or foils, thermal deposition, sputtering, electro/ electroless plating. The above stated known processes have the following drawbacks. The use of preforms or foils is comparatively easy but their cutting in desired shape and handling are rather difficult. Availability of preforms or foils of the desired interlayer element/ alloy in the desired thickness may not be always possible. Thermal deposition or sputtering are good techniques for controlling the thickness of the interlayer but are relatively costly and not suitable for large scale production purposes and for handling large dimension parts. Availability of targets of the desired interlayer element/ alloy and vacuum chambers may not be always possible. Electro/ electroless plating are rather low cost processes but involve a complete set-up of the desired interlayer to be plated and standardisation of process and sometimes involve use of hazardous chemicals. Handling of parts of large dimensions and providing interlayer by selective plating is rather difficult and expensive. The main object of the present invention is to provide a novel metallic paste composition useful as an interlayer in diffusion soldering which obviates the drawbacks as detailed above. Another object of the present invention is to provide a process of diffusion soldering using the novel metallic paste composition as an interlayer which enables joining of a wide spectrum of metals of varied geometries. Yet another object is to provide a novel metallic paste for diffusion soldering which is environment friendly. Still another object is to provide a novel metallic paste which is easy to apply. Still another object is to provide a novel metallic paste which when used as an interlayer in diffusion soldering, results in a stable and mechanically strong joint. In the present invention, which relates to improvements in and related to diffusion soldering, comprises suitable selection of metallic/ alloy constituents taking into account the metals/ alloys to be joined, obtaining respective proportions of the constituents from the phase diagrams based on the temperature stability of the joint required and preparation of the material for providing interlayer: the constituents of the interlayer are selected and mixed in such a way that a smooth metallic paste is formed and once this is achieved then the metallic paste can be applied for providing interlayer by screen printing, stamping, applying with brush and the like. Accordingly the present invention provides a novel metallic paste composition useful as an interlayer in diffusion soldering which comprises:- i) first metallic element, liquid at room temperature : 20 to 50 wt %; ii) second metallic element in powder form capable of forming intermetallic compound with atleast one of the metals to be joined : 20 to 80 wt %; iii) third metallic element in powder form capable of forming intermetallic compound with the metal with which the second metallic element powder does not form an intermetallic compound : 0 to 80 wt %. In an embodiment of the present invention the first metallic element, liquid at room temperature of the order of 20 to 35° C, used may be such as Gallium. In another embodiment of the present invention the metallic elements used may be metals or alloys. The novel metallic paste composition of the present invention is not a mere admixture but a synergistic mixture having properties which are different from the mere aggregation of the properties of the individual ingredients and there is no chemical reaction involved in the preparation of the said composition. The present invention comprises of suitable selection of the constituents of the interlayer taking into account the metals/ alloys to be joined, examination of phase diagrams of participating materials with respect to temperature stability of joint required, obtaining respective proportions of the constituents from the phase diagrams taking into account the intermetallic phases required to govern the properties of the joint in order to provide the desired temperature stability and in some cases the viscosity/ wettability properties. The 1st metallic element/ alloy should be liquid at near room temperature of the order of 20 to 35 °C; 2nd metallic element/ alloy should be powder and 3rd metallic element/ alloy should be powder (if required) is selected taking into account the capability of forming intermetallics with either of the metals/ alloys to be joined or with the 1st metallic element/ alloy or with each other or with all, preparation of the material for providing interlayer by immersion and stirring of fine powders of different metals/ alloys in a metal/ alloy of relatively low melting point which is liquid at near room temperature. The constituents of the interlayer are selected and mixed in such a way that a smooth metallic paste is formed and once this is achieved then the metallic paste can be applied by for providing interlayer by screen printing, stamping, applying with brush and the like. In our copending patent application no. NF 160/02, we have described and claimed a process for diffusion soldering. The present invention provides a novel metallic paste composition useful in diffusion soldering which is used as follows: Cleaning by known methods the defined areas of metals to be joined. Coating a thin layer of novel metallic paste as an interlayer by screen printing, stamping, applying with brush and the like on one or both the defined areas, contacting the said defined areas face-to-face and heating the same at appropriate temperature, which is normally above the melting point of one or more interlayer material constituents, and pressure, in the range from zero to several MPa, for appropriate duration of time, in the range of few seconds to several hours in order to attain the desired intermetallic phases for forming the joints. The present invention can also be used for sealing purposes such as sealing of 1C packages, microelectronic modules and systems and the like. In the prior art one of the major constraint in joining different metals was the availability of the foil or plating bath or deposition target/ chamber whereby one can use foil or plated or deposited materials which may react with both the metals to be joined. With the invention of this interlayer metallic paste composition one can join a wide spectrum of metals by preparation of the material for providing interlayer taking into account the metals to be joined, selecting their constituents based on the intermetallic phase required to govern the joint. The inventive step is the use of an element which is liquid at near room temperature as the base and mixing the other constituents in the same resulting into a metallic paste. The novelty is to provide an interlayer using this metallic paste which improves the process of diffusion soldering and allows it to be used for a wide variety and size of components which was hitherto not possible. Using the material for providing interlayer of the present invention, diffusion soldering can be carried out at specified temperature, pressure and time as mentioned above, the molten pre-applied material reacts with the defined areas thereby connecting them with intermetallics formed by the process of isothermal solidification. These intermetallics are in accordance with the phase diagrams of the defined reacting materials and the property of joints is governed by the property of these phases. The following example is given by way of illustration and therefore should not be construed to limit the scope of the present invention. Example 1: A typical case of joining copper and nickel pieces by diffusion soldering using the novel metallic paste composition of the present invention is illustrated. From standard available phase diagram it comes out that an interlayer material with aluminium- gallium- nickel (Al-Ga- Ni) is to be formed for attaining thermally stable joints. The contribution of different elements to the interlayer material composition by weight is primarily obtained from the standard binary phase diagrams of Al-Ni and Ni-Ga. The wt % of (i) Gallium (liquid metallic element): 44.87 wt %; (ii) nickel (powder form): 37.82 wt % and (iii) aluminium (powder form): 17.31 wt %. The interlayer material is formulated by immersion of fine Al and Ni powder into liquid Ga and stirring the same to obtain it in the form of a paste. The interlayer material is then applied to the defined area of a Cu piece by brush which is to be connected to the defined area of a Ni piece. The two pieces are then brought into contact with each other by application of 0.5 MPa pressure. This assembly is heated up to 700 °C and the temperature maintained for 5 min in vacuum. This results in the formation of desired intermetallics thereby joining the defined areas of Cu and Ni pieces. The main advantages of the present invention are: 1. It can be used to join a wide spectrum of metals by preparing the interlayer material accordingly. 2. The related process for fabrication of joints is environment friendly as it avoids the use of toxic elements such as Pb, Cd, Tl etc. and effluxes containing chlorofluorocarbons. 3. Since the interlayer material does not contain materials like organic binders, thinners, screening agents and the like so there are no burn- outs during processing. 4. The interlayer material is easy to apply by methods such as applying by brush, screen printing, stamping etc. so the process becomes relatively simple and low cost. 5. The sizes and shapes of the parts to be joined do not matter in this novel method of applying the interlayer. 6. The shelf life of the interlayer material can be changed by proper selection of materials. It may be varied from a few minutes to several months. This is a unique novel feature opening the vast window of room temperature diffusion soldering using such material. 7. The joints produced using such an interlayer material and the related fabrication process are stable at temperatures higher than the fabrication temperature and their mechanical strength is also high. We claim: 1. A novel metallic paste composition useful as an interlayer in diffusion soldering which comprises: i) first metallic element, liquid at room temperature : 20 to 50 wt%; ii) second metallic element in powder form capable of forming intermetallic compound with atleast one of the metals to be joined : 20 to 80 wt %; iii) third metallic element in powder form capable of forming intermetallic compound with the metal with which the second metallic element powder does not form an intermetallic compound : 0 to 80 wt %. 2. A novel metallic paste composition as claimed in claim 1, wherein the metallic element, liquid at room temperature of the order of 20 to 35° C, used is such as gallium. 3. A novel metallic paste composition as claimed in claim 1-2, wherein the metallic elements used is such as a metal or alloy. 4. A novel metallic paste composition useful in diffusion soldering substantially as herein described with reference to the example.

Documents:

355-DEL-2002-Abstract-30-11-2007.pdf

355-del-2002-abstract.pdf

355-DEL-2002-Claims-30-11-2007.pdf

355-del-2002-claims.pdf

355-DEL-2002-Correspondence-Others-30-11-2007.pdf

355-del-2002-correspondence-others.pdf

355-del-2002-correspondence-po.pdf

355-DEL-2002-Description (Complete)-30-11-2007.pdf

355-del-2002-description (complete).pdf

355-DEL-2002-Form-1-30-11-2007.pdf

355-del-2002-form-1.pdf

355-del-2002-form-18.pdf

355-del-2002-form-2.pdf

355-DEL-2002-Form-3-30-11-2007.pdf

355-del-2002-form-3.pdf