SINTERING COMPOSITION

Abstract

A sintering composition, consisting essentially of: a solvent; and a metal complex dissolved in the solvent, wherein: the sintering composition contains at least 60 wt. % of the metal complex, based on the total weight of the sintering composition; and the sintering composition contains at least 20 wt. % of the metal of the metal complex, based on the total weight of the sintering composition.

Claims

1-52. (canceled)

53. A sintering composition, consisting essentially of: a solvent; and a metal complex dissolved in the solvent, wherein: the sintering composition contains at least 60 wt. % of the metal complex, based on the total weight of the sintering composition; and the sintering composition contains at least 20 wt. % of the metal of the metal complex, based on the total weight of the sintering composition.

54. The sintering composition of claim 53, consisting of the solvent and the metal complex dissolved in the solvent; and/or wherein the metal complex substantially decomposes over a temperature range of less than 20° C., preferably less than 15° C., more preferably less than or equal to 10° C.; and/or wherein the sintering composition contains the metal complex in an amount of at least 70 wt. % based on the total weight of the sintering composition, preferably at least 75 wt. % based on the total weight of the sintering composition; and/or wherein the solvent is supersaturated with the metal complex; and/or wherein the sintering composition contains at least 25 wt. % metal based on the total weight of the sintering composition, preferably at least 30 wt. % metal based on the total weight of the sintering composition, more preferably at least 35 wt. % metal based on the total weight of the sintering composition; and/or wherein the metal complex substantially decomposes at a temperature of less than 200° C., preferably less than 190° C.; and/or wherein the solvent and/or the ligand of the metal complex has a boiling point of less than 300° C., preferably less than 250° C. and more preferably less than 200° C.; and/or wherein the metal of the metal complex is selected from one or more of silver, gold, platinum, palladium, nickel, copper and molybdenum, preferably copper and/or silver; and/or wherein the solvent comprises an organic solvent, preferably wherein the solvent is selected from one or more of propane-1-2-diol, terpineol, triethylene glycol, glycerol and 2-methyl-1,3-propanediol, more preferably terpineol or glycerol; and/or wherein the metal complex is a metal carboxylate, preferably wherein the carboxylate has the structure RCOO.sup.− wherein R is a branched or unbranched hydrocarbon chain having 14 carbon atoms or less, preferably 12 carbon atoms or less and more preferably 10 carbon atoms or less; and/or wherein the metal complex is selected from one or more of silver neodecanoate, silver 2-ethyl hexanoate, silver oxalate, silver lactate, silver hexafluoroacetylacetonate cyclooctadiene copper formate tetrahydrate and copper acetate, preferably silver neodecanoate or copper formate tetrahydrate; and/or wherein the viscosity of the sintering composition is from 60,000 to 120,000 cP, preferably from 70,000 to 80,000 cP.

55. The sintering composition of claim 53, in the form of: a gel, preferably a metallic gel; or a free-standing film or foil; or applied, deposited, printed or laminated on a wafer; or applied, deposited, printed or laminated on a substrate fabricated from glass and/or ceramic and/or a metal and/or a polymeric film and/or a composite material used as a printed circuit board.

56. A method of manufacturing the sintering composition of claim 53, the method comprising: providing a solvent; providing a metal complex; and dissolving the metal complex in the solvent at a temperature of from 100° C. to 180° C. to form a solution.

57. The method of claim 56, wherein the solution is supersaturated; and/or wherein the method further comprises cooling the solution to a temperature of from 30 to 60° C., preferably to a temperature of from 40 to 50° C.; and/or wherein the method further comprises milling the solution; and/or wherein dissolving the metal complex in the solvent is carried out at a temperature of from 110° C. to 150° C., preferably from 120° C. to 140° C., even more preferably about 130° C.; and/or wherein the sintering composition is free of any metal particles and/or free of any particles of the metal complex.

58. A method of forming a sintered metal deposit on a substrate, the method comprising: providing a substrate; disposing the sintering composition of claim 53 onto the substrate; drying the composition at a temperature of from 140 to 200° C. to form a dried composition; and sintering the dried composition at a temperature of from 150° C. to 300° C., wherein the sintering is carried out at a temperature higher than the temperature of the drying.

59. The method of claim 58, wherein the substrate is a flexible substrate, preferably comprising one of more of PET, PC, PI, TPU, PEN and PEEK; or wherein the substrate is a rigid substrate, preferably comprising FR4, glass, ceramic, such as Al.sub.2O.sub.3, SiC or SiO.sub.2, a semiconductor material, such as Si, SiC, Ge or GaN, or a metal foil, such as a metal foil formed of Cu, Ag, Au, Al, Ni or an alloy.

60. The method of claim 58, wherein disposing the sintering composition onto the substrate comprises stencil printing and/or screen printing the sintering composition onto the substrate; and/or wherein: the metal complex of the sintering composition comprises silver, and the drying is carried out at a temperature of from 170° C. to 190° C. for from 30 to 90 minutes; or the metal complex of the sintering composition comprises copper, and the drying is carried out at a temperature of from 150° C. to 170° C. for from 15 to 60 minutes under a nitrogen atmosphere; and/or wherein the sintering is carried out at a temperature of from 180° C. to 295° C., preferably from 200° C. to 290° C., more preferably from 210° C. to 285° C., even more preferably from 240° C. to 280° C., still even more preferably from 250° C. to 270° C.; and/or wherein the sintering step is performed at a pressure of from 2 MPa to 15 MPa, preferably from 4 MPa to 12 MPa.

61. The method of claim 58, wherein the sintered metal deposit comprises a joint between a die and a substrate; or wherein the sintered metal deposit comprises an electrical circuit or an interconnect; or wherein the sintered metal deposit comprises a coating.

62. The method of claim 58, wherein the sintered metal deposit is substantially free of metal complex and/or substantially free of organic ligand and/or substantially free of organic material and/or substantially free of carbon; and/or wherein the sintered metal deposit has a die shear strength of greater than or equal to 30 MPa, preferably greater than or equal to 40 MPa, and more preferably greater than or equal to 50 MPa; and/or wherein: after a thermal shock test for 1000 cycles, from +150° C. to −50° C. with a dwell time of 3 minutes, the percentage void in the sintered metal deposit is 0.5% or less, preferably 0.3% or less; and/or after a thermal shock test for 3000 cycles, from +150° C. to −50° C. with a dwell time of 3 minutes, the percentage void in the sintered metal deposit is 1.5% or less, preferably 1.0% or less; and/or wherein the peel strength of the sintered metal deposit is greater than or equal to 10 N/mm, preferably greater than or equal to 15 N/mm, and more preferably greater than or equal to 20 N/mm; and/or wherein the sintered metal deposit is solderable and/or high-current capable; and/or wherein the sintered metal deposit is flexible; and/or wherein the sintered metal deposit, preferably a silver deposit, exhibits a sheet resistance of less than 12 mΩ/sq/mil.

63. A sintered metal deposit formed using the sintering composition of claim 53, preferably wherein the sintered metal deposit is formed using a method of forming a sintered metal deposit on a substrate, the method comprising: providing a substrate; disposing the sintering composition onto the substrate; drying the composition at a temperature of from 140 to 200° C. to form a dried composition; and sintering the dried composition at a temperature of from 150° C. to 300° C., wherein the sintering is carried out at a temperature higher than the temperature of the drying.

64. The sintered metal deposit of claim 63, wherein the sintered metal deposit comprises a joint between a die and a substrate; or wherein the sintered metal deposit comprises an electrical circuit or an interconnect; or wherein the sintered metal deposit comprises a coating.

65. A method of die attachment, the method comprising: providing a substrate and a die; disposing the sintering composition of claim 53 between the substrate and the die; drying the composition at a temperature of from 140 to 200° C. to form a dried composition; and sintering the dried composition at a temperature of from greater than 150° C. to 300° C. at a pressure of from 2 MPa to 15 MPa to attach the die to the substrate, wherein the sintering is carried out at a temperature higher than the temperature of the drying.

66. The method of claim 65, wherein the sintering is carried out at a pressure of from 4 MPa to 12 MPa.

67. A method of forming an electrical circuit or an interconnect on a substrate, the method comprising: providing a substrate; disposing the sintering composition of claim 53 onto the substrate in the form a circuit or interconnect; drying the composition at a temperature of from 140 to 200° C. to form a dried composition; and sintering the dried composition at a temperature of from 150° C. to 300° C. to form the circuit or interconnect, wherein the sintering is carried out at a temperature higher than the temperature of the drying.

68. The method of claim 67, wherein the substrate is either rigid or flexible.

69. A method of manufacturing a soldered assembly, the method comprising: forming a sintered metal deposit on a substrate according to the method of claim 58; depositing a solder paste on the sintered metal; contacting a component with the deposited solder paste; and reflowing the solder paste to provide a soldered assembly.

70. The method of claim 69, wherein the component is selected from an electronic component, an LED, a metallized printed circuit board and heat sink; and/or wherein the substrate is selected from a carrier film, an interposer or a circuit board or a heat sink.

71. Use of the sintering composition of claim 53 in a method selected from: die attachment, wafer-to-wafer bonding, hermetic and near hermetic sealing, dispensing and the production of circuitry or interconnect lines.

72. Use of the sintering composition of claim 53 in a method of forming a sintered metal deposit on a substrate, the method comprising: providing a substrate; disposing the sintering composition onto the substrate; drying the composition at a temperature of from 140 to 200° C. to form a dried composition; and sintering the dried composition at a temperature of from 150° C. to 300° C., wherein the sintering is carried out at a temperature higher than the temperature of the drying, to produce electrical/electronic devices, interconnects, assemblies, circuits, or printed cables and wire harnesses; preferably wherein the assemblies and/or circuits are flexible or thermoformed and/or injection molded 3D electronic structures and/or preferably wherein the assemblies and/or circuits are solderable and can carry high current density.

Description

[0111] The invention will now be described in relation to the following non-limiting Figures, in which:

[0112] FIGS. 1a and 1b show an FESEM cross-section of joints made using a metallic gel according to the present invention.

[0113] FIG. 2a shows a microscopic image of a failure mode on the substrate side of an assembled sample according to an embodiment of the present invention.

[0114] FIG. 2b shows a microscopic image of a failure mode on the ribbon side of an assembled sample according to an embodiment of the present invention.

[0115] FIG. 3a shows an FESEM micrograph of a failure mode on the substrate side of an assembled sample according to an embodiment of the present invention.

[0116] FIG. 3b shows an FESEM micrograph of a failure mode on the ribbon side of an assembled sample according to an embodiment of the present invention.

[0117] FIG. 4a shows X-ray analysis of the percentage void of an assembled sample according to an embodiment of the present invention after being subjected to a thermal shock test for 0 cycles.

[0118] FIG. 4b shows X-ray analysis of the percentage void of an assembled sample according to an embodiment of the present invention after being subjected to a thermal shock test for 1000 cycles.

[0119] FIG. 4c shows X-ray analysis of the percentage void of an assembled sample according to an embodiment of the present invention after being subjected to a thermal shock test for 3000 cycles.

[0120] FIGS. 5a, 5b and 5c show FESEM cross-sections of assembled samples according to embodiments of the present invention after being subjected to a thermal shock test for 3000 cycles.

[0121] FIG. 6a shows a peel strength failure mode on the DBC side of an assembled sample in accordance with an embodiment of the present invention.

[0122] FIG. 6b shows a peel strength failure mode on the ribbon side of an assembled sample in accordance with an embodiment of the present invention.

[0123] FIGS. 7a, 7b and 7c show SEM cross-sections of the soldering on a metallic gel on an FR4 substrate in accordance with an embodiment of the present invention.

[0124] FIG. 8a shows an FESEM cross-section of an assembled sample in accordance with an embodiment of the present invention.

[0125] FIGS. 8b and 8c show FESEM cross-sections of the die interface of assembled samples in accordance with an embodiment of the present invention.

[0126] FIGS. 8d and 8e show FESEM cross-sections of the substrate interface of assembled samples in accordance with an embodiment of the present invention.

[0127] FIGS. 9a-9c show images of SnBi solder paste printed and reflowed under nitrogen/air on the printed surface of a metallic gel according to the present invention, which was printed on an FR4/PI substrate.

[0128] The invention will now be described in relation to the following non-limiting examples.

[0129] Materials and Equipment:

[0130] Materials:

[0131] Silver neodecanoate, 95%, was purchased from Gelest INC.

[0132] Propane-1, 2-diol, tri-ethylene glycol and 2-Methyl 1-3 propanediol were purchased from SDFCL.

[0133] Terpineol (mixture of isomers) was purchased from Loba chemie.

[0134] Copper (II) formate tetrahydrate was purchased from Alfa Aesar. 3-(Dimethylamino)-1,2-propanediol was purchased from Tokyo Chemical Industry.

[0135] Glycerol (98%) was purchased from Merck.

[0136] 1-ethyl-2-pyrrolidinone was purchased from Advent.

[0137] All of the above materials are analytical grade reagents and were used without any further purification.

[0138] Equipment:

[0139] Die shear was performed on a Dage 4000 PXY.

[0140] Die attachment was performed on a Carver (3891CEB.4NE1001).

[0141] Viscosity was measured with a Brookfield viscometer (HB DV-III) Spindle CP51.

[0142] X-ray analysis was done in a Phoenix X-ray (Microme/X-180).

[0143] Peel strength was done with an IMADA peel tester (model MX2-110).

EXPERIMENTAL

(1) Synthesis of Silver Metallic Gel

Example 1-I

[0144] 12 g of silver neodecanoate was weighed into a two-neck round bottom flask equipped with thermometer and condenser. To the silver precursor, 4.05 g of terpineol and 0.45 g propane-1-2-diol were added. The reaction mixture was then heated at 85° C. with constant stirring until all of the silver neodecanoate was fully dissolved. The resultant mixture was then allowed to cool and was then milled to get a homogenous metallic gel. As silver compounds are very much photosensitive, the particle free metallic gel was kept in an opaque or amber coloured bottle under refrigerated conditions. The results are shown in Table 1.

TABLE-US-00001 TABLE 1 Component Weight % Silver neodecanoate 72.73 Propane-1-2-diol 2.72 Terpineol 24.55

Example 1-II

[0145] 12 g of silver neodecanoate was weighed into a two-neck round bottom flask equipped with thermometer and condenser. To the silver precursor, 4 g of terpineol was added. The reaction mixture was then heated at 85° C. with constant stirring until all of the silver neodecanoate was fully dissolved. The resultant mixture was then allowed to cool and was then milled to get a homogenous metallic gel. As silver compounds are very much photosensitive, the particle free metallic gel paste was kept in an opaque or amber coloured bottle under refrigerated conditions. The results are shown in Table 2.

TABLE-US-00002 TABLE 2 Component Weight % Silver Neodecanoate 75 Terpineol 25

[0146] The metal content of the metallic gel can be varied by varying the weight percentage of the silver precursor in the formulation.

Example 1-III

[0147] 12 g of silver neodecanoate was weighed into a two-neck round bottom flask equipped with thermometer and condenser. To the silver precursor, 2.25 g of terpineol and 2.25 g of trigol were added. The reaction mixture was then heated at 85° C. with constant stirring until all of the silver neodecanoate was fully dissolved. The resultant mixture was then allowed to cool and was then milled to get a homogenous metallic gel. The metallic gel was kept in an amber coloured bottle in refrigerated conditions. The results are shown in Table 3.

TABLE-US-00003 TABLE 3 Component Weight % Silver precursor 72.72 Terpineol 13.64 Trigol 13.64

Example 1-IV

[0148] 12 g of silver neodecanoate was weighed into a two-neck round bottom flask equipped with thermometer and condenser. To the silver precursor, 2.25 g of terpineol and 2.25 g of 2-methyl-1,3-propanediol were added. The reaction mixture was then heated at 85° C. with constant stirring until all of the silver neodecanoate was fully dissolved. The resultant mixture was then allowed to cool and was then milled to get a homogenous metallic gel. The results are shown in Table 4.

TABLE-US-00004 TABLE 4 Component Weight % Silver precursor 72.72 Terpineol 13.64 2-Methyl 1-3 propanediol 13.64

(2) Synthesis of Metallo-Organic Paste

Example 2-I

[0149] 9.2 g of silver neodecanoate was weighed in a container. To which 2.25 g of tri-ethylene glycol was added. The mixture was then mixed in a planetary mixer at 1000 rpm for 1 minute. The slurry was then milled to obtain a homogenous paste. The paste was kept in an amber coloured bottle in refrigerated conditions. The results are shown in Table 5.

TABLE-US-00005 TABLE 5 Component Weight % Silver precursor 80.35 Tri-ethylene glycol 19.65

[0150] Metallic Gel and its Form Factors:

[0151] Thermal Characterization:

[0152] Thermogravimetric analysis (TGA) of the particle free molecular paste or metallic gel showed the complete evaporation of solvents and decomposition of the metal-organic precursor below 250° C. Total weight loss of 72% at 250° C. reveals the fact that on heating the metallic gel at 250° C., the organic moieties fully detached from the elemental silver and evaporated off leaving no carbon residue but only bulk silver metal. Thus, the thermal decomposition of the silver precursor leads to the deposition of only pure silver metal with no carbon residue. At TGA at temperatures above 250° C., the percentage residue is close to the theoretical weight percentage of silver in silver neodecanoate (28 wt. %) based on the chemical formula. This apparently indicates that thermal decomposition of the silver neodecanoate has completely taken place, leaving only pure silver.

[0153] Differential scanning calorimetry (DSC) of the particle free metallic gel showed an endothermic peak at 100-120° C. which indicates melting of the isomers of silver neodecanaote along with the slow evaporation of the solvents. The isomers of silver neodecanoate undergo a solid to liquid phase transition prior to the thermal decomposition which occurs above 200° C. Two exothermic peaks at 205° C. and 228° C. in the DSC indicate that the decomposition of the silver precursor is a two-step decomposition process.

[0154] Viscosity and Stability of the Paste:

[0155] The viscosity of the paste was measured using a Brookfield viscometer (HB DV-III) Spindle CP51 at 1 rpm. The viscosity of the paste was 76 Pa.Math.s. The viscosity was measured at each alternative day. There was no change in the viscosity of the paste after 1 month of manufacturing, indicating the good stability of the paste.

[0156] Particle Free Molecular Paste or Metallic Gel; Form Factors and its Application in Die Attachment:

[0157] (i) Printing Paste and its Application in Die Attachment:

[0158] The attachment of semiconductor or other die elements can be accomplished by printing the metallic gel or particle free molecular paste on to the substrates such as DBC (Direct Bond Copper), DPC (Direct Plate Copper), MCPCB (Metal Core PCBs), FR4, Copper lead-frames, flexible PCBs with the help of a DEK printer.

[0159] The printed substrate was dried at 180° C. for 60 minutes in air. The pre-dried material is then allowed to cool at normal room temperature. Our study has shown that the pre-dried print material does not contain any organic impurity obtained from the decomposing reaction of the metal precursor. The complete removal of the organic impurity before the die placement provides a carbon free joint between the die and substrate. Die elements such as gold coated Si dies and Ag ribbons were placed on the pre-dried printed area. The assembly was then wrapped in an aluminium foil. The vehicle was then subjected to 260° C. at 10 MPa pressure for 1 minute in Carver press.

[0160] The assembled samples were then subjected to X-ray analysis for void measurement. An X-ray of the assembled samples showed a void of <1%. The low percentage void can be attributed to the wetting of die and substrate surfaces due to the solid to liquid phase transition before decomposition of the silver precursor.

[0161] FIGS. 1a and 1b show FESEM micrographs of the cross-section of the assembled samples. The SEM cross-section shows a good densification of silver. The SEM cross-section also reveals good diffusion of silver to die and also on the substrate interface, which of course is the contributing factor for the higher joint strength. The bond-line thickness measured varies from 2.5 microns to 3 microns.

[0162] The joint strength of the assembled samples was evaluated using Dage Die shear machine. The assembled 3 mm*3 mm Au coated Si dies show a joint strength>50 MPa. The die shear value could not be exactly quantified as because part of the dies was shattering and the majority of the die was stuck to the substrate. Data were also collected for high temperature die shear done at 260° C. It was seen that there was no deterioration in joint strength when the die shear was done at higher temperature.

[0163] Joint strength was further evaluated by measuring the peel strength of the Ag ribbon (2 mm width and 85 microns thick) attached to gold coated DBC substrate using IMADA peel tester. The peel strength obtained was much stronger than the joints made by the reported nanoparticle based interconnect material. The failure mode of the joints shows a bulk failure. Microscopic images (FIGS. 2a and 2b) show a diffused layer of silver both on the silver ribbon and DBC side.

[0164] The microstructure of the diffused layer of silver on both the interfaces was analyzed using FESEM. Micrographs of FESEM (FIGS. 3a and 3b) show the diffused silver layer on the surface of ribbon and DBC has an elongated spike like structure. This reveals the fact that the peeling off the ribbon from the substrate has created a huge stress on the diffused silver layers of ribbon and DBC which made the silver layer to protrude out of the surfaces.

[0165] The assembled samples were subjected to a thermal shock test for 1000 cycles (+150 to −50° C. dwell time of 3 minutes) for 3000 cycles. X-ray analysis shows that the percentage void slowly increases from 0.3 to 1% with increasing number of thermal shock cycles (FIGS. 4a, 4b, and 4c).

[0166] It was observed that there was no deterioration in die shear strength after completion of 3000 cycles. It was seen that during die shear most part of the die was shattered and was stuck to the substrate after 3000 cycles, which reveals the excellent joint strength. FIGS. 5a, 5b and 5c show the FESEM cross-sections of the 3000TS. The cross-sections reveal a good diffusion of silver layer at both the interfaces with no cracks within the silver layer and interfaces.

[0167] Our study shows that with increase in sintering pressure and sintering time, the peel strength increases. In case of low pressure die attachment, higher joint strength can be achieved by increasing the sintering time. In our case, a maximum of 20 N/mm can be achieved at 300° C. at 10 MPa for 1 minute. Table 6 shows the peel strength at different sintering pressure and temperature.

TABLE-US-00006 TABLE 6 Sintering Time Peel Strength (sec) (N/mm) Sintering Temperature @ 10 MPa 300 60 17-20 300 30 17-20 300 15 12-17 260 60 15-18 260 30 10-14 200 60 10-14 200 30  7-10 Sintering Temperature @ 6 MPa 300 60  9-12 300 30  9-11 300 15  8-10 260 60  9-12 260 30 7-8 200 120 <4 200 90 No strength

[0168] (ii) Free Standing Silver Film/Foil and its Application in Die Attachment:

[0169] Preparation of Free Standing Silver Films:

[0170] The particle free molecular paste was printed on glass substrate. The printed glass substrate was then heated at 150° C. for 140 minutes. It was then allowed to cool at room temperature. The glass substrate was then dipped in a water bath for 10 minutes. This allows the print to detach itself from the glass. The thin silver film was then dried at room temperature for 60 minutes. The thickness of the silver film varies with the thickness of the stencil. Basically, a silver film of thickness which varies from 3 microns to 15 microns was synthesised. The synthesised silver films are flexible and further, these films were used for die attach applications.

[0171] The synthesised film can be used as a die attach interconnect material. The film was placed as interconnect material between the die (3*3 mm Au coated Si die) and substrate (gold coated DBC). The assembled sample was then wrapped in an aluminium foil and then subjected to 260° C. at 6 MPa pressure for 1 minute in Carver press. X-ray analysis showed that the assembled sample did not show the presence of any void.

[0172] The die shear obtained was around 40 MPa. The failure mode obtained was an interfacial failure. A diffused layer of silver was seen on the die side but no layer of silver is seen on the substrate side.

[0173] Wafer Lamination and its Application in Die Attachment:

[0174] Particle free molecular paste was printed on Au coated 2″ silicon wafer using a DEK printer. The printed wafer was then allowed to dry at 180° C. for 75 minutes in a box oven. The laminated wafer was then mounted on a UV tape and diced using ADT 7100 dicing machine. The coated singulated die was then used for die attach in Au coated DBC using Datacon die bonder. The UV tape did not show any remains of the silver coating on the UV tape. The coated Si die was then attached to the DBC substrate at 250° C. using a sintering pressure of 6 MPa for 1 minute.

[0175] X-ray analysis revealed the presence of 0.3% void. The joint strength of the assembled sample at 250° C. at 6 MPa sintering pressure was around 40 MPa. Failure mode analysis of the assembled sample showed a bulk failure.

[0176] Metalorganic Paste, Die Attach Application:

[0177] The invented metalorganic paste was printed on a DBC. The printed substrate was dried at 180° C. for 60 minutes. The pre-dried material was then allowed to cool at normal room temperature. Die elements such as gold coated Si dies and Ag ribbons were placed on the pre-dried printed area. The assembly was then wrapped in an aluminium foil. The vehicle was then subjected to 260° C. at 10 MPa pressure for 1 minute in a Carver press. The joint strength of the assembled samples was evaluated using a Dage Die shear machine. The assembled 3 mm*3 mm Au coated Si dies show a joint strength>50 MPa. The die shear value could not be exactly quantified as because part of the dies were shattering and most of the die was stuck to the substrate. Joint strength was further evaluated by measuring the peel strength of the Ag ribbon (2 mm width and 85 micron thick) attached to gold coated DBC substrate using IMADA peel tester. The peel strength obtained was around 8-10 N/mm. The failure mode of the joints shows a bulk failure as shown in FIGS. 6a and 6b.

[0178] Soldering on Silver Metallic Gel:

[0179] The disclosure also reveals the soldering on metallic gel. It has been seen that the invented metallic gel has 5B addition to PI, PET, FR4, glass, ceramic, metal and polymeric substrates. In the current invention, the metallic gel was printed on a FR4/polyimide substrate which was then cured at 180° C. for 1 hour. On the printed surface of the metallic gel, (Sn/Bi) solder paste was printed and reflowed under nitrogen/air. Good wetting properties of solder on metallic gel printed on the polymeric substrate were seen (see FIGS. 9a-9c).

[0180] The soldered assembly was cross-sectioned for a FESEM micrograph. From FIGS. 7a, 7b and 7c, it can be easily seen that the solder paste nicely diffuses through the metallic gel layer and no cracks were also seen at the solder and gel interface. EDAX done at the interfaces shows almost 70% Ag and 30% (wt/wt) Sn. The percentage composition clearly reveals the formation of Ag.sub.3Sn.

(3) Synthesis of Copper Metallic Gel

Example 3-I

[0181] 5 g copper (II) formate tetrahydrate was weighed and 0.2 g 3-(dimethylamino)-1,2-propanediol was added to it. This mixture was then heated at 130° C. with occasional stirring for 10 minutes and it was then cooled to room temperature. 0.03 g silver neodecanoate and 1 g glycerol were then added to the previous mixture and it was then mixed for 2-3 minutes. The whole mixture was then milled in a three roll mill and it was then homogenized in an orbital mixture at 1000 RPM for 10 minutes.

Example 3-II

[0182] 5 g copper (II) formate tetrahydrate was weighed and 0.2 g 3-(dimethylamino)-1,2-propanediol was added to it. This mixture was then heated at 130° C. with occasional stirring for 10 minutes and it was then cooled to room temperature. 0.7 g glycerol and 0.3 g diethylene glycol monomethyl ether were then added to the previous mixture and it was then mixed for 2-3 minutes. The whole mixture was then milled in a three roll mill and it was then homogenized in an orbital mixture at 1000 RPM for 10 minutes.

Example 3-III

[0183] 5 g copper (II) formate tetrahydrate was weighed and 0.2 g 3-(dimethylamino)-1,2-propanediol was added to it. This mixture was then heated at 130° C. with occasional stirring for 10 minutes and it was then cooled to room temperature. 1 g terpeneol was then added to the previous mixture and it was then mixed for 2-3 minutes. The whole mixture was then milled in a three roll mill and it was then homogenized in an orbital mixture at 1000 RPM for 10 minutes.

Example 3-IV

[0184] 5 g copper (II) formate tetrahydrate was weighed and 0.2 g 3-(dimethylamino)-1,2-propanediol was added to it. This mixture was then heated at 130° C. with occasional stirring for 10 minutes and it was then cooled to room temperature. 1 g glycerol was then added to the previous mixture and it was then mixed for 2-3 minutes. The whole mixture was then milled in a three roll mill and it was then homogenized in an orbital mixture at 1000 RPM for 10 minutes.

Example 3-V

[0185] 5 g copper (II) formate tetrahydrate was weighed and 0.2 g 3-(dimethylamino)-1,2-propanediol was added to it. This mixture was then heated at 130° C. with occasional stirring for 10 minutes and it was then cooled to room temperature. 0.5 g glycerol was then added to the previous mixture and it was then mixed for 2-3 minutes. The whole mixture was then milled in a three roll mill and it was then homogenized in an orbital mixture at 1000 RPM for 10 minutes.

Example 3-VI

[0186] 4.5 g copper (II) formate tetrahydrate was weighed and 0.2 g 3-(dimethylamino)-1,2-propanediol was added to it. This mixture was then heated at 130° C. with occasional stirring for 10 minutes and it was then cooled to room temperature. 0.5 g Cu nano powder and 1 g glycerol were then added to the previous mixture and it was then mixed for 2-3 minutes. The whole mixture was then milled in a three roll mill and it was then homogenized in an orbital mixture at 1000 RPM for 10 minutes.

Example 3-VII

[0187] 8 g silver metallic gel (see section (1) of Experimental section) and 2 g of Example 3-IV were weighed and then mixed and homogenized using an orbital mixture for 3 times at 1000 RPM for 1 minute.

Example 3-VIII

[0188] 5 g copper (II) formate tetrahydrate was weighed; 0.2 g 3-(dimethylamino)-1,2-propanediol was added to it. This mixture was then heated at 130° C. with occasional stirring for 10 minutes and it was then cooled to room temperature. 1 g glycerol and 0.31 g 1-ethyl-2-pyrrolidinone were then added to the previous mixture and it was then mixed for 2-3 minutes. The whole mixture was then milled in a three roll mill and it was then homogenized in an orbital mixture at 1000 RPM for 1 minute.

Example 3-IX

[0189] 5 g copper (II) formate tetrahydrate was weighed; 0.2 g 3-(dimethylamino)-1,2-propanediol was added to it. This mixture was then heated at 130° C. with occasional stirring for 10 minutes and it was then cooled to room temperature. 1 g 1-ethyl-2-pyrrolidinone was then added to the previous mixture and it was then mixed for 2-3 minutes. The whole mixture was then milled in a three roll mill and it was then homogenized in an orbital mixture at 1000 RPM for 1 minute.

[0190] Copper Gel, Die Attach Application:

[0191] Copper gel was printed on a DBC. The printed substrate was then dried at 160° C. for 30 minutes in nitrogen atmosphere. The pre-dried material was then allowed to cool at room temperature. Die elements such as gold coated Si dies were placed on the pre-dried printed area. The assembly was then wrapped in an aluminium foil with a graphite sheet for cushioning effect. The vehicle was then subjected to 260° C. at 10 MPa pressure for 2 minutes in a Carver press. The joint strength of the assembled samples was evaluated using a Dage Die shear machine. The assembled 3 mm*3 mm Au coated Si dies show a joint strength>70 MPa. The die shear value could not be exactly quantified because part of the dies shattered and most of the die was stuck to the substrate.

[0192] FESEM micrographs of the cross-section of the assembled samples (FIGS. 8a-e) show a good densification of copper. SEM cross-section also reveals good diffusion of copper to die and on the substrate interface, which of course is the contributing factor for the higher joint strength. The bond-line thickness for a print using 200 micron stencil is around 11 microns.

[0193] Soldering on Copper Metallic Gel:

[0194] It has been seen that the invented metallic copper gel has good addition to PI, PET and FR4 substrates if it is laminated at a minimum pressure of 5 MPa at 150-200° C. In the current invention, the copper gel was printed on a FR4/polyimide substrate which was then pre-dried at 160° C. for 30 minutes under nitrogen, then laminated in a Carver press at 5 MPa pressure at 200° C. (PI and FR4)/150° C. (PET) for 2 minutes. On the laminated surface of metallic copper gel, solder paste was printed and reflowed under nitrogen. Good wetting properties of solder on metallic copper gel printed on polymeric substrate were seen. When 1026 components were soldered, the joint strength was 4-5 kg with a bulk failure.

[0195] The foregoing detailed description has been provided by way of explanation and illustration, and is not intended to limit the scope of the appended claims. Many variations in the presently preferred embodiments illustrated herein will be apparent to one of ordinary skill in the art and remain within the scope of the appended claims and their equivalents.

[0196] The present invention will now be described with reference to the following numbered clauses. [0197] 1. A sintering composition comprising: [0198] a solvent; and [0199] a metal complex dissolved in the solvent. [0200] 2. The sintering composition of clause 1, in the form of a gel, preferably a metallic gel. [0201] 3. The sintering composition of clause 1 or clause 2, wherein the sintering composition is for die attach. [0202] 4. The sintering composition of any one of the preceding clauses, wherein solvent is supersaturated with the metal complex. [0203] 5. The sintering composition of any one of the preceding clauses, wherein the sintering composition is substantially free of polymers, and preferably substantially free of organic polymers. [0204] 6. The sintering composition of any one of the preceding clauses, wherein the sintering composition is substantially free of metal particles and/or substantially free of particles of the metal complex. [0205] 7. The sintering composition of any one of the preceding clauses, wherein the metal complex comprises an organo-metal complex. [0206] 8. The sintering composition of clause 7, wherein the metal of the organo-metal complex is selected from one or more of Ag, Au, Pt, Pd, Ni, Cu, and Mo, preferably Cu and/or Ag. [0207] 9. The sintering composition of any one of the preceding clauses, wherein the metal complex comprises silver neodecanoate. [0208] 10. The sintering composition of any one of the preceding clauses, wherein the solvent has a boiling point of less than 300° C., preferably less than 250° C. and more preferably less than 200° C. [0209] 11. The sintering composition of any one of the preceding clauses, wherein the metal complex substantially decomposes at a temperature of less than 300° C., preferably less than 250° C. and more preferably less than 200° C. [0210] 12. The sintering composition of any one of the preceding clauses, wherein the sintering composition comprises the metal complex in an amount of 60 wt. % or more, preferably 70 wt. % or more, more preferably 75 wt. % or more, based on the total weight of the sintering composition. [0211] 13. The sintering composition of any one of the preceding clauses, wherein the sintering composition comprises 20 wt. % metal or more, preferably 25 wt. % metal or more, more preferably 30 wt. % metal or more, even more preferably 35 wt. % metal or more, based on the total weight of the sintering composition. [0212] 14. The sintering composition of any one of the preceding clauses, wherein the solvent comprises an organic solvent, preferably wherein the solvent is selected from one or more of propane-1-2-diol, terpineol, trigol or 2-methyl-1,3-propanediol, preferably terpineol. [0213] 15. The sintering composition of any one of the preceding clauses, wherein the viscosity of the sintering composition is between 70,000 and 80,000 cP [0214] 16. The sintering composition of any one of the preceding clauses, wherein the sintering composition is substantially free of metal oxide. [0215] 17. The sintering composition of any one of the preceding clauses, in the form of a free standing film or foil, and/or laminated on a wafer. [0216] 18. A sintering paste comprising: [0217] a solvent, and [0218] a metal complex suspended in the solvent. [0219] 19. The sintering paste of clause 18, comprising the sintering composition of any one clauses 1 to 17. [0220] 20. The sintering paste of clause 18 or clause 19, wherein the solvent comprises triethylene glycol. [0221] 21. The sintering paste of any one of clauses 18 to 20, wherein the metal complex is in an amount of 70 wt. % or more, preferably 80 wt. % or more, based on the total weight of the sintering paste. [0222] 22. The sintering paste of any one of clauses 18 to 21, wherein the sintering paste is for die attach. [0223] 23. The sintering paste of any one of clauses 18 to 22, in the form of a free standing film or foil, and/or laminated on a wafer. [0224] 24. A method of manufacturing the sintering composition of any of clauses 1-17, the method comprising: [0225] at least partially dissolving a metal complex in a solvent to form a solution, and [0226] milling the solution. [0227] 25. The method of clause 24, wherein the dissolving is carried out at a temperature of between 60° C. and 100° C., preferably between 70° C. and 80° C. [0228] 26. The method of clause 24 or 25, wherein the solution is cooled before milling. [0229] 27. A method of manufacturing the sintering paste of any one of clauses 18 to 23, the method comprising:

[0230] providing a slurry comprising a metal complex and a solvent, and [0231] milling the slurry. [0232] 28. Use of the sintering composition of any one of clauses 1 to 17, and/or the sintering paste of any of clauses 18 to 23 in a method selected from: die attachment, wafer-to-wafer bonding, hermetic and near hermetic sealing, dispensing and the production of interconnect lines. [0233] 29. A method of forming a joint between two or more work pieces, the method comprising:

[0234] providing two or more work pieces to be joined, [0235] providing the sintering composition of any one of clauses 1 to 17 and/or the sintering paste of any one of clauses 18 to 23 in the vicinity of the two or more work pieces, and [0236] heating the sintering composition and/or sintering paste to at least partially evaporate the solvent. [0237] 30. The method of clause 29, wherein the solvent is substantially evaporated, preferably wherein the solvent is completely evaporated. [0238] 31. The method of clause 29 or clause 30, wherein the at least some of metal complex of the sintering composition and/or sintering paste decomposes, preferably wherein substantially all of the metal complex of the sintering composition and/or sintering paste decomposes. [0239] 32. The method of any one of clauses 29 to 31, wherein the heating is carried out at a temperature of less than 300° C., preferably less than 270° C., more preferably less than 250° C. and most preferably less than 230° C. [0240] 33. The method of any one of clauses 29 to 32, wherein the heating results in a joint substantially free of carbon. [0241] 34. The method of any one of clauses 29 to 33, wherein the metal of the metal complex is at least partially sintered. [0242] 35. A sintered joint formed using the sintering composition of any one of clauses 1 to 17 and/or the sintering paste of any one of clauses 18 to 23, preferably wherein the sintered joint is formed using the method of any one of clauses 29 to 34. [0243] 36. The sintered joint of clause 35, wherein the joint is substantially free of solvent, and preferably substantially free of carbon. [0244] 37. The sintered joint of clause 35 or 36, wherein the joint is substantially free of a metal complex.