ENGINEERED MATERIALS FOR ELECTRONICS ASSEMBLY

Abstract

A solder material for use in electronic assembly, the solder material comprising: solder layers; and a core layer comprising a core material, the core layer being sandwiched between the solder layers, wherein: the thermal conductivity of the core material is greater than the thermal conductivity of the solder.

Claims

1-48. (canceled)

49. A solder material for use in electronic assembly, the solder material comprising: solder layers; and a core layer comprising a core material, the core layer being sandwiched between the solder layers, wherein: the thermal conductivity of the core material is greater than the thermal conductivity of the solder.

50. The solder material of claim 49, wherein the thickness of each solder layer is from greater than 50 to 99 ?m, preferably from 55 to 95 ?m, more preferably from 60 to 90 ?m.

51. The solder material of claim 49, wherein the solder comprises one or both of SnSb alloy (e.g. 95% Sn, 5% Sb) or Innolot alloy (SnAg3.7Cu0.65Bi3.0Sb1.43Ni0.15).

52. The solder material of claim 49, wherein: the thickness of the core layer is from 150 to 300 ?m, the thickness of each solder layer is from greater than 50 to 100 ?m, preferably from 55 to 100 ?m, and the core material comprises one or more of copper and silver.

53. The solder material of claim 49, wherein the core layer comprises two or more core sublayers separated by one or more further solder layers, the two or more core sublayers formed of core material, the core material of one sublayer having a different coefficient of thermal expansion to the core material of another sublayer.

54. The solder material of claim 53, wherein the core material of one core sublayer is different from the core material of another core sublayer.

55. The solder material of claim 53, comprising two sublayers.

56. The solder material of claim 55, wherein the core material of one core sublayer comprises copper and the core material of the other core sublayer comprises nickel.

57. The solder material of claim 53, comprising three sublayers.

58. The solder material of claim 57, wherein the coefficients of thermal expansion of the core materials of the core sublayers increase across the thickness of the solder material.

59. The solder material of claim 57, wherein the three sublayers comprise an inner sublayer and two outer sublayers, core material of one core sublayer comprises copper, the core material of another core sublayer comprises nickel and the core material of another core sublayer comprises copper-tungsten alloy.

60. The solder material of claim 57, wherein the core material of one core sublayer comprises silver, the core material of another core sublayer comprises nickel and the core material of another core sublayer comprises molybdenum.

61. The solder material of claim 49, wherein: the solder material is not in the form of a cuboid having a length, a width and a thickness, the thickness being perpendicular to the plane of the core layer, the length being 10 mm and the width being 10 mm; and/or the thickness of the core layer is not 0.2 mm, 0.3 mm or 0.4 mm; and/or the solder layers do not each have a thickness of 0.05 or 0.1 mm; and/or the solder material does not comprise Sn20% In2% Ag; and/or the core material does not comprise copper.

62. A multilayered structure for use in electronic assembly, the multilayered structure comprising: two outer solder layers, each outer solder later comprising solder material; and a core layer sandwiched between the two outer solder layers, wherein: the core layer comprises two outer core sublayers and optionally one or more central core sublayers: the two core sublayers, and central core layers if present, being separated from each other by one or more solder layers: the outer core sublayers and inner core sublayers comprise core material: the core material of one outer core sublayer has a different coefficient of thermal expansion than the core material of the other outer core sublayer: and the thermal conductivity of the core materials is greater than the thermal conductivity of the solder materials.

63. The multilayered structure of claim 62, wherein the core comprises at least one central core sublayers and the coefficient of thermal expansion of the core materials of the outer and inner core sublayers increases across the thickness of the core.

64. A method of forming a solder joint comprising: providing the solder material of claim 49; or, a multilayered structure for use in electronic assembly, the multilayered structure comprising: two outer solder layers, each outer solder later comprising solder material; and a core layer sandwiched between the two outer solder layers, wherein: the core layer comprises two outer core sublayers and optionally one or more central core sublayers: the two core sublayers, and central core layers if present, being separated from each other by one or more solder layers: the outer core sublayers and inner core sublayers comprise core material: the core material of one outer core sublayer has a different coefficient of thermal expansion than the core material of the other outer core sublayer: and the thermal conductivity of the core materials is greater than the thermal conductivity of the solder materials, the multilayered structure in the vicinity of two or more work pieces to be joined, and heating the solder material to form a soldered joint.

65. The method of claim 64, wherein: the two or more work pieces to be joined comprise at least three work pieces, the work pieces have different thicknesses, different solder materials are used to join different work pieces, and the thicknesses of the solder materials are adjusted to reduce a mismatch in coefficient of thermal expansion between the work pieces.

66. The method of claim 64, wherein: the core layer comprises two or more core sublayers separated by one or more further solder layers, the two or more core sublayers formed of core material, the core material of one sublayer having a different coefficient of thermal expansion to the core material of another sublayer, the sublayers arranged such that the coefficient of thermal expansion of the core material of the sublayers increases across the thickness of the solder material to provide a side having a higher coefficient of thermal expansion and a side having a lower coefficient of thermal expansion; the two or more work pieces to be joined have contact materials having different coefficients of thermal expansion; and the solder material is placed between the two or more work pieces, with the work piece having the contact material with the lower coefficient of thermal expansion in contact with the side having a lower coefficient of thermal expansion and the work piece having contact material with the higher coefficient of thermal expansion in contact with the side having a higher coefficient of thermal expansion.

67. The method of claim 64, further comprising: providing an additional layer of core material; laminating the additional layer of core material on a layer of solder; providing an additional layer of solder; and laminating the additional layer of solder on the additional layer of core material.

68. The method of claim 67, further comprising: providing another additional layer of core material; laminating the another additional layer of core material on a layer of solder or a layer of additional solder; providing another additional layer of solder; and laminating the additional layer of solder on the another additional layer of core material.

Description

[0103] The invention will now be described in relation to the following non-limiting drawings in which:

[0104] FIG. 1 shows a schematic of the assembly of a typical electronic device.

[0105] FIG. 2 shows cross-sectional schematics of alternative arrangements of the solder material according to the present invention.

[0106] FIG. 3 shows a cross-sectional schematic of a solder material according to the present invention.

[0107] FIG. 4 shows a cross-sectional schematic of a solder material according to the present invention.

[0108] FIG. 5 shows a microscopy image of a cross section of a solder material according to the present invention.

[0109] FIG. 2 shows cross-sectional views of two types of solder material according to the present invention. The solder materials comprise a core layer 9 sandwiched between solder layers 10. The solder material shown in the top picture has solder only at the top and at the bottom side. There is no solder on the sides. The other solder material has solder on all sides of the core.

[0110] FIGS. 3 and 4 shows cross-sectional views of two types of solder material according to the present invention. In the solder material of FIG. 3, the core layer 9 comprises two core sublayers 11 separated by a further solder layer 12. The two core sublayers 11 are formed of core material. The core material of the top sublayer has a different coefficient of thermal expansion to the core material of the bottom sublayer. In a preferred embodiment, the core material of the top core sublayer is nickel and the core material of the bottom core sublayer is nickel. As a result, the CTE decreases from top to bottom. In the solder material of FIG. 4, the core layer 9 comprises three core sublayers 11 separated by a further solder layer 12. The three core sublayers 11 are formed of core material. The core material of the top sublayer has a different coefficient of thermal expansion to the core material of the middle sublayer and the bottom sublayer. The coefficients of thermal expansion of the core materials of the core sublayers may increase or decrease across the thickness of the solder material. In a preferred embodiment, the core material of the top core sublayer is molybdenum, the core material of the middle core sublayer is nickel and the core material of the bottom core sublayer is silver. As a result, the CTE decreases from top to bottom. In another preferred embodiment, the core material of the top core sublayer is copper-tungsten alloy, the core material of the middle core sublayer is nickel and the core material of the bottom core sublayer is copper. As a result, the CTE increases from top to bottom.

[0111] The invention will now be described in relation to the following non-limiting examples.

EXAMPLE 1

[0112] A solder material (preform) was prepared by a high-pressure lamination process. FIG. 5 shows a microscopy image of a cross section of the preform. The central core is 300 ?m thick and formed of copper. The solder on both sides is Sn20% In2% Ag. The solder thickness varies from 50 to 100 ?m. The effective thermal conductivity of this sample is about 130 W/m.Math.K, measured by a nano-flash transient measurement technique.

EXAMPLE 2

[0113] A number of preforms were prepared in a similar manner to Example 1 but with varying thicknesses of the core (Keff=400 W/m.Math.K) and solder layers (Keff=54 W/m.Math.K). The thermal performance of the preforms was evaluated. Table 1 shows estimated thermal resistances and equivalent thermal conductivities. The thermal resistance of the hick interfaces is much lower (equivalent Keff is much higher) as compared to solder alone.

TABLE-US-00001 TABLE 2 Selected examples of Cu-core preforms with their estimated thermal resistance and equivalent thermal conductivity. Solder Core thickness Thermal Equivalent Length Width thickness on each Resistance Keff (mm) (mm) (mm) side (mm) (C/W) (W/m .Math. K) 10 10 0.2 0.1 0.0420 95.2 10 10 0.2 0.05 0.0235 127.6 10 10 0.3 0.1 0.0445 112.3 10 10 0.3 0.05 0.0260 153.7 10 10 0.4 0.1 0.0470 127.6 10 10 0.4 0.05 0.0285 175.3

[0114] The invention will now be further described with reference to the following numbered clauses:

[0115] 1. A solder material comprising: [0116] a core comprising a core material; and [0117] solder at least partially coating the core.

[0118] 2. The solder material of clause 1 for use in electronic assembly.

[0119] 3. The solder material of clause 1 or clause 2, wherein the core is in the form of a layer.

[0120] 4. The solder material of clause 3, wherein the thickness of the core layer is from 100 to 500 ?m, preferably from 200 to 400 ?m, more preferably from 150 to 300 ?m.

[0121] 5. The solder material of clause 3 or clause 4, wherein solder is in the form of layers, and wherein the core is sandwiched between two solder layers.

[0122] 6. The solder material of clause 6, wherein the thickness of the solder layer is from 25 to 150 ?m, preferably from 50 to 100 ?m.

[0123] 7. The solder material of any preceding clause in the form of a foil, a strip, a film, a ribbon or a preform.

[0124] 8. The solder material of any preceding clause, wherein the melting point of the core material is greater than the reflow temperature of the solder.

[0125] 9. The solder material of any preceding clause, wherein the thermal conductivity of the core material is greater than the thermal conductivity of the solder.

[0126] 10. The solder material of clause 9, wherein the core material has a thermal conductivity of greater than or equal to 65 W/m.Math.K, preferably greater than 65 w/m.Math.K, more preferably greater than 70 W/m.Math.k, even more preferably greater than 75 W/m.Math.K.

[0127] 11. The solder material of any preceding clause, wherein the core material comprises a metal and/or an alloy.

[0128] 12. The solder material of any preceding clause, wherein the core material comprises one or more of: copper, silver, nickel, molybdenum, beryllium, cobalt, iron, copper-tungsten alloy, nickel-silver alloy, copper-zinc alloy and copper-nickel-zinc alloy.

[0129] 13. The solder material of any preceding clause, wherein the solder is lead-free.

[0130] 14. The solder material of any preceding clause, wherein the solder comprises one or more of: In, SnIn alloy (e.g. 5-58% Sn, 42-95% In), SnBi alloy (e.g. 42-60% Sn, 40-58% Bi), BiIn alloy (e.g. 5-67% Bi, 33-95% In), AgIn alloy (e.g. 3% Ag, 97% In), SnAg alloy (e.g. 90-97.5% Sn, 2.5-10% Ag), SnCu alloy (e.g. 99.3-99.6% Sn, 0.4-0.7% Cu), InGa alloy (e.g. 99.3-99.5% In, 0.5-0.7% Ga), SnBiAgCu alloy (e.g. 50% Sn, 47% Bi, 1% Ag, 2% Cu), SnBiZn alloy (e.g. 65.5% Sn, 31.5% Bi, 3% Zn), SnInAg alloy (e.g. 77.2% Sn, 20% In, 2.8% Ag), SnBiAgCuln alloy (e.g. 82.3% Sn, 2.2% Bi, 3% Ag, 0.5% Cu, 12% In), SnZn alloy (e.g. 91% Sn, 9% Zn), SnCuInGa alloy (e.g. 92.8% Sn, 0.7% Cu, 6% In, 0.5% Ga), SnCuAg alloy (e.g. 95.5% Sn, 3.8% Ag, 0.7% Cu), SnAgSb alloy (e.g. 95% Sn, 3.5% Ag, 1.5% Sb) and SnCuSb alloy (e.g. 4-95% Sn, 1-2% Cu, 4% Sb).

[0131] 15. The solder material of any preceding clause, wherein the core material comprises copper and the solder comprises Sn-20In-2Ag alloy.

[0132] 16. The solder material of any preceding clause, wherein the core and the solder are in the form of layers, and wherein the solder layers are coated on either side of the core layer.

[0133] 17. The solder material of clause 16, wherein the thickness of the core layer is from 100 to 500 ?m, preferably from 200 to 400 ?m, more preferably from 150 to 300 ?m.

[0134] 18. The solder material of clause 16 or clause 17, wherein the thickness of the solder layer is from 25 to 150 ?m, preferably from 50 to 100 ?m.

[0135] 19. The solder material of any preceding clause, wherein the core is completely coated with the solder.

[0136] 20. The solder material of any preceding clause having an effective thermal conductivity of greater than 65 W/m.Math.K, preferably greater than 80 W/m.Math.K, more preferably greater than 100 W/m.Math.K, even more preferably greater than 130 W/m.Math.K.

[0137] 21. Use of the solder material of any preceding clause in a soldering method selected from Surface Mount Technology (SMT) soldering, die attach soldering, thermal interface soldering, hand soldering, laser and RF induction soldering, and thermos-sonic soldering.

[0138] 22. Use of the solder material of any of clauses 1 to 20 for die-attach (Level I), substrate attach (Level II) or package to heatsink attach (Level III).

[0139] 23. An interconnect comprising the solder material of any of clauses 1 to 20.

[0140] 24. An IGBT, MOSFET, LED or microprocessor comprising the solder material of any of clauses 1 to 20, or the interconnect of clause 23.

[0141] 25. A method of forming a solder joint comprising: [0142] providing the solder material of any of clauses 1 to 20 in the vicinity of two or more work pieces to be joined, and [0143] heating the solder material to form a soldered joint.

[0144] 26. A method of manufacturing the solder material of any of clauses 1 to 20, the method comprising: [0145] providing two of more layers of solder, [0146] providing a layer of core material, and [0147] laminating the layers of solder on either side of the layer of core material.

[0148] 27. The method of clause 26, wherein the layer of core material is in the form of a ribbon and/or the layer of solder is in the form of a ribbon.

[0149] 28. The method of clause 27, wherein the ribbons are provided by casting, extrusion or drawing.

[0150] 29. The method of any of clauses 26 to 28, wherein the layers are laminated in a co-drawing process, preferably a high-pressure co-drawing process.

[0151] 30. The method of any of clauses 26 to 29, wherein the laminated layers are diced and/or stamped.

[0152] 31. A method of manufacturing the solder material of any of clauses 1 to 20, the method comprising: [0153] providing a layer of core material, and [0154] coating the core material with solder.

[0155] 32. The method of clause 31, wherein surface of the layer of core material is cleaned prior to it being coated with the solder.

[0156] 33. The method of clause 31 or clause 33, wherein coating the core material with solder comprises passing the core material through a molten solder bath.

[0157] 34. The solder material of any of clauses 1 to 20 in the form of a preform.

[0158] 35. The solder material of clause 34 wherein the preform provides increasing CTE from top to bottom to reduce the stresses at the interfaces of the adjoining materials with solder.

[0159] 36. The solder material of clause 34 wherein the preform provides decreasing CTE from top to bottom to reduce the stresses at the interfaces of the adjoining materials with solder.

[0160] 37. The solder material of any of clauses 34 to 36, wherein the preform can be used for level I, Level II or Level III interconnects.

[0161] 38. The solder material of any of clauses 34 to 37, wherein the preform can be used for packaging and assembly of IGBT, MOSFET, LED, Microprocessor and other electronic devices.

[0162] 39. The solder material of any of clauses 34 to 38, wherein the preform can be used in assembly of multi-chip modules with difference size of components and components with different heat generation rates.

[0163] 40. The solder material of any of clauses 34 to 39, wherein the preforms can be used in assembly of multi-chip modules with difference thickness of components and preform thickness is selected to adjust for component thickness.

[0164] 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.