COATED ROUND WIRE

Abstract

A round wire comprising a wire core with a surface, the wire core having a coating layer superimposed on its surface, wherein the wire core itself is a silver-based wire core, wherein the coating layer is a double-layer comprised of a 1 to 100 nm thick inner layer of palladium or nickel and an adjacent 1 to 250 nm thick outer layer of gold, wherein the outer layer of gold exhibits at least one of the following intrinsic properties A1) and A2): A1) the average grain size of the crystal grains in the outer layer of gold, measured in longitudinal direction, is in the range of 0.1 to 0.8 m; A2) 60 to 100% of the crystal grains in the outer layer of gold are oriented in <100> direction, and 0 to 20% of the crystal grains in the outer layer of gold are oriented in <111> direction.

Claims

1. A round wire comprising a wire core with a surface, the wire core having a coating layer superimposed on its surface, wherein the wire core itself is a silver-based wire core, wherein the coating layer is a double-layer comprised of a 1 to 100 nm thick inner layer of palladium or nickel and an adjacent 1 to 250 nm thick outer layer of gold, wherein the outer layer of gold exhibits at least one of the following intrinsic properties A 1) and A2): A 1) the average grain size of the crystal grains in the outer layer of gold, measured in longitudinal direction, is in the range of from 0.1 to 0.8 m; A2) 60 to 100% of the crystal grains in the outer layer of gold are oriented in <100> direction, and O to 20% of the crystal grains in the outer layer of gold are oriented in <111> direction, each % with respect to the total number of crystal grains with orientation parallel to the drawing direction of the wire.

2. The round wire of claim 1 having an average diameter is in the range of from 8 to 80 m.

3. The round wire of claim 1, wherein the silver-based wire core consists of a silver-based material in the form of (a) doped silver, (b) a silver alloy or (c) a doped silver alloy.

4. The round wire of claim 3, wherein the doped silver is a silver-based material consisting of (a1) silver in an amount in the range of from >99.49 to 99.997 wt.-%, (a2) at least one doping element other than silver in a total amount of from 30 to <5000 wt.-ppm and (a3) further components in a total amount of from O to 100 wt.-ppm.

5. The round wire of claim 3, wherein the silver alloy is a silver-based material consisting of (b1) silver in an amount in the range of from 89.99 to 99.5 wt.-%, (b2) at least one alloying element in a total amount in the range of from 0.5 to 10 wt.-% and (b3) further components in a total amount of from O to 100 wt.-ppm.

6. The round wire of claim 5, wherein the silver alloy comprises palladium as the only alloying element.

7. The round wire of claim 6, wherein the silver alloy has a palladium content of 1 to 2 wt.-%.

8. The round wire of claim 3, wherein the doped silver alloy is a silver-based material consisting of (c1) silver in an amount in the range of from >89.49 to 99.497 wt.-%, (c2) at least one doping element in a total amount of from 30 to <5000 wt.-ppm, (c3) at least one alloying element in a total amount in the range of from 0.5 to 10 wt.-% and (c4) further components in a total amount of from O to 100 wt.-ppm, wherein the at least one doping element (c2) is other than the at least one alloying element (c3).

9. The round wire of claim 1, wherein the coating layer superimposed on the surface of the wire core is a double-layer comprised of a 1 to 30 nm thick inner layer of palladium or nickel and an adjacent 1 to 200 nm thick outer layer of gold.

10. The coated round wire of claim 1 exhibiting at least one of the following intrinsic properties A3) to AS): A3) the average grain size of the crystal grains in the wire core, measured in longitudinal direction, is in the range of from 0.7 to 1.1 m; A4) the fraction of twin boundaries, measured in longitudinal direction of the wire core, is in the range of from 5 to 40%; AS) 20 to 70% of the crystal grains of the wire core are oriented in <100> direction, and 3 to 40% of the crystal grains of the wire core are oriented in <111> direction, each % with respect to the total number of crystal grains with orientation parallel to the drawing direction of the wire.

11. The coated round wire of claim 1, wherein the gold layer comprises at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium in a total proportion in the range of from 10 to 300 wt.-ppm, based on the weight of the wire.

12. The coated round wire of claim 11, wherein the total proportion of the at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium is in the range of from 300 to 9500 wt.-ppm, based on the weight of the gold of the gold layer.

13. The coated round wire of claim 11, wherein antimony is alone present within the gold layer.

14. A process for the manufacture of a coated round wire of claim 1, the process comprising at least the steps (1) to (5): (1) providing a silver-based precursor item, (2) elongating the precursor item to form an elongated precursor item, until an intermediate diameter in the range of from 30 to 200 m is obtained, (3) applying a double-layer coating of an inner layer of palladium or nickel and an adjacent outer layer of gold on the surface of the elongated precursor item obtained after completion of process step (2), (4) further elongating the coated precursor item obtained after completion of process step (3) until a desired final diameter and a double-layer comprised of an inner layer of palladium or nickel having a desired final thickness in the range of from 1 to 100 nm and an adjacent outer layer of gold having a desired final thickness in the range of from 1 to 250 nm is obtained, and (5) finally strand annealing the coated precursor obtained after completion of process step (4) at an oven set temperature in the range of from >370 to 520 C. for an exposure time in the range of from 0.8 to 10 seconds to form the coated round wire, wherein step (2) may include one or more sub-steps of intermediate batch annealing of the precursor item at an oven set temperature of from 200 to 650 C. for an exposure time in the range of from 30 to 300 minutes, and wherein the further elongating of step (4) comprises die drawing with the following drawing parameters B1) to B4) prevailing: B1) the drawing speed is within the range of 500 to 700 meters per minute, B2) the cone angle of each drawing die is within the range of 70 to 90 degrees, B3) the bearing length at each drawing die is 30 to 40% of the diameter of the respective circular drawing die opening, B4) the circular cross-sectional area reduction of the coated precursor item at each drawing die is in the range of 7 to 15%.

15. The process of claim 14, wherein the palladium or nickel layer and the gold layer are applied by electroplating.

Description

TEST METHODS

[0075] All tests and measurements were conducted at T=20 C. and a relative humidity RH=50%.

A. Electron Backscattered Diffraction (EBSD) Pattern Analysis for Determination of the Crystallographic Orientation and Grain Size of the Crystal Grains of the Outer Gold Layer:

[0076] The wire was placed on a sample holder and fixed using conductive copper tape and observed in FESEM (field emission scanning electron microscope) with a 70 angled holder to the normal FESEM sample holding table surface. The FESEM was further equipped with an EBSD detector. The electron back-scattering patterns (EBSP) containing the wire surface crystallographic information of outer gold layer were obtained.

[0077] These patterns were further analyzed for crystal grain orientation fraction, average crystal grain size, etc. (using a software called EBSD program developed by Oxford Instruments). Points of similar orientation were grouped together to form the texture component.

[0078] The wires were first potted using cold-mounting epoxy resin and then polished (cross-sectioned) by standard metallographic technique. A multi-prep semi-automatic polisher was used with low force and optimal speed to grind and polish the sample with minimum deformation strain on the sample surface. Finally, the polished surface is ion-milled to remove a thin layer to further reduce the deformation strain to close to zero. The crystal grain size was measured using grain boundary delineation concept defining critical misorientation angle (>15) with EBSD tool according to the ASTM E2627-13 (2019) standard. To accurately measure grain size, grain boundaries were first detected with highest degree of grain boundary delineation.

[0079] To distinguish different texture components, a maximum tolerance angle of 10 was used. The wire drawing direction was set as a reference orientation. The <100> and <111> texture percentages were calculated by measurement of the percentage of crystals with <100> and <111> plane of orientation parallel to the reference orientation.

[0080] The EBSD pattern analysis was performed at five different locations per sample to obtain average values.

B. Electron Backscattered Diffraction (EBSD) Pattern Analysis for Determination of the Crystallographic Orientation and Twin Boundaries of the Crystal Grains of the Wire Core:

[0081] The main steps adopted to measure wire texture were sample preparation, getting good Kikuchi pattern and component calculation:

[0082] The wires were first potted using epoxy resin and polished as per standard metallographic technique. Ion milling was applied in the final sample preparation step to remove any mechanical deformation of the wire surface, contamination and oxidation layer. The ion-milled cross-sectioned sample surface was sputtered with gold. Then ion milling and gold sputtering were carried out for two further rounds. No chemical etching or ion-etching was carried out.

[0083] The sample was loaded in a FESEM (field emission scanning electron microscope) with a 70 angled holder to the normal FESEM sample holding table surface. The FESEM was further equipped with an EBSD detector. The electron back-scattering patterns (EBSP) containing the wire crystallographic information were obtained.

[0084] These patterns were further analyzed for crystal grain orientation fraction, average crystal grain size, etc. (using a software called EBSD program developed by Oxford Instruments). Points of similar orientation were grouped together to form the texture component.

[0085] To distinguish different texture components, a maximum tolerance angle of 10 was used. The wire drawing direction was set as a reference orientation. The <100> and <111> texture percentages were calculated by measurement of the percentage of crystals with <100> and <111> plane of orientation parallel to the reference orientation.

[0086] Twin boundaries (also called 23 CSL twin boundaries) were excluded in the average crystal grain size calculation. The twin boundary was described by a 60 rotation about 111> plane of orientation between the neighboring crystallographic domains. The number of scanning points of the area of interest depends on the step size, which was less than of the observed finest-crystal grain size (about 100 nm).

[0087] The EBSD pattern analysis was performed at five different locations per sample to obtain average values.

C. Linear Intercept Method for Determination of the Sizes of the Crystal Grains of the Wire Core:

[0088] The wires were first potted using cold-mounting epoxy resin and then polished (cross-sectioned) by standard metallographic technique. A multi-prep semi-automatic polisher was used with low force and optimal speed to grind and polish the sample with minimum deformation strain on the sample surface. Finally, the polished sample was chemically etched using ferric chloride to reveal the crystal grain boundary. The crystal grain size was measured using linear intercept method under optical microscopy with a magnification of 1000, according to the ASTM E112-12 Standard.

D. Evaluation of Flowery Bonded Ball and Wire Sway:

D.1) Preparation of FAB:

[0089] It was worked according to the procedures described in the KNS Process User Guide for FAB (Kulicke & Soffa Industries Inc, Fort Washington, PA, USA, 2002, 31 May 2009) in ambient atmosphere. FAB was prepared by performing conventional electric flame-off (EFO) firing by standard firing (single step, 20 m wire, EFO current of 50 mA, EFO time 315 s, BSR ratio 2.3 (Bonded Ball to wire diameter ratio, using IConnKNS bonder).

D.2) Ball Bonding:

[0090] The formed FAB descended to an Al-0.5 wt.-% Cu bond pad from a predefined height (tip of 203.2 m) and speed (contact velocity of 6.4 m/sec). Upon touching the bond pad, a set of defined bonding parameters (bond force of 100 g, ultrasonic energy of 95 mA and bond time of 15 ms) took into effect to deform the FAB and formed the bonded ball. After forming the ball, the capillary rose to a predefined height (kink height of 152.4 m and loop height of 254 m) to form the loop. After forming the loop, the capillary descended to the lead to form the stitch. After forming the stitch, the capillary rose and the wire clamp closed to cut the wire to make the predefined tail length (tail length extension of 254 m). For each sample a meaningful number of 2500 bonded wires were optically inspected using a microscope with a magnification of 1000. The percentage of defects was determined.

D.3) Evaluation of Bonded Ball with Respect to Flowery Bonded Ball: [0091] + Poor: 15% of the bonded balls are not round but deformed [0092] ++ Good: 10% to <15% of the bonded balls are not round but deformed [0093] +++ Very good: <10% of the bonded balls are not round but deformed

D.4) Evaluation of Wire Sway:

[0094] + Poor: <25% of the wires deflect towards neighbor wire in the loop [0095] ++ Good: <5% of the wires deflect towards neighbor wire in the loop [0096] +++ Excellent: wires show no loop deflections

Wire Examples

[0097] For all wire examples a quantity of 98.5 wt % silver (Ag) and 1.5 wt % palladium (Pd) of at least 99.99% purity (4N) for each metal were melted in a crucible. Then wire core precursor items in the form of 8 mm rods were continuously cast from the melt. The rods were then drawn in several drawing steps to form wire core precursors having circular cross-sections with diameters of 2 mm. The wire core precursors were intermediate batch annealed at an oven set temperature of 500 C. for an exposure time of 60 minutes. The rods were further drawn in several drawing steps to form wire core precursors having diameters of 46 m.

[0098] All wire core precursors were electroplated with a double-layer coating of an inner layer of nickel and an adjacent outer layer of gold. To this end, the wire core precursor while being wired as cathode was moved through a 60 C. warm nickel electroplating bath and, subsequently, through a 61 C. warm gold electroplating bath. The nickel electroplating bath comprised 90 g/l (grams per liter) Ni(SO.sub.3NH.sub.2).sub.2, 6 g/l NiCl.sub.2 and 35 g/l H.sub.3BO.sub.3, whereas the gold electroplating bath (based on MetGold Pure ATF from Metalor) had a gold content of 13.2 g/l and an antimony content of 20 wt.-ppm (based on MetGold Pure ATF from Metalor).

[0099] Thereafter the coated wire precursors were further drawn to final diameters of 20 m according to the drawing parameters indicated in table 1. All wire samples were finally strand annealed at an oven set temperature of 410 C. for an exposure time of 0.9 seconds, immediately followed by quenching the so-obtained coated wires in water containing 0.07 vol.-% of surfactant. All 20 m thick wire samples had a 9 nm thick inner layer of nickel and an adjacent outer 90 nm thick layer of gold.

[0100] Table 1 gives an overview on the drawing parameters as well as evaluation of the inventive samples S1 to S3 and comparative samples C1 to C5.

TABLE-US-00001 TABLE 1 Circular cross- sectional area Cone reduction of Size of Wire angle of the coated Crystallographic crystal drawing each Bearing precursor item orientation of outer grains of Flowery speed drawing length at each layer of gold [%] outer gold bonded Wire Wire [m/min] die [] [%] drawing die [%] <100> <111> layer [m] ball sway S1 600 70 30 9.5 90 7.4 0.19 +++ +++ S2 600 80 40 9.5 91 1.0 0.26 +++ +++ S3 600 90 40 9.5 82 2.0 0.17 +++ +++ C1 600 60 20 9.5 40 30 0.39 ++ + C2 600 100 50 9.5 35 32 0.23 ++ +++ C3 300 80 40 4.5 20 35 0.42 +++ ++ C4 900 80 40 9.5 35 32 0.40 ++ + C5 900 60 20 18.0 30 25 0.41 + +