COATED WIRE

Abstract

A 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 nickel or palladium and an adjacent 1 to 250 nm thick outer layer of gold, characterized in that the wire exhibits a total carbon content of 40 wt.-ppm.

Claims

1. A 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 nickel or palladium and an adjacent 1 to 250 nm thick outer layer of gold, characterized in that the wire exhibits a total carbon content of:5 40 wt.-ppm.

2. The wire of claim 1 having an average cross-section in the range of from 50 to 5024 m2.

3. The wire of claim 1 having a circular cross-section with an average diameter in the range of from 8 to 80 m.

4. The wire of claim 1, wherein the inner layer of nickel or palladium is 1 to 30 nm thick.

5. The wire of claim 1, wherein the outer layer of gold is 20 to 200 nm thick.

6. The wire of claim 1, wherein the outer 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 10 to 100 wt.-ppm based on the weight of the wire.

7. The wire of claim 6, wherein the total proportion of said at least one member is in the range of 300 to 3500 wt.-ppm based on the weight of the gold of the gold layer.

8. The wire of claim 1, wherein the wire's total carbon content is in the range of 10 to 30 wt.-ppm.

9. The wire of claim 1, wherein the wire exhibits a carbon content of:5 3 mg carbon per square meter of the wire surface.

10. The wire of claim 9, wherein the carbon content of the wire surface is in the range of 0.5 to 2.5 mg carbon per square meter.

11. A process for the manufacture of a coated wire of claim 1, wherein the process comprises 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 cross-section in the range of from 706 to 31400 m2 or 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 nickel or palladium 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 cross-section or diameter and a double-layer comprised of an inner layer of nickel or palladium having a desired final thickness in the range of 1 to 100 nm and an adjacent outer layer of gold having a desired final thickness in the range of 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 200 to 600 C. for an exposure time in the range of from 0.4 to 0.8 seconds and quenching it to form the coated 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 400 to 800 C. for an exposure time in the range of from 50 to 150 minutes, and wherein the application of the gold layer in step (3) is performed by electroplating it from a gold electroplating bath comprising gold.

12. The process of claim 11, wherein the nickel or palladium layer is applied by electroplating.

13. The process of claim 11, wherein the quenching is performed in water or, preferably, in an aqueous quenching solution comprising or consisting of water and at least one quenching additive, the latter being present in an amount resulting in a total organic carbon content of the aqueous quenching solution of:5 10 mg per liter.

14. The process of claim 13, wherein the total organic carbon content of the aqueous quenching solution is in the range of >0.0001 to 10 mg per liter.

15. The process of claim 13, wherein at least one quenching additive is selected from the group consisting of anionic surfactants and water-soluble organic solvents.

Description

EXAMPLES

[0053] Analytical Methods A. to C.

[0054] A. Determination of the Total Carbon Content of Wire

[0055] The total carbon (TC) content of the wire was determined based on ASTM E 1019 using a CS 844 analyzer from LECO. For each analysis 0.5 g of the respective wire and 1.5 g of Leco Cu-Accelerator were placed in a crucible made of aluminum oxide. The crucible was heated to 1500 C. for 45 s in a stream of oxygen (3 l/min) to oxidize the carbon in the wire. The oxidized carbon was detected via infrared absorption.

[0056] B. Determination of the Carbon Content of the Wire Surface

[0057] The carbon content of the wire surface was determined using a RC 612 Multiphase Determinator from LECO. A piece of wire exhibiting a surface area of 0.22 dm.sup.2 was placed in a nickel crucible and heated in a stream of oxygen (0.5 l/min) at 600 C. for 120 s. Oxidized carbon was detected via infrared absorption.

[0058] C. Determination of the Total Organic Carbon (TOC) Content of an Aqueous Quenching Solution

[0059] TOC is determined by ASTM D7573.

[0060] Wire Ball-Wedge Bonding Procedure and Evaluation of Bonder Stoppage

[0061] Ball-wedge bonding has multiple steps: first EFO firing, melting of wire tip, formation of free air ball (FAB), first bond (ball bond) to bond pad, looping, second bond (wedge bond or stitch bond) on substrate or plated fingers, tail cut of wire.

[0062] Here, the FAB descended to an AI-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 to form the bonded ball. After forming the bonded 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 1250 wires were bonded.

[0063] Evaluation of Bonder Stoppages:

[0064] Poor: 1 bonder stoppage during formation of 1250 wire bonds. Unwinding of the wire from the spool is difficult, or the length of tail cut portion is inconsistent leading to short tail, which means that there is not enough wire left for EFO firing and the bonder is stopping. This is disadvantageous, because threading the wire needs to be carried out to restart the bonding cycle.

[0065] Good: zero bonder stoppage during formation of 1250 wire bonds

[0066] Wire Examples 1 to 8

[0067] 98.5 wt.-% of silver (Ag) and 1.5 w.-t % of palladium (Pd), each exhibiting at least 99.99 wt.-% purity (4N), were melted in a crucible. Then a wire core precursor item in the form of 8 mm rods was continuous cast from the melt. The rods were then drawn in several drawing steps to form a wire core precursor having a circular cross-section with a diameter of 2 mm. The wire core precursor was 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 a wire core precursor having a circular cross-section with a diameter of 46 m. The wire core precursor was then 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.

[0068] Thereafter the coated wire precursor was further drawn to a final diameter of 20 m, followed by a final strand annealing at an oven set temperature of 430 C. for an exposure time of 0.6 seconds, immediately followed by quenching the so-obtained coated wires in water (deionized water type II from Bestchem) or in an aqueous quenching solution (deionized water type II from Bestchem plus isopropanol and/or anionic surfactant (sodium stearate)) as shown in Table 1. The contact time of each wire with the aqueous quenching solution was 0.3 s. After quenching the wires were spooled at 300 m length.

[0069] The 20 m thick wires had an 9 nm thick inner layer of nickel and an adjacent outer 90 nm thick layer of gold.

TABLE-US-00001 TABLE 1 TOC of Carbon aqueous content of quenching wire Anionic solution TC of wire surface No of Bonding Ex. surfactant Isopropanol [mg/l] [wt.-ppm] [mg/m.sup.2] Stoppages 1 ./. ./. <0.5 21.5 0.8 0 2 + ./. 1.1 23.1 1.2 0 3 + ./. 4.3 25.0 1.4 0 4 + + 6.8 26.5 1.5 0 5 ./. + 9.3 28.9 1.7 0 6 + + 67.4 45.5 2.5 7 7 + + 335.7 67.0 3.3 23 8 + + 685.1 99.5 5.3 59 Examples 1 to 5: according to the invention Examples 6 to 8: comparative examples