WO2020218968A1

WO2020218968A1 - Coated wire

Info

Publication number: WO2020218968A1
Application number: PCT/SG2019/050233
Authority: WO
Inventors: Shern Jiang CHING; Yean Mee PUN; Wei Chih Pan; Murali Sarangapani; Chin Yeung WONG; Sylvia SUTIONO; Dan Su; Miew Wan LO
Original assignee: Heraeus Materials Singapore Pte. Ltd.
Priority date: 2019-04-26
Filing date: 2019-04-26
Publication date: 2020-10-29
Also published as: KR20210133276A; JP2022528929A; WO2020218969A1; TW202039940A; JP7216841B2; KR102595696B1; CN113518687A; CN113518687B; TWI735981B

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 wire core or a silver-based wire core, wherein the coating layer is a 1 to 1000 nm thick single-layer of gold or a double-layer comprised of a 1 to 100 nm thick inner layer of palladium and an adjacent 1 to 250 nm thick outer layer of gold, characterized in that 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 10 to 100 wt.-ppm, based on the weight of the wire.

Description

Coated wire

The invention relates to a coated wire comprising a silver or silver-based wire core and a coating layer superimposed on the surface of the wire core. The invention further relates to a process for manufacturing such coated wire.

The use of bonding wires in electronics and microelectronics applications is well-known state of the art. While bonding wires were made from gold in the beginning, nowadays less expensive materials are used such as copper, copper alloys, silver and silver alloys. Such wires may have a metal coating.

With respect to wire geometry, most common are bonding wires of circular cross-section and bonding ribbons which have a more or less rectangular cross-section. Both types of wire geometries have their advantages making them useful for specific applications.

It is an object of the invention to provide a coated silver or silver-based wire suitable for use in wire bonding applications, the wire being outstanding in particular in spherical FAB (free air ball) formation. The coated silver or silver-based wire to be provided shall enable to effectively suppress the occurrence of OCB (off-centered ball) phenomena during ball bonding.

A contribution to the solution of said object is provided by the subject-matter of the category forming claims. The dependent sub-claims of the category-forming claims represent preferred embodiments of the invention, the subject-matter of which also makes a contribution to solving the objects mentioned above.

In a first aspect, the invention relates to a wire comprising a wire core (hereinafter also called “core” for short) with a surface, the wire core having a coating layer superimposed on its surface, wherein the wire core itself is a silver wire core or a silver-based wire core, wherein the coating layer is a 1 to 1000 nm thick single-layer of gold or a double-layer comprised of a 1 to 100 nm thick inner layer of palladium and an adjacent 1 to 250 nm thick outer layer of gold, characterized in that 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 10 to 100 wt.-ppm (weight-ppm), preferably 10 to 40 wt.-ppm, based on the weight of the wire. At the same time, in an embodiment, the total proportion of the at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium may be in the range of 300 to 3500 wt.-ppm, preferably 300 to 2000 wt.-ppm, most preferably 600 to 1000 wt.-ppm, based on the weight of the gold of the gold layer.

The wire of the invention is preferably a bonding wire for bonding in microelectronics. It is preferably a one-piece object. Numerous shapes are known and appear useful for wires of the invention. Preferred shapes are - in cross-sectional view - round, ellipsoid and rectangular shapes. For the invention, the term“bonding wire” comprises all shapes of cross-sections and all usual wire diameters, though bonding wires with circular cross-section and thin diameters are preferred. The average cross-section is in the range of for example from 50 to 5024 pm² or preferably 1 10 to 2400 pm²; accordingly in case of the preferred circular cross-sections, the average diameter is in the range of, for example, from 8 to 80 pm or preferably 12 to 55 pm.

The average diameter or, simply stated, the diameter of a wire or wire core can be obtained by the“sizing method”. According to this method the physical weight of the wire for a defined length is determined. Based on this weight, the diameter of a wire or wire core is calculated using the density of the wire material. The diameter is calculated as arithmetic mean of five measurements on five cuts of a particular wire.

The wire core is a silver wire core or it is silver-based; i.e. the wire core consists of (a) silver, i.e. pure silver, or it consists of a silver-based material in the form of (b) doped silver, (c) a silver alloy or (d) a doped silver alloy.

The term“pure silver” used herein means pure silver consisting of (a1 ) silver in an amount in the range of from 99.99 to 100 wt.-% (weight-%) and (a2) further components (components other than silver) in a total amount of from 0 to 100 wt.-ppm.

The term“doped silver” used herein means a silver-based material consisting of (b1 ) silver in an amount in the range of from > 99.49 to 99.997 wt.-%, (b2) at least one doping element other than silver in a total amount of from 30 to < 5000 wt.-ppm and (b3) further components

(components other than silver and the at least one doping element) in a total amount of from 0 to 100 wt.-ppm. In a preferred embodiment, the term“doped silver” used herein means doped silver consisting of (b1 ) silver in an amount in the range of from > 99.49 to 99.997 wt.-%, (b2) at least one doping element selected from the group consisting of calcium, nickel, platinum, palladium, gold, copper, rhodium and ruthenium in a total amount of from 30 to < 5000 wt.-ppm and (b3) further components (components other than silver, calcium, nickel, platinum, palladium, gold, copper, rhodium and ruthenium) in a total amount of from 0 to 100 wt.-ppm.

The term“silver alloy” used herein means a silver-based material consisting of (d ) silver in an amount in the range of from 89.99 to 99.5 wt.-%, preferably 97.99 to 99.5 wt.-%, (c2) at least one alloying element in a total amount in the range of from 0.5 to 10 wt.-%, preferably 0.5 to 2 wt.-% and (c3) further components (components other than silver and the at least one alloying element) in a total amount of from 0 to 100 wt.-ppm. In a preferred embodiment, the term“silver alloy” used herein means a silver alloy consisting of (d ) silver in an amount in the range of from 89.99 to 99.5 wt.-%, preferably 97.99 to 99.5 wt.-%, (c2) at least one alloying element selected from the group consisting of nickel, platinum, palladium, gold, copper, rhodium and ruthenium in a total amount in the range of from 0.5 to 10 wt.-%, preferably 0.5 to 2 wt.-% and (c3) further components (components other than silver, nickel, platinum, palladium, gold, copper, rhodium and ruthenium) in a total amount of from 0 to 100 wt.-ppm.

The term“doped silver alloy” used herein means a silver-based material consisting of (d1 ) silver in an amount in the range of from > 89.49 to 99.497 wt.-%, preferably 97.49 to 99.497 wt.-%,

(d2) at least one doping element in a total amount of from 30 to < 5000 wt.-ppm, (d3) at least one alloying element in a total amount in the range of from 0.5 to 10 wt.-%, preferably 0.5 to 2 wt.-% and (d4) further components (components other than silver, the at least one doping element and the at least one alloying element) in a total amount of from 0 to 100 wt.-ppm, wherein the at least one doping element (d2) is other than the at least one alloying element (d3). In a preferred embodiment, the term“doped silver alloy” used herein means a doped silver alloy consisting of (d1 ) silver in an amount in the range of from > 89.49 to 99.497 wt.-%, preferably 97.49 to 99.497 wt.-%, (d2) at least one doping element selected from the group consisting of calcium, nickel, platinum, palladium, gold, copper, rhodium and ruthenium in a total amount of from 30 to < 5000 wt.-ppm, (d3) at least one alloying element selected from the group consisting of nickel, platinum, palladium, gold, copper, rhodium and ruthenium in a total amount in the range of from 0.5 to 10 wt.-%, preferably 0.5 to 2 wt.-% and (d4) further components (components other than silver, calcium, nickel, platinum, palladium, gold, copper, rhodium and ruthenium) in a total amount of from 0 to 100 wt.-ppm, wherein the at least one doping element (d2) is other than the at least one alloying element (d3).

This disclosure mentions“further components” and“doping elements”. The individual amount of any further component is less than 30 wt.-ppm. The individual amount of any doping element is at least 30 wt.-ppm. All amounts in wt.-% and wt.-ppm are based on the total weight of the core or its precursor item or elongated precursor item.

The core of the wire of the invention may comprise so-called further components in a total amount in the range of from 0 to 100 wt.-ppm, for example, 10 to 100 wt.-ppm. In the present context, the further components, often also referred as“inevitable impurities”, are minor amounts of chemical elements and/or compounds which originate from impurities present in the raw materials used or from the wire core manufacturing process. The low total amount of 0 to 100 wt.-ppm of the further components ensures a good reproducibility of the wire properties. Further components present in the core are usually not added separately. Each individual further component is comprised in an amount of less than 30 wt.-ppm, based on the total weight of the wire core.

The core of the wire is a homogeneous region of bulk material. Since any bulk material always has a surface region which might exhibit different properties to some extent, the properties of the core of the wire are understood as properties of the homogeneous region of bulk material. The surface of the bulk material region can differ in terms of morphology, composition (e.g. sulfur, chlorine and/or oxygen content) and other features. The surface is an interface region between the wire core and the coating layer superimposed on the wire core. Typically, the coating layer is completely superimposed on the wire core’s surface. In the region of the wire between its core and the coating layer superimposed thereon a combination of materials of both, the core and the coating layer, can be present.

The coating layer superimposed on the surface of the wire core is a 1 to 1000 nm thick, preferably 20 to 300 nm thick single-layer of gold or a double-layer comprised of a 1 to 100 nm thick, preferably 1 to 30 nm thick inner layer of palladium and an adjacent 1 to 250 nm thick, preferably 20 to 200 nm thick outer layer of gold. In this context the term“thick” or“coating layer thickness” means the size of the coating layer in perpendicular direction to the longitudinal axis of the core.

The single or 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, preferably 10 to 40 wt.-ppm, based on the weight of the wire (wire core plus coating layer). At the same time, in an embodiment, the total proportion of said at least one member may be in the range of 300 to 3500 wt.-ppm, preferably 300 to 2000 wt.-ppm, most preferably 600 to 1000 wt.-ppm, based on the weight of the gold of the gold layer.

It is preferred that antimony is present within the gold layer. It is even more preferred that antimony is alone present within the gold layer, i.e. without the simultaneous presence of bismuth, arsenic and tellurium. In other words, in a preferred embodiment, the gold layer comprises antimony in a proportion in the range of 10 to 100 wt.-ppm, preferably 10 to 40 wt.- ppm, based on the weight of the wire (wire core plus coating layer), without bismuth, arsenic and tellurium being present within the gold layer; at the same time, in an even more preferred embodiment, the proportion of the antimony may be in the range of 300 to 3500 wt.-ppm, preferably 300 to 2000 wt.-ppm, most preferably 600 to 1000 wt.-ppm, based on the weight of the gold of the gold layer.

In an embodiment, the at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium may exhibit a concentration gradient within the gold layer, said gradient increasing in the direction towards the wire core, i.e. in perpendicular direction to the longitudinal axis of the wire core.

Applicant has found that the presence of the at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium within the gold layer comes along with a number of surprising and advantageous effects. For example, the gold layer is distinguished by exhibiting a bright shiny yellow gold appearance; formation of spherical and axi-symmetrical FAB is enabled and, when ball bonding the coated wire of the invention, the occurrence of OCB phenomena can be suppressed or even be prevented. It is unknown in what chemical form or as what chemical species said at least one member is present in the gold layer, i.e. whether it is present in the gold layer in elemental form or in the form of a chemical compound.

In another aspect, the invention relates also to a process for the manufacture of the coated wire of the invention in any of its embodiments disclosed above. The process comprises at least the steps (1 ) to (5):

(1 ) providing a silver or 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 pm² or an intermediate diameter in the range of from 30 to 200 pm is obtained, (3) applying a single-layer of gold or a double-layer coating of an inner layer of 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 single-layer of gold having a desired final thickness in the range of 1 to 1000 nm or a double-layer comprised of an inner layer of 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 200 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 Ό for an exposure time in the range of from 0.4 to 0.8 seconds 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 Ό 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 and at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium.

The term“strand annealing” is used herein. It is a continuous process allowing for a fast production of a wire with high reproducibility. In the context of the invention, strand annealing means that the annealing is done dynamically while the coated precursor to be annealed is pulled or moved through a conventional annealing oven and spooled onto a reel after having left the annealing oven. Here, the annealing oven is typically in the form of a cylindrical tube of a given length. With its defined temperature profile at a given annealing speed which may be chosen in the range of, for example, from 10 to 60 meters/minute the annealing time/oven temperature parameters can be defined and set.

The term“oven set temperature” is used herein. It means the temperature fixed in the temperature controller of the annealing oven. The annealing oven may be a chamber furnace type oven (in case of batch annealing) or a tubular annealing oven (in case of strand annealing).

This disclosure distinguishes between precursor item, elongated precursor item, coated precursor item, coated precursor and coated wire. The term“precursor item” is used for those wire pre-stages which have not reached the desired final cross-section or final diameter of the wire core, while the term“precursor” is used for a wire pre-stage at the desired final cross- section or the desired final diameter. After completion of process step (5), i.e. after the final strand annealing of the coated precursor at the desired final cross-section or the desired final diameter a coated wire in the sense of the invention is obtained.

The precursor item as provided in process step (1 ) is a silver precursor item or it is silver-based; i.e. the precursor item consists of (a) silver, i.e. pure silver, (b) doped silver, (c) a silver alloy or (d) a doped silver alloy. As regards the meaning of the terms“pure silver”,“doped silver”,“silver alloy” and“doped silver alloy” reference is made to the afore made disclosure.

In the embodiment of a silver precursor item the latter is typically in the form of a rod having a diameter of, for example, 2 to 25 mm and a length of, for example, 2 to 100 m. Such silver rod can be made by continuous casting silver using an appropriate mold, followed by cooling and solidifying.

In the embodiment of a silver-based precursor item the latter can be obtained by alloying, doping or alloying and doping silver with the desired amount of the required components. The doped silver or silver alloy or doped silver alloy can be prepared by conventional processes known to the person skilled in the art of metal alloys, for example, by melting together the components in the desired proportional ratio. In doing so, it is possible to make use of one or more conventional master alloys. The melting process can for example be performed making use of an induction furnace and it is expedient to work under vacuum or under an inert gas atmosphere. The materials used can have a purity grade of, for example, 99.99 wt.-% and above. The melt so-produced can be cooled to form a homogeneous piece of silver-based precursor item. Typically, such precursor item is in the form of a rod having a diameter of, for example, 2 to 25 mm and a length of, for example, 2 to 100 m. Such rod can be made by continuous casting the silver-based melt using an appropriate mold, followed by cooling and solidifying.

In process step (2) the precursor item is elongated to form an elongated precursor item, until an intermediate cross-section in the range of from 706 to 31400 pm² or an intermediate diameter in the range of from 30 to 200 pm is obtained. Techniques to elongate a precursor item are known and appear useful in the context of the invention. Preferred techniques are rolling, swaging, die drawing or the like, of which die drawing is particularly preferred. In the latter case the precursor item is drawn in several process steps until the desired intermediate cross-section or the desired intermediate diameter is reached. Such wire die drawing process is well known to the person skilled in the art. Conventional tungsten carbide and diamond drawing dies may be employed and conventional drawing lubricants may be employed to support the drawing.

Step (2) of the process of the invention may include one or more sub-steps of intermediate batch annealing of the elongated precursor item at an oven set temperature in the range of from 400 to 800 O for an exposure time in the range of from 50 to 150 minutes. Said optional intermediate batch annealing may be performed, for example, with a rod drawn to a diameter of 2 mm and coiled on a drum.

The optional intermediate batch annealing of process step (2) may be performed under an inert or reducing atmosphere. Numerous types of inert atmospheres as well as reducing

atmospheres are known in the art and are used for purging the annealing oven. Of the known inert atmospheres, nitrogen or argon is preferred. Of the known reducing atmospheres, hydrogen is preferred. Another preferred reducing atmosphere is a mixture of hydrogen and nitrogen. Preferred mixtures of hydrogen and nitrogen are 90 to 98 vol.-% nitrogen and, accordingly, 2 to 10 vol.-% hydrogen, wherein the vol.-% total 100 vol.-%. Preferred mixtures of nitrogen/hydrogen are equal to 93/7, 95/5 and 97/3 vol.-%/vol.-%, each based on the total volume of the mixture.

In process step (3) a coating in the form of a single-layer of gold or of a double-layer coating of an inner layer of palladium and an adjacent outer layer of gold is applied on the surface of the elongated precursor item obtained after completion of process step (2) so as to superimpose the coating over said surface.

The skilled person knows how to calculate the thickness of such coating on an elongated precursor item to finally obtain the coating in the layer thickness disclosed for the embodiments of the wire, i.e. after finally elongating the coated precursor item. The skilled person knows numerous techniques for forming a coating layer of a material according to the embodiments on a silver or silver-based surface. Preferred techniques are plating, such as electroplating and electroless plating, deposition of the material from the gas phase such as sputtering, ion plating, vacuum evaporation and physical vapor deposition, and deposition of the material from the melt. In case of applying said double-layer comprised of inner palladium layer and outer gold layer, it is preferred to apply the palladium layer by electroplating. The gold layer is applied by electroplating. Gold electroplating is performed making use of a gold electroplating bath, i.e. an electroplating bath that allows for a silver or silver-based or a palladium cathode surface to be electroplated with gold. In other words, the gold electroplating bath is a composition allowing for direct application of gold in elemental, metallic form onto a silver or silver-based or a palladium surface wired as cathode. The gold electroplating bath comprises gold and at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium; hence, the gold electroplating bath is a composition allowing for the deposition of not only the elemental gold but also allowing for depositing said at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium within the gold layer. It is unknown what chemical species said at least one member is, i.e. whether it is present in the gold layer in elemental form or as a chemical compound. The gold

electroplating bath can be made by adding said at least one member in a suitable chemical form to an aqueous composition containing gold as dissolved salt or dissolved salts. Examples of such aqueous compositions into which the at least one member can be added are Aurocor® K 24 HF made by Atotech and Auruna® 558 and Auruna® 559 made by Umicore. Alternatively, one can use a gold electroplating bath which already comprises at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium, like for example MetGold Pure ATF made by Metalor. The concentration of the gold in the gold electroplating bath can be in the range of, for example, 8 to 40 g/l (grams per liter), preferably 10 to 20 g/l. The

concentration of the at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium in the gold electroplating bath can be in the range of, for example, 15 to 50 wt.-ppm, preferably 15 to 35 wt.-ppm.

Electroplating application of the gold layer is performed by guiding the uncoated elongated precursor item or the palladium-coated elongated precursor item wired as a cathode through the gold electroplating bath. The so obtained gold-coated precursor item exiting the gold

electroplating bath may be rinsed and dried before process step (4) is performed. The use of water as a rinsing medium is expedient, with alcohol and alcohol/water mixtures being further examples of rinsing media. The gold electroplating of the uncoated elongated precursor item or the palladium-coated elongated precursor item passing through the gold electroplating bath can take place at a direct voltage in the range of, for example, 0.2 to 20 V at a current in the range of, for example, 0.001 to 5 A, in particular 0.001 to 1 A or 0.001 to 0.2 A. Typical contact times may be in the range of, for example, 0.1 to 30 seconds, preferably 2 to 8 seconds. The current densities used in this context can be in the range of, for example, 0.01 to 150 A/dm². The gold electroplating bath may have a temperature in the range of, for example, 45 to 75Ό, preferably 55 to 65Ό.

The thickness of the gold coating layer can be adjusted as desired essentially via the following parameters: chemical composition of the gold electroplating bath, contact time of the elongated precursor item with the gold electroplating bath, current density. In this context, the thickness of the gold layer can generally be increased by increasing the concentration of the gold in the gold electroplating bath, by increasing the contact time of the elongated precursor item wired as cathode and the gold electroplating bath, and by increasing the current density.

Applicant has no knowledge whether the aforementioned beneficial effects result from the presence of said at least one member in the gold electroplating bath or whether its mere presence within the gold layer is key.

In process step (4) the coated precursor item obtained after completion of process step (3) is further elongated until (4) a desired final cross-section or diameter of the wire having a single layer of gold with a desired final thickness in the range of 1 to 1000 nm, preferably 20 to 300 nm, or a double-layer comprised of an inner layer of palladium having a desired final thickness in the range of 1 to 100 nm, preferably 1 to 30 nm, and an adjacent outer layer of gold having a desired final thickness in the range of 1 to 250 nm, preferably 20 to 200 nm, is obtained.

Techniques to elongate the coated precursor item are the same elongation techniques like those mentioned above in the disclosure of process step (2).

In process step (5) the coated precursor obtained after completion of process step (4) is finally strand annealed at an oven set temperature in the range of from 200 to 600 Ό, preferably 350 to 500 Ό for an exposure time in the range of from 0.4 to 0.8 seconds to form the coated wire.

In a preferred embodiment, the finally strand annealed coated precursor, i.e. the still hot coated wire is quenched in water which, in an embodiment, may contain one or more additives, for example, 0.01 to 0.2 volume-% of additive(s). The quenching in water means immediately or rapidly, i.e. within 0.2 to 0.6 seconds, cooling the finally strand annealed coated precursor from the temperature it experienced in process step (5) down to room temperature, for example by dipping or dripping. After completion of process step (5) and the optional quenching the coated wire of the invention is finished. In order to fully benefit from its properties, it is expedient to either use it immediately for wire bonding applications, i.e. without delay, for example, within no longer than 28 days after completion of process step (5). Alternatively, in order to keep the wire’s wide wire bonding process window property and in order to prevent it from oxidative or other chemical attack, the finished wire is typically spooled and vacuum sealed immediately after completion of process step (5), i.e. without delay, for example, within <1 to 5 hours after completion of process step (5) and then stored for further use as bonding wire. Storage in vacuum sealed condition should not exceed 12 months. After opening the vacuum seal the wire should be used for wire bonding within no longer than 28 days.

It is preferred that all process steps (1 ) to (5) as well as spooling and vacuum sealing are carried out under clean room conditions (US FED STD 209E cleanroom standards, 1 k standard).

A third aspect of the invention is a coated wire obtainable by the afore disclosed process according to any embodiment thereof. It has been found that the coated wire of the invention is well suited for use as a bonding wire in wire bonding applications. Wire bonding technique is well known to the skilled person. In the course of wire bonding it is typical that a ball bond (1 ^st bond) and a stitch bond (2^nd bond, wedge bond) are formed. During bond forming a certain force (typically measured in grams) is applied, supported by application of ultrasonic energy (typically measured in mA). The mathematical product of the difference between the upper and the lower limits of the applied force and the difference between the upper and the lower limits of the applied ultrasonic energy in a wire bonding process defines the wire bonding process window:

(Upper limit of applied force - Lower limit of applied force) (Upper limit of applied ultrasonic energy - Lower limit of applied ultrasonic energy) = Wire bonding process window.

The wire bonding process window defines the area of force/ultrasonic energy combinations which allow formation of a wire bond that meets specifications, i.e. which passes the

conventional tests like conventional pull tests, ball shear test and ball pull test to name only few.

Examples

Preparation of FAB: 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, 17.5 pm wire, EFO current of 50 mA, EFO time 125 ps).

Test methods A and B:

All tests and measurements were conducted at T = 200 and a relative humidity FtH = 50 %.

A. FAB morphology

The formed FAB was examined by scanning electron microscope (SEM) with a magnification of 1000.

Evaluation:

++++ = Excellent (spherical axi-symmetrical ball)

+++ = Good (spherical axi-symmetrical ball)

++ = Satisfactory (ball is not perfectly round, but no obvious tilt (< 2 degree) with respect to wire axis)

+ = Inferior (ball is not perfectly round, but no obvious plateau on the FAB surface, tilt 5 to

10 degree with respect to wire axis)

B. OCB occurrence

The formed FAB descended to an AI-0.5wt.-%Cu bond pad from a predefined height (tip of 203.2 pm) and speed (contact velocity of 6.4 pm/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 pm and loop height of 254 pm) 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 pm). 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.

A quantity of silver (Ag) and, optionally, palladium (Pd) or palladium (Pd) and gold (Au) of at least 99.99 % purity (“4N”) in each case 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 Ό for an exposure time of 60 mi nutes. 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 pm. The wire core precursor was then electroplated with a single-layer of gold or with a double-layer coating of an inner layer of palladium and an adjacent outer layer of gold. To this end the wire core precursor while being wired as cathode was moved either through a 61 Ό warm gold electroplating bath or through a 53Ό war m palladium electroplating bath and, subsequently, through a 61 Ό warm gold electroplati ng bath. The palladium electroplating bath (based on [Pd(NH₃)₄]Cl2, with pH7 buffer) had a palladium content of 1.45 g/l (grams per liter), 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.

Thereafter the coated wire precursor was further drawn to a final diameter of 17.5 pm, followed by a final strand annealing at an oven set temperature of 220 Ό for an exposure time of 0.6 seconds, immediately followed by quenching the so-obtained coated wires in water containing 0.07 vol.-% of surfactant.

By means of this procedure, several different samples 1 to 1 1 of palladium and gold coated silver and silver-based wires and an uncoated reference silver wire of 4N purity (Ref) were manufactured.

Table 1 below shows the composition of the uncoated and the coated wires.

Presence of Sb, Au, Pd was determined by ICP (inductively Coupled Plasma. Layer thicknesses were measured on cross-sectioning by STEM (Scanning Transmission Electron Microscopy). Table 2 below shows certain test results.

Forming gas purged” means that the FAB was purged with 95/5 vol.-%/vol.-%

nitrogen/hydrogen during its formation, whereas“under atmosphere” means that FAB formation was performed under air atmosphere.

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 wire core or a silver-based wire core, wherein the coating layer is a 1 to 1000 nm thick single-layer of gold or a double-layer comprised of a 1 to 100 nm thick inner layer of palladium and an adjacent 1 to 250 nm thick outer layer of gold, characterized in that 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 10 to 100 wt.-ppm, based on the weight of the wire.

2. The wire of claim 1 , 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 300 to 3500 wt.-ppm, based on the weight of the gold of the gold layer.

3. The wire of claim 1 or 2 having an average cross-section in the range of from 50 to 5024 pm².

4. The wire of claim 1 or 2 having a circular cross-section with an average diameter in the range of from 8 to 80 pm.

5. The wire of any one of the preceding claims, wherein the at least one member selected from the group consisting of antimony, bismuth, arsenic and tellurium exhibits a concentration gradient within the gold layer, said gradient increasing in perpendicular direction to the longitudinal axis of the wire core.

6. The wire of any one of the preceding claims, wherein antimony is present within the gold layer.

7. The wire of claim 6 without the simultaneous presence of bismuth, arsenic and tellurium within the gold layer.

8. A process for the manufacture of a coated wire of any one of the preceding claims, wherein the process comprises at least the steps (1 ) to (5):

(1 ) providing a silver or 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 pm² or an intermediate diameter in the range of from 30 to 200 pm is obtained,

(3) applying a single-layer of gold or a double-layer coating of an inner layer of palladium and an adjacent outer layer of gold on the surface of the elongated precursor item obtained after completion of process step (2),

9. The process of claim 8, wherein the palladium layer is applied by electroplating.