Coated round wire

Abstract

A round wire includes a wire core having a surface, the wire core having a coating superimposed on its surface, wherein the wire core itself is a silver-based wire core, and wherein the coating is a bilayer consisting of a palladium or nickel inner layer of 1 nm to 100 nm thickness and an adjacent gold outer layer of 1 nm to 250 nm thickness, wherein the gold outer layer exhibits at least one of the following inherent properties A1) and A2): A1) the average grain size of the grains in the gold outer layer, measured along the longitudinal direction, is in the range of 0.1 μm to 0.8 μm; A2) 60% to 100% of the grains in the gold outer layer are along the longitudinal direction. <100> Orientation, and 0% to 20% of the grains in the gold outer layer are oriented along the direction. <111> Orientation, each % is relative to the total number of grains having an orientation parallel to the drawing direction of the wire.

Description

[0001] This invention relates to a coated round wire comprising a silver-based wire core and a coating superimposed on the surface of the wire core. The invention also relates to a process for manufacturing such coated round wires.

[0002] The use of bonding wires in electronic and microelectronic applications is a well-known technical practice. Although bonding wires were initially made of gold, cheaper materials such as copper, copper alloys, silver, and silver alloys are now used. These wires may have a metallic coating.

[0003] Regarding wire geometries, the most common are joint lines with a circular cross-section and joint strips with more or less rectangular cross-sections. Both types of wire geometries have their advantages, making them suitable for specific applications.

[0004] One object of the present invention is to provide a coated round silver wire suitable for wire bonding applications, which, in addition to meeting basic requirements such as wire flexibility, feasibility of forming FAB (airless ball) in an air atmosphere, corrosion resistance, and oxidation resistance, is particularly outstanding in preventing flower-shaped bonding balls and in terms of stable looping behavior with respect to wire swing.

[0005] The contribution to the solution to the stated objective is provided by the subject matter of the class-formed claims. The dependent claims of the class-formed claims represent preferred embodiments of the invention, and their subject matter also contributes to solving the aforementioned objective.

[0006] This invention relates to a round wire comprising a wire core (hereinafter also referred to as "core") having a surface, the wire core having a coating superimposed on its surface, wherein the wire core itself is a silver-based wire core, and wherein the coating is a bilayer consisting of a palladium or nickel inner layer of 1 nm to 100 nm thickness and an adjacent gold outer layer of 1 nm to 250 nm thickness, wherein the gold outer layer exhibits at least one of the following inherent properties A1) and A2) (see "Test Method A" as described below):

[0007] A1) The average grain size of the gold outer layer, measured along the longitudinal direction, is in the range of 0.1 μm to 0.8 μm, preferably 0.1 μm to 0.4 μm, and most preferably 0.15 μm to 0.25 μm;

[0008] A2) 60% to 100%, preferably 80% to 100%, of the grains in the gold outer layer are along...

[0009] <100> Orientation, and 0% to 20%, preferably 0% to 10%, of the grains in the gold outer layer are oriented along the direction. <111> Orientation, each % is relative to the total number of grains having an orientation parallel to the drawing direction of the wire.

[0010] This article uses the term "intrinsic properties". Intrinsic properties refer to the properties that an object possesses itself (independent of other factors), while extrinsic properties depend on the object's relationship or interaction with other external factors.

[0011] The wire of the present invention is preferably a bonding wire for bonding in microelectronics. The wire is preferably a one-piece object. For the purposes of this invention, the term "bonding wire" includes a bonding wire having a circular cross-section and a small diameter. The average diameter is, for example, in the range of 8 μm to 80 μm or preferably 12 μm to 55 μm.

[0012] The average diameter, or simply the diameter of the wire or wire core, can be obtained through a "sizing method." This method determines the physical weight of a wire of a defined length. Based on this weight, the diameter of the wire or wire core is calculated using the density of the wire material. The diameter is calculated as the arithmetic mean of five measurements taken at five notches on a particular wire.

[0013] The wire core is a silver-based core; that is, the wire core is composed of silver-based material in the form of (a) silver doping, (b) silver alloy or (c) silver alloy doping.

[0014] As used herein, the term “silver doped” means a silver-based material consisting of: (a1) silver in an amount ranging from >99.49 wt% to 99.997 wt%, (a2) at least one dopant element other than silver in a total amount ranging from 30 wt.-ppm to <5000 wt.-ppm%, and (a3) ​​additional components (other than silver and at least one dopant element) in a total amount ranging from 0 wt.-ppm to 100 wt.-ppm. In a preferred embodiment, the term “doped silver” as used herein means doped silver consisting of: (a1) silver in an amount ranging from >99.49% to 99.997% by weight, (a2) at least one doping element selected from the group consisting of calcium, nickel, platinum, palladium, gold, copper, rhodium and ruthenium in a total amount ranging from 30 wt.-ppm to <5000 wt.-ppm, and (a3) ​​additional components (other than silver, calcium, nickel, platinum, palladium, gold, copper, rhodium and ruthenium) in a total amount ranging from 0 wt.-ppm to 100 wt.-ppm.

[0015] As used herein, the term "silver alloy" means a silver-based material consisting of: (b1) silver in an amount ranging from 89.99% to 99.5% by weight, preferably from 97.99% to 99.5% by weight; (b2) at least one alloying element in a total amount ranging from 0.5% to 10% by weight, preferably from 0.5% to 2% by weight; and (b3) other components (other than silver and at least one alloying element) in a total amount ranging from 0 wt.-ppm to 100 wt.-ppm. In a preferred embodiment, the term "silver alloy" as used herein means a silver alloy comprising: (b1) silver in an amount ranging from 89.99 wt% to 99.5 wt%, preferably from 97.99 wt% to 99.5 wt%; (b2) at least one alloying element selected from the group consisting of nickel, platinum, palladium, gold, copper, rhodium, and ruthenium, in a total amount ranging from 0.5 wt% to 10 wt%, preferably from 0.5 wt% to 2 wt%; and (b3) additional components (other than silver, nickel, platinum, palladium, gold, copper, rhodium, and ruthenium) in a total amount ranging from 0 wt.-ppm to 100 wt.-ppm. Silver alloys containing palladium as the sole alloying element are most preferred, particularly those silver alloys having a palladium content of 1 wt% to 2 wt%, especially 1.5 wt%.

[0016] As used herein, the term "doped silver alloy" means a silver-based material consisting of: (c1) silver in an amount ranging from >89.49 wt% to 99.497 wt%, preferably from 97.49 wt% to 99.497 wt%; (c2) at least one doping element in a total amount ranging from 30 wt.-ppm to <5000 wt.-ppm; (c3) at least one alloying element in a total amount ranging from 0.5 wt% to 10 wt%, preferably from 0.5 wt% to 2 wt%; and (c4) additional components (other than silver, at least one doping element, and at least one alloying element) in a total amount ranging from 0 wt.-ppm to 100 wt.-ppm, wherein the at least one doping element (c2) is different from the at least one alloying element (c3). In a preferred embodiment, the term "silver-doped alloy" as used herein means a silver-doped alloy consisting of: (c1) silver in an amount ranging from >89.49% to 99.497% by weight, preferably from 97.49% to 99.497% by weight; (c2) at least one dopant element selected from the group consisting of calcium, nickel, platinum, palladium, gold, copper, rhodium, and ruthenium in a total amount ranging from 30 wt.-ppm to <5000 wt.-ppm; (c3) at least one alloying element selected from the group consisting of nickel, platinum, palladium, gold, copper, rhodium, and ruthenium in a total amount ranging from 0.5% to 10% by weight, preferably from 0.5% to 2% by weight; and (c4) additional components (other than silver, calcium, nickel, platinum, palladium, gold, copper, rhodium, and ruthenium) in a total amount ranging from 0 wt.-ppm to 100 wt.-ppm, wherein the at least one dopant element (c2) is different from the at least one alloying element (c3).

[0017] This disclosure refers to "additional components" and "doping elements". The individual amount of any additional component is less than 30 wt.-ppm. The individual amount of any doping element is at least 30 wt.-ppm. All amounts, expressed in weight percent and wt.-ppm, are based on the total weight of the core or its precursor article or elongated precursor article.

[0018] The core of the wire of the present invention may contain so-called additional components in total amounts ranging from 0 wt.-ppm to 100 wt.-ppm, for example, from 10 wt.-ppm to 100 wt.-ppm. In the context of the present invention, additional components (also commonly referred to as "unavoidable impurities") are impurities originating from the raw materials used or from trace amounts of chemical elements and / or compounds arising from the wire core manufacturing process. The low total amount of additional components, from 0 wt.-ppm to 100 wt.-ppm, ensures good reproducibility of the wire properties. Additional components present in the core are generally not added individually. Based on the total weight of the wire core, the amount of each individual additional component is less than 30 wt.-ppm.

[0019] The core of a wire is a homogeneous region of bulk material. Since any bulk material always has surface areas that can exhibit different properties to some extent, the properties of the wire core are understood as the properties of the homogeneous region of the bulk material. The surface of the bulk material region can differ in morphology, composition (e.g., sulfur, chlorine, and / or oxygen content), and other characteristics. This surface is the interface region between the wire core and the coating superimposed on it. Typically, the coating is completely superimposed on the surface of the wire core. In the region of the wire between the core and the coating superimposed thereon, a combination of materials from both the core and the coating may exist.

[0020] The coating superimposed on the surface of the wire core is a double layer, consisting of an inner palladium or nickel layer that is 1 nm to 100 nm thick, preferably 1 nm to 30 nm thick, and an adjacent outer gold layer that is 1 nm to 250 nm thick, preferably 20 nm to 200 nm thick. In this context, the term "thickness" or "coating thickness" refers to the magnitude of the coating along a direction perpendicular to the longitudinal axis of the core.

[0021] The coated round wire or its gold outer layer of the present invention exhibits at least one of the following inherent properties A1) and A2):

[0022] A1) The average grain size of the gold outer layer, measured along the longitudinal direction, is in the range of 0.1 μm to 0.8 μm, preferably 0.1 μm to 0.4 μm, and most preferably 0.15 μm to 0.25 μm;

[0023] A2) 60% to 100%, preferably 80% to 100%, of the grains in the gold outer layer are along... <100> Orientation, and 0% to 20%, preferably 0% to 10%, of the grains in the gold outer layer are oriented along the direction. <111> Orientation, each % is relative to the total number of grains having an orientation parallel to the drawing direction of the wire.

[0024] Preferably, the coated round wire of the present invention exhibits both inherent properties A1) and A2).

[0025] In a preferred embodiment, the coated round wire of the present invention also exhibits at least one of the following inherent properties (A3) to A5) (see “Test Methods B and C” below):

[0026] A3) The average grain size of the grains in the wire core, measured along the longitudinal direction, is in the range of 0.7 μm to 1.1 μm;

[0027] A4) The fraction of twin boundaries measured along the longitudinal direction of the wire core is in the range of 5% to 40%;

[0028] A5) 20% to 70% of the grains in the wire core are along <100> Orientation, and 3% to 40% of the grains in the wire core are oriented along the direction. <111> Orientation, each % is counted relative to the total number of grains with an orientation parallel to the drawing direction of the wire.

[0029] Preferably, the coated round wire of the preferred embodiment exhibits at least two of the inherent properties A3) to A5). More preferably, the wire of the preferred embodiment exhibits all of the inherent properties A3) to A5).

[0030] In an advantageous embodiment, the gold layer comprises at least one component selected from the group consisting of antimony, bismuth, arsenic, and tellurium, and the total proportion of the at least one component, based on the weight of the wire of the present invention, is in the range of 10 wt.-ppm to 300 wt.-ppm, preferably from 10 wt.-ppm to 150 wt.-ppm. Meanwhile, in embodiments, based on the weight of the gold in the gold layer, the total proportion of the at least one component selected from the group consisting of antimony, bismuth, arsenic, and tellurium can be in the range of 300 wt.-ppm to 9500 wt.-ppm, preferably from 300 wt.-ppm to 5000 wt.-ppm, and most preferably from 600 wt.-ppm to 3000 wt.-ppm.

[0031] In an advantageous embodiment, it is preferred that antimony is present within the gold layer, and even more preferably, antimony is present only within the gold layer, i.e., bismuth, arsenic, and tellurium are not present simultaneously. In other words, in the most preferred variant of this advantageous embodiment, the gold layer contains antimony in a proportion based on the weight of the wire (wire core plus coating) ranging from 10 wt.-ppm to 300 wt.-ppm, preferably from 10 wt.-ppm to 150 wt.-ppm, while bismuth, arsenic, and tellurium are absent within the gold layer; meanwhile, in embodiments, the proportion of antimony based on the weight of gold in the gold layer can range from 300 wt.-ppm to 9500 wt.-ppm, preferably from 300 wt.-ppm to 5000 wt.-ppm, and most preferably from 600 wt.-ppm to 3000 wt.-ppm.

[0032] In an advantageous embodiment, at least one component selected from the group consisting of antimony, bismuth, arsenic and tellurium can exhibit a concentration gradient within the gold layer, said gradient increasing in the direction toward the wire core (i.e., in the direction perpendicular to the longitudinal axis of the wire core).

[0033] In another aspect, the invention also relates to a process for manufacturing the coated round wire of the invention in any of the disclosed embodiments above. The process comprises at least steps (1) to (5):

[0034] (1) Provide silver-based precursor items

[0035] (2) Stretch the precursor article to form an elongated precursor article until an intermediate diameter in the range of 30 μm to 200 μm is obtained.

[0036] (3) Apply a double coating of palladium or nickel inner layer and adjacent gold outer layer to the surface of the elongated precursor article obtained after process step (2).

[0037] (4) Further elongate the coated precursor article obtained after process step (3) until the desired final diameter and bilayer are obtained, the bilayer consisting of an inner palladium or nickel layer having a desired final thickness in the range of 1 nm to 100 nm and an adjacent outer gold layer having a desired final thickness in the range of 1 nm to 250 nm, and

[0038] (5) Finally, the coated precursor obtained after process step (4) is subjected to strand annealing at an oven temperature set in the range of >370°C to 520°C, preferably >400°C to 460°C, for an exposure time in the range of 0.8 seconds to 10 seconds, preferably 0.8 seconds to 2 seconds, to form coated round wire.

[0039] Step (2) may include one or more of the following sub-steps: intermediate batch annealing of the precursor article at an oven setting temperature of 200°C to 650°C for an exposure time ranging from 30 minutes to 300 minutes, preferably at an oven setting temperature of 300°C to 500°C for an exposure time ranging from 60 minutes to 180 minutes, and

[0040] The further elongation in step (4) includes die drawing with the following drawing parameters B1) to B4):

[0041] B1) Drawing speed is in the range of 500 m / min to 700 m / min.

[0042] B2) The cone angle of each die is in the range of 70 to 90 degrees.

[0043] B3) The bearing length at each die is 30% to 40% of the diameter of the corresponding circular die opening.

[0044] B4) The circular cross-sectional area of ​​the coated precursor article at each die is reduced by 7% to 15%.

[0045] This document uses the term "stretch annealing." It is a continuous process that allows for the rapid production of materials with high reproducibility. In the context of this invention, stretch annealing means that annealing is performed dynamically as the coated precursor to be annealed is pulled or moved through a conventional annealing oven and wound onto a reel after leaving the annealing oven. Here, the annealing oven is typically in the form of a cylindrical tube of a given length. The annealing time / oven temperature parameters can be defined and set using its defined temperature profile at a given annealing rate selectable, for example, from 10 m / min to 60 m / min.

[0046] The term "oven set temperature" is used in this article. It refers to the temperature fixed in the temperature controller of the annealing oven. The annealing oven can be a chamber furnace type oven (in the case of batch annealing) or a tubular annealing oven (in the case of strand annealing).

[0047] This disclosure distinguishes between precursor articles, elongated precursor articles, coated precursor articles, coated precursors, and coated round wires. The term "precursor article" is used for those early stages of round wires that have not yet reached the desired final diameter of the wire core, while the term "precursor" is used for the early stages of wires at the desired final diameter. After process step (5) is completed, i.e., after the final stranding annealing of the coated precursor at the desired final diameter, the coated round wires in the sense of this invention are obtained.

[0048] The precursor article provided in process step (1) is a silver-based precursor article; that is, the precursor article consists of (a) silver doping, (b) a silver alloy, or (c) a silver-doped alloy. For the meaning of the terms “silver doping,” “silver alloy,” and “silver-doped alloy,” refer to the aforementioned disclosure.

[0049] In embodiments of silver-based precursor articles, the silver-based precursor articles can be obtained by alloying, doping, or alloying and doping with silver in desired amounts of the desired components. Doped silver or silver alloys or silver-doped alloys can be prepared by conventional processes known to those skilled in the art of metal alloys, for example, by melting the components together in a desired proportional ratio. In doing so, one or more conventional master alloys can be used. The melting process can be performed, for example, using an induction furnace, and advantageously, under a vacuum or in an inert gas atmosphere. The materials used can have a purity grade of, for example, 99.99% by weight or higher. The resulting melt can be cooled to form a homogeneous sheet of the silver-based precursor article. Typically, such precursor articles are in the form of round bars having a diameter of, for example, 2 mm to 25 mm and a length of, for example, 2 m to 100 m. Such bars can be produced by continuously casting a silver-based melt using a suitable mold, followed by cooling and solidification.

[0050] In process step (2), the silver-based precursor article is drawn to form an elongated precursor article until an intermediate diameter in the range of 30 μm to 200 μm is obtained. Techniques for drawing the precursor article are known and appear useful in the context of this invention. Preferred techniques are rolling, forging, die drawing, etc., with die drawing being particularly preferred. In the latter case, the precursor article is drawn in several process steps until the desired intermediate cross-section or desired intermediate diameter is achieved. Such wire drawing processes are well known to those skilled in the art. Conventional tungsten carbide and diamond dies can be used, and conventional drawing lubricants can be used to support the drawing process.

[0051] Step (2) of the process of the present invention may include one or more of the following sub-steps: intermediate batch annealing of the elongated precursor article at an oven setting temperature in the range of 200°C to 650°C for an exposure time in the range of 30 minutes to 300 minutes, preferably at an oven setting temperature in the range of 300°C to 500°C for an exposure time in the range of 60 minutes to 180 minutes. For example, the optional intermediate batch annealing may be performed on a bar that has been drawn to a diameter of 2 mm and wound on a roller.

[0052] Optional intermediate batch annealing of process step (2) can be performed under an inert or reducing atmosphere. Many types of inert atmospheres and reducing atmospheres are known in the art and are used to purge the annealing oven. Among known inert atmospheres, nitrogen or argon is preferred. Among known reducing atmospheres, hydrogen is preferred. Another preferred reducing atmosphere is a mixture of hydrogen and nitrogen. A preferred hydrogen and nitrogen mixture is 90 vol% to 98 vol% nitrogen and correspondingly 2 vol% to 10 vol% hydrogen, wherein the total vol% is 100 vol%. Preferred nitrogen / hydrogen mixtures are equal to 93 / 7 vol% / vol%, 95 / 5 vol% / vol%, and 97 / 3 vol% / vol%, each based on the total volume of the mixture.

[0053] In process step (3), a coating in the form of a double coating of palladium or nickel inner layer and an adjacent gold outer layer is applied to the surface of the elongated precursor article obtained after process step (2) is completed, so as to superimpose the coating on the surface.

[0054] Those skilled in the art know how to calculate the thickness of such a coating on an elongated precursor article to ultimately (i.e., after the coated precursor article has been ultimately elongated) obtain a coating at the layer thickness disclosed in the embodiment for wire. Those skilled in the art are familiar with various techniques for forming a coating of the material according to the embodiment on a silver-based surface. Preferred techniques include material deposition from vapor deposition (such as electroplating and electroless plating), such as sputtering, ion plating, vacuum evaporation, and physical vapor deposition, as well as material deposition from melt. In the case of applying the bilayer consisting of a palladium or nickel inner layer and a gold outer layer, it is preferred to apply the palladium or nickel layer by electroplating.

[0055] The gold layer is preferably applied by electroplating. Gold plating is performed using a gold plating bath (i.e., a plating bath that allows gold to be plated onto the surface of a palladium or nickel cathode). In other words, a gold plating bath is a composition that allows the element, in metallic form, gold to be directly applied to the surface of a palladium or nickel cathode on which the wiring is used as a cathode.

[0056] The gold plating is performed by guiding an elongated precursor article coated with palladium or nickel, with wiring serving as the cathode, through a gold plating bath. Before performing process step (4), the thus-obtained gold-coated precursor article leaving the gold plating bath can be rinsed and dried. Water is advantageous as the rinsing medium, with alcohols and alcohol / water mixtures being further examples. Gold plating of the elongated precursor article coated with palladium or nickel through the gold plating bath can be performed at a DC voltage, for example, in the range of 0.2V to 20V, and at a current, for example, in the range of 0.001A to 5A, particularly 0.001A to 1A or 0.001A to 0.2A. The contact time is typically in the range of, for example, 0.1 seconds to 30 seconds, preferably 2 seconds to 8 seconds. The current density used in this context can be, for example, 0.01A / dm³. 2 Up to 150A / dm 2 Within a certain range. The gold plating bath may have a temperature in the range of, for example, 45°C to 75°C, preferably 55°C to 65°C.

[0057] The thickness of the gold coating can be adjusted as needed, primarily by the following parameters: the chemical composition of the gold plating bath, the contact time between the elongated precursor article and the gold plating bath, and the current density. In this context, the thickness of the gold layer can typically be increased by increasing the gold concentration in the gold plating bath, by increasing the contact time between the elongated precursor article, which is used as a cathode, and the gold plating bath, and by increasing the current density.

[0058] In the embodiments, the process of the present invention is a process for manufacturing the coated round wire of the present invention as disclosed in the advantageous embodiments above. Here, the application of the gold layer in step (3) is performed by electroplating the gold layer from a gold plating bath containing gold and at least one component selected from the group consisting of antimony, bismuth, arsenic and tellurium. Thus, in the embodiments described, the gold plating bath is a composition that allows not only the deposition of elemental gold but also the deposition of at least one component selected from the group consisting of antimony, bismuth, arsenic and tellurium within the gold layer. It is not known what chemical species the at least one component is, i.e., whether it exists in the gold layer in elemental form or as a compound. In the embodiments described, the gold plating bath can be prepared by adding the at least one component in a suitable chemical form (e.g., compounds such as Sb2O3, BiPO4, As2O3 or TeO2) to an aqueous composition containing gold as a dissolving salt or a type of dissolving salt. An example of such an aqueous composition in which at least one component can be added is produced by Atotech. K 24HF and manufactured by Umicore 558 and 559. Alternatively, a gold plating bath already containing at least one component selected from the group consisting of antimony, bismuth, arsenic, and tellurium can be used, such as a gold plating bath based on, for example, MetGold Pure ATF manufactured by Metalor. The concentration of gold in the gold plating bath can be, for example, in the range of 8 g / L to 40 g / L, preferably 10 g / L to 20 g / L. The concentration of at least one component selected from the group consisting of antimony, bismuth, arsenic, and tellurium in the gold plating bath can be, for example, in the range of 15 wt.-ppm to 50 wt.-ppm, preferably 15 wt.-ppm to 35 wt.-ppm.

[0059] In process step (4), the coated circular precursor article obtained after process step (3) is further drawn until a desired final diameter of a wire with a double layer is obtained, the double layer consisting of a palladium or nickel inner layer having a desired final thickness in the range of 1 nm to 100 nm, preferably 1 nm to 30 nm, and an adjacent gold outer layer having a desired final thickness in the range of 1 nm to 250 nm, preferably 20 nm to 200 nm. The drawing technique used in step (4) includes die drawing or is die drawing. Conventional tungsten carbide and diamond drawing dies can be used, and conventional drawing lubricants can be used to support die drawing. However, it is important that during the die drawing in process step (4), the following drawing parameters B1) to B4) are dominant:

[0060] B1) Drawing speed is in the range of 500 m / min to 700 m / min.

[0061] B2) The cone angle of each die is in the range of 70 to 90 degrees.

[0062] B3) The bearing length at each die is 30% to 40% of the diameter of the corresponding circular die opening.

[0063] B4) The circular cross-sectional area of ​​the coated precursor article at each die is reduced by 7% to 15%.

[0064] Unbound by theory, it is believed that the interaction of all drawing parameters B1) to B4) together is a key (if not a decisive key) to obtaining the beneficial properties of the coated round wire of the present invention.

[0065] Those skilled in the art will understand that in step (4), die pulling refers to pulling the coated precursor article through one or more die sets, such as one, two, or three die sets. A die set comprises multiple dies arranged in succession, such as 9 to 22 dies arranged in succession. In other words, in step (4), die pulling means pulling the coated precursor article through a succession of dies, wherein the dies have tapered inlets, wherein the circular opening of the dies becomes narrower along the pulling direction, and wherein the coated precursor article becomes thinner as it passes through the opening of the dies, thereby exiting the final die with the final wire diameter.

[0066] The drawing parameter B1 mentioned in this article refers to a drawing speed in the range of 500 m / min to 700 m / min. To avoid any misunderstanding, this drawing speed is the speed at which the coated precursor article leaves the circular opening of the final drawing die, i.e., at the final wire diameter before the final stranding annealing in process step (5).

[0067] The drawing parameter B2 mentioned in this article refers to the cone angle (entry angle) of each drawing die in the range of 70 to 90 degrees.

[0068] The drawing parameter B3 mentioned herein refers to the bearing length at each drawing die, which is 30% to 40% of the diameter of the corresponding circular drawing die opening. The circular opening of each drawing die forms a cylindrical hollow space, characterized by its diameter and the length of the cylinder (=bearing length).

[0069] The drawing parameter B4 mentioned in this article refers to the reduction in the circular cross-sectional area of ​​the coated precursor article at each drawing die, ranging from 7% to 15%. In other words, the drawing parameter B4) describes the gradual narrowing experienced by the coated precursor article at each drawing die.

[0070] In process step (5), the coated precursor obtained after process step (4) is finally subjected to strand annealing at an oven set temperature in the range of >370°C to 520°C for an exposure time in the range of 0.8 seconds to 10 seconds, preferably 0.8 seconds to 2 seconds, to form the coated round wire of the present invention. In a preferred embodiment, the oven set temperature range is even narrower, i.e., the range is then >400°C to 460°C.

[0071] In a preferred embodiment, the coated precursor, which has undergone final strand annealing, i.e., the still-hot coated round wire, is quenched in water. In one embodiment, the water may contain one or more additives, such as 0.01 vol% to 0.2 vol% of additives. Quenching in water means immediately or rapidly (i.e., within 0.2 to 0.6 seconds), for example by immersion or dripping, cooling the coated precursor, which has undergone final strand annealing, from the temperature it experienced in process step (5) to room temperature.

[0072] After completing process step (5) and optional quenching, the coated round wire of the present invention is completed. To fully benefit from its properties, it is advantageous to use it immediately (i.e., without delay, for example, within 25 to 70 days, preferably within 60 days, after the completion of process step (5)). Alternatively, in order to maintain the wire's wide wire bonding process window and to prevent its oxidation / sulfurization or other chemical corrosion, the finished wire is typically wound and vacuum-sealed immediately after the completion of process step (5) (i.e., without delay, for example, within <1 hour to 5 hours after the completion of process step (5)) and then stored for further use as a bonding wire. Storage under vacuum-sealed conditions should not exceed 12 months. After opening the vacuum seal, the wire should be used for wire bonding within 25 to 70 days, preferably within 60 days.

[0073] Preferably, all process steps (1) to (5), as well as winding and vacuum sealing, are performed under cleanroom conditions (US FEDSTD 209E Cleanroom Standard, 1k Standard).

[0074] A third aspect of the invention is a coated round wire obtainable by the process disclosed above according to any embodiment thereof. It has been found that the coated round wire of the present invention is well-suited for use as a bonding wire in wire splicing applications. Wire splicing techniques are well known to those skilled in the art. In the process of wire splicing, ball joints (first joints) and pin joints (second joints, wedge joints) are typically formed. During joint formation, a force (typically measured in grams) is applied, which is supported by the application of ultrasonic energy (typically measured in mA). The coated round wire of the present invention exhibits a fairly wide wire splicing process window.

[0075] The following non-limiting examples illustrate the invention. These examples are for illustrative purposes only and are not intended to limit the scope of the invention or the claims in any way.

[0076] Test methods

[0077] All tests and measurements were conducted at T=20℃ and RH=50%.

[0078] A. Electron backscattering diffraction (EBSD) diagram used to determine the crystallographic orientation and grain size of the gold outer layer. Case Analysis :

[0079] The wire was placed on a sample holder and secured with conductive copper tape, and observed in a FESEM (Field Emission Scanning Electron Microscope) with a holder angled at 70° to the surface of a normal FESEM sample stage. The FESEM was also equipped with an EBSD detector. Electron backscattered pattern (EBSP) containing crystallographic information of the wire surface, including the gold outer layer, was obtained.

[0080] Further analysis of these patterns revealed grain orientation fractions, average grain size, and other parameters (using software called the EBSD program developed by Oxford Instruments). Points with similar orientations were grouped together to form texture components.

[0081] First, the wire was encapsulated with cold-mounted epoxy resin and then polished (cross-section) using standard metallographic techniques. The sample was ground and polished using a multi-preparation semi-automatic polishing machine at low force and optimal speed to minimize deformation strain on the sample surface. Finally, the polished surface was ion-milled to remove a thin layer, further reducing deformation strain to near zero. Grain size was measured using a grain boundary delineation concept defined by a critical orientation deviation angle (>15°) relative to the EBSD tool, according to ASTM E2627-13 (2019). To accurately measure grain size, grain boundaries were first detected using the highest degree of grain boundary delineation.

[0082] To differentiate between different texture components, a maximum tolerance angle of 10° was used. The wire drawing direction was set as the reference orientation. Measurements were taken of wires parallel to the reference orientation. <100> and <111> Calculated by the percentage of crystals with orientation planes <100> and <111> Texture percentage.

[0083] EBSD pattern analysis was performed at five different locations for each sample to obtain the average value.

[0084] B. Electron backscattering diffraction (EBSD) diagram used to determine the crystallographic orientation of the grains and twin boundaries of the wire core. Case Analysis :

[0085] The main steps involved in measuring wire texture are sample preparation, obtaining a good Kikuchi pattern, and composition calculation.

[0086] First, the wire is encapsulated with epoxy resin and polished using standard metallographic techniques. In the final sample preparation step, ion milling is applied to remove any mechanical deformation of the wire surface, contaminants, and oxide layer. The sample cross-sectional surface is then ion-milled using gold sputtering. Two more rounds of ion milling and gold sputtering are then performed. No chemical etching or ion etching is performed.

[0087] The sample was mounted in a FESEM (Field Emission Scanning Electron Microscope) with a holder angled at 70° to the surface of the normal FESEM sample stage. The FESEM was also equipped with an EBSD detector. Electron backscattered spectrum (EBSP) containing crystallographic information of the wire was obtained.

[0088] Further analysis of these patterns revealed grain orientation fractions, average grain size, and other parameters (using software called the EBSD program developed by Oxford Instruments). Points with similar orientations were grouped together to form texture components.

[0089] To differentiate between different texture components, a maximum tolerance angle of 10° was used. The wire drawing direction was set as the reference orientation. Measurements were taken of wires parallel to the reference orientation. <100> and <111> Calculated by the percentage of crystals with orientation planes <100> and <111> Texture percentage.

[0090] Twin boundaries (also known as ∑3CSL twin boundaries) are excluded from the average grain size calculation. Twin boundaries are defined by the boundary surrounding adjacent crystallographic domains. <111> The orientation plane is rotated 60° for description. The number of scan points in the region of interest depends on the step size, which is less than 1 / 5 of the smallest grain size observed (approximately 100 nm).

[0091] EBSD pattern analysis was performed at five different locations for each sample to obtain the average value.

[0092] C. Linear intercept method for determining the grain size of wire cores :

[0093] First, the wire was encapsulated with cold-mounted epoxy resin and then polished (cross-section) using standard metallographic techniques. The sample was ground and polished using a multi-preparation semi-automatic polishing machine at low force and optimal speed to minimize deformation strain on the sample surface. Finally, the polished sample was chemically etched with ferric chloride to expose the grain boundaries. Grain size was measured using the linear intercept method under a 1000x magnification optical microscope according to ASTM E112-12 standard.

[0094] D. Evaluation of flower-shaped joint balls and wire sway :

[0095] D.1) Preparation of FAB :

[0096] This was performed in an ambient atmosphere according to the procedure described in the "KNS Process User Guide for FAB" (Kulicke & Soffa Industries Inc., Fort Washington, PA, USA, 2002, May 31, 2009). FAB was prepared by EFO calcination using standard calcination (single step, 20 μm wire, 50 mA electron flame quenching (EFO) current, 315 μs EFO time, BSR ratio 2.3 (bonding ball to wire diameter ratio, using an IConn-KNS bonding machine)).

[0097] D.2) Ball joint :

[0098] The formed FAB descends from a predefined height (203.2 μm tip) and speed (6.4 μm / s contact speed) to the Al-0.5 wt% Cu bonding pad. Upon contact with the bonding pad, a set of defined bonding parameters (100 g bonding force, 95 mA ultrasonic energy, and 15 ms bonding time) act to deform the FAB and form a bonding ball. After ball formation, capillary rises to a predefined height (152.4 μm junction height and 254 μm loop height) to form a loop. After loop formation, capillary descends to the lead to form a pin. After pin formation, capillary rises, and the wire clamp closes to cut the wire to form a predefined tail length (254 μm tail length extension). For each sample, 2500 meaningful bonding wires were optically inspected using a microscope at 1000x magnification. The defect percentage was determined.

[0099] D.3) Evaluation of paired spheres relative to flower-shaped paired spheres :

[0100] +Poor: ≥15% of the bonded spheres are not round but deformed.

[0101] ++Good: ≥10% to <15% of the bonded spheres are not round but deformed.

[0102] +++ Excellent: <10% of the bonded spheres are not round but deformed. D.4) Assessment of wire sway :

[0103] +Poor: <25% of the wire deflects towards the adjacent wire in the loop.

[0104] ++Good: <5% of the wire deflects toward the adjacent wire in the loop.

[0105] +++Excellent: The cable did not exhibit loop deflection.

[0106] Wire Example

[0107] For all wire examples, 98.5% by weight of silver (Ag) and 1.5% by weight of palladium (Pd) (each metal having a purity of at least 99.99% (“4N”)) were melted in a crucible. Wire core precursor articles in the form of 8mm rods were then continuously cast from the melt. The rods were then drawn in several drawing steps to form wire core precursors with a circular cross-section having a diameter of 2mm. The wire core precursors were intermediately batch-annealed at an oven setting 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 with a diameter of 46μm.

[0108] All wire core precursors were electroplated using a double-layer coating of a nickel inner layer and an adjacent gold outer layer. For this purpose, the wire core precursors were moved through a 60°C nickel plating bath while being wired as the cathode, and subsequently through a 61°C gold plating bath. The nickel plating bath contained 90 g / L Ni(SO3NH2)2, 6 g / L NiCl2, and 35 g / L H3BO3, while the gold plating bath (based on Metalor's Met Gold Pure ATF) had a gold content of 13.2 g / L and an antimony content of 20 wt.-ppm (based on Metalor's MetGold Pure ATF).

[0109] Subsequently, the coated wire precursor was further drawn to a final diameter of 20 μm according to the drawing parameters indicated in Table 1. Finally, all wire samples were stranded and annealed at an oven setting of 410 °C for 0.9 seconds, followed by quenching in water containing 0.07 vol% surfactant, thus obtaining the coated wire. All 20 μm thick wire samples had a 9 nm thick nickel inner layer and an adjacent 90 nm thick gold outer layer.

[0110] Table 1 provides an overview of the drawing parameters and evaluation of the present invention samples S1 to S3 and comparative samples C1 to C5.

[0111] Table 1

[0112]

Claims

1. A round wire comprising a wire core having a surface, the wire core having a coating superimposed on its surface, wherein the wire core itself is a silver-based wire core, wherein the coating is a bilayer consisting of a palladium or nickel inner layer of 1 nm to 100 nm thickness and an adjacent gold outer layer of 1 nm to 250 nm thickness, wherein the gold outer layer exhibits the following inherent properties A1) and A2): A1) The average grain size of the gold outer layer, measured along the longitudinal direction, is in the range of 0.1µm to 0.8µm; A2) 60% to 100% of the grains in the gold outer layer are along... <100> Orientation, and 0% to 20% of the grains in the gold outer layer are oriented along... <111> Orientation, wherein each percentage value is based on the total number of grains having an orientation parallel to the drawing direction of the wire.

2. The round wire according to claim 1, wherein the round wire has an average diameter in the range of 8µm to 80µm.

3. The round wire according to claim 1 or 2, wherein the silver-based core is composed of a silver-based material in the form of (a) silver-doped, (b) silver alloy or (c) silver alloy-doped.

4. The round wire according to claim 3, wherein the doped silver is a silver-based material consisting of: (a1) silver in an amount ranging from >99.49 wt% to 99.997 wt%, (a2) at least one doping element other than silver in a total amount ranging from 30 wt.-ppm to <5000 wt.-ppm, and (a3) ​​other components in a total amount ranging from 0 wt.-ppm to 100 wt.-ppm.

5. The round wire according to claim 3, wherein the silver alloy is a silver-based material comprising: (b1) silver in an amount ranging from 89.99% to 99.5% by weight, (b2) at least one alloying element in a amount ranging from 0.5% to 10% by weight, and (b3) other components in a amount ranging from 0 wt. to 100 wt. to ppm.

6. The round wire according to claim 5, wherein the silver alloy comprises palladium as the sole alloying element.

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

8. The round wire according to claim 3, wherein the doped silver alloy is a silver-based material composed of: (c1) silver in an amount ranging from >89.49% to 99.497% by weight, (c2) at least one doping element in a total amount ranging from 30 wt.-ppm to <5000 wt.-ppm, (c3) at least one alloying element in a total amount ranging from 0.5% to 10% by weight, and (c4) other components in a total amount ranging from 0 wt.-ppm to 100 wt.-ppm, wherein the at least one doping element (c2) is different from the at least one alloying element (c3).

9. The round wire according to claim 1 or 2, wherein the coating superimposed on the surface of the wire core is a double layer consisting of a palladium or nickel inner layer of 1 nm to 30 nm thickness and an adjacent gold outer layer of 1 nm to 200 nm thickness.

10. The round wire according to claim 1 or 2, wherein the round wire exhibits at least one of the following inherent properties A3) to A5): A3) The average grain size of the grains in the wire core, measured along the longitudinal direction, is in the range of 0.7µm to 1.1µm; A4) The fraction of twin boundaries measured along the longitudinal direction of the wire core is in the range of 5% to 40%; A5) 20% to 70% of the grains in the wire core along <100> Orientation, and 3% to 40% of the grains in the wire core are oriented along... <111> Orientation, wherein each percentage value is based on the total number of grains having an orientation parallel to the drawing direction of the wire.

11. The round wire according to claim 1 or 2, wherein the gold outer layer comprises at least one component selected from the group consisting of antimony, bismuth, arsenic and tellurium, and the total proportion of the at least one component is in the range of 10 wt.-ppm to 300 wt.-ppm based on the weight of the wire.

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

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

14. A method for manufacturing round wire according to any one of the preceding claims, the method comprising at least steps (1) to (5): (1) Provide silver-based precursor items. (2) Stretch the precursor article to form an elongated precursor article until an intermediate diameter in the range of 30µm to 200µm is obtained. (3) Apply a double coating of palladium or nickel inner layer and an adjacent gold outer layer to the surface of the elongated precursor article obtained after step (2) is completed. (4) Further elongation is performed after step (3) to obtain the coated precursor article until the desired final diameter and bilayer are obtained, the bilayer consisting of an inner palladium or nickel layer having a desired final thickness in the range of 1 nm to 100 nm and an adjacent outer gold layer having a desired final thickness in the range of 1 nm to 250 nm, and (5) Finally, the coated precursor obtained after step (4) is subjected to strand annealing at an oven setting temperature in the range of >370°C to 520°C for an exposure time in the range of 0.8 seconds to 10 seconds to form the coated round wire. Step (2) may include one or more of the following sub-steps: intermediate batch annealing of the precursor article at an oven set temperature of 200°C to 650°C for an exposure time ranging from 30 minutes to 300 minutes, and The further elongation in step (4) includes die drawing primarily using the following drawing parameters B1) to B4): B1) Drawing speed is in the range of 500 m / min to 700 m / min. B2) The cone angle of each die is in the range of 70 to 90 degrees. B3) The bearing length at each die is 30% to 40% of the diameter of the corresponding circular die opening. B4) The circular cross-sectional area of ​​the coated precursor article at each die is reduced by 7% to 15%.

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

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