TWI551361B - Method for obtaining a palladium surface finish for copper wire bonding on printed circuit boards and ic-substrates and the products prepared therefrom - Google Patents

Description

Method for obtaining palladium surface treatment for copper wire bonding on printed circuit boards and integrated circuit substrates, and articles prepared therefrom

The present invention relates to a method of bonding a copper wire to a substrate, particularly a printed circuit board and an integrated circuit substrate, the printed circuit board and the integrated circuit substrate having a layer comprising a copper joint portion and a palladium or palladium alloy layer. The copper wire is bonded to the substrate of the above layer assembly.

In the manufacture of printed circuit board (PCB) and integrated circuit (IC) substrates, it is desirable to bond the electronic component to a selected bond area of the copper structure produced on one or both sides of the substrate (as a joint of the joint) pad). The interconnect must be reliable in terms of bond strength, i.e., the thermal stress applied to the bond interconnect should never cause damage to the interconnect.

Wire bonding is a preferred method of connecting a wafer to an integrated circuit substrate in an integrated circuit package and it accounts for more than 70% of commercial integrated circuit production. The main wire bonding method currently used for integrated circuit substrates is gold wire bonding, in which gold wires are bonded to the electrolytically deposited nickel and gold layers. Alternatively, the gold wire is bonded to the surface of nickel, palladium and gold (ENEPIG). In all cases, the copper wire is bonded to the final gold layer. Recently, copper wire bonding technology has been introduced as an alternative to gold wire bonding in the integrated circuit substrate industry. The current copper bonding standard is to use a copper wire bond bonded to a layer sequence consisting of a first electrolytic nickel layer and a second gold layer deposited on the copper wire bonding portion of the substrate.

The mechanical reliability of wire bonding in microelectronic packages depends to a large extent on the formation and development of intermetallic compounds at the interface between the bonding wedge and the bonding pad on the substrate (printed circuit board, PCB or integrated circuit substrate). , which is necessary for successful bonding.

The main reason why it is difficult to bond gold or copper wires to the surface of the copper bond pad is that the copper metal plating layer has a higher tendency to oxidize.

The wire bonding portion is usually made of copper. If it remains exposed or exposed to the atmosphere and moisture, the soldering and wire bonding properties of the copper layer degrade due to surface oxidation or corrosion. Thus, to maintain soldering or wire bonding properties, the exposed or exposed copper layer is typically nickel plated or electroless plated. The electroplated nickel layer protects copper from corrosive atmospheres for a long time. Also, the nickel layer protects the copper from being dissolved by the solder during the solder assembly step by acting as a diffusion barrier layer. In addition, the electroplated nickel layer functions as an interface film to prevent diffusion of the subsequently plated copper layer and the gold layer from each other. Subsequently, a wire bonding gold of a thickness of about 0.5 μm is electroplated electrolytically or electrodelessly to impart properties that contribute to the wire bonding process. Such processes are described, for example, in US 5,235,139 and US 6,733,823.

US 2007/0104929 relates to a method of electroplating a printed circuit board comprising the steps of: (a) providing a printed circuit board having a predetermined circuit pattern, the printed circuit board having a wire bonding portion for mounting a semiconductor on an upper surface thereof, and a soldering portion for connecting the external component to the printed circuit board; (b) forming a photo solder resist layer on the printed circuit board except for the wire bonding portion and the soldering portion; (c) bonding the wire Forming a palladium or palladium alloy electroless plating layer on the joint portion and the welded portion; and (d) immersing the palladium or palladium alloy plating layer in a substituted impregnation gold plating solution containing a water-soluble gold compound to palladium or palladium alloy A gold or gold alloy electrodeless plating layer is formed on the plating layer.

US 2006/055023 relates to wafer carriers comprising a laminate and an oxidation barrier. The anti-oxidation layer is an electroless metal coating or an organic anti-oxidation film formed on the surface of the bonding finger pad or other contact using a simple rapid film coating technique.

US 5,175,609 relates to a multi-layer metallurgical mat for corrosion and stress resistant interconnection (which comprises a chromium layer, a nickel layer and a precious or relatively precious metal layer deposited sequentially according to interconnect metallurgy) or a multilayer metallurgical mat (which includes A novel structure and method for a chromium layer, a soluble precious metal layer, a nickel layer, and a precious or relatively precious metal layer deposited sequentially by metallurgy.

EP 0 697 805 A1 relates to a method for manufacturing a printed circuit board by providing a circuit board having a copper circuit pattern, a hole and a land; covering the circuit pattern with a solder mask; and plating the circuit board with an electrodeless electrode The solution is contacted for a period of time sufficient to provide a final palladium treatment layer that is thick enough to protect the copper deposits in the voids and on the joint from oxide formation and relatively smooth and flat to provide excellent solderability and excellent wire bonding capability. . Copper circuit patterns, holes, and landings are typically provided by electroplating copper on a circuit board. Next, the palladium layer can be placed directly on the copper or on the electrodeless electroplated nickel layer initially deposited on the copper.

Accordingly, it is an object of the present invention to provide a layer deposited on a substrate having a wire bonding portion made of copper which ensures a very reliable formation of wire bonding and no gold layer. A metal layer as used herein means at least one metal layer formed of a palladium or palladium alloy formed on a wire bonding portion, which is suitable for use as a copper wire bonding surface.

Another object of the present invention is to provide a method of forming the layer while forming a stable wire bond on the metal layer, more specifically copper wire bonding.

Another object of the present invention is to provide an integrated circuit substrate having a copper structure, wherein the copper structure is coated with a metal layer.

Another object of the present invention is to provide a printed circuit board having a copper structure wherein the copper structure is coated with a metal layer.

To achieve these objectives, the present invention provides a metal layer assembly comprising

(i) at least one wire bonding portion layer made of copper or a copper alloy; and deposited thereon

(ii) a metal layer which is a palladium or palladium alloy layer;

(iii) a copper wire bonded to a palladium or palladium alloy layer.

The palladium or palladium alloy layer deposited in the step (ii) has a thickness of from 0.01 to 5.0 μm, preferably from 0.05 to 2.0 μm and more preferably from 0.1 to 0.5 μm.

In a preferred embodiment of the method, the palladium layer consists of pure palladium. In another preferred embodiment, the layer assembly on the wire bonding portion does not contain a nickel or nickel alloy layer, but only a palladium or palladium alloy layer directly plated on the wire bonding portion.

Palladium is deposited from an electrodeless (autocatalytic) electroplating palladium bath containing

Palladium ion source;

▇ wrong mixture;

▇Reducing agent.

Heretofore, a nickel or nickel alloy layer is usually electroplated on a wire bonding portion layer made of copper or a copper alloy. The thickness of the nickel layer is usually from 0.5 to 10.0 μm.

Electroplating of nickel or nickel alloy layers is no longer required in the process of the invention.

The inventors have found that it is particularly advantageous to plate a pure palladium layer on the copper or copper alloy of the wire bonding portion. According to one embodiment (in which a nickel or nickel alloy layer is not deposited on the wire bonding portion made of copper or a copper alloy before plating the palladium or palladium layer), pure palladium is even more preferable. The pure palladium layer of the present invention is a layer having a palladium content of more than 99.0 wt.%, preferably more than 99.5 wt.% palladium or more preferably more than 99.9 wt.% or more than 99.99 wt.% palladium.

In another embodiment of the method, the palladium plating layer is an alloy layer comprising 90 to 99.9 wt.% palladium and 0.1 to 10.0 wt.% phosphorus or boron.

In a preferred embodiment of the invention, the metal layer comprises only one palladium or palladium alloy layer, such as a palladium-phosphorus layer, which is formed directly on the copper or copper alloy wire bond portion of the substrate.

Typically, the copper or copper alloy surface of the joint portion is prepared by prior treatment with an acid cleaner followed by reduction of the surface copper oxide in a microetch bath.

For the purposes of the present invention, another activation step can be advantageously applied to the copper or copper alloy wire bond portion prior to depositing the palladium or palladium alloy layer. The activation solution can comprise a palladium salt that produces a thin palladium layer. This layer is extremely thin and typically does not cover the entire copper or copper alloy wire bond portion. It is not considered to be a different layer in the layer assembly, but rather as an activation which forms a metal seed layer other than the palladium/palladium alloy layer. The thickness of the seed layer is typically a few angstroms. The seed layer is electroplated to a copper or copper alloy layer by an immersion exchange process.

The activation solution is particularly preferred if the palladium layer is deposited by an electrodeless (autocatalytic) plating method.

The palladium/palladium alloy layer is preferably plated onto a copper or copper alloy structure disposed on an integrated circuit substrate or a printed circuit board.

The integrated circuit substrate includes a carrier body and a copper structure disposed on one or both sides of the carrier body. Therefore, a palladium or palladium alloy layer is coated on the metal structure. Subsequently, the copper wire is bonded to the palladium or palladium alloy layer.

The copper wire is preferably a pure copper wire. Alternatively, it may be a copper alloy wire. Further, the copper wire or copper alloy wire may be coated with gold or palladium, preferably palladium. The thickness of the palladium coating can be, for example, between 5 and 50 nm, preferably between 10 and 25 nm. The gold coating can have the same thickness range.

The copper structure includes a wire bond portion (also referred to as a bond pad) for connecting the electronic component to the substrate; a wire for electrical connection between the pads and between the metallized hole and the pad; and other conductor regions , for example, a grounded area, a shielded area, and the like. The palladium or palladium alloy layer is preferably applied only to the solder and joint portions and not to the wires and other conductor regions.

The layer assembly of the present invention provides the most reliable joint connection between the electronic component and the substrate as compared to the prior art.

The layer assembly of the present invention also does not contain a gold layer plated onto the palladium or palladium alloy layer. This not only significantly reduces the total process cost. The inventors have also discovered that the elimination of the gold layer greatly enhances the bonding efficiency.

The method of the present invention preferably comprises depositing a layer of palladium or palladium alloy by electroless (autocatalytic) electroplating.

Electrodeless (autocatalytic) electroplating involves depositing a metal by means of a reducing agent contained in an electroless plating solution to deposit a metal, which is thus oxidized. The substrate metal will therefore oxidize and thereby dissolve. In this case, the reducing agent contained in the plating solution is not used.

Further, it is preferred to deposit a pure palladium layer by contacting at least one copper wire portion of the substrate with a solution containing a source of palladium ions and a reducing agent which does not contain phosphorus and causes deposition of a pure palladium layer.

Electrodeless (autocatalytic) electroplating bath compositions suitable for depositing a pure palladium layer are described, for example, in US 5,882,736. The electroplating bath contains a palladium salt, one or more nitriding complexing agents, and a formic acid or formic acid derivative, but does not contain a hypophosphite and/or an amine borane compound. The pH of the solution is above 4. A primary amine, a secondary amine or a tertiary amine or a polyamine is preferably used as the nitriding wrong agent. It is, for example, an ethyl-diamine; 1,3-diamino-propane, 1,2-bis(3-amino-propyl-amino)-ethane; 2-diethyl-amino group- Ethyl-amine; and di-ethyl-triamine. In addition, di-ethyl-triamine-pentaacetic acid; nitro-acetic acid; N-(2-hydroxy-ethyl)-extended ethyl-diamine; ethyl-diamine-N,N- Diacetic acid; 2-(dimethyl-amino)-ethyl-amine; 1,2-diamino-propyl-amine; 1,3-diamino-propyl-amine; 3-(methyl -amino)-propyl-amine; 3-(dimethyl-amino)-propyl-amine; 3-(diethyl-amino)-propyl-amine; bis(3-amino-propyl , an amine; 1,2-bis(3-amino-propyl)-alkyl-amine; di-ethyl-triamine; tri-ethyl-tetraamine; tetraethyl-pentamine; Ethyl-hexamine; and any desired mixture of such nitriding complexing agents. However, sulfur-containing compounds are not used as stabilizers with the wronging agent.

More preferably, the solution for depositing a pure palladium layer in an electrodeless manner is an aqueous solution and contains a palladium salt such as palladium chloride or palladium sulfate, and a hypophosphite-free compound (for example, formic acid, mineral acid) as a reducing agent. (such as sulfuric acid and hydrochloric acid) or a mineral base (such as sodium hydroxide or potassium hydroxide), a complexing agent (such as an amine compound such as ethylenediamine) and, if desired, a stabilizing compound. Alternatively, other electrodeless electroplating palladium deposition solutions and methods that can be used are well known in the art and are described in U.S. Patent Nos. 5,292,361, 4,424,241, 4,341,846, 4,279,951, and 4,255,194.

In another preferred embodiment of the present invention, a conformal mask is disposed on one or both sides of the substrate, except that at least one of the copper wire portions on the one or both sides is coated with a palladium or palladium alloy layer. In addition, the conformal mask covers all areas on one or both sides of the substrate. The conformal mask may preferably be a solder mask, such as an exposable and developable mask. The mask may be based on, for example, epoxy resin and laminated, spin coated, rolled or otherwise applied to the surface of the circuit carrier. It is then exposed to actinic light and developed to expose the area of the surface on which the deposited layer will be assembled. These areas will set the wire bonding portion. The conformal mask may be formed such that the exposed area on the surface of the substrate is larger than the wire bonding portion formed in the copper structure, thereby exposing a portion of the dielectric surface area of the substrate, or causing the exposed area on the surface of the substrate to be smaller than the copper structure The wire bonding portion is formed in the middle to expose only the wire bonding portion formed in the copper structure.

According to the method of the present invention, the exposed wire bonding portion made of copper or a copper alloy is plated with a layer of pure palladium or palladium alloy.

The palladium alloy layer is preferably a palladium-phosphorus layer having a phosphorus content of 0.1 to 10 wt.%, more preferably 0.5 to 7 wt.%. The pure palladium layer of the present invention is a layer having a palladium content of more than 99.0 wt.%, preferably more than 99.5 wt.% palladium or more preferably more than 99.9 wt.% or more than 99.99 wt.% palladium. A pure palladium layer is preferred.

A palladium or palladium alloy is electroplated on the exposed wire bonding portion to form a palladium or palladium alloy plating layer.

An electrodeless palladium or palladium alloy plating layer is formed on the wire bonding portion in more detail below.

The reducing agent used in the plating solution determines whether or not the pure palladium or palladium alloy (palladium-phosphorus, palladium-boron) is electroplated. For example, a typical electrodeless electroplating palladium solution suitable for use in the present invention includes, but is not limited to, palladium sulfate as a source of palladium; sodium hypophosphite or dimethylamine borane, formaldehyde or formic acid as a reducing agent; Lactic acid and succinic acid as a buffer. In order to obtain a dense structure of the palladium plating layer, the pH of the electrodeless electroplating palladium solution is preferably in the range of 4.5 to 5.5.

The palladium or palladium alloy electroplating process is carried out at about 45 ° C to 80 ° C for 1 minute to 60 minutes to obtain a palladium or palladium alloy having a thickness in the range of 0.05 to 5.0 μm, more preferably 0.1 to 1.0 μm and more preferably 0.1 to 0.5 μm. Plating.

The concentration of the complexing agent in the plating bath depends on the palladium content. Typically, the molar ratio of the cross-linking agent to palladium is from 5:1 to 50:1, wherein the concentration of the cross-linking agent in the electroplating bath is from 0.05 grams to 100 grams per liter of plating bath.

The pH of the coating solution is usually greater than 4. In the case where the pH is lower than 4, the solution becomes unstable and tends to self-decompose due to the generation of hydrogen. At a pH slightly below 4, a poorly viscous black palladium layer is deposited on the metal surface, and at a pH below about 2, palladium precipitates out of solution. In this case, the resulting precipitate on the substrate was black and insufficiently sticky.

The pH of the coating solution is preferably in the range of 5 to 6. At pH values greater than 7, the alkaline bath enhances the tendency of palladium to deposit on the metal surface in a cemented manner (i.e., without bright gloss or adhesion to the substrate). In addition, the alkali coating solution can attack organically resistant films, such as solder masks, that are applied to the circuit board.

If a pure palladium layer is deposited directly on the copper or copper alloy layer of the wire bonding portion, the copper surface is preferably pretreated. To achieve this, etching cleaning is usually carried out in an oxidizing acidic solution such as a solution of sulfuric acid and hydrogen peroxide. Preferably, the cleaning is then carried out in an acidic solution such as a sulfuric acid solution. After washing, the surface is preferably activated with a solution containing palladium such as a palladium sulfate solution containing other acids such as sulfuric acid, methanesulfonic acid and phosphoric acid. Typically, the substrate is immersed in an activation bath at 25 ° C to 30 ° C for 1 to 4 minutes.

This activation step is preferred for subsequent deposition steps, such as in an electrodeless (autocatalytic) palladium bath containing a reducing agent that is catalytically activated by the presence of palladium metal. Therefore, at least some palladium must be deposited on the copper surface by an impregnation process in which the more expensive metal palladium replaces the less expensive copper. This initial immersion palladium deposition can also occur if the copper surface is directly immersed in an autocatalytic palladium bath, and once a very small amount of palladium is deposited, the process proceeds in an autocatalytic deposition manner. The autocatalytic palladium bath composition was found to be deactivated by excess copper ions. If the initial immersion palladium deposition is carried out in an autocatalytic palladium bath, copper ion enrichment inevitably occurs. Thus, in a preparative environment, an impregnated palladium bath (the previously described activation solution) may be preferably employed in an additional step prior to the autocatalytic palladium bath to extend the useful life of the autocatalytic palladium bath.

Next, in a preferred embodiment, the surface can be rinsed and then pretreated in an activated palladium bath followed by an electrodeless (autocatalytic) palladium bath; in another case, it can be immediately electrodeless (autocatalytic) The palladium bath treatment was carried out without pretreatment in an activated palladium bath.

The following examples are provided to illustrate various aspects of the invention.

Instance

A single-sided PCB (array size 61.8 x 113.8 mm) was used as the substrate. The array consists of two single cards (50 x 50 mm). The total copper thickness is 30 μm (+/- 5 μm). Wire bonding can be performed on any area of the substrate.

The substrate material was Hitachi MCL-E679FGB-(S), FR4 18/80, 300 μm.

The plating sequence used is shown in the table below:

Table 1A: Electroplating of pure palladium on a copper wire bond portion (according to the invention)

Table 1B: Palladium plating on copper pads using an immersion method (comparative)

Table 2: Electroplating of nickel and pure palladium on copper wire bonding

Table 3: Electroplating of nickel and palladium-phosphorus alloys on copper wire joints

Table 4: Electroplating of nickel, pure palladium and gold on copper wire joints (comparative)

Components used according to Table 1 A/B-4:

Test conducted: Pull test

After depositing the metal layers as described in Table 1 A/B-4, the electroplated wire bonding portions were bonded to a portion of the copper pad having a Heraeus Maxsoft copper wire (0.8 μm in diameter) at 165 °C. The copper bonding process was performed using a Microenvironment Copper Kit using a Kulicke and Soffa Max Ultra wire bonder. The forming gas (5%/95% H ₂ /N ₂ ) protects the free-air ball during formation. The bonding tool was a Kulicke and Soffa CuPRAplus capillary with a 1.25 mil chamfer diameter (CD) and a 60° internal chamfer (ICA). Bonding is performed in the presence of ultrasonic (US) processing.

Prior to bonding, the palladium surface is cleaned by plasma etching methods known in the art.

The pull test was performed with the Dage 4000 T p pull tester.

The test results are shown in Table 5. Ten junction samples were tested and an average is provided in Table 5. The test "table number" indicates the plating sequence as shown in Table 1 A/B-4 above. The thickness of the resulting metal layer is expressed in μm. The US processing time varies between 90 seconds and 140 seconds. The corresponding pull strength values expressed in grams of carat force are provided in Table 5.

US (ultrasonic) processing time

Table 5: Results of the average pull strength values obtained

As evident from the information in Table 5, the pull strength value of sample No. 1A was highest in all US processing times. The higher the pull strength value, the better the bond between the substrate and the copper wire, which is desirable. A bond strength value below 2.1 is considered insufficient to form a reliable bond between the substrate and the copper wire, and a value between 2.2 and 3.9 is considered acceptable. A value equal to or higher than 4.0 is considered excellent. When the palladium layer was electroplated using the immersion palladium plating method (sample No. 1B), the bonding efficiency was remarkably weak as compared with the electrodeless (autocatalytic) plating method of the present invention.

All values of sample No. 1 were higher than 5, that is, sample No. 1 was excellent. This example corresponds to a particularly preferred embodiment of the invention wherein the palladium layer is a pure palladium layer and is deposited directly onto the copper wire bond portion without an intermediate nickel or nickel alloy layer.

Sample No. 2 and No. 3 were also obtained by the method of the present invention and contained an additional layer of nickel interposed between the substrate (more specifically, the wire bond portion) and the palladium layer. The resulting average draw strength value is lower than the preferred embodiment, but is still acceptable.

The addition of the gold layer (sample No. 4) as the final layer of the assembly according to the experiment corresponding to the comparative example unexpectedly caused the resulting average tensile strength value to be significantly lowered. Therefore, the bonding efficiency is poor. Three of the eight values are considered "failed" and five of the eight values are considered "acceptable" and there are no excellent values.

So far, Xianxinjin is the most suitable layer for wire bonding.

Thus, the layer sequence provided by the method of the present invention produces excellent copper wire bonding results in terms of the resulting average draw strength value, indicating excellent bonding between the copper wire and the treated surface of the substrate.

Other joint tests were conducted on samples prepared according to the plating methods of Table 1A (electrodeless (autocatalytic) plating method, the present invention) and 1B (immersion plating method, comparison) to determine that the method of the present invention produced better bonding efficiency.

Test the Cu wire bonding ability of the four samples listed below (with two different palladium layer thicknesses):

Table 1A:

Electrodeless plating Pd (autocatalytic) 90 nm (Pd thickness 90 nm)

Electrodeless plating Pd (autocatalytic) 150 nm (Pd thickness 150 nm)

Table 1B:

Impregnated Pd 100 nm, (Pd thickness 100 nm)

Impregnated Pd 150 nm, (Pd thickness 150 nm)

The test conditions were "as is" (just electroplated and not heat treated) and after heat treatment at 150 ° C for 4 hours. Electrodeless (autocatalytic) all samples plated with palladium were able to bond with Cu under both test conditions. In the sample impregnated with Pd, only the sample "impregnated Pd 150 nm" can be joined under "as is" conditions. The sample "impregnated Pd 100 nm" was not bondable under both test conditions, and the sample "impregnated Pd 150 nm" was not bondable after heat treatment "150 ° C for 4 hours". The sample failed to bond during the wire bonding process. Therefore, no copper wire can be fixed on the surface and the copper wire drawing test cannot be performed. In addition, the two samples impregnated with Pd showed significant discoloration after heat treatment.

Unless otherwise stated, the joining equipment and parameters are as described above, with detailed parameters set as follows. Copper wire used: Hereaus Cu Maxsoft, Φ = 20 μm, breaking load 7.3 g, temperature: 200 °C.

Table 6: Parameter settings for wire bonding test

Each sample was subjected to 30 pulls. The pull test criteria are based on the DVS 2811 standard: minimum breaking force >50% line breaking load (>4.95 g); coefficient of variation CV (ratio of standard deviation to average) <0.15 (<15%). All samples were pretreated with argon plasma for 10 minutes prior to bonding.

Table 7: Drawing strength (g) of surface treatment; aging = as it is

Table 8: Drawing strength (g) of surface treatment; aging = 4 hours, 150 ° C

As is apparent from Tables 7 and 8, the palladium layer obtained by the electrodeless (autocatalytic) palladium deposition method has a higher bonding efficiency than the palladium layer obtained by the impregnation deposition. After aging and at a thickness of 100 nm, the impregnated electroplated palladium layer is completely unbondable and therefore not suitable for use in the bonding method of the present invention.

Claims (7)

A method of forming a copper wire on a substrate having at least one wire bonding portion made of copper or a copper alloy, the method comprising the steps of: (i) providing at least one made of copper or a copper alloy a substrate for bonding the wire bonding portion; (ii) forming a palladium layer having a palladium content of more than 99.9 wt.% on the at least one wire bonding portion, or a palladium comprising 90.0-99.9 wt.% palladium and 0.1-10.0 wt.% phosphorus An alloy layer; and subsequently (iii) bonding a copper wire to the palladium or palladium alloy layer, and wherein the palladium or palladium alloy layer of step (ii) is formed directly on the copper or copper alloy joint portion. The method of claim 1, wherein the palladium or palladium alloy layer has a thickness of from 0.05 to 5.0 μm. The method of claim 1 or 2, wherein the palladium or palladium alloy layer has a thickness of from 0.1 to 0.5 μm. The method of claim 1, wherein the palladium is deposited from an electrodeless (autocatalytic) electroplating palladium bath comprising: a source of palladium ions; a binder; a reducing agent. A layer assembly comprising: (i) at least one wire bonding portion layer made of copper or a copper alloy; and deposited directly thereon: (ii) a palladium layer having a palladium content of more than 99.9 wt.%, or a palladium alloy layer comprising 90.0-99.9 wt.% palladium and 0.1-10.0 wt.% phosphorus; and (iii) wire bonding to the palladium or palladium alloy layer Copper on the top. A printed circuit board having a layer assembly as claimed in claim 5, and additionally, a solder portion for connecting an external component to the printed circuit board. An integrated circuit having a layer assembly as claimed in claim 5.