WO2014102140A1

WO2014102140A1 - A method for plating a substrate with a metal

Info

Publication number: WO2014102140A1
Application number: PCT/EP2013/077413
Authority: WO
Inventors: Anders REMGÅRD
Original assignee: Pk Plating Technology Ab
Priority date: 2012-12-28
Filing date: 2013-12-19
Publication date: 2014-07-03
Also published as: TW201432089A

Abstract

The invention relates to a method for plating a substrate with a metal. The method comprises the steps of: - mixing: a metal salt, a complexing agent, a surfactant, and an acid in water thereby forming a non-autocatalytic aqueous plating bath, and - subjecting the substrate to the non-autocatalytic aqueous plating bath. The method may further include an additional plating step.

Description

A METHOD FOR PLATING A SUBSTRATE WITH A METAL

TECHNICAL FIELD

The present invention relates to a method for plating a substrate with an aqueous plating bath. The invention also relates to an object coated according to the method as well as an aqueous plating bath for plating a substrate with a metal.

BACKGROUND OF THE INVENTION

There are several well known technologies for plating of metals on substrates. For instance, electroplating, immersion plating and autocatalytic electroless plating may be used.

Electroplating involves the formation of an electrolytic cell wherein a plating metal represents an anode and a substrate represents a cathode, and an external electrical charge is supplied to the cell to faciliate the coating of the substrate. Immersion (displacement ) plating is the deposition of a metallic coating on a base metal from a solution that contains the coating metal. A first metal ion is displaced by a second metal ion that has a lower oxidation potential than the displaced metal ion. In immersion plating, a reducing agent is not required to reduce the metal ions to metal, as the base metal acts as reducing agent. The thickness of deposits obtained by immersion plating is limited because deposition stops when the entire surface of the base metal is coated.

Autocatalytic electroless plating refers to the autocatalytic or chemical reduction of metal ions plated to a base substrate. The process differs from immersion plating in that deposition of the metal is autocatalytic or continuous. One attractive benefit of autocatalytic electroless plating over electroplating is the ability to plate a substantially uniform metallic coating onto a substrate having an irrregular shape. Electroless coatings are also virtually nonporous, which allows for greater corrosion resistance than electroplated substrates. In general, electroless plating baths comprise metal salts, complexing agents, reducing agents and different additives for increasing brightness, stability and deposition rate.

Typical applications for autocatalytic electroless plating are conductive and environmental protective layers on microwave components, solderable and bondable surfaces on PCB^'s (Printed Circuit Boards) and wafers, the plating of solar cells, catalytic beds and interconnects for multi-layer three-dimensional silicon architecture in multi-wafer stacks. The materials and surfaces of the components in these applications comprise nickel silicon, copper and copper alloys.

Nickel plated surfaces are very prone to be passive, and therefore an activation is commonly used prior to silver plating. The activation is performed using electroplating or non-electroplating methods. For electroplating, the simultaneous activation/ nickel strike procedure, known as the Wood^'s nickel strike, is commonly used. This is an electroplating process using a solution comprising nickel chloride and hydrochloric acid. For non- electroplating processes, direct autocatalytic silver plating may be used on freshly deposited electroless nickel surfaces or on surfaces treated in diluted acids. However, the process is initially very slow and uneven which often leads to a plating with bad coverage, bad adhesion and thickness variations within the silver layer. To overcome this disadvantage, an intermediate layer of gold or palladium may be formed on the substrate surface using processes of immersion type or autocatalytic type. However for surfaces having complex geometries, such as narrow corners in cavities, it is difficult to get good coverage of gold or palladium. Moreover, the gold or palladium layers can be sensitive to concentrated gas flows during the subsequent autocatalytic silver plating, i. e. the layers can be etched away.

For direct plating on silicon solar cells, a seed layer of electroless nickel is commonly used. In order to reduce contact resistance between the conductive fingers and the emitter it is possible to exchange nickel for silver. Silver is the best conductive material, but in order to be competitive, there is a need for good adhesion between silicon and silver. Copper and alloys of copper can be directly silver plated using autocatalytic electroless plating if the surface is clean and deoxidised. However in narrow corners, cavities and/or on rough surfaces, the gassing during the autocatalytic electroless plating may result in unsatisfactory coverage. For instance, this may be observed during autocatalytic silver plating of the corrugations of flexible waveguides. Accordingly, there is a need for a method allowing for improved deposition of metals on substrates.

DESCRIPTION

It is an object of the present invention to overcome or at least mitigate some of the disadvantages associated with the prior art. The above-mentioned object is achieved by a method for plating a substrate with a metal. The method comprises the steps of:

- mixing:

a metal salt,

a complexing agent,

a surfactant, and

an acid in water thereby providing a non-autocatalytic aqueous plating bath, and

- subjecting the substrate to the non-autocatalytic aqueous plating bath.

The substrate may be subjected to the aqueous plating bath by immersion into the aqueous plating bath for a time period of a desired length such as from 30 seconds to ten minutes, from 30 seconds to one minute or from four to six minutes.

It has surprisingly been found that the method described herein results in excellent plating of substrates with simple geometry as well as substrates with complex geometries having high specific surface area, such as narrow corners, pores, or on irregular surfaces in general. The fact that also geometries with high specific surface area can be plated is easily observed by the coated surface taking on a more intensive white colour which is believed to be due to the presence of metal nuclei such as silver nuclei in these areas under the initial stage of an additional plating step which may be autocatalytic silver plating. This is contrary to what is commonly achieved by using intermediate layers of gold or palladium, where subsequent autocatalytic silver plating always has a slower start in such areas. A slow plating start in such areas may be detrimental to the plating of the substrate. For example, when the start is slow, the substrate surface may become passive or an autocatalytic silver bath containing cyanide may etch away a thin gold layer in such areas. Both cases result in poor coverage. The non-autocatalytic aqueous plating bath may be obtained by mixing the metal salt, the complexing agent, the surfactant and the acid in water. The amount of acid added allows for convenient adjustment of the bath pH. After mixing the components of the bath, close visual inspection may reveal the formation of rod like clusters, cluster like particles and/or nuclei like particles. The clusters, cluster like particles and/or nuclei like particles are more easily observed after the bath has been allowed to stand for some time or during the plating process. It is believed that the clusters, cluster like particles and/or nuclei like particles contain the metal such as silver. While not wishing to be bound by any specific theory, it is believed that the clusters, cluster like particles and/or nuclei like particles have a beneficial impact on the plating result possibly by adhering to substrate parts such as corners, pores or in the bottom of rough surfaces in general.

Part from the displacement function of the bath, it has a catalytic ability, but not auto- catalytic, i.e. silver deposition can occur on pure silver surfaces with high surface energy, like porous silver surfaces, but the maximum thickness of the deposited layer will be less than 0.2 micrometers. Further it^'s believed that during plating, an equilibrium is achieved between growth and size reduction of the rod like clusters within the bath. Accordingly, there is also provided a method for plating a substrate comprising the steps of:

- subjecting the substrate to a non-autocatalytic aqueous plating bath obtainable by mixing a metal salt, a complexing agent, a surfactant and an acid in water. The non-autocatalytic aqueous plating bath in the method described herein may be seen as a pre-dip aqueous bath intended for treating the substrate. In this document, the expressions "non-autocatalytic aqueous plating bath" and "pre-dip aqueous bath" are used interchangeably. The thus deposited metal layer has very limited surface roughness due to complete or very limited presence of microvoids, pores, cavities or irregularities thereby enhancing the substrate properties with respect to, for instance, adhesion, reactivity, printed circuit board bonding values and metal layer thickness tolerance values. The improved adhesion facilitates plating of additional layers. The increased reactivity of a previously rather passive surface, such as a nickel surface, enables and/or facilitates further plating. The improved metal thickness tolerance values are advantageous since it has been found that the method described herein allows for producing the same or essentially the same metal layer thickness on printed circuit boards regardless of pad size. This is in contrast to traditional plating methods in which a small pad is associated with a thick plated metal layer and vice versa. The method described herein allows for treatment such as activation of substrates, such as substrate surfaces, by depositing a layer of metal onto the substrate whereby further plating of the substrate is facilitated. The deposited metal may have a barrier function. Further, the metal layer deposited onto the substrate, which may be a silver layer, provides excellent coverage and is uniform, non-porous and exhibits excellent electrical properties. The deposited metal layer may have a thickness of about 0.03 to 0.15 micrometers. The method may be applied to different substrates, such substrates comprising or consisting of nickel, silicon, tin, copper and copper alloys. Subsequently, a further layer of metal may be applied onto the treated substrate by performing an additional plating step resulting in a thicker coating of the substrate.

The method described herein may be seen as a method for treating a substrate surface or as a method for sealing a substrate such as a substrate surface. Thus, the method described herein may be used for sealing of a substrate. In particular, the method described herein may be used for sealing of a metal substrate or for sealing of a metal coating on a substrate. The sealing may include sealing of pores, microvoids or cavities of a substrate or a metal coating of a substrate. As an example, a thin porous copper layer on a zmk zincate treated aluminium substrate may be sealed using the method described herein and further plated on top of the deposited silverlayer, with a more dense copperlayer.This gives better environmental resistance and better electrical conductivity for the plated layer. Moreover, a copper layer on a zink containing substrate may be sealed using the method described herein.

Before being subjected to the non-autocatalytic aqueous plating bath, the substrate may be prepared by cleaning or etching.

The deposited metal layer resulting from use of the pre-dip aqueous bath, although very thin, provides an improved barrier function, as it is virtually non-porous. This gives several important benefits:

Increased tarnish resistance

Reduction and/or elimination of microvoids Increased thickness uniformity

Possibility to reduce overall metal thickness such as silver thickness or reduction of overall thickness of a plated coating, such as silver coating It is well known that silver plated copper substrates are prone to tarnish, both from exposure to humidity and H₂S, if CuO is present on the surface of the substrate. The solubility of copper in silver is low, therefore migration of copper to the silver surface mainly follows pores which may be found at grain boundaries. This means that, if the silver layer is non-porous, it can act as a migration barrier for copper although being very thin. The pre-dip aqueous bath creates a surface covered by a sealing layer of silver, presumably by increasing the rate of nucleation and lateral growth of the silver deposit on the copper surface. The migration barrier therefore provides an increased tarnish resistance. The metal deposit, such as a silver deposit, from the pre-dip aqueous bath will also act as a barrier from the opposite direction, i. e. protecting the Cu-Ag interface from galvanic attacks from the subsequent autocatalytic silver plating, which decreases creation of microvoids under the autocatalytic silver deposition. The method described herein is advantageous from an economical as well as from an environmental point of view since it allows for decreasing the overall coating thickness such as the thickness of a silver coating as a result of the barrier function of the pre-dip aqueous bath. For instance, this applies to PCB^'s, i.e. printed circuit boards, as well as to microwave components.

The substrate subjected to the non-autocatalytic aqueous plating bath in the method described herein may comprise or consist of silicon, nickel, tin, copper or a copper alloy.

The metal salt of the non-autocatalytic aqueous plating bath in the method described herein may include one, two or more metal salts. The metal salt may be a silver salt and may be selected from the group consisting of AgN0₃, AgCN, AgCI0 , Ag₂S0₄,AgS0₄, Ag₂0,KAg(CN)₂, NaAg(CN)₂, Ag(C₂H₃0₂),Ag-succinimide complex or mixtures thereof.

The complexing agent of the non-autocatalytic aqueous plating bath in the method described herein may include one, two or more complexing agents. The complexing agent may be selected from the group consisting of EDTA, Rochelle^'s salt, citric acid, sodium citrate, succinic acid, proprionic acid, glycolic acid, sodium acetate, lactic acid, sodium pyrophosphate, pyridum-3-sulfonic acid, potassiumtartrate, Quadrol, sodium phosphate potassium citrate sodium borate, sodium cyanide, potassium cyanide,

triethylenetetraamine, methylamine and mixtures thereof.

Some complexing agents may also have some reductive capacity. Such weak reducing agents are not able to provide an autocatalytic electroless plating process as disclosed herein and do not fall within the meaning of the term "reducing agent" as used herein. Accordingly, the presence of a weak reducing agent is not sufficient to make an aqueous plating bath work in an autocatalytic way. By monitoring the plating process it can easily be determined if the reducing agent is a weak reducing agent or a strong reducing agent. If the plating process stops by itself, the reducing agent is a weak reducing agent.

However, if the plating process is self-propagating and continuous the reducing agent is a strong reducing agent. Generally, a strong reducing agent results in the formation of a thick and uniform metal layer on the substrate.

It will be appreciated that some complexing agents may also function as weak reducing agents. Thus, the complexing agents described herein include complexing agents that are non-reducing agents as well as complexing agents having a low reducing capacity.

Complexing agents that may function as reducing agents in an autocatalytic plating process are not included in the scope of the present invention.

The acid of the aqueous plating bath in the method described herein may include one, two or more acids. The acid may be a carboxylic acid such as, for instance, citric acid. In case the complexing agent is an acid, it may be the same or different from the complexing agent.

The surfactant of the non-autocatalytic aqueous plating bath in the method described herein may be a non-ionic surfactant. For instance, the surfactant may be nonylphenol oxylate.

The non-autocatalytic aqueous plating bath in the method described herein may further comprise a polymer based on oxyethylene, i.e. a polyalkylene oxide compound, such as polyethylene glycol (PEG). The polymer may have an average molecular weight from 100 to 4000.

The non-autocatalytic aqueous plating bath in the method described herein may further comprise boric acid.

The non-autocatalytic aqueous plating bath in the method described herein may be operated at a temperature from 15 to 95°C such as from 15 to 65°C. The pH of the aqueous plating bath in the method described herein may be between 4 and 12 such as between 5 and 8. It will be appreciated that between "4 and 12 such as between 5 and 8" is understood tobe equivalent to "from 4 to 12 such as from 5 to 8". Further, the pH may range from 5 to 10, from 8 to 10 or from 9 to 10. The pH may also be about 9.5. Preferred pH values, in particular for silver plating, include values from 8 to 10, from 9 to 10, about 6 and about 9.5.

The method described herein may provide for forming a metal deposit, a metal layer or a metal film on a substrate. The method described herein may further comprise an additional plating step. The additional plating step may be autocatalytic electroless plating, immersion plating or electroplating. The additional plating step takes place directly after the step of subjecting the substrate to the pre-dip aqueous bath as described herein, or after subsequent rinsing. The additional plating step such as autocatalytic electroless plating may involve plating of silver.

Due to the absence of or very limited substrate surface roughness of the substrate having been subjected to the non-autocatalytic aqueous plating bath in the method described herein subsequent additional plating steps are facilitated. For instance, the enhanced substrate reactivity-which may also be denominated as substrate activation- leads to faster overall processing. In addition, the improved substrate adhesion allows for making a thinner layer of metal than would otherwise be possible. For instance, a silver layer with a thickness from 0.03 to 0.15 micrometers may easily be formed using the method described herein. This is in contrast to the thick layers produced by galvanic methods wherein the layer thickness commonly is several micrometers. The method described herein is therefore advantageous from an economic as well as from an environmental point of view. In addition, the use of a pre-dip aqueous bath as described herein before the additional plating step such as autocatalytic metal plating provides an increased uniformity of the resulting overall metal thickness, as the autocatalytic reaction will start simultaneously over the metal plated substrate.

Thus, there is provided a method for plating a substrate with a metal comprising the steps of:

-mixing:

a metal salt,

a complexing agent,

a surfactant, and

an acid in water thereby forming a non-autocatalytic aqueous plating bath,

- subjecting the substrate to the non-autocatalytic aqueous plating bath, followed by -autocatalytic electroless plating.

There is also provided a method for plating a substrate with a metal comprising the steps of:

- subjecting the substrate to a non-autocatalytic aqueous plating bath obtainable by mixing a metal salt, a complexing agent, a surfactant, and an acid in water, followed by

- autocatalytic electroless plating.

The autocatalytic electroless plating may involve plating with silver. The autocatalytic electroless plating may be performed using a plating bath that is operated such that at least two phases are present in the bath. This is described in WO 2006/065221.

-mixing:

a metal salt,

a complexing agent,

a surfactant, and

an acid in water thereby forming a non-autocatalytic aqueous plating bath, - subjecting the substrate to the non-autocatalytic aqueous plating bath, followed by -immersion plating.

There is also provided a method for plating a substrate comprising the steps of:

- subjecting the substrate to the non-autocatalytic aqueous plating bath obtainable by mixing a metal salt, a complexing agent, a surfactant, and an acid in water, followed by

- immersion plating.

The immersion plating may involve plating with gold.

-mixing:

a metal salt,

a complexing agent,

a surfactant, and

an acid in water thereby forming a non-autocatalytic aqueous plating bath,

- subjecting the substrate to the non-autocatalytic aqueous plating bath, followed by

- electroplating.

- subjecting the substrate to the non-autocatalytic aqueous plating bath obtainable by mixing a metal salt, a complexing agent, a surfactant, and an acid in water, followed by - electroplating.

It has been found that substrates containing tin may conveniently be electroplated using the method described herein. The tin containing substrate may then be placed in the non- autocatalytic aqueous plating bath followed by electroplating in the same plating bath using electrodes.

It has surprisingly been found that the clusters, cluster like, or nuclei like particles of the precipitate resulting from the pre-dip aqueous bath also affect the subsequent plating step, such as autocatalytic plating, during the entire plating process. This phenomenon can be observed as a "self-healing" capability for the process, as it has the capability to cover defects and fill them with adherent silver, not only in the initial stage but during the entire plating process. This is important both for supressing microvoids in, for instance, a copper-silver interface in printed circuit boards applications and for covering difficult geometries where metal ion access is difficult for substrate parts such as electroless nickel plated components. On silicon, the cluster particles and/or nuclei particles from a silver pre-dip bath may subsequently additionally be autocatalytic silver plated and heat- treated for sintering and formation of a silver silicide with excellent adhesion and low contact resistance. A typical application for this is plating of silicon based solar cells. Thus, treatment of a substrate with an aqueous plating bath according to the method described herein will have a beneficial effect also on additional plating steps to which the substrate is subjected.

The method described herein may further include a heat treatment step. Since metal layers, in particular silver layers, are prone to tarnish it may be advisable to add an tarnish resistant layer on top of the metal layer. Compositions providing such tarnish resistant layers are known in the art. For instance, EVABRITE™ WST may be used. Accordingly, the method described herein may include a step of adding an tarnish resistant layer or film onto the metal layer formed in the method.

There is also provided an object coated in accordance with the method described herein. For instance, the object may be a microwave component, a printed circuit board, a medical device such as an electrode for electrocardiography, an antenna such as an antenna for mobile telephones, solar cells, catalytic beds or wafers. The object may consist of or comprise silicon, nickel, tin, copper or a copper alloy. There is also provided a silver silicide formed according to the method described herein.

There is also provided an non-autocatalytic aqueous plating bath. The non-autocatalytic aqueous plating bath may be prepared by mixing a metal salt, a complexing agent, a surfactant, and an acid in water.

The non-autocatalytic aqueous plating bath is also obtainable by mixing a metal salt, a complexing agent, a surfactant and an acid in water. As described herein, close visual inspection of the non-autocatalytic aqueous plating bath may reveal the presence of clusters, cluster like particles and/or nuclei like particles.

The metal salt of the non-autocatalytic aqueous plating bath may include one, two or more metal salts. The metal salt may be a silver salt selected from from the group consisting of AgN0₃, AgCN, AgCI0₄, Ag₂S0₄,AgS0₄, Ag₂0,KAg(CN)₂, NaAg(CN)₂, Ag(C₂H₃0₂) and Ag-succinimide complex.

The complexing agent of the aqueous plating bath may include one, two or more complexing agents. The complexing agent may be selected from from the group consisting of EDTA, Rochelle^'s salt, citric acid, sodium citrate, succinic acid, proprionic acid, glycolic acid, sodium acetate, lactic acid, sodium pyrophosphate, pyridum-3-sulfonic acid, potassiumtartrate, Quadrol, sodium phosphate potassium citrate sodium borate, sodium cyanide, potassium cyanide, triethylenetetraamine and methylamine.

The complexing agent may be as described herein.

The surfactant of the non-autocatalytic aqueous plating bath may include one, two or more surfactants. The surfactant may be a non-ionic surfactant. For example, the surfactant may be nonylphenol oxylate.

The non-autocatalytic aqueous plating bath may further comprise a polymer based on oxyethylene, i.e. a polyalkylene oxide compound, such as polyethylene glycol (PEG). The polymer may have an average molecular weight from 100 to 4000.

The non-autocatalytic aqueous plating bath may further comprise boric acid.

The non-autocatalytic aqueous plating bath may be operated at a temperature from 15 to 95°C such as from 15 to 65°C.

The pH of the non-autocatalytic aqueous plating bath may be between 4 and 12 such as between 5 and 8. It will be appreciated that between "4 and 12 such as between 5 and 8" is understood to include from "4 to 12 such as from 5 to 8". Further, the pH may range from 5 to 10, from 8 to 10 or from 9 to 10. The pH may also be about 9.5. Preferred pH values, in particular for silver plating, include values from 8 to 10, from 9 to 10, about 6 and about 9.5.

The acid of the non-autocatalytic aqueous plating bath may comprise one, two or more acids. The acid may be a carboxylic acid such as citric acid. In case the complexing agent is an acid, the acid may be the same or different from the complexing agent.

Further aspects

There is provided a non-autocatalytic method for plating a substrate with a metal using an aqueous plating bath, said bath comprising at least one metal, at least one complexing agent, at least one acid and at least one surfactant, wherein at least some of the metal is present in the form of clusters.

There is also provided a non-autocatalytic aqueous plating bath, said bath comprising at least one metal salt, at least one complexing agent, at least one acid and at least one surfactant wherein at least some of the metal salt is present in the form of clusters. The aqueous bath may further comprise at least one polyalkylene oxide compound. The aqueous plating bath may be used as a pre-dip aqueous bath meaning that the bath is used for providing a first coating or metal layer prior to adding an additional metal layer in an additional plating step. The pre-dip aqueous bath thus activates the surface of the substrate prior to the additional plating step. The pre-dip bath contains nucleis of metal clusters such as silver clusters with the capability to adsorb to the substrates and to provide an active surface for an additional plating step such as autocatalytic silver plating. Depending on type of base substrate, the pre-dip bath can combine cluster adsorption with displacement reactions.

The clusters or cluster particles may have a size of about 100 nanometers or less.

Further, there is provided a non-autocatalytic aqueous plating bath, said bath comprising at least one metal salt, at least one complexing agent, at least one acid and at least one surfactant wherein at least some of the metal salt is present in the form of clusters. The aqueous bath may further comprise at least one polyalkylene oxide compound. The aqueous plating bath may be used as a pre-dip aqueous bath meaning that the bath is used for providing a first coating or metal layer prior to adding an additional metal layer in an additional plating step. The pre-dip aqueous bath thus activates the surface of the substrate prior to the additional plating step. The pre-dip bath contains nucleis of metal clusters such as silver clusters with the capability to adsorb to the substrates and to provide an active surface for an additional plating step such as autocatalytic silver plating. Depending on type of base substrate, the pre-dip bath can combine cluster adsorption with displacement reactions.

The invention is illustrated, but not limited, by the following examples.

EXAMPLES

A silver predip bath was made up from silvercyanide and sodiumcyanide, resulting in a silver content of 3 g/l.

0.1 g/l nonylphenol oxylate

0.2 g/l PEG

pH was adjusted to 9 by addition of a carboxylic acid

Examples for the use of the silver predip is showed below: 1.

A flexible waveguide of beryllium copper was silver plated as below:

Acidic cleaner, ( Ronaclean PC 960 ), 40C, 5 min

Rinse

Microetch, ( Circuposit Etch 3330 ) 25 C, 3 min

Rinse

Silver predip, ( activation and barrier function ), rt, 20 min, pH 6.0

Rinse

Autocatalytic silver, ( ESM( Electroless Silver Mix ) 200, PK Plating ) 63 C, 30 min

Rinse

Examination in microscope at 40 x magnification verifies that the coverage was very good. The waveguide was heated in a oven 1 h at 260 C to check adhesion. Adhesion was very good.

2.

A copper plated PCB sample, with pad size from 0.8 to 8.0 mm, was silver plated as below: Acidic Cleaner, ( Ronaclean PC 960 ), 40C, 5 min

Rinse

Microetch, ( Circuposit Etch 3330 ) 25 C, 3 min

Rinse

Silver predip, ( activation and barrier function ), 3 C, 4Θ 4 min, pH §τ -9

Rinse

Autocatalytic silver, ( ES 100, PK Plating ) 3 C, 206 min, pH 10.3

Rinse

The silver thickness was examined by X-ray for all pads and the resulting silver thickness was found to be 14 - 2 m

3.

A zinc substrate was plated as below:

Activating, 1 % sulfuric acid

rinse

Electroplating copper cyanide strike until covered

rinse

Silver predip( pore sealing ), 40 C, 30 s, pH 9

rinse

Electroplating acid copper 2 μητι

rinse

Examination in microscope at 100 x magnification, shows no sign of pores.

4.

A microwave component made of aluminium was plated with nickel and silver as below:

Etching NaOH

Rinsing

Etching HN0₃

Rinsing

Zincate

Etching HN0₃

Rinsing

Zincate

Rinsing

Electroless nickel ( Durni-Coat DNC 471 ), 90C, 1 h 15 min, pH 4.6

Rinse

Silver predip, ( activation and barrier function ), rt, 4 min, pH 7.0

Rinse

Autocatalytic silver, ( ESM 200, PK Plating ) 63 C, 2 h, pH 10.3, Rinse

Examination after plating showed:

Excellent coverage and adhesion.

Nickel thickness 12 pm

Silver thickness 2.7 pm

Electrical testing showed lower losses, compared to same component plated with electrolytic silver. 5.

An Ag-silicon contact layer, for subsequent front side metallization of a crystalline silicon solar cell was performed as below:

ARC-opening by laser

Etching oxide ( HF ) 6 vol%, 2 min

Rinse

Ag-Seed ( Silver PreDip 2 min, 60 C, pH 6.6 )

Rinse

Autocatalytic silver barrier, ( ESM 300, PK Plating ) 5 min 60 C, pH 10.2

Rinse

Heat treatment, (Ag-silicide formation and Ag sintering ) 400 C, 10 min

After heat treatment, the silver layer showed excellent adhesion and could easily be further plated.

Claims

1. A method for plating a substrate with a metal, said method comprises the steps of:

- mixing:

a metal salt,

a complexing agent,

a surfactant, and

an acid in water thereby forming a n n-non-autocatalytic aqueous plating bath, and

- subjecting the substrate to the non-autocatalytic aqueous plating bath.

2. A method according to claim 1 , wherein the non-autocatalytic aqueous plating bath is catalytic and comprises rod like clusters.

3. A method according to claim 1 or 2, characterized in that the substrate

comprises or consists of silicon, nickel, tin, copper or a copper alloy.

4. A method according to any one of the previous claims, characterized in that the metal salt is a silver salt.

5. A method according to claim 4, characterized in that the silver salt is selected from the group consisting of AgN0₃, AgCN, AgCI0₄, Ag₂S0₄,AgS0₄,

Ag₂0,KAg(CN)₂, NaAg(CN)₂, Ag(C₂H₃0₂) and Ag-succinimide complex.

6. A method according any one of the previous claims, characterized in that the complexing agent is selected from the group consisting of EDTA, Rochelle's salt, citric acid, sodium citrate, succinic acid, proprionic acid, glycolic acid, sodium acetate, lactic acid, sodium pyrophosphate, pyridum-3-sulfonic acid,

potassiumtartrate, Quadrol, sodium phosphate potassium citrate sodium borate, sodium cyanide, potassium cyanide, triethylenetetraamine and methylamine.

7. A method according any one of the previous claims, characterized in that the acid is a carboxylic acid.

8. A method according any one of the previous claims, characterized in that the surfactant is a non-ionic surfactant.

9. A method according any one of the previous claims, characterized in that the non-autocatalytic aqueous plating bath further comprises a polyalkylene oxide compound.

10. A method according to any one of the previous claims, characterized in that the non-autocatalytic aqueous plating bath further comprises boric acid.

11. A method according any one of the previous claims, characterized in that the plating bath is operated at a temperature from 15 to 95°C.

12. A method according any one of the previous claims, characterized in that the pH of the plating bath ranges from 4 to 12, from 5 to 10 or from 9 to 10.

13. A method according any one of the previous claims, characterized in that the method further comprises an additional plating step selected from the group consisting of autocatalytic electroless plating, immersion plating or electroplating..

14. A method according to claim 13, characterized in that the additional plating step is autocatalytic electroless plating.

15. A method according to claim 14, characterized in that the additional plating step is autocatalytic electroless plating with silver.

16. A method according any one of the previous claims, characterized in that the method further comprises a heat treatment step.

17. A method according to any one of the previous claims comprising sealing of the substrate.

18. An object coated according to the method as defined in any one of claims 1 to 17.

19. An non-autocatalytic aqueous plating bath for plating a substrate with a metal, said non-autocatalytic aqueous plating bath being obtainable by mixing a metal salt, a complexing agent, a surfactant and an acid in water.

20. An non-autocatalytic aqueous plating bath according to claim 19, characterized in that the metal salt is a silver salt selected from the group consisting of AgN0₃, AgCN, AgCI0₄, Ag₂S0₄,AgS0₄, Ag₂0,KAg(CN)₂, NaAg(CN)₂, Ag(C₂H₃0₂) and Ag- succinimide complex.

21. An non-autocatalytic aqueous plating bath according to claim 19 or 20,

characterized in that the complexing agent is selected from the group consisting of EDTA, Rochelle^'s salt, citric acid, sodium citrate, succinic acid, proprionic acid, glycolic acid, sodium acetate, lactic acid, sodium pyrophosphate, pyridum-3- sulfonic acid, potassiumtartrate, Quadrol, sodium phosphate potassium citrate sodium borate, sodium cyanide, potassium cyanide, triethylenetetraamine and methylamine.

22. An non-autocatalytic aqueous plating bath according to any one of claims 19 to 21 , characterized in that the acid is a carboxylic acid.

23. An non-autocatalytic aqueous plating bath according to any one of claims 19 to 22, characterized in that the surfactant is a non-ionic surfactant.

24. An non-autocatalytic aqueous plating bath according to any one of claims 19 to 23 further comprising polyalkylene oxide compound.

25. A non-autocatalytic aqueous plating bath according to any one of claims 19 to 24 further comprising boric acid.

26. A non-autocatalytic aqueous plating bath according to any one of claims 19 to 25, characterized in that the pH of the non-autocatalytic aqueous plating bath from 4 to 12, from 5 to 10 or from 9 to 10.