WO1996011751A1

WO1996011751A1 - Noble metal coating method by immersion

Info

Publication number: WO1996011751A1
Application number: PCT/US1995/013162
Authority: WO
Inventors: Rudolf P. Sedlak
Original assignee: Rd Chemical Company
Priority date: 1994-10-17
Filing date: 1995-10-16
Publication date: 1996-04-25
Also published as: AU3955595A; TW293985B

Abstract

The invention features a composition for immersion plating comprising an aqueous solution of a noble metal salt and a complexing agent for ions of the noble metal, said solution having a pH of from about 0 to 5.5. The noble metal is preferably palladium in the form of palladium nitrate. The complexing agent is preferably oxalic acid. The composition is used in a method of immersion plating which comprises contacting a substrate which is preferably a copper or copper alloy component of a printed circuit board with the composition for a period of time sufficient to coat palladium onto the copper surface in a uniform manner. The methodology provides for a high degree of uniformity, good adhesion between the palladium and the copper as well as a relatively thin coating. Further, the palladium coating is carried out in a relatively short period of time and provides a longlived solderable finish.

Description

NOBLE METAL COATING METHOD BY IMMERSION

Field of the Invention

The present invention relates to a composition, a method for immersion coating of noble metals onto circuit boards and to coated circuit boards. More particularly, the invention relates to a method of creating a solderable palladium layer upon the surface of the circuit board using immersion coating with a precisely defined aqueous solution.

Background of the Invention

Most printed circuit boards (PCBs) manufactured through 1985 had electroplated tin/lead on the top of the circuitry. This tin/lead coating was ideally 63% tin and 37% lead, although the metal composition varied widely, even on the same printed circuit board, due to manufacturing variations. The tin/lead coating served two purposes: 1) to facilitate the manufacturing process; and 2) to preserve the solderability of the PCB. Most of the PCB surface was covered with a clear impervious coating, called a "Solder Mask." The holes and pads, the areas used to electrically connect the devices on the PCB, remained exposed, i.e. free of the solder mask, to allow connection of the active components (e.g., semiconductors). These areas must retain their solderability, since the bare printed circuit board was manufactured well in advance of the time that the (transistor or semiconductor) components were soldered to the board. Loss of solderability renders the PCB useless.

Later, as the dimensions of the circuitry became smaller, the tin/lead coating beneath the solder mask, and covering most of the circuitry, created a problem. The tin/lead coating melted during the process of attaching the components, ran, and electrically shorted the circuitry. To address this problem, the Solder-Mask- Over-fiare-Copper (SMOBC) board was developed. Production of SMOBC boards involves stripping the electroplated tin/lead from the board prior to application of the solder mask. However, the SMOBC board required an additional process to cover the exposed pads with a solderable coating. The process of Hot A.ir Leveling (HAL) of solder was developed to solve this problem.

After application of the solder mask, the PCB was dipped into molten solder, and excess molten solder blown off the PCB surface with hot air. This process coated only the exposed copper holes and connector pads. PCBs produced by this method did not short circuit during attachment of components.

In contrast to the SMOBC board, the more modern PCB is used to interconnect devices (semiconductors) that are surface mounted rather than attached by pins inserted in the holes in the board. Surface mounted connections require a very flat surface for proper electrical connection. Because the height of solder left on the connector pads by the HAL process varies, boards produced by this process are not sufficiently flat for high speed, high quality, surface mounting. Therefore, there is a clear need in the field for a new type of finish which provides a sufficiently flat surface. The industry has produced some new finishes which are made by including various organic tarnish inhibitors, using nickel followed by gold plating, and using selective tin/lead plating

(electrolytic or immersion) . Each of these methods have some failing in that they are either too expensive, too complex, unreliable, and/or provide insufficient protection for an extended shelf life. It is known that electroplated palladium provides an acceptable finish for connectors. However, the surfaces to be coated on a PCB are not electrically connected, making the use of electroplating impractical. Thus, there is a need for an acceptable, non-electrolytic method of plating upon the exposed copper of PCBs.

Two types of "wet" technology may be used to achieve non-electrolytic plating: electroless plating and immersion plating. These techniques differ in the source of the electrons (reducing agent) used to reduce the metal in the plating solution to the elemental state. Electroless plating uses a component of the plating solution as the reducing agent. Immersion plating uses the substrate metal (such as copper in the case of printed circuit boards) as the reducing agent, causing some of the substrate to dissolve into the plating bath. Immersion plating is much preferred as there is less tendency to "poison" the plating system (i.e., stop plating before depletion of metal from the plating bath solution) or to plate improperly (e.g., plating on surfaces upon which plating is not desired) where it is not wanted.

The use of palladium as a catalyst for electroless plating is well known in the literature (see for example U.S. Patent numbers 3,650,913; 4,182,784; and 4,717,421). In such uses, palladium is usually applied in a colloidal form. Electroless plating using palladium as a catalyst is carried out in the presence of stannous and stannic chlorides. Such palladium-catalyst systems are the basis for the standard method of initiating electroless copper plating and is a key process for interconnecting the layers of most PCBs. However, these conventional palladium-catalyst systems do not plate selectively upon an inorganic substrate in the presence of an organic substrate. For example, attempts to provide a solderable finish upon the exposed copper substrate of a PCB by this electroless plating methods results in deposition of metal over the entire PCB surface, including the organic polymer supporting the copper foil. This renders the PCB useless.

Canestaro, U.S. Patent No. 4,431,685 discloses immersion coating of a noble metal such as palladium onto a copper layer using an acidic aqueous solution containing about 0.1 to 0.2 gm/1 of PdCl₂ or AuCl₂ and about 0.5% HC1. Zeller, U.S. Patent No. 4,770,899 discloses a method of coating copper conductors on polyamide with a corrosion resistant metal such as cobalt including an intermediate treatment with a solution of palladium chloride. However, the prior art systems have some disadvantages. For example, they may plate slowly, have a very low maximum plating thickness, produce porous to semi-porous layers (and thus must be deposited relatively thickly in order to provide a layer which is impermeable to corrosives) , or cease plating well before the system has run out of metal, especially when the substrate is copper. Moreover, prior art systems for providing an electroless, precious metal coating are expensive, use baths which must be replaced periodically (i.e. are not permanent baths) , and produce significant amounts of hazardous waste, including solutions which contain chelated heavy metals.

SUMMARY OF THE INVENTION A composition for immersion plating a solderable noble metal layer (preferably palladium) onto a metal substrate (e.g. copper of a printed circuit board) is disclosed. The composition is an aqueous solution which comprises a noble metal salt, one or more complexing agents for ions of the noble metal, and a pH of from about 0 to 5.5. The composition of the invention is formulated so that contacting the aqueous solution with a metal substrate (which is composed of a metal other than a noble metal) , results in the selective reduction of the noble metal in the aqueous solution onto the metal substrate.

Use of the above composition for immersion plating enables faster plating rates, better uniformity, greater ultimate thickness, better conformity of the solderable surface (i.e. the noble metal) to the substrate, greater selectivity (i.e. the noble metal (e.g. palladium) will plate only onto the substrate metal (e.g. copper) and not onto a non-metal layer supporting the substrate metal) , and consequently better protection of solderability. Moreover, the method of the subject invention provides a long-lived, solderable finish which, compared to the solderable finish produced by known methods (e.g., HAL), is essentially congruent to the substrate, i.e., the solderable noble metal layer introduces no irregularities upon the PCB surface.

Another aspect of the invention features a method of immersion plating which comprises contacting a substrate with an immersion plating composition comprising an aqueous solution of a noble metal salt and a complexing agent for the noble metal. Preferably, the pH of the aqueous solution is from about 0 to 5.5.

A further aspect of the invention features a printed circuit board (PCB) which has a substrate metal layer (e.g. copper) , a non-metal substrate (e.g. an organic, polymeric material) which supports the substrate metal, and a noble metal coating (e.g. palladium) selectively plated on the substrate metal layer. The PCB is produced by contacting the substrate metal with an immersion plating composition comprising an aqueous solution of a noble metal salt and a complexing agent for the noble metal. Preferably, the aqueous solution has a pH of from about 0 to 5.5. The preferred substrate metal is copper or a copper alloy. The preferred noble metal is palladium. By "immersion plating" is meant a method for plating a first metal in an aqueous plating bath upon a second metal present on a substrate, where the substrate metal acts as the reducing agent for the first metal present in the plating bath. The process requires that the first and second metals be different. Preferably, the first metal is palladium and the second metal is copper or an alloy thereof.

By "noble metal" is meant an elemental metal selected from the group palladium, platinum, gold, iridium, and rhodium.

By "complexing agent" is meant a compound which coordinates (i.e. complexes) with a metal ion present in a solution in the form of a complex or a complex ion. "Complexing agents" as used herein is meant to include, but are not limited to, compounds recognized in the art as chelating agents. Chelating agents are multidentate ligands which attach to a central metal by more than one coordinating atom. Where the noble metal is preferably palladium, the preferred complexing agents are oxalic acid, glycolic acid, and citric acid, with oxalic acid being most preferred.

An important object of the invention is to provide a method for immersion plating of a solderable palladium coating onto copper contacts of a printed circuit board. An advantage of the invention is that the methodology provides a fast and efficient method for coating palladium onto the copper contacts of printed circuit boards. Another advantage of the invention is that the methodology is environmentally friendly in that substantial amounts of toxic waste are not created.

A feature of the invention is that the palladium coating is created in a substantially thinner layer than the layer created by other coating methodologies, (e.g., one-third the thickness or less than that provided by conventional methodology) . Moreover, the thinner layer produced by the invention is not as porous as layers created by other methods, and is impermeable to corrosives, thus maintaining solderability.

Another advantage is that the coating conforms closely to the outer surface of the substrate metal (e.g., copper) layer being coated. Yet another advantage is that the coating layer maintains the solderability of the copper contacts on a printed circuit board over a substantial period of time. These and other objects, advantages and features of the present invention will become apparent to those skilled in the art upon reading this disclosure.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS It must be noted that as used herein and in the appended claims, the singular forms "a," "and," and "the" include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to "an immersion plating composition" or "an immersion plating bath composition" includes mixtures of different compositions and reference to "the method of immersion plating" includes reference to equivalent steps and methods known to those skilled in the art, and so forth. Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. Although any methods and materials similar or equivalent to those described herein can be used in the practice or testing of the invention, the preferred methods and materials are now described. All publications and patents mentioned herein are incorporated herein by reference to describe and disclose specific information for which the reference was cited in connection with.

The present invention features compositions and methods for immersion plating, as well as products (e.g. a plated PCB) produced with the disclosed compositions and methods. The immersion plating composition comprises an aqueous solution of a noble metal salt and a complexing agent for ions of the noble metal. Preferably, the solution has a pH of from about 0 to 5.5. The noble metal is preferably palladium, more preferably in the form of palladium nitrate. The noble metal complexing agent is preferably oxalic acid, particularly where the noble metal is palladium. The preferred substrate metal is copper or a copper alloy.

The composition is used in a method of immersion plating which comprises contacting a substrate metal, preferably a copper or copper alloy component of a printed circuit board, with the composition for a period of time sufficient to coat palladium onto the substrate metal surface in a uniform, smooth, flat manner. The methodology provides for a high degree of uniformity, good adhesion between the palladium and the copper as well as a relatively thin coating. Further, the palladium coating is carried out in a relatively short period of time.

The invention will now be described in further detail. For simplicity, the "first metal" or noble metal illustrated is palladium and the "second metal" or substrate metal is a copper foil positioned on a polymeric base which forms a part of a printed circuit board (PCB) .

The noble metal employed in the composition of the present invention is a metal different from the metal of the metallic substrate. Examples of some suitable noble metals are palladium, platinum, gold, iridium, and rhodium. Preferably, the noble metal is platinum. More preferably, palladium is particularly preferred.

The immersion plating composition and method of the invention can be used to coat any substrate which is sufficiently electropositive to reduce the noble metal in solution to free metal. The preferred substrate is copper and/or a copper alloy, such as bronze, brass, ALLOY 42™ (iron-nickel alloy), or KOVAR™ (iron-nickel- cobalt alloy) .

Most preferably, the substrate metal surface is for use in integrated printed circuit boards or carriers. The metal (copper) substrate is supported by a polymeric material. Suitable polymeric support materials include both thermoplastic and thermosetting polymers. Typical thermosetting polymeric materials to which the copper foil is laminated include epoxy, phenolic based materials, polyamides and polyimides. The dielectric materials may be molded articles of the polymers containing fillers and/or reinforcing agents such as glass-filled epoxy or phenolic based materials. Examples of some phenolic type materials include copolymers of phenol, resorcinol, and cresol. Examples of some suitable thermoplastic polymeric materials include polyolefins such as polypropylene, polysulfones, polycarbonates, nitrile rubbers, and ABS polymers. Methods for bonding of the substrate metal such as copper or an alloy thereof to a polymeric support is well-known in the art. Immersion palladium plating baths are known (Canestaro, USPN 4,431,685) and consist of little more than palladium chloride dissolved in HC1. However these baths do not produce a layer of palladium of sufficient thickness, particularly where the substrate is copper. The coating produced by the method of Canestaro was so thin that the color of the copper substrate could be seen beneath the palladium coating. Such a thin palladium coating would not be acceptable to facilitate attachment of components to the circuit board.

The use of immersion plating compositions containing a "complexed" noble metal system results in significantly better quality noble metal plating, particularly in terms of uniformity, thickness, and selectivity for adhesion to the substrate. The noble metal coating of the present invention conforms to the substrate metal such that the noble metal coating does not extend beyond a point on a given surface of the substrate metal more than an average height of about 4.0 to 6.0 millionths of an inch, preferably not more than about 3.0 millionths of an inch, more preferably not more than about 2.5 millionths of an inch.

The noble metal complexing agent is selected according to the desired resultant reduction potential between the complexed noble metal and the substrate metal to be oxidized. The pH of the plating bath composition should also be taken into consideration when selecting the complexing agent. Preferably, the complexing agent will be selected to decrease the availability of the noble metal in the aqueous plating bath solution for reduction by the substrate, thus insuring that the noble metal plates only upon the metal substrate, and not upon the organic substrate due to the presence of any random reduction functionality remaining in the organic substrate. The complexing agent, however, should not associate with the noble metal so strongly that reduction of the noble metal is completely prevented and plating ceases.

Selection of a complexing agent for the noble metal is particularly important where, for example, the PCB has two or more substrates, only one of which is to be plated with the noble metal. Preferably, the complexing agent is selected so as to adjust the reduction potential between the noble metal ion and the substrate to be plated such that plating of the noble metal upon an organic functionality of the PCB is avoided. For example, where the PCB has an organic substrate supporting a copper substrate, the noble metal complexing agent is selected so that the difference in the reduction potential between the noble metal and the organic substrate is lower than the difference in the reduction potential between the noble metal and the copper substrate, so that the noble metal only plates on the copper substrate. Preferred noble metal complexing agents have a pKa approximately equal to or less than the operating pH of the bath, such that the complexing agent functions as a pH buffer as well as a complexing agent. The pKa of the complexing agent should also be selected such that the final plating bath composition comprises a supply of non-protonated complexing agent available to bind the noble metal ions. For example, if the pKa of the complexing agent is too high, (i.e., the acid is too weak) , all or most of the complexing sites on the complexing agent will be occupied by hydrogen ions, and thus will be unable to effectively chelate the noble metal in the plating bath solution.

As discussed above, immersion plating uses the metal substrate as the reducing agent. During plating, some of the metal substrate is dissolved in the plating bath solution. It is important to the operability of the present invention that the noble metal complexing agent, or other components which may act as complexing agents present in the plating bath composition (e.g. chloride ions) , do not form stable, soluble complexes with substrate metal ions which represent a partially oxidized state of the substrate metal. In contrast, association of the complexing agent with substrate metal ions which represent a fully oxidized state of the substrate metal may be desirable, as complexing of such substrate metal ions effectively makes the substrate more electropositive (i.e. the potential of the metal substrate is reduced) , and can serve to increase the selectivity of the noble metal plating upon the substrate metal. For example, where the noble metal is palladium and the metal substrate is copper, copper ions accumulate in the plating bath by the following reactions:

Cu° (substrate) + Pd⁺⁺ (in solution)—> Cu⁺⁺ Cu°(substrate) + Cu⁺⁺—> 2Cu⁺ 2Cu° (substrate) + Pd⁺⁺—> 2Cu⁺

The cuprous ion (Cu⁺) represents a partially oxidized state of the copper substrate, while the cupric ion (Cu⁺⁺) represents the fully oxidized state of the copper substrate. Thus, in this example, while it is desirable for the complexing agent to bind the cupric ion, it is undesirable for the complexing agent to bind to the cuprous ion. Binding of the complexing agent to the cupric ion will serve to drive the plating reaction forward and increase plating selectivity. In contrast, binding of the complexing agent with the cuprous ion

(such that the cuprous ion is chelated and/or stabilized in the aqueous plating bath solution) will eventually result in cessation of the plating process and, as a result, production of an unacceptably thin noble metal coating upon the metal substrate. Examples of suitable complexing agents for the noble metal include oxalic acid, glycolic (hydroxy acetic acid) , and citric acid, preferably oxalic acid and glycolic acid, and more preferably oxalic acid. Where palladium is the noble metal and copper is contained in the substrate to be plated, the preferred noble metal complexing agent is oxalic acid.

The amount of complexing agent in the aqueous solution can vary widely. For example from about 0.1 to 10 molar excess of that needed to complex the noble metal, preferably from about 3 to 4 molar excess is present. More preferably, the complexing agent for the noble metal is present in a 1:1 molar ratio of complexing agent to noble metal ion. At a minimum, the amount of complexing agent present in the composition is an amount sufficient to complex essentially all noble metal ions present in the plating composition.

The plating composition of the invention has a pH of from about 0 to 5.5, preferably from about 1.6 to about 3.6 more preferably from about 2.0 ± 0.50, even more preferably from about 2.0 ± 0.25. The preferred pH range depends in part on the complexing agents present in the composition and is readily determined by routine experimentation. Any water-soluble noble metal salt can be used in the immersion plating composition of the invention. Non- limiting examples of such salts include palladium nitrate, platinum nitrate, gold nitrate and the like. Preferably, the noble metal salt is palladium nitrate. The amount of soluble noble metal salt in the plating composition can vary up to saturation. Preferably, the amount of soluble noble metal salt is from about 0.001 M to about saturation, more preferably from about 0.005 M to about 0.04 M, and most preferably from about 0.02 M to about 0.025 M. In general, the amount of noble metal ion present in the plating composition is preferably the amount of noble metal ion necessary to provide for an acceptably thick, acceptably uniform noble metal coating upon the metal substrate within a commercially acceptable amount of time and at temperatures which are readily obtainable and commercially feasible.

The immersion plating composition of the invention can contain optional ingredients conventionally known in the art, such as dyes, surfactants (e.g. FLORAD™, DOWFAX 2A1™) , chelating agents, brighteners, leveling agents and the like. Where a surfactant is employed, the surfactant is preferably an anionic or amphoteric surfactant (e.g. DOWFAX 2A1™ and FLORAD™, respectively) , and may be present in the immersion plating bath at various concentrations ranging, for example, from about 10 ppm to 100 ppm, and preferably at concentrations below the critical micelle concentration (CMC) of the surfactant to minimize co-deposition of the surfactant with the noble metal to be plated. To carry out the method of the invention, a printed circuit board (PCB) comprising a polymeric material supporting copper foil may be first cleaned in a mild etchant comprising a persulfate in dilute sulfuric acid, and rinsed thoroughly. The cleaned board is then immersed immediately into a plating bath comprising an aqueous solution of a soluble noble metal salt and a complexing agent for the noble metal. The solution must have a pH of from about 0 to 5.5, preferably 2.0 ± 0.25. The PCB is immersed in the bath for a time sufficient to achieve the desired degree of noble metal thickness, usually from about 1 minute to about 10 minutes, preferably about 2 minutes to about 5 minutes, more preferably about 3 minutes. In general, the amount of time required to accomplish an acceptably thick, acceptably uniform noble metal coating may vary substantially, but will generally be an amount of time that is feasible commercially, such as to allow for satisfactory production levels. However, the PCB may be left in the immersion bath for hours without adversely affecting the quality of the coating of noble metal. The temperature of the immersion bath may be from about 70^*F to about 170^*F, preferably about 115^*F to 125^*F, more preferably about 120^*F. However, the immersion plating is may be conducted at various temperatures, including ambient temperatures of room temperature to 80^*C and normal pressures. Higher and lower temperatures and pressures can be used, and will generally be selected so as to provide for a plating process that is completed within an commercially acceptable amount of time and provides an acceptably thick and uniform noble metal coating upon the metal substrate.

The resulting noble metal (palladium) coating is from about 1 millionth of an inch (thickness) to about 5 millionths of an inch, preferably from about 2.0 millionths of an inch to about 3.0 millionths of an inch, normally about 2.5 millionths of an inch, as measured by X-ray fluorescence.

The noble metal coating produced by the subject method is flat and conforms to the contour of the substrate metal (copper) surface, such that the noble metal coating does not deviate from any given plane of the metal substrate more than 4.0 to 6.0 millionths of an inch, preferably not more than 3.0 millionths of an inch, more preferably not more than 2.5 millionths of an inch. In contrast, the solderable finish produced by HAL, which adds 2 to 3 orders of magnitude of roughness to the surface of the metal substrate, the method of the present invention adds essentially no roughness to the substrate metal surface. Moreover, the palladium coating produced by the method of the present invention provides a long- lived solderable finish which remains solderable even after multiple passes (e.g., three or more) through a soldering oven or steam aging of the noble metal-coated substrate. The present method provides for good wetting of the solder upon the PCB and the attached component (i.e., a good bond between the solder, the substrate metal surface, and the component) .

Use of immersion plating compositions as a permanent immersion plating bath The plating composition of the subject invention can also provide a permanent bath for immersion plating of a noble metal coating onto a substrate metal surface. A permanent bath allows for multiple rounds of noble metal plating onto a substrate metal surface within the same plating bath solution. It may be necessary to filter out substrate metal precipitates from the bath either continuously or between plating reactions. Otherwise, the plating bath composition may be used until all or nearly all of the noble metal in the plating bath composition is depleted from the bath.

For example, where the noble metal is palladium, the substrate metal is copper, and the chelating agent for the noble metal is oxalic acid, the copper which dissolves from the substrate metal into the plating bath composition will precipitate as copper oxalate at approximately 100 ppm to 500 ppm copper. Precipitation of the copper at this level allows for filtration of the copper from the plating bath and continuous use of the plating bath composition in immersion plating.

Use of noble metal-coated substrate metals as a catalyst in conventional electroless plating

The noble metal-coated substrate metal produced using the method of the present invention may be used directly in soldering components onto the PCB. Alternatively, the noble metal-coated substrate metal produced by the subject method may be used to underlay a second electroless metal coating (e.g. , nickel, palladium, gold) . In this latter application, the noble metal plated upon the metal substrate serves as a catalyst for the subsequent immersion plating process.

Conventional electroless deposition of a metal on a substrate may require pre-treatment or sensitization of the substrate to render it catalytic to the reception of such a deposit. Various metals have been found to exhibit such catalytic properties which are useful in chemical plating including the precious metals gold, silver, members of the platinum family, and certain less precious metals including cobalt, nickel, copper, and iron. Palladium is generally the most satisfactory of these catalyst metals for the activation of non- conductive substrates. This applies both to substrates such as PCBs where the plated deposit is intended to afford conductive paths in electronic circuits, as well as to plastic articles whose surfaces are to be covered with a metal finish for decorative or protective purposes. The catalytic compositions may be prepared as an aqueous bath, a colloidal suspension of the catalyst which is applied just prior to immersion of the substrate into the electroless plating bath, or as a solid surface which is fixed upon the surface of the substrate just prior to immersion in the electroless plating bath.

One class of catalysts used in preparation of a substrate for electroless plating employs tin and palladium in combination. Conventional stabilized tin/palladium baths have been prepared by using either the chloride salt or bromide salt of palladium and/or tin dissolved in HC1 to produce the working bath. Preparation of such palladium catalysts is described in US Pat. No. 4,717,421, US Pat. No. 3,650,913, and US Pat. No. 4,182,784, incorporated herein by reference to disclose and describe such catalysts and their use in coating PCBs. For example, the palladium catalyst can be prepared by making a mixture of stannous halide, an alkali metal halide and water, reacting this mixture with a palladium halide salt at an elevated temperature, adding a stannous salt of an organic carboxylic acid to the reaction mixture, and continuing the heating of the mixture until the reaction is complete.

An example of a conventional electroless plating method using a palladium/tin catalyst involves: l) immersing the substrate in a low concentration bath containing the tin/palladium catalyst; 2) accelerating the activated/sensitized surface with an acid dip;

3) rinsing the substrate in deionized water; and

4) immersing the substrate in an electroless nickel bath containing nickel ions, hypophosphite, stabilizers, and buffering agents. Alternatively, the tin/palladium catalyst may be prepared as a colloidal suspension which is applied to the substrate to be plated just prior immersion in the electroless plating bath, as described in US Pat. No.s 3,650,913 and 4,182,784.

The noble metal-plated substrate produced by the method of the subject invention may be used in place of conventional tin/palladium catalysts. For example, the palladium-coated copper substrate produced by the subject invention may be used in place of steps 1) - 4) outlined immediately above, and thus may be directly immersed in an electroless plating bath for deposition of a selected metal ion (e.g. nickel, copper) upon the palladium-coated surface. The use of a noble metal-coated substrate in the conventional electroless plating methods described above is advantageous in that the noble-metal coating is long-lived, and thus need not be used in electroless plating immediately after preparation. Moreover, the noble metal-coated substrate can provide for even, smooth deposition of the electroless metal (e.g. copper, nickel) upon the substrate. Exemplary materials which may be produced according to this embodiment of the invention (e.g. selective immersion plating of palladium upon a metal substrate, followed by conventional electroless nickel or copper plating) include PCBs and plastic- or ceramic-packaged semiconductors. Suitable electroless plating baths are described in US Pat. No.s 3,095,309 and 2,532,283.

EXAMPLES The following examples are put forth so as to provide those of ordinary skill in the art with a complete disclosure and description of how to carry out the synthesis of the invention and is not intended to limit the scope of what the inventors regard as their invention. Efforts have been made to ensure accuracy with respect to numbers used (e.g. , amounts, temperature, etc.), but some experimental error and deviation should be accounted for. Unless indicated otherwise, parts are parts by weight, molecular weight is weight average molecular weight, temperature in degrees Centigrade and pressure is at or near atmospheric.

Example 1 (Comparative Example)

Conventional immersion plating baths consist of little more than palladium chloride dissolved in HCl (USPN 4,31,685). However, when such solutions are used to immersion plate copper, the palladium deposits on the substrate are unacceptably thin, such that the copper color of the substrate can be seen through the palladium coating.

Example 2

In an attempt to solve the problem of Example 1, the pH of the solution was increased to about 4.0 by the addition of ammonia. Extra ammonium chloride was added to stabilize the bath from hydrolysis and to prevent precipitation of the palladium. This more basic solution produced a uniformly thicker palladium coating which completely covered the bare copper substrate than the method of Example 1. However, the palladium coating was not acceptably dense or uniform.

Example 3

A surfactant was added to the solution of Example 2 to provide an more dense, more uniform palladium coating. Only soluble anionic surfactants proved acceptable as other type of surfactants (cationic, nonionic) stopped plating. However, when the palladium chloride/ammonium chloride bath of Example 2 or Example 3 was used to coat the copper substrate on a PCB, the palladium plated onto the organic substrate of the PCB as well as the copper, likely due to the presence of reducing entities within the organic substrate. This non-selective plating caused the circuitry to short out, rendering the PCB useless.

Example 4

Complexing agents for palladium were added to the immersion plating bath of Example 3 in order to raise the reduction potential required to cause the palladium to plate out of solution. This approach capitalized upon the differential in the potential between the copper and the organic substrate of the PCB, causing the palladium to plate only on the copper substrate. The complexing agent tried was ethylenediamine tetraacetic acid (EDTA) . EDTA proved to be too strong of a palladium chelator and stopped all plating.

Example 5

Quadrol (1,1' ,1" ,1'' '-(ethylenedinitrilo)tetra-2- propanol) (BASF) , a less strong analog of EDTA, was substituted for EDTA in the plating composition of Example 4. This achieved the desired effect of selective plating of the palladium upon the copper substrate. However, another undesirable effect resulted. The plating was covered with blue dots, which were presumably a copper salt or residue. (As discussed above, immersion plating can result in the dissolution of metal ions from the substrate.)

Example 6

The procedure of Example 5 was repeated with the addition of citric acid to the plating composition. Citric acid acts as chelating agent for copper, provided a solution to this problem, i.e. the blue dots did not appear on the plating. However, chelation and/or stabilization of the cuprous ion present in aqueous plating bath solution (as a result of dissolution of the substrate metal in the bath) poisoned the plating process, resulting in an unacceptably thin palladium coating.

Example 7

In view of the unacceptable results of the examples above, an alternate plating bath composition was prepared. This plating bath was composed of palladium nitrate (Pd(N0₃)₂) and oxalic acid, a complexing agent for the palladium ion. The pH of the plating bath composition was adjusted by addition of ammonium hydroxide (NH₄OH) to a pH of about 2.0. This plating bath composition produced a uniform, acceptably thick palladium coating on the copper substrate, without plating onto the organic substrate of the PCB or producing undesirable precipitates on the PCB surface. Addition of a surfactant to the plating bath composition served to improve the density and uniformity of the plating coating on the copper metal substrate, although the addition of surfactant proved non-essential.

Example 8

In general the amounts of the reagents present in the bath are not critical, as long as there is a sufficient amount to achieve the desired results. The pH of the bath is critical to proper plating. A detailed example of the successful plating of a PCB is described below.

A printed circuit board (PCB) comprising an epoxy material supporting copper foil was cleaned by dipping into RD-84, a mild etchant comprising a persulfate in dilute (3%) sulfuric acid, for about 1-5 minutes at room temperature. The PCB was rinsed thoroughly with running tap water, bath rinsed in tap water, and sprayed rinsed with deionized (D.I.) water. The PCB was then immersed into the warm (120"F) immersion plating bath for 6 minutes. The immersion plating bath was composed of:

0.887 g/liter Pd(N0₃)₂

0.8 g/liter NH₄N0₃ 0.017 g/liter FC99 (also known as FLORAD™ (3M) ; an anionic, fluorocarbon-based surfactant)

0.672 g/liter oxalic acid (The pH of the above solution was adjusted to 2.0 with NH₄OH.) The PCB was removed from the plating bath, rinsed in tap water and then D.I. water, and blown dry. This resulted in a palladium coating of 2.5 millionths (μ) inches (thickness) . This palladium coating is approximately 3 orders of magnitude thinner than conventional solder coatings of the copper substrate produced by, for example, using HAL.

Example 9

A printed circuit board (PCB) similar to the PCB of Example 8 is cleaned by dipping into a mild etchant comprising a persulfate in dilute (3%) sulfuric acid, for about 1 minute (1 minute to about 1 minute, 15 seconds) at room temperature. The PCB is rinsed thoroughly with running tap water, bath rinsed in tap water, and sprayed rinsed with deionized (D.I.) water. The PCB is then immersed into the warm (120^*F) immersion plating bath for 6 minutes. The immersion plating bath is composed of:

0.4 - 1.3 g/liter Pd(N0₃)₂

0.4 - 1.2 g/liter NH₄N0₃ 0.008 - 0.026 g/liter FC99 (an anionic surfactant)

0.336 - 1.01 g/liter oxalic acid (The pH of the above solution is adjusted to 2.0 with NH₄0H. )

The PCB is removed from the plating bath, rinsed in tap water and then D.I. water, and blown dry.

Example 10

A printed circuit board (PCB) similar to the PCB of Example 8 is cleaned by dipping into a mild etchant comprising a persulfate in dilute (3%) sulfuric acid, for about 1 minute (1 minute to about 1 minute, 15 seconds) at room temperature. The PCB is rinsed thoroughly with running tap water, bath rinsed in tap water, and sprayed rinsed with deionized (D.I.) water. The PCB is then immersed into the warm (120"F) immersion plating bath for 6 minutes. The immersion plating bath is composed of:

0.4 - 1.3 g/liter Pd(N0₃)₂ 0.4 - 1.2 g/liter NH₄N0₃

0.008 - 0.026 g/liter FC99 (an anionic surfactant)

0.336 - 2.0 g/liter glycolic acid (The pH of the above solution is adjusted to 2.0 with NH₄OH.)

Example 11

A printed circuit board (PCB) similar to the PCB of Example 8 is cleaned by dipping into a mild etchant comprising a persulfate in dilute (3%) sulfuric acid, for about 1 minute (1 minute to about 1 minute, 15 seconds) at room temperature. The PCB is rinsed thoroughly with running tap water, bath rinsed in tap water, and sprayed rinsed with deionized (D.I.) water. The PCB is then immersed into the warm (120^*F) immersion plating bath for 6 minutes. The immersion plating bath is composed of: 0.4 - 1.3 g/liter Pt(N0₃)₂ 0.4 - 1.2 g/liter NH₄N0₃

0.008 - 0.026 g/liter FC99 (an anionic surfactant) 0.336 - 1.01 g/liter oxalic acid

(The pH of the above solution is adjusted to 2.0 with NH₄0H.)

The PCB is removed from the plating bath, rinsed in tap water and then D.I. water, and blown dry. Example 12

A printed circuit board (PCB) comprising an epoxy material supporting a foil made of ALLOY 42™ (42% nickel, 58% iron) is cleaned by dipping the PCB into an appropriate etchant solution, for about l minute (1 minute to about 1 minute, 15 seconds) at room temperature. The PCB is rinsed thoroughly with running tap water, bath rinsed in tap water, and sprayed rinsed with deionized (D.I.) water. The PCB is then immersed into the warm (120^*F) immersion plating bath for 6 minutes. The immersion plating bath is composed of:

0.4 - 1.3 g/liter Pd(N0₃)₂

0.4 - 1.2 g/liter NH₄N0₃

(The pH of the above solution is adjusted to 2.0 with NH₄OH. )

Example 13

A substrate composed of copper foil is cleaned by dipping into RD-84, a mild etchant comprising a persulfate in dilute (3%) sulfuric acid, for about 1-5 minutes at room temperature. The PCB is rinsed thoroughly with running tap water, bath rinsed in tap water, and sprayed rinsed with deionized (D.I.) water. The PCB is then immersed into the warm (120^*F) immersion plating bath for 6 minutes. The immersion plating bath is composed of:

0.4 - 1.3 g/liter Pd(N0₃)₂ 0.4 - 1.2 g/liter NH₄N0₃ 0.008 - 0.026 g/liter FC99 (an anionic, fluorocarbon-based surfactant) 0.336 - 1.01 g/liter oxalic acid (The pH of the above solution is adjusted to 2.0 with NH₄OH.)

The palladium-plated copper is removed from the plating bath, rinsed in tap water and then D.I. water, and blown dry. The palladium-coated copper is then immersed in a conventional electroless plating bath, described in US Pat. No. 2,532,283, which contains: 3 parts by weight NiCl₂ 1 part by weight NaH₂P0₃.H₂0 0.9 part by weight NaC₂H₂0₂

These components were dissolved in water and the pH adjusted to 4 - 6 units.

Following procedures similar to those described above, other noble metals can be plated onto a selected metal substrate.

The invention now being fully described, it will be apparent to one of ordinary skill in the art that many changes and modifications can be made thereto without departing from the spirit or scope of the appended claims.

What is claimed is:

Claims

1. A composition for immersion plating a solderable noble metal layer onto a substrate which is sufficiently electropositive to reduce the metal in the composition, said composition comprising an aqueous solution of a noble metal salt, and a complexing agent for said noble metal, said solution having a pH of from about 0 to 5.5.

2. The composition of claim 1 wherein the noble metal is palladium, and the complexing agent is selected from the group consisting of oxalic acid, glycolic acid, and citric acid.

3. The composition of claim 1 wherein the complexing agent is oxalic acid the pH is from about 1.0 to about 3.0 and the noble metal salt is palladium nitrate.

4. The composition of claim 1 wherein the complexing agent is present in an amount of about 1:1 to about 10:1 molar excess of that required to complex with the noble metal in the composition.

5. The composition of claim 3 wherein the pH is about 2.0 ± 0.25 and the composition further comprises an anionic surfactant.

6. A method of immersion plating which comprises contacting a metal substrate with an immersion plating composition comprising an aqueous solution of a noble metal salt and a complexing agent for the noble metal, said solution having a pH of from about 0 to 5.5.

7. The method of claim 6 wherein the noble metal is palladium, and the substrate is a copper or copper alloy surface of a printed circuit board wherein the board is comprised of a non-metal and further wherein the metal substrate is cleaned with a mild etchant before immersion plating.

8. The method of claim 6 wherein the complexing agent is selected from the group consisting of oxalic acid, glycolic acid, and citric acid.

9. The method of claim 6 wherein the complexing agent is oxalic acid the pH is from about 1.6 to about 3.6 and the noble metal salt is palladium nitrate.

10. The method of claim 9 wherein the complexing agent is present in an amount of about 1:1 to about 10:1 molar excess of that required to complex with the noble metal in the composition.

11. The method of claim 10 wherein the pH is about 2.0 ± 0.25.

12. A printed circuit board having a noble metal coating selectively plated on a substrate metal supported by said board, said noble metal coating produced by a method comprising: contacting the substrate metal with an immersion plating composition comprising an aqueous solution of a noble metal salt and a complexing agent for the noble metal, said solution having a pH of from about 0 to 5.5.

13. The printed circuit board of claim 12 wherein the noble metal is palladium and the substrate metal is copper or a copper alloy.

14. The printed circuit board of claim 12, wherein the noble metal coating conforms to the substrate metal such that the noble metal coating does not extend beyond a point on a given surface of the substrate metal more than an average height of about 4.0 to 6.0 of millionths of an inch.

15. The printed circuit board of claim 12, wherein the noble metal coating conforms to the substrate metal such that the noble metal coating does not extend beyond a point on a given surface of the substrate metal more than an average height of about 2.0 millionths of an inch.

16. The printed circuit board of claim 12 wherein the noble metal coating is from about 1 to 5 millionths of an inch in thickness.

17. The printed circuit board of claim 16 wherein the noble metal coating is from about 1 to 2 millionths of an inch in thickness.

18. A method of electroless plating comprising: contacting a substrate metal having a palladium-coated surface produced by the method of claim 6 with an electroless plating bath composition comprising an aqueous solution of a metal ion, wherein said palladium in the palladium-coated surface functions as a catalyst for electroless plating of the metal ion onto said palladium-coated substrate metal.

19. The method of claim 18, wherein said metal ion is selected from the group consisting of nickel, palladium, and gold.