An electroless silver plating bath and method of use is presented within. The electroless silver plating bath is designed to plate only on the desired metal substrate while preventing plating on areas other than those which are to be plated. The invention uses heavy metal based stabilizers in the electroless silver plating bath to prevent extraneous plating. The ability to control the amount of stabilizer present in the plating bath allows for elimination of extraneous plating and allows for a stable bath. The electroless silver plating bath is very stable and yet plates at an acceptable rate. The electroless silver plating bath prevents corrosion on the underlying metal that is plated on by using the stabilizers as described herein. The silver plating bath presented herein is useful for a wide variety of applications including those in electronic packaging, integrated circuits (IC) and in manufacturing of light emitting diodes (LEDs).

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An electroless silver plating composition comprising: a. A source of silver ions; b. A stabilizer comprising a heavy metal ion; c. A surfactant; d. A complexor; e. And a buffer.2. A composition according to claim 1, wherein the stabilizer consists of a heavy metal ion selected from Group IIIA metals, Group IVA metals, Group VA metals, Group VIA metals, the lanthanide series, and combinations thereof.3.

A composition according to claim 2, wherein the stabilizer is selected from the group consisting of lead chloride, lead acetate, lead lactate, lead citrate, bismuth citrate, tin sulfate, thallium nitrate, telluric acid, antimony chloride, potassium antimony (III) tartarate, lanthanum (III) nitrate, europium (III) nitrate, indium (III) nitrate, seleneous acid, sodium selenite, and combinations thereof.4. A composition according to claim 3, wherein the stabilizer or combinations thereof, comprises lead.5. A composition according to claim 3, wherein the stabilizer or combinations thereof, comprises bismuth.6. A composition according to claim 3, wherein the stabilizer or combinations thereof, comprises antimony.7.

A composition according to claim 1, wherein the concentration of the heavy metal ion in the electroless silver composition is from 0.1 mg/L to 1,000 mg/L.8. A composition according to claim 7, wherein the concentration of the heavy metal ion in the electroless silver composition is from 0.5 mg/L to 100 mg/L.9. A composition according to claim 8, wherein the concentration of the heavy metal ion in the electroless silver composition is from 1 mg/L to 10 mg/L.10. A composition according to claim 1, wherein the pH is between 9 and 11.11. A composition according to claim 10, wherein the pH is between 9.5 and 10.5.12. A composition according to claim 11, wherein the pH is between 10.1 and 10.4.13. A method of plating electroless silver over a metal barrier layer comprising the steps: a) optionally preparing a copper or copper alloy for plating; b) plating a copper or copper alloy with a metal barrier layer using electroless nickel or electroless cobalt, c) optionally providing an immersion silver strike layer over the metal barrier layer; and then d) plating electroless silver using an electroless silver composition over the metal barrier layer and optional strike layer; wherein the electroless silver composition contains a stabilizer that comprises heavy metal ions.14.

The method according to claim 13, wherein the stabilizer consists of a heavy metal ion selected from Group IIIA metals, Group IVA metals, Group VA metals, Group VIA metals, the lanthanide series, and combinations thereof.15. The method according to claim 14, wherein the stabilizer is selected from the group consisting of lead chloride, lead acetate, lead lactate, lead citrate, bismuth citrate, tin sulfate, thallium nitrate, telluric acid, antimony chloride, potassium antimony (III) tartarate, lanthanum (III) nitrate, europium (III) nitrate, indium (III) nitrate, seleneous acid, sodium selenite, and combinations thereof.16. The method according to claim 15, wherein the stabilizer or combinations thereof, comprises lead.17. The method according to claim 15, wherein the stabilizer or combinations thereof, comprises bismuth.18. The method according to claim 15, wherein the stabilizer or combinations thereof, comprises antimony.19. The method according to claim 13, wherein the concentration of the heavy metal ion in the electroless silver composition is from 0.1 mg/L to 1,000 mg/L.20.

The method according to claim 19, wherein the concentration of the heavy metal ion in the electroless silver composition is from 0.5 mg/L to 100 mg/L.21. The method according to claim 20, wherein the concentration of the heavy metal ion in the electroless silver composition is from 1 mg/L to 10 mg/L.22. The method according to claim 13, wherein the barrier layer metal is nickel.23. The method according to claim 13, wherein the electroless silver composition does not cause extraneous plating.24. The method according to claim 13, wherein the electroless silver plating rate increases linearly with immersion time.25.

The method according to claim 13, wherein the electroless silver plating composition has a pH between 9 and 11.26. The method according to claim 25, wherein the electroless silver plating composition has a pH between 9.5 and 10.5.27.

The method according to claim 26, wherein the electroless silver plating composition has a pH between 10.1 and 10.4.28. FIELD OF THE INVENTIONThe present invention relates generally to an electroless silver plating composition that is both stable and prevents extraneous plating. The invention uses heavy metal based stabilizers which are both measurable and controllable in solution. The process of plating on a substrate using the invention described herein substantially prevents plating in areas other than the metal surface where plating is desired. This invention provides for an autocatalytic reaction, opposed to the galvanic reaction that typically occurs between silver and the metal to be plated upon BACKGROUND OF THE INVENTIONThere are several well-known methods for the plating of metals, such as electroplating, immersion plating and autocatalytic electroless plating. Among all the plating methods, autocatalytic electroless plating has the capability to plate a substantially uniform metallic coating onto a substrate having an irregular shape. Electroless coatings are also virtually nonporous, which allows for greater corrosion resistance than electroplated plated or immersion plated substrates.

Thus, electroless plating methods are widely used in the printed circuit board (PCB), integrated circuit (IC), and light emitting diode (LED) industries. Most common plating methods involve electroless nickel plating, electroless copper plating, and electroless gold plating.Plating a copper or copper alloy surface with electroless nickel followed by immersion gold (ENIG) is an industry standard that typically produces a reliable deposit that is useful in various applications. While ENIG has proven to be very reliable, it is not without issues. The gold plating step can be excessively corrosive to the nickel deposit causing deterioration of the nickel at the grain boundaries which compromises the integrity of the deposit. The gold plating step is additionally very expensive in comparison to other plating steps. Electroless silver plating has become a desirable alternative to plating immersion gold over nickel plated surfaces due to cost restraints and a desire to reduce potential corrosion at the grain boundaries found on the nickel surface. Black line nickel is a well-known issue within the industry when the traditional coating of ENIG is employed.

Not only is silver an economically responsible choice over gold but a silver bath formulation that plates completely by an electroless mechanism (not by immersion/exchange reaction) is much less corrosive to the underlying metal surface. The electroless silver plated surface is additionally useful in applications such as LEDs where surface reflectivity is important.Electroless silver plating is a well-known process. However, application of electroless silver plating in industries such as PCB, IC, and LED manufacturing is limited due to several fundamental issues of the process. Some of the issues are:a) The plating baths tend to spontaneously decompose forming silver particles throughout the solution.

This decomposition causes loosely adherent, very fine silver metal particles on the deposit and short bath life.b) An immersion reaction occurs during plating due to the difference of reduction potential between Ag and the metal substrates, such as copper and nickel. This reaction causes severe metal substrate corrosion (see FIG.

1), which brings problems such as adhesion loss, poor solderability, and wirebond failures.c) Undesirable extraneous Ag plating is a widely experienced problem (See FIG. It is well known that electroless silver process has the tendency for the silver to plate not only on the desired metal substrate lines and pads, but also to deposit on portions of the dielectric substrate or insulator located between lines and pads.

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This problem is especially pronounced when dealing with very fine lines that are only separated by very small intervals, resulting bridging and short circuiting. Controlling the undesirable extraneous plating is a significant problem.Although electroless silver technology is well known, electroless silver plating has not become a widely used commercial technology due to issues mentioned above.There have been attempts to cure the problem of extraneous plating while maintaining a stable silver plating bath as set out in U.S. 5,322,553, US Patent Application 20 A1, and International Publication WO 2006/065221 A1. These patents are hereby incorporated by reference in their entirety.In U.S.

5,322,553 the inventors found that a thiosulfate salt in combination with a sulfite salt in an electroless silver plating solution allowed for a uniform deposit. By using this redox system there is no need for any additional type of reducing agent and the bath does not contain ammonia or cyanide ions. While the inventors show that a reasonable silver deposit may be plated over nickel, the bath is said to be sensitive to silver concentration such that if the concentration is out of the desired range then the bath is difficult to control and uncontrolled plating may occur in the container holding the solution or on areas of the substrate where plating is not desired.

This is likely due to use of sulfur stabilization which is difficult to control and makes the plating solution very sensitive and often times unstable.In U.S. 20 A1, the inventors have used an immersion silver plating bath over electroless nickel to increase solderability in electronics packaging applications. While the method is useful, the silver thickness is limited due to the immersion type reaction and corrosion of the nickel surface is still an issue if the article to be plated is left in the plating solution for extended time.In International Publication WO 2006/065221 A1 describes an electroless silver plating bath which must operate with two phases present to deposit a uniform silver deposit. The bath is stabilized based on a multi-phase process using non-ionic surfactants where the bath is operated above the cloud point of the surfactants. While the process of this invention results in a desirable deposit, bath control is critical. The bath should be kept warm to prevent decomposition and unwanted deposition, which is not practical when considering commercially viable options.

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