Electrolysis: processes – compositions used therein – and methods – Electrolytic coating – Displacement or replacement coating
Reexamination Certificate
2000-10-03
2002-04-23
Valentine, Donald R. (Department: 1741)
Electrolysis: processes, compositions used therein, and methods
Electrolytic coating
Displacement or replacement coating
C205S220000, C205S223000, C205S263000, C228S209000, C427S343000
Reexamination Certificate
active
06375822
ABSTRACT:
FIELD OF THE INVENTION
This invention relates generally to a method of treating a surface which treatment enhances the solderability of the surface. The method is particularly useful in the fabrication and assembly of printed circuit boards.
BACKGROUND OF THE INVENTION
Soldering is generally used for making mechanical, electromechanical, or electronic connections to a variety of articles. The distinction between expected functions of the joints is important because each application has its own specific requirements for surface preparation. Of the three soldering applications, making electronic connections is the most demanding.
In the manufacture of electronic equipment utilizing printed circuits, connections of electronic components to the printed circuits are made by soldering of the leads of the components to the through-holes, surrounding pads, lands and other points of connection (collectively, “Areas of Connection”). Typically the connection occurs by wave soldering techniques.
To facilitate this soldering operation, the printed circuit fabricator is required to arrange that the through-holes, pads, lands and other points of connection are receptive to the subsequent soldering processes. Thus these surfaces must be readily wettable by the solder and permit an integral conductive connection with the leads or surfaces of the electronic components. Because of these needs, printed circuit fabricators have devised various methods of preserving and enhancing the solderability of surfaces.
One means of arranging good solderability of the surfaces in question is to provide the surfaces with a pre-coating of solder. In printed circuit fabrication, however, this method has several drawbacks. Since it is not easy to selectively provide these areas with solder, all conductive areas of the board must be solder plated. This can cause severe problems with the subsequent application of solder mask.
Another means of arranging good solderability of these surfaces is to plate them with a final finish coating of a precious metal such as gold, palladium or rhodium. U.S. Pat. No. 5,235,139 (Bengston, et. al.), the teachings of which are incorporated herein by reference, proposes a method for achieving this previous metal final finish. Bengston et al. proposes plating the copper areas to be soldered with electroless nickel-boron, followed by a precious metal coating such as gold. See also U.S. Pat. No. 4,940,181 to Juskey, Jr. et al., the teachings of which are incorporated herein by reference for a similar process which teaches the plating of electroless copper, followed by electrolytic copper, followed by nickel followed by gold as a solderable surface. These processes work well but are time consuming and expensive.
Various attempts have been made to selectively apply solder to the necessary areas only. One such method involves use of organic etch resists over the solder plated areas of connection followed by selective stripping of tin-lead from the copper traces before application of the solder mask. See U.S. Pat. No. 4,978,423 to Durnwith et. al. See also U.S. Pat. No. 5,160,579 to Larson, the teachings of which are incorporated herein by reference, for other known selective solder processes.
Soldering directly to copper surfaces has been difficult and inconsistent. These problems are due mainly to the inability of keeping the copper surfaces clean and free of oxidation throughout the soldering operation. Various organic treatments have been developed to preserve copper surfaces in a readily solderable state. For example, see U.S. Pat. No. 5,173,130 (Kinoshita) which teaches the use of certain 2-alkylbenzimidazoles as copper pre-fluxes to preserve the solderability of the copper surfaces. Treatments such as those taught in Kinoshita have proven successful but there is still need to improve the reliability of the process.
U.S. Pat. No. 5,935,640, the teachings of which are incorporated herein by reference in their entirety teaches a process for improving the solderability of a surface through the use of immersion silver deposits. The immersion silver plating bath used there comprises an imidazole. Generally silver plated surfaces can be stored for some period of time, ranging from about one to sixty days depending upon the storing conditions. As the silver plated surface ages, its solderability tends to get worse.
The present invention proposes a process and plating composition, which improves the storability and solderability of silver plated surfaces. Thus surfaces plated with the composition and process of this invention may be stored for extended periods of time while retaining good solderability characteristics.
SUMMARY OF INVENTION
The present invention proposes a composition and process for treating a surface, particularly a copper or copper alloy surface, in order to enhance and/or preserve the solderability of that surface through immnersion silver plating upon that surface. Specifically the invention proposes plating silver upon the surface through use of an immersion silver plating composition. The surface is thereafter separately contacted with a solution comprising an organic heterocyclic mercapto compound, and optionally but preferably an alkali metal hydroxide. In the alternative, but less preferably, the organic heterocyclic mercapto compound may be incorporated into the silver plating composition, if solubility permits.
DETAILED DESCRIPTION OF THE INVENTION
It has been discovered that immersion silver deposits provide excellent solderability finishes, which are particularly useful in the fabrication of printed circuit boards. It has further been discovered that the long term solderability of silver deposits can be improved by subsequent treatment of the plated silver surface with a composition comprising an organic heterocyclic mercapto compound, and optionally but preferably an alkali metal hydroxide or less preferably by inclusion of the heterocyclic mercapto compound into the silver plating solution, solubility permitting.
Thus the present invention proposes in one embodiment a silver plating solution comprising an organic heterocyclic mercapto compound. In this regard the silver plating solution would typically have the following composition:
a. a soluble source of silver ions;
b. an acid;
c. an organic heterocyclic mercapto compound;
d. optionally, an imidazole;
e. optionally, an oxidant;
f. water
This embodiment may be restricted by the solubility of the organic heterocyclic mercapto compound in the silver plating solution and therefore is less preferable to including the organic heterocyclic mercapto compound in a subsequent treatment step as noted below. In any case the silver plating solution of this invention is preferably an immersion plating solution. Immersion plating is a process which results from a replacement reaction whereby the surface being plated dissolves into solution and at the same time the metal being plated deposits from the plating solution onto the surface. The immersion plating generally initiates without prior activation of the surfaces. The metal to be plated is generally more noble than the surface metal. Thus immersion plating is usually significantly easier to control and significantly more cost effective than electroless plating, which requires sophisticated auto catalytic plating solutions and processes for activation of the surfaces prior to plating.
In the most preferred embodiment of the invention, the original surface is plated using either the silver plating solution noted in the first embodiment (i.e. a silver plating solution comprising an organic heterocyclic mercapto compound) or most preferably a known silver plating solution (i.e. a silver plating solution not comprising an organic heterocyclic mercapto compound). Such known silver plating solutions include MacDermid Sterling™ silver plating solution available from MacDermid, Incorporated 245 Freight Street, Waterbury, Conn. 06702. In either case the then plated silver surface is further contacted with a solution comprising an organic heterocyclic mercapto compound and option
Carmody & Torrance LLP
Leader William T.
Valentine Donald R.
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