Coating processes – Measuring – testing – or indicating
Reexamination Certificate
2001-04-21
2003-02-25
Barr, Michael (Department: 1762)
Coating processes
Measuring, testing, or indicating
C427S443100
Reexamination Certificate
active
06524642
ABSTRACT:
FIELD OF THE INVENTION
The invention relates to a process for the electroless metal-plating of a substrate in a plating bath containing an aging composition comprising by-products generated in the course of a plating cycle.
BACKGROUND OF THE INVENTION
Electroless metal-plating of substrates is well known Typically, the substrate is a material such as stainless steel, aluminum, nonconductive surface, etc. The plating metal is typically nickel, boron, cobalt, alloys of nickel or cobalt, copper or alloys of copper.
Fabricators of electroless metal-plated substrates have found that there is a desirable age to be established for electroless metal-plating baths, depending on the particular deposit properties produced by electroless plating. The desirable age of an electroless nickel plating bath is measured in plating cycles, and for the purpose of the discussion which follows, it is assumed that one plating cycle is, in the case of the plating metal being nickel, equal to 6 g/l nickel consumed and replenished. For example, for the plating of aluminum memory disks, the desirable bath age is in the range of about 2 to about 5 plating cycles, for job shop applications, the desirable bath age is in the range of about 2 to about 10 plating cycles.
When a fresh electroless metal-plating bath is initially prepared, such bath has experienced no plating cycles and therefore is undesirable for the electroless metal-plating of a substrate wherein the fabricator desires that the substrate be electrolessly metal-plated in a bath that has experienced at least 0.5 plating cycles. When using a fresh bath, the fabricator's costs are increased because the initial substrates which are electrolessly metal-plated in the fresh bath are unsatisfactory. This results in increased production time, extra costs for the electroless metal-plating bath components as well as for additional costs for waste treatment
In the case of electroless metal-plating, the present methods typically involve three different modes of chemical usage:
Bath MakeUp: wherein a metal source, e.g., nickel, a reducing agent, e.g., sodium hypophosphite monohydrate, a chelating agent, e.g., malic acid, a pH adjuster, e.g., sodium hydroxide, and a stabilizer, e.g., lead acetate trihydrate, are combined in water to make-up a plating bath.
Steady State Plating In Respect To Only Consumables: wherein the plated objects are periodically removed from the plating bath and consumables are replenished whenever their concentrations decrease to predetermined threshold levels. In the course of the plating operation, reaction by-products, in addition to the adjunct ions of the consumable ions, accumulate in the bath.
Steady State Plating In Respect To All Ingredients: wherein the plated objects are periodically removed from the plating bath and consumables are replenished whenever their concentration decreases to a predetermined threshold level. In addition, “bleed and feed” operations are concurrently carried out such that a volume of the bath is continuously or intermittently withdrawn from the bath in relation to the level of build-up of by-products beyond a desired maximum concentration in the bath and the ingredients of the Bath Make-up that were withdrawn from the bath are continuously or intermittently added to the bath.
In order to accommodate the three modes of chemical usage, a minimum of three liquid components is required, i.e., bath make-up solution, consumables' replenishment solution and pH adjusting solution, e.g., 28 wt. % ammonium hydroxide. In practice, the foregoing three liquid components are usually further subdivided such that (a) imbalances incurred during plating operations can be offset by small adjustments with such subdivided components and (b) solubility problems inherent in highly concentrated components may be avoided by separating certain ions between components. Accordingly, current commercial practice involves the following:
Bath Make-Up Solution: (1) nickel sulfate component, and (2) reducing agent, chelating agent, pH adjuster and stabilizer component Consumables Replenishment Solution: (1) nickel sulfate component, and (2) reducing agent and stabilizer component pH Adjusting Solution: e.g., 28 wt. % ammonium hydroxide.
From a commercial point of view, it is desirable to bypass mode 2 and proceed directly from mode 1 to mode 3. If such bypass can be achieved, not only does the bath operate at steady state conditions from start to finish with respect to all principal consumables and by-products, but the resultant Ni-P deposits on the plated substrates also exhibit steady state characteristics in respect to their physical properties. To accomplish this goal, a fourth liquid component is required which would be added during the Bath Make-Up mode and supplied in lieu of that volume of water used in the Bath Make-Up that the fourth liquid component would displace. Such fourth liquid component comprises an aging composition which is described in greater detail below.
Objects of the Invention It has been found that for many applications, the most satisfactory electrolessly metal-plated substrates are those which have been plated in a bath which has been “aged”, i.e., prepared in a manner so as to contain byproducts which are present in the bath after the bath has experienced a particular bath age, measured in plating cycles.
It is an object of the present invention to provide an aging composition which, when incorporated in the fresh electroless metal-plating bath, will establish a bath having the desired bath age, for the particular substrate and metal in question, such that when electroless metal-plating operations are conducted, there will be no wasted plated substrates, i.e., plated objects not meeting desired physical and chemical specifications. In such an “aged” bath, all the parts will be plated with the utmost consistency from the start to the completion of the plating operation.
It is a further object of the present invention to provide a process whereby substrates may be continuously electrolessly metal-plated in a bath having the desired bath age without any significant waste.
It has been found that in the case of the electroless nickel metal-plating of aluminum memory disks, the process of the present invention results in disks which: (1) are extremely smooth such that polishing pads suffer less wear in post-plating polishing operations; and (2) have “ski-jumps” at the outer diameters of the disks rather than “roll-offs”; in this regard, it should be noted that a polishing machine can readily polish a “ski-jump” down to a level surface, whereas the polishing down of the entire surface of the surface of the disk to level it off to that of the “roll-off” is practically impossible.
The above objects will become apparent from the detailed description of the invention which follows.
SUMMARY OF THE INVENTION
By way of summary, the invention encompasses a process for the electroless metal-plating of a substrate in an “aged” metal-plating bath using “bleed and feed” features described below. The plating bath will contain metal-plating ions, one or more reducing agents, one or more complexing agents, one or more stabilizers and one or more pH adjusters. Prior to commencement of plating operations, an aging composition is added to the bath. Such aging composition comprises reaction by-products, generated and accumulated in the course of a plating cycle, and will be present in the bath in a concentration corresponding to the concentration of the by-products present in the bath at a desired bath age.
Once plating operations have commenced, the aged bath is subjected to “bleed and feed” operations. Such “bleed and feed” operations are well known in the prior art and involve the continuous or intermittent discharge of a predetermined volume of the bath and the continuous or intermittent replenishment of the bath with one or more replenishing solutions comprising a source of the metal-plating ions, the reducing agents, the complexing agents, the stabilizers and the pH adjusters. The “bleed and feed” operations are
Andre Robert C.
Du Jerry G.
Leibman Dolly
Mistry Nilesh
Ruffini Alan J.
Barr Michael
Kalow & Springut LLP
OMG Fidelity, Inc.
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