Method for Plating Conductive Plastics

FIG. 1 is a flow chart showing the steps of the preferred embodiment of the invention.

The method of the present invention may be used to great advantage to plate on essentially any conductive plastic, including graphite-reinforced composites. (In this description, the term "conductive plastic" is intended to include both any plastic which is itself conductive and any composite containing a conductive material.) One type of such composites is graphite-reinforced epoxy composites. The following detailed description of the preferred embodiment of the method of the invention is specifically directed toward plating on graphite-reinforced epoxy composites, but it is of course to be understood that the method of the invention may be applied to other types of conductive plastics without departing from the spirit and scope of the invention as defined in the claims.

If the conductive plastic component to be plated is oily or greasy, the first step in the plating process is to clean the surfaces of the component, or at least the surfaces which are to be plated or which must be protected from the plating solution, to remove any surface oil or grease. The cleaning process is carried out using a solvent or solution which does not appreciably attack epoxy. Preferably, the initial cleaning process is carried out with a hot alkaline cleaning solution. The solution may be applied manually or, if the configuration or size of the component make manual cleaning difficult, the solution may be flowed or pumped over the surfaces to be cleaned. The flowed or pumped solution may be recycled. Following the initial cleaning, areas which are not to be plated and which may be exposed to plating solution during the plating process should be masked. Any of a large number of known masking processes may be used. These include using a masking tape and painting, spraying, or dipping the maskant onto the component.

After carrying out the initial cleaning and masking steps or after determining that these intitial steps are unnecessary, the area to be plated is abraded by abrasive blasting to produce suitable mechanical bonding sites. The abrasive blasting may be carried out using a variety of materials, including 180 to 240 grit aluminum oxide at 60 to 100 pounds per square inch gauge as necessary to remove the surface epoxy layer. During the abrasive blasting, care must be taken to avoid excessive abrasive blasting which could reduce the gauge of the composite material. In most cases, when the area to be plated has been machined through the weave layers, the step of abrasive blasting is omitted since suitable mechanical bonding sites are already present. When a type of conductive plastic other than a graphite-reinforced epoxy composite is being plated, the step of abrasive blasting may be omitted if the area to be plated includes suitable mechanical bonding sites without the blasting.

After the initial steps of cleaning, masking, and abrading have been carried out or determined to be unnecessary, the area to be plated is cleaned with an alkaline cleaning solution as described above in connection with the initial cleaning process. The area is then sensitized to provide a base for firm adhesion of the metal to be electrolessly plated onto the plastic. When the plastic being plated is a graphite-reinforced epoxy composite, the step of sensitizing is preferably carried out as follows.

The area to be plated is wetted with a dilute solution of hydrochloric acid by flowing said solution over the area. This dilute solution is preferably about 9% hydrochloric acid. The entire area is kept wet with the dilute solution for about 2 to 4 minutes. At the end of this period, without allowing the area to dry, a catalyst is flowed over the area, preferably a palladium chloride catalyst. An example of a suitable catalyst is a solution containing the palladium catalyst sold under the name of Cuposit 9F by the Shipley Company, Inc., of Newton, Mass. The area is kept wet with the catalyst solution for about 2 to 4 minutes. Then the area is rinsed twice with cold water, with each rinse being carried out for approximately 2 minutes. The rinsed area is then wetted with a stannous accelerator for about 2 to 4 minutes by flowing the accelerator over the area. An example of a suitable accelerator solution is a solution of about 17% of the accelerator sold under the name Accelerator 19 by the Shipley Company, Inc. After application of the accelerator, the area is rinsed with cold water for about 2 to 3 minutes. The area is kept wet until the next step is initiated. During the sensitizing process, each wetting and each rinsing of the area to be plated may be carried out by flowing the liquid used for wetting or rinsing over the area to be plated, and each liquid may be recycled to reduce the cost of the operation.

An important feature of the method of the invention is the flowing of the catalyst solution and the accelerator solution at a very low velocity to ensure effective and even action on the entire area to be plated. Each of the solutions is flowed extremely slowly over the area at a velocity sufficiently low to allow the solution to act on the area effectively and evenly. The slow flowing of the catalyst solution allows the catalyst to adhere evenly to the surface of the area. Experiments in which the catalyst solution was sprayed or sponged on the area resulted in spotty rather than smooth and even plating on the area. Flowing the solutions at a very slow rate ensures that sufficient solution is provided to act on the entire area and allows the chemicals in each solution to act evenly on the area without simply being washed away by the solution itself. In addition, flowing, as opposed to spraying or sponging, ensures that there is constant and even contact between the solution and the area throughout the entire period of time during which the solution is being applied. The result is the very high quality plated surface that is a significant advantage of the present invention.

In the preferred embodiment of the invention, each solution that is applied to the area to be plated is recycled to help minimize the cost of the process. This is carried out by positioning a container of the solution below the area to be plated. The solution is pumped out of the container at a very low rate as required to provide the proper flow velocity. The solution is pumped to a location generally above the area to be plated and allowed to flow over the area at the desired velocity and back down into the container. This recycling procedure allows maximum use to be obtained from a given quantity of solution. Crucial concentrations of various chemicals in the solution are monitored to insure that they remain within acceptable ranges, and additional chemicals are added as needed. When the solution in the container can no longer be used, it may be sent to another location for processing to recover valuable chemicals.

In the preferred embodiment, the catalyst solution is prepared as follows. A mixture is prepared including 25% water, 50% dilute hydrochloric acid (35 to 37%), and 25% Cuposit 9F catalyst. The solution is maintained at a pH of about 7. The palladium concentration is controlled to between 0.22 and 0.40 grams per liter. This concentration may be maintained by adding Cuposit 9F concentrate as required. The stannous tin content is between 10 and 50 grams per liter.

The accelerator speeds the deposit of a layer of metal onto the plastic during the next step of striking, described below. The accelerator lays down a layer of tin no more than about one molecule thick. The plating metal readily adheres to this tin layer. The accelerator also protects against catalyst drag into the electroless bath used in the striking process.

Following the sensitizing process, a striking process is carried out to provide a preliminary deposit of the metal being plated on the area to be plated. The striking process is preferably begun without allowing the area to dry following the sensitizing process. This helps prevent contamination of the area. The striking process is accomplished by wetting the area to be plated with an electroless plating solution and keeping the area wet with such plating solution until the area is essentially completely covered with the preliminarly deposit of metal. This wetting of the area is preferably accomplished by flowing the electroless plating solution over the area to be plated. The solution is flowed at a velocity sufficiently low to allow the metal in the solution to plate onto the area evenly over the entire area. Preferably, the solution is recycled in the manner described above to reduce the cost of the plating process. The electroless plating solution may be a copper solution or a nickel solution.

An example of a suitable copper solution is a solution including 72.5% deionized or distilled water, 12.5% of the solution sold by the Shipley Company, Inc., under the name of Electroless Copper Mix 328A, 12.5% of the solution sold by Shipley under the name Electroless Copper Mix 328Q, and 2.5% of the solution sold by Shipley under the name Electroless Copper Mix 328C. The electroless solution prepared by mixing the above in the required proportions is applied to the area to be plated and the area is kept wet with the mixture for about 10 to 20 minutes. Following this time, the area is rinsed with cold water for 2 to 3 minutes.

One of the advantages of the present invention is that nickel may be deposited directly onto the conductive plastic without first providing a layer of copper. When it is desired to apply nickel directly, the striking step is carried out by wetting the area to be plated with an electroless nickel plating solution, such as one which contains a nickel-phosphorus alloy. One example is a solution prepared with Niposit 65 nickel mix, manufactured by the Shipley Company, Inc. The area is kept wet with the nickel solution for about 8 to 15 minutes and then rinsed. When a nickel solution, as opposed to a copper solution, is used, it may be necessary to "jump start" the electroless plating process to initiate the coverage of the area to be plated with the preliminary deposit of nickel. The jump start is accomplished by very briefly, essentially momentarily, applying a very low current to the area to be plated. The current should, of course, be sufficiently low to prevent any overheating or burning of the plastic.

Following the striking process, the desired buildup of copper or nickel on the area is provided by plating the area to the desired thickness. This may be accomplished in a number of ways. One alternative is to continue the electroless plating process by keeping said area wet with th electroless plating solution until the desired thickness has been achieved. This may be done by flowing the electroless plating solution or continuing to flow said solution over the area. When the buildup is accomplished by continuing the electroless plating process, care must be taken to ensure that the solution is replenished as required. This applied to a situation in which the area is immersed in the solution as well as to a situation in which the solution is flowed over the area and recycled.

Although a significant advantage of the present invention is that it does not require immersion of the component being plated at any point in the process, each step that may be accomplished by flowing a solution or water over the area to be plated may also be accomplished by immersing the area to be plated. Such steps of course include the final step of plating to the desired buildup. If immersion plating by electrolytic means is chosen as the means for carrying out this final step, great care must be taken because the composite is conductive and prone to overheat with the passage of excess current. Therefore, conventional immersion plating techniques must be modified in order to prevent burning of the composite at the contact points. The modified immersion plating process is as follows.

Cathode lead contact is made at at least one contact point. The area to be plated is immersed in a suitable plating solution. While the area is immersed, the solution is agitated to minimize temperature and concentration gradients. A very low current, preferably not more than 2 amps per contact, is applied for a short period of about 4 minutes. Then the contact point or points are moved and the plating is continued with an increased current. Of course, the increased current must still be kept sufficiently low to prevent burning of the composite at the contact points. The number of contact points required is determined largely by the size of the area to be plated. On large areas, several contact points, essentially the more the better, are necessary. The use of multiple contact points is known in the art but is especially important when the component being plated is made from a conductive plastic.

A number of different plating solutions may be used to accomplish immersion plating. One example is a copper plating solution of plating grade copper sulphate in a concentration of 24 to 30 ounces per gallon, 96% sulfuric acid in a concentration of 6 to 8 ounces per gallon, and chloride ion in a concentration of 20 to 60 parts per million. The additive sold under the name EK-1H by the Harshaw Chemical Company of Cleveland, Ohio may be used in this solution. This additive contributes to the brightness and smoothness of the surface of the finished product. It tends to smooth down high points and fill in holes so that the final surface is smooth and even.

A third alternative technique for accomplishing the final buildup is to brush plate the area to be plated to the desired thickness. Because conductive plastics are prone to overheat with the passage of current, the cathode lead contact point or points should be prepared before initiating the brush plating process. The preparation includes plating the ar4as chosen as contact points to approximately 0.1 mil at a voltage not exceeding 4 volts. This initial plating of the contact points helps to prevent burning at the contact points and allows the brush plating process to be carried out at a somewhat higher voltage. The total area of the contact point or points depends on the total area to be plated. A larger area to be plated requires a correspondingly larger total contact area in order to avoid burning the conductive plastic. The configuration of the contact point area, apart from its size, is unimportant.

It will obvious to those skilled in the art to which this invention is addressed that the invention may be used to advantage in a variety of situations. Therefore, it is also to be understood by those skilled in the art that various changes, modifications, and omissions in form and detail may be made without departing from the spirit and scope of the present invention as defined in the following claims.