Claims
- 1. A method of treating polymeric plastic substrate prior to plating on a surface thereof, which comprises etching the substrate in an aqueous acid bath, activating the substrate surface by contact with an acidic tin-palladium complex activator, rinsing the activated surface with an aqueous medium and thereby forming on said surface tin and palladium hydroxides, and accelerating said activated and rinsed surface with a solution comprising about 10 to 30 grams per liter of HCl, approximately 1/2 to 40 grams per liter of NiCl.sub.2 .multidot. 6H.sub.2 O and about 1/4 to 2 grams per liter of NaHF.sub.2.
- 2. A plastic treating method as defined in claim 1, in which during the accelerating step the substrate is contacted by the solution for between 50 seconds and three minutes and the solution is maintain at a temperature between approximately 150.degree. and 140.degree. F.
CROSS REFERENCE TO RELATED CASES
This application is a continuation-in-part of application Ser. No. 399,674 filed Sept. 21, 1973, and now abandoned.
It is known in the art to which this invention pertains that plastic parts prior to electroless plating and subsequent electroplating are pre-treated by a sequence of steps basically plating.
In the accelerating step, to which the instant invention is particularly directed, it has been proposed after activation and before metallization to employ a solution which will remove the excess stannous hydroxide from the substrate surface. Typical examples of such solutions are dilute acids such as perchloric acid, sulfuric acid or phosphoric acid, and alkaline materials such as sodium hydroxide, sodium carbonate or sodium pyrophosphate.
It has also been suggested by the prior art that more readily controllable acceleration can be accomplished with the use of an acidic dilute palladium chloride solution following activation. It has been found, however, that the effectiveness of the accelerator solution just mentioned is impeded by trace amounts of hexavalent chromium ion carried by the substrate from the prior etching operation, and in an endeavor to rectify this condition it has been proposed to control the hexavalent chromium ion contamination of the accelerating solution by the periodic addition of a suitable source of stannous ions, to thereby effect a conversion of the hexavalent chromium to the trivalent condition.
However, in none of the known literature references is there revealed the successful accomplishment of an essentially complete removal of the stannous hydroxide which results from the water rinse following activation, and at the same time substantially zero removal of the palladium hydroxide also formed by the water rinse. As was stated, the palladium ions are requisite for their catalytic reactive potential in the subsequent electroless plating step.
The instant inventive concept is particularly directed to a method and composition effective to accelerate the surfaces of a polymeric substrate subsequent to the activation thereof and prior to electroless plating, the accelerating solution promoting the removal of the stannous ions which are codeposited with palladium during the activating step, the accelerator being possessed of the property of activating the stainless steel rack contacts, whereby an immersion deposit of a metal catalytic to the subsequent electroless metallization solution is formed thereon.
An accelerating solution as provided by this invention basically comprises specified molar concentrations of a relatively dilute inorganic acid and a metal salt desirably selected from the group consisting of nickel, cobalt and ruthenium. More specifically, the inorganic acid is relatively dilute hydrochloric acid, fluoboric acid, sulfuric acid or equivalents or mixtures thereof. A preferred metal salt meeting the requirements just mentioned is nickel chloride, although there may be used cobalt chloride, ruthenium chloride or any salt from the platinum group of the Periodic Chart and which has a reduction potential less than that of iron.
It is also an important function of an accelerator solution to prevent the re-immersion of dissolved tin ions on the accelerated plastic. During rinsing following activation, primarily due to oxygen dissolved in the rinse water, or during subsequent transfer of the activated and rinsed plastic surface through the air, it is frequently found that a particular portion of the stannous ions retained on the plastic surface are oxidized to the stannic state. In aqueous solutions, these stannic ions rather quickly assume what has been referred to in the prior art as being in a colloidal form. The stannic colloid is very powerful and has heretofore been unrecoginzed as an agent in inhibiting or precluding the catalytic effect of the adsorbed palladium ions on the subsequent electroless metallization solution. In a significant number of cases, a sufficiently high ionic strength of the inorganic acid or acid salt is sufficient to counteract the deleterious effect of the re-immersed stannic ion. In a relatively few particularly severe instances, however, it may be desirable and even advantageous to make an addition to the acidic accelerator of a compound having the capability of ionizing in order to serve as a source of fluoride ions. Among the compounds suitable for this purpose are hydrofluoric acid, sodium fluoride, sodium acid fluoride, ammonium acid fluoride, ammonium fluoride, lithium fluoride, potassium acid fluoride and fluosilic acid. The action of acidic accelerators based on the concepts of the instant invention will be more fully understood when reference is made to the examples appearing hereinafter.
By proceeding in accordance with this invention, excess stannous hydroxide from the activating step is removed from the microscopic cavities in the polymeric surface, the stannous hydroxide being the result of a hydrolysis reaction when the plastic part is rinsed with water after the activating step.
Hydrolysis of the palladium chloride also occurs during the water rinsing step, with consequent formation of palladium hydroxide. Palladium ions have a catalytic effect in the subsequent electroless nickel plating operation, whereas the stannous ions are not catalytic. By this invention, there is achieved selective removal of the stannous ions from the palladium ions by the use, under carefully controlled time and temperature conditions, of a solution which at present preferably comprises relatively dilute hydrochloric acid, nickel chloride hexahydrate and sodium bifluoride.
US Referenced Citations (11)
Continuation in Parts (1)
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Number |
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399674 |
Sep 1973 |
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Patent Grant
4035227
