Method of Applying Catalytic Solution for Use in Electroless Deposition
An improved method of activating a surface to receive electroless metal plating thereon, particularly for use in activating through holes in printed circuit substrates, in which the activating solution comprising a palladium tin colloid in an acidic aqueous matrix is sparged with nitrogen gas to slow the oxidation of stannous tin contained therein. A dynamic flood conveyorized system to perform said activation is described.
1 . A method for activating a surface to receive electroless plating thereon comprising:
(a) transporting the surface through a selectively closed enclosure;
(b) providing a means to contain a liquid activator in the selectively closed enclosure and pumping the liquid activator such that the liquid activator contacts the surface when the surface is being transported through the selectively closed enclosure; and
(c) introducing a substantially deoxygenated gas into the selectively closed enclosure;
wherein the liquid activator solution comprises colloidal palladium particles and stannous ions and wherein the deoxygenated gas inhibits the oxidation the stannous ions in the liquid activator.
2 . The method according to claim 1 , wherein the substantially deoxygenated gas is selected from the group consisting of hydrogen, helium, argon, nitrogen, carbon dioxide, and mixtures of the foregoing.
3 . The method according to claim 2 wherein said substantially deoxygenated gas comprises nitrogen gas.
4 . The method according to claim 3 wherein the nitrogen gas is introduced at a rate of 0.1-9,000 liters/hour.
5 . The method according to claim 3 wherein the nitrogen gas is introduced by means of bubbling or sparging the gas through the liquid activator.
6 . The method according to claim 3 , comprising the step of pumping nitrogen gas into the selectively closed enclosure by means of a porous pipe.
7 . The method according to claim 3 , comprising the step of spraying the nitrogen gas into the selectively closed enclosure by means of a spraying nozzle.
8 . The method according to claim 3 wherein the nitrogen gas is obtained by purification of ambient air through pressure swing adsorption.
9 . The method according to claim 3 wherein the nitrogen gas is of a purity range of at least 85% by weight.
10 . The method according to claim 1 , further comprising the step of pumping the liquid activator so as to flood the enclosure such that the activator contacts the surface when the surface is transported through the selectively closed enclosure.
11 . The method according to claim 1 , further comprising the step of pumping the liquid activator through a spraying nozzle such that the activator contacts the surface when the surface is being transported through the selectively closed enclosure.
12 . The method according to claim 1 wherein the selectively closed enclosure comprises two rollers in contact with each other at the entrance and exit of the enclosure.
13 . The method according to claim 1 further comprising treating the surface with an electroless plating bath after the surface leaves the enclosure.
14 . The method according to claim 13 wherein said electroless plating baths is selected from the group consisting of copper electroless plating baths, nickel electroless plating baths, and tin electroless plating baths.
15 . A conveyorized mechanism for activating a surface to be electrolessly plated, said mechanism comprising.
(a) a conveyor for transporting said surface;
(b) a selectively closed enclosure comprising;
(i) at least a portion of the conveyor;
(ii) a reservoir for containing a liquid activator;
(iii) a pump and piping capable of transporting the liquid activator from the reservoir to the conveyor area;
(iv) a selectively closed mechanism for allowing the surface to enter and exit the enclosure while substantially maintaining the liquid activator in the enclosure;
(v) a means for bubbling a deoxygenated gas into the liquid activator; and
(vi) walls establishing the extent of such enclosure and substantially containing components (i)-(v); and
(c) a source of deoxygenated gas.
16 . A mechanism according to claim 15 wherein the deoxygenated gas comprises nitrogen.
17 . A mechanism according to claim 15 wherein the selectively closed mechanism comprises pairs of pinch rollers.
18 . A mechanism according to claim 15 wherein the source of deoxygenated gas generates nitrogen gas from atmospheric air using pressure swing adsorption.
19 . A mechanism according to claim 15 wherein the means for bubbling deoxygenated gas comprises a porous pipe.
20 . A mechanism according to claim 16 wherein the liquid activator comprises water, colloidal palladium particles and stannous ions.