Coating processes – Electrical product produced – Integrated circuit – printed circuit – or circuit board
Reexamination Certificate
2002-08-29
2003-07-29
Barr, Michael (Department: 1762)
Coating processes
Electrical product produced
Integrated circuit, printed circuit, or circuit board
C427S097100, C427S304000, C427S305000, C427S306000, C427S314000, C427S316000, C427S318000, C427S319000, C427S437000, C427S438000, C427S443100
Reexamination Certificate
active
06599563
ABSTRACT:
FIELD OF THE INVENTION
The invention describes a method and apparatus for improving interfacial chemical reactions in electroless depositions of metals, and more particularly for producing a through-hole-plated Printed Circuit Board (PCB).
BACKGROUND OF THE INVENTION
U.S. Pat. Nos. 4,279,948, 4,265,943, and 4,209,331, which are hereby referenced in their entirety, summarize the prior art of using electroless copper to prepare non-conductive through-hole walls for electrolytic copper plating, said through-hole walls predominantly consisting of electrically insulating glass-epoxy composites.
In this application the terms process tanks, process solutions, process steps and process stations are interchangeable. The term “substrate” is meant to denote workpiece, panel, board, PCB, with or without through-holes, these terms being interchangeable.
Indeed, the substrate, with its through holes, has to undergo a long, intricate sequence of process steps before the hole walls are electrically conductive and electroplatable. Many of these process steps require bath temperatures weal above ambient for the desired chemical reactions to take place. In current practice, the through-hole panels enter the process solutions mostly following cold water rinses, resulting in significant heat transfer time delays before the panel/solution interface reaches the “threshold” temperature, below which, the desired reaction will not take place satisfactorily.
U.S. Pat. Nos. 3,532,518 and 3,011,920 are referenced herewith to show the crucial importance of adequate catalytic preparation of the polymer hole walls for electroless copper to take place on non-conductive substrates.
Perhaps the most critical step in the PCB production process, is the electroless copper bath, known also as chemical copper. In order to achieve complete, void-free coverage of the through-hole walls, the electroless copper bath must operate at a sufficiently elevated temperature to provide the necessary “activation energy” for the reduction of cupric and/or cuprous ions in solution to metallic copper. Indeed, electroless copper deposition, whether obtained via formaldehyde, or via the environmentally friendlier hypophosphite as the main reducer, is understood to be the result of a complex sequence of intermediate reactions that take place at the interface of the Pd-catalyzed non-conducting polymer hole walls and the electroless Cu solution. Excessively low interfacial temperatures will retard copper initiation to a point where coverage will be incomplete, resulting in rejects.
More importantly, significant delays in chemical copper initiation are especially harmful in hypophosphite-reduced baths, because unlike formaldehyde baths, they are not autocatalytic, and Cu deposition is understood to essentially cease after the Pc layer over the hole wall surface is coated with a continuous coating of Cu. Contamination, or poisoning, of the Pd catalytic layer on the hole wall occasioned by slow initiation of copper deposition due to low interfacial temperature in the electroless copper bath may cause deposition of poorly conductive copper oxides, areas with no deposit at all, or a combination of both. Also, while formaldehyde type electroless copper baths operate at temperatures in the range of 25° C., the recommended operating temperature of hypophosphite-based electroless copper systems is 70° C., perceived by both equipment manufacturers and PCB producers as restrictive and not user-friendly.
A technical document entitled “Process Operating Guide, M system” by MacDermid Inc., is referenced herewith, as indicative of the elaborate process steps required for satisfactory PCB production, with some process steps using solutions that require elevated temperatures. With some few exceptions, PCB production facilities currently use automatic plating machines, wherein a computer-programmed hoist carries panels to be plated through the various process steps, with the panels mounted on vertical jigs, also called racks. The software that directs the movement of the hoist is called timeway. The jigs, with the mounted panels, enter and exit the process solutions in a vertical position.
Relatively recently, some PCB producers have changed to horizontal, conveyorized equipment, wherein PCB panels move through the process solutions in a horizontal, as opposed to vertical mode. Horizontal machines are gaining attention, principally because of reduction in labor expenses one such horizontal PCB machine is called Uniplate LB and can be obtained from Atotech.
SUMMARY OF THE INVENTION
A central consideration behind the present invention hinges on the fact that one deals mainly with interfacial chemical reactions, as opposed to predominantly bulk reactions, when practicing Pd activation and electroless depositions on a substrate. The prior art use of large bath volumes is in most cases superfluous, such that the reduction in bath volume in the present invention results in considerable savings due to reduction in waste disposal volumes and inherent environmental problems.
Thus, the present invention provides according to a first of its aspects, an improved method for manufacturing a PCB, wherein the board to be plated is pre-heated prior to its immersion in an electroless plating solution. The pre-heating is carried out at a temperature that is needed to bring about the desired chemical reaction at the panel-solution interface, allowing the bath of that process step to operate significantly below the temperature that would have been needed if the panel had not been pre-heated, and below the solution temperature of current practice.
According to another aspect of the present invention, the electroless plating apparatus for plating a workpiece, operates in a vertical mode and it comprises a heating station, with the panel to be plated returning to the heating station as dictated by the temperature required for a given process step. Alternatively, the heating station is incorporated into the hoist system of such apparatus.
In still another aspect of the present invention, the electroless plating apparatus operates in a horizontal mode and comprises at least one heating station. The station element where the preheating is to take place is stationary and the panel to be plated can only move in a forward direction, as dictated by conveyorized, moving belt type equipment. Preheating therefore needs to be limited to the most critical process steps such as for example activation or electroless plating.
In a preferred embodiment, a workpiece to be plated is preheated in an appropriate hot chemical solution, thus entering the subsequent process tank without the usual water rinses, as accepted in the industry. By enveloping the workpiece in a desirable, compatible hot liquid layer that will preferably participate in the reaction, one can greatly enhance the desired effect of a given process. This embodiment is referred to hereinbelow as Participant Liquid Layer (PLL) technology.
Thus, the method of the present invention provides an economical approach for interfacial chemical reactions, enabling a reduction in bath volume, and therefore a reduction in waste disposal volumes, which is environmentally desirable. The method can be used with both horizontal and vertical type plating apparatuses, so that it is easily and conveniently adaptable to current manufacturing practices. Use of the PLL technology, as stated, enhances the results of the process.
Additional features and advantages of the present invention will become apparent from the following description.
DETAILED DESCRIPTION OF THE INVENTION
The preheating of the through-hole panel to be plated can be accomplished by numerous techniques, such as for example IR, thermal laser, hot air, high pressure steam, pressurized hot water spray, microwave, etc. With tine, new technologies will be available to impart the desired temperature to the copper-clad panel with the through-holes in it. Desirably, the heating should be focused or directed into the holes, in relation to the method chosen, reducing the heat sink effect of t
Barr Michael
J.G. Systems Inc.
Langer, Pat. Atty. Edward
Shiboleth Yisraeli Roberts Zisman & Co.
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