Etching a substrate: processes – Forming or treating electrical conductor article
Patent
1999-08-02
2000-12-05
Beck, Shrive
Etching a substrate: processes
Forming or treating electrical conductor article
216 78, 427305, 427328, 427264, 427271, 427 98, 427405, 427421, 427437, 427438, B05D 138, B05D 310
Patent
active
061562181
DESCRIPTION:
BRIEF SUMMARY
TECHNICAL FIELD
This invention relates to a pretreatment solution for electroless nickel plating, method for pretreatment and method for electroless nickel plating.
BACKGROUND ART
Many printed circuit board are finished by electroless nickel/gold plating. This step follows etching of copper circuit and coating of solder resist, and deposits electrolessly plated nickel and gold films, selectively on exposed copper portions.
Electroless nickel plating follows several pretreatment steps, in which boards are immersed in a series of pretreatment solutions. One of the pretreatment steps is activator, in which Pd catalyst is attached on the exposed copper portions in order to initiate deposition of electroless nickel plating, which is followed by electroless gold plating.
Such an electroless nickel plating step, however, is frequently accompanied by a phenomenon in which nickel is deposited not only on the copper circuits but also on nearby insulator sections.
This phenomenon is called "bridging", "excess plating" or "bleeding", depending on the communities and the organizations.
Recently, printed circuit boards are increasingly required to have circuits with narrower width of line and space. This affects that probability of defects caused by bridging, such as short-circuit, highly increases.
Probability of occurrence of the bridging phenomenon significantly increase in boards prepared by build-up methods, which involve attachment of Pd catalyst on resin surface, electroless copper plating, electrolytic copper plating and formation of circuits by etching of the deposited copper layer. Pd catalyst is basically dissolved in copper chloride and iron chloride solutions used as etchant for circuit patterning. But, if it partially remains, bridging of electrolessly plated nickel are easily occurred on the residual Pd catalyst. As a result, it causes serious problem, because even one short-circuit can damage whole pattern.
Prevention of this phenomenon needs very strict control of the electroless nickel plating solution composition and stirring condition.
It is an object of the present invention to establish a technique which prevents "bridging" by electroless nickel/gold plating between the copper patterns.
The inventors of the present invention have found, after extensive studies to solve the above problems, that the "bridging" phenomenon can be efficiently prevented by immersing the board in a solution containing a thiosulfate immediately before the pretreatment for electroless nickel plating, comprising a series of steps of degreasing, soft etching, acid treatment and activator treatment.
Furthermore, they have also found that use of permanganate treatment and neutralization steps before the thiosulfate treatment is effective, when the Pd catalyst is deposited on the side walls of the holes, prepared in the boards by drilling or the other methods.
SUMMARY OF THE INVENTION
The present invention provides a pretreatment solution for the electroless nickel plating of a printed circuit board. The pretreatment solution according to the present invention contains a thiosulfate as a major ingredient. Preferably, the circuit board has an etched copper pattern, and is immersed in, or sprayed with, the pretreatment solution before the circuit board is electrolessly plated with nickel. The pretreatment solution can also include additives, such as, a pH adjuster, a complexing agent, a surfactant, and a corrosion inhibitor.
According to another aspect of the present invention, a method is provided for pretreating a circuit board. The method includes immersing the circuit board in a pretreatment solution containing a thiosulfate. Alternatively, the pretreatment solution can be sprayed onto the circuit board. Preferably, the circuit board is treated with a permanganate and neutralized before the circuit board is immersed in, or sprayed with, the pretreatment solution.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
Methods for electrolessly forming nickel alloys, such as Ni--P and Ni--B, and pure nickel are within the sc
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patent: 3857724 (1974-12-01), Bakos
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patent: 5648125 (1997-07-01), Cane
English Abstract of the Japanese publication No. 09-176864.
English Abstract of the Japanese publication No. 08-269726.
Hisumi Yoshiyuki
Okubo Toshikazu
Barr Michael
Beck Shrive
Japan Energy Corporation
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