Coating processes – Electrical product produced
Reexamination Certificate
2002-12-11
2004-08-24
Barr, Michael (Department: 1762)
Coating processes
Electrical product produced
C427S123000, C427S125000, C427S437000, C427S438000, C427S443100, C427S404000, C427S405000, C427S240000, C427S346000
Reexamination Certificate
active
06780456
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a method of manufacturing an electronic part, an electronic part, and an electroless plating method. Particularly, the present invention relates to a method of manufacturing a chip-type electronic part such as a monolithic capacitor, a noise filter, or the like, and an electronic part manufactured by the manufacturing method.
2. Description of the Related Art
In the field of electronic parts, a surface of an electrode is often plated with nickel or gold for improving a bonding ability, solderability, a conductive resin bonding ability, heat resistance, corrosion resistance, etc., obtaining a highly functional electronic part having excellent reliability.
As is generally known, plating methods are roughly divided according to coating methods into electrolytic plating in which electrolysis is performed by passing a current through a plating solution containing metal ions to deposit a metal on a work piece, and electroless plating. Electroless plating includes an auto-catalytic type in which electrons produced by oxidation reaction of a reducing agent added to a plating solution are used for metal deposition reaction, and a substitutional type using substitution reaction between a base metal and metal ions in a solution.
In the auto-catalytic electroless plating, a surface of an electrode must be made catalytically active to oxidation reaction of the reducing agent. Therefore, a member to be plated is conventionally dipped in a catalyst solution containing Pd (palladium) to treat the surface of a Cu electrode so that the surface of the electrode becomes catalytically active.
However, as described above, when the member to be plated is dipped in the catalyst solution containing Pd, Pd adheres to portions other than the electrode to be made catalytically active, and thus Ni plating proceeds with Pd serving as nuclei to possibly deposit Ni on the portions other than the electrode. Furthermore, a degreasing step and an etching step are required as pre-treatments for applying a Pd catalyst, thereby complicating the manufacturing process. Therefore, this type of plating is principally performed by electrolytic plating.
On the other hand, it has recently been found that by using a boron-based compound as a reducing agent, electroless plating can be performed directly on an electrode without performing a catalytic treatment with Pd. Therefore, a technique based on this finding is proposed, in which a Ni—B layer, a Ni—P layer and an Au layer are successively laminated on a Cu electrode by electroless plating (Japanese Unexamined Patent Application Publication No. 10-135607).
In this conventional technique, electroless plating can be performed on a work piece without application of a Pd catalyst thereto, and thus the Ni—B layer, the Ni—P layer and the Au layer are successively laminated by electroless plating. Therefore, a Ni-based film and Au-based film can be formed only on the surface of the electrode without the formation of a plating film on portions other than the electrode.
However, the electroless plating by the above-described plating methods has the following problems.
(1) A multi-terminal electronic part having a plurality of terminals has difficulties in uniformly supplying electric power to the terminals, thereby causing variations in thickness of plating films due to non-uniformity in thickness of the plating films of the terminals. In this case, a necessary and minimum thickness must be secured, and thus plating conditions must be set to permit the formation of a plating film based on the minimum thickness. Therefore, the thickness of the plating film is increased as a whole, and particularly in Au plating, the cost is increased due to the high cost of a material itself. In Ni plating, a Ni plating film is likely to be separated from the electrode due to film stress.
(2) Also, in a small-size part such as a chip-type electronic part, so-called barrel plating is widely put into practical used. On the other hand, further miniaturization of electronic parts is required at present.
Namely, electronic parts are possibly caught in many holes formed in a barrel inner wall with miniaturization of electronic parts, and thus the opening diameter of the holes must be decreased. However, a decrease in the diameter of the holes worsens a flow of a plating solution.
In barrel plating, plating is performed in a barrel device in which many conductive media are charged for securing conductivity of the electrode. Therefore, the diameter of the conductive media used must be decreased to 0.8 mm or less with further miniaturization of electronic parts, and thus inexpensive media (so-called steel shots) containing relatively large conductive media of about 1 mm in diameter cannot be used. Namely, from the economical viewpoint, it is preferable to charge inexpensive steel shots having irregular shapes into the barrel device for plating. However, the opening diameter of the barrel holes must be decreased with miniaturization of electronic parts, and consequently expensive small-diameter conductive media or steel balls having a uniform shape must be used because the conductive media are possibly caught in the small-diameter holes, thereby causing an increase in the manufacturing cost.
(3) When an Au film is formed by electrolytic barrel plating, Au is deposited not only on electrodes of electronic parts but also on the conductive media. Therefore, expensive Au is excessively consumed, and particularly in Au plating for bonding, a large thickness is required to increase the manufacturing cost.
(4) Furthermore, in a low-resistance electronic part such as a varistor or the like, which comprises a ceramic material as a base material, electrons flow to the surface of the ceramic base material in electrolytic plating, and thus a plating metal is abnormally deposited on the surface of the ceramic base material. Particularly in barrel plating, a potential distribution is complicated to cause difficulties in avoiding abnormal deposition of the plating metal on the ceramic base material.
On the other hand, the electroless plating method disclosed in Japanese Unexamined Patent Application Publication No. 10-135607 is capable of plating without applying a Pd catalyst on an electrode surface, and is thus capable of forming a desired plating film only on the electrode. However, for example, when substitutional Au plating is performed directly on the surface of a Ni—B layer, there is a fear that sufficient adhesion cannot be held between the Ni—B layer and the Au layer, thereby causing the need to form a Ni—P layer having good adhesion to Au on the Ni—B layer. Therefore, the two plating films, i.e., the Ni—B layer and the Ni—P layer, must be formed between the electrode and the Au layer to cause the problem of complicating the production process. Furthermore, in forming the Ni—B layer by electroless plating, an expensive reducing agent such as dimethylaminoborane ((CH
3
)
2
NHBH
3
; referred to as “DMAB” hereinafter) must be used, causing the problem of increasing the manufacturing cost.
SUMMARY OF THE INVENTION
The present invention has been achieved in consideration of the above problems, and an object of the present invention is to provide an electroless plating method capable of forming a desired plating film only on a desired portion at a low cost, a method of manufacturing an electronic part, and an electronic part with excellent reliability which is manufactured by the manufacturing method at a low cost.
As described above, in barrel plating on a small-size electronic part such as a chip-type electronic part based on the electroless plating method, plating must be performed in a barrel in which small-diameter conductive media are charged, and a plating metal is also deposited on the surfaces of the conductive media to accordingly cause the need for excessive metal, thereby increasing the manufacturing cost. Furthermore, a multi-terminal electronic part causes the problem of widening a thickness distribution, and the like
Kunishi Tatsuo
Numata Toshi
Saitoh Junichi
Sakabe Yukio
Barr Michael
Dickstein , Shapiro, Morin & Oshinsky, LLP
Murata Manufactruing Co., Ltd.
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