Coating processes – Electrical product produced – Metal coating
Reexamination Certificate
1999-07-13
2001-07-31
Beck, Shrive (Department: 1762)
Coating processes
Electrical product produced
Metal coating
C427S156000, C427S264000, C427S272000, C427S273000, C427S282000, C427S343000, C427S405000, C427S435000, C205S135000, C205S223000
Reexamination Certificate
active
06268017
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a plating process and, more particularly, to a method for providing a partial plating onto a relatively small article such as a lead pin capable of being used for electronic devices.
2. Description of the Related Art
Conventionally, a lead pin adapted to be incorporated in an electronic device, such as a semiconductor package, is plated over the entire surface of the lead pin. Various characteristics or performances are usually required for such a lead pin, so that it has been a general practice to provide a composite or multilayer plating onto the lead pin, which is formed by stacking various metal coatings one on top of the other. One of the typical conventional composite platings is a gold plating including a nickel base layer and a gold uppermost layer deposited on the nickel base layer.
The lead pin, the entire surface of which is coated with the gold plating including the nickel base layer, can be suitably used and incorporated in a semiconductor package named “a multichip module” in which plural semiconductor chips are mounted on a single substrate. In the multichip module, the lead pin is normally fixed at one end thereof to the surface of the module substrate by a gold/tin alloy solder (an Au/Sn solder).
The multichip module is adapted to be mounted on the surface of a circuit board through numbers of lead pins arranged in an array. When maintenance, such as chip replacement, is required for the multichip module, an operator usually performs the maintenance to a multichip module fully disengaged from the circuit board.
To facilitate such a disengagement of the module from the circuit board, each lead pin is generally connected at an end thereof, opposite to the fixed end on the substrate, to the circuit board through a tin/lead alloy solder (an Sn/Pb solder), the melting point of which is lower than that of the Au/Sn solder. In this structure, the lead pins are heated to melt only the Sn/Pb solder, which permits the module to be readily disengaged from the circuit board.
When the Sn/Pb solder for connecting each lead pin to the circuit board is molten, the molten Sn/Pb solder may flow on the gold plated surface of the lead pin to reach the Au/Sn solder for connecting the lead pin to the module substrate. Therefore, if one multichip module is repeatedly disengaged from and mounted to the circuit board during maintenance, the melting point of the Au/Sn solder may be lowered due to the mixing of a gold/tin alloy and a tin/lead alloy. When the melting point of the Au/Sn solder is lowered, not only the Sn/Pb solder but also the Au/Sn solder are molten during subsequent maintenance work, which may cause the lead pin to fall off from the module substrate.
To solve the problem of the falling-off of the lead pin, it may be beneficial to provide means for impeding the flow of the molten Sn/Pb solder to a gold plated portion defined between the solders on the opposed ends of the lead pin. Such means may be effectively structured by forming an annular exposed region having no gold uppermost layer, such as a nickel base-layer surface region or a lead-pin material surface region, over a predetermined length on the gold plated portion between the solders. In general, a nickel base-layer surface or a lead-pin material surface is more readily oxidized in comparison with the gold plated surface and is thereby provided with less wetability for solder, so that the flow of the molten Sn/Pb solder can be impeded at the annular surface region of the nickel base-layer or the lead-pin material.
Both the annular nickel base-layer surface region and the annular lead-pin material surface region may be formed by completing a gold plating process only in the opposed end regions of the lead pin, or by sanding the surface of the gold plated portion between the solders of the opposed ends to partially remove the uppermost gold layer after completion of the gold plating process. However, these processes are difficult to precisely perform on a minute lead pin, such as the lead pin of the multichip module, and also be unsatisfactory for mass production of the lead pin.
SUMMARY OF THE INVENTION
It is, therefore, an object of the present invention to provide a method for precisely providing a partial plating to a desired location on the surface of a minute article.
It is another object of the present invention to provide a partial plating method satisfying mass production of a lead pin of an electronic device.
In order to accomplish the above objects, the present invention provides a method for providing a partial plating to an article, comprising the steps of plating an entire surface of the article; covering a first part of a plated surface of the article by a gel material or member; and subjecting a second part of the plated surface of the article, which extends out of the gel member, to a metal-coating removing agent, to remove a metal coating of the second part of the plated surface.
In the above method, the plating step may include providing a composite plating on the entire surface of the article, and the subjecting step may include removing an uppermost metal layer of the composite plating by the metal-coating removing agent.
Also, the gel member may be formed from a non-oily clay or paper-mache.
The present invention also provides a method for providing partial plating to a lead pin, comprising the steps of plating an entire surface of the lead pin; covering a first part of a plated surface of the lead pin by a gel member; and subjecting a second part of the plated surface of the lead pin, which extends out of the gel member, to a metal-coating removing agent, to remove a metal coating of the second part of the plated surface.
In the above method, the plating step may include providing a composite plating on the entire surface of the lead pin, and the subjecting step may include removing an uppermost metal layer of the composite plating by the metal-coating removing agent.
The composite plating may include a nickel base layer and a gold uppermost layer deposited on the nickel base layer.
The method may further comprise the step of forming a bulging portion at one end region of the plated surface of the lead pin, wherein the bulging portion is supported during the subjecting step to embed the first part into the gel member.
The bulging portion may be formed of a solder ball.
The method may be adapted to simultaneously provide partial platings to a plurality of lead pins, and may further comprise the step of providing a support plate including plural through holes for respectively receiving the plurality of lead pins, the bulging portion of each of the lead pins being supported on an edge of the support plate defining each of the through holes.
The support plate may be treated with a water-repellent finishing, and the subjecting step may include removing the metal coating of the second part of each of the lead pins located between the support plate and the gel member.
The water-repellent finishing may comprise a nickel plating containing polytetrafluoroethylene particles.
The method may further comprise the step of subjecting the lead pin to vibration while subjecting the second part of the plated surface of the lead pin to the metal-coating removing agent.
REFERENCES:
patent: 4539839 (1985-09-01), Horiguchi et al.
patent: 4946711 (1990-08-01), Hawker et al.
patent: 5486135 (1996-01-01), Arpaio
patent: 5580432 (1996-12-01), Shibata et al.
patent: 2105749 (1983-03-01), None
patent: 62-113457 (1987-05-01), None
Arai Ryoji
Futaki Kazuyiki
Miyanoo Yoshinobu
Nishimura Kouji
Takei Nobuyuki
Barr Michael
Beck Shrive
Fujitsu Takamisawa Component Limited
Staas & Halsey , LLP
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