Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Metallic housing or support
Reexamination Certificate
1999-09-13
2001-09-11
Picardat, Kevin M. (Department: 2823)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Metallic housing or support
C438S106000, C438S121000
Reexamination Certificate
active
06287896
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention is related to a method for manufacturing lead frames and a lead frame material including an intermediate layer and a top layer for improving the reliability of wire bonding such as anti-corrosion, anti-oxidation, solderability, and less bending cracking in the semiconductor device.
2. Related Art
One of the popular approaches to electrically interconnect an IC unit to circuitry external of the IC device takes the form of a lead frame made of copper or copper alloy. The lead frame includes die pads on which semiconductor chips are fixed. The inner leads are connected to semiconductor chips and outer leads are connected to printed circuit board. However, the surface of the lead frame is easily oxidized by contact with air. The oxidation of lead frame will cause a drop in bonding strength between the inner lead and the bonding wires which connect bonding pads of the semiconductor chips to the corresponding inner leads of the lead frames. Conventional means for preventing the drop in bonding strength by the oxidation is to form a layer of SnPb or Ag on the surface of lead frames.
A pre-plated frame (PPF) is another technique to avoid the oxidation of copper in air. In this technique, the copper base is blanket plated with a protective coating over the surface of the lead frame before package fabrication. The protective coating should meet the requirements of reliability such as good solderability and non-cracking.
The blanket frames are typically plated with a layer of nickel (Ni) over the surface of the lead frames to form a protective coating. Nickel plating was intended to serve as a barrier against the the diffusion of copper atoms and prohibit formation of copper oxides on the surface of the lead frame. Furthermore, a solder layer of palladium (Pd) was formed by deposited a palladium or palladium
ickel alloy on top of the nickel layer in order to enhance solderability of the outer lead, as disclosed in U.S. Pat. Nos. 5,436,082, 5,454,929, 5,486,721, European Patent Application No. 0335608 and European Patent Application No. 0384586.
Unfortunately, the nickel layer in thickness of less than 40 microinches contains pores through which migration and diffusion of copper to the surface of the lead frame takes place. However, the nickel layer in thickness of greater than 40 microinches tends to crack when the lead are eventually bent. An attempt to reduce effects of diffusion of copper through a less than 40 microinches thick nickel layer was made by successively deposited a nickel layer, a layer of palladium or soft gold (Au) strike, a layer of palladium-nickel alloy, a layer of palladium and a layer of gold as disclosed in U.S. Pat. No. 5,360,991. However, the manufacturing process of in succession from the five-layered-structure (Au/Pd/Pd—Ni/Ni or Au strike/Ni/lead frame) is quite complicated and hard processed.
In recent years, the price of palladium is almost as that of gold; therefore, the conventional pre-plated frame with a solder layer of palladium has a drawback in that the manufacturing cost thereof is high.
SUMMARY OF THE INVENTION
The present invention is directed toward resolving the above problems. Namely, an object of the present invention is to provide lead frame having high reliability for wire bonding. The protective coating of the lead frame avoid the oxidation of the copper base and can be easily facilitated by soldering without bending or cracking. Meanwhile, the process steps are not complicated. The lead frame can be produced with a low manufacturing cost.
Accordingly, the present invention provides a method for manufacturing lead frames comprising the steps of: providing a base plate, forming a intermediate layer on the base plate, and forming a top layer on the intermediate layer. The intermediate layer is composed of a layer of nickel-cobalt alloy and a layer of nickel or nickel alloy and the intermediate layer in a thickness sufficient to inhibit further diffusion of the base metal to the surface of the leads. The top layer consisting of gold or gold alloy.
In order to accomplish this object, lead frame material of the present invention comprising the copper or copper alloy base plate. The layer of nickel-cobalt alloy formed on the base plate, wherein the layer of nickel-cobalt alloy has 5 to 30 weight percent cobalt and a thickness of 3 to 20 microinches. The layer of nickel or nickel alloy formed on the layer of nickel-cobalt alloy, wherein the layer of nickel alloy has less than 15 weight percent phosphorus or boron, the layer of nickel or nickel alloy has a thickness of 10 to 80 microinches. The layer of gold or gold alloy formed on the layer of nickel or nickel alloy, wherein the layer of gold alloy is composed of gold and at least one metal selected from the group consisting of palladium, silver, tin and copper, the layer of gold alloy has at least 60 weight percent gold and the layer of gold or gold alloy has a thickness of 0.1 to 5 microinches.
A semiconductor device package is also provided according to object of the present invention comprising at least one semiconductor device; and lead frames sealed within a protective package in which the lead frame including a device mounting pad and lead frame leads to be bonded to the semiconductor device. The lead frame is consisting of: a base plate, a layer of nickel-cobalt alloy having 5 to 30 wt. % of cobalt (preferably 5 to 20 wt. %) and a thickness of 3 to 20 microinches (preferably 3 to 7 microinches), a layer of nickel or nickel alloy, which is a nickel-phosphorus or nickel-boron alloy with phosphorus or boron content less than 15 wt. %. (preferably 8 to 12 wt. %), has a thickness of 10 to 80 microinches (preferably 10 to 20 microninches)., and a layer of gold or gold alloy, which is composed of gold and at least one metal selected from the group consisting of palladium, silver, tin and copper and has at least 60 weight percent gold, has a thickness of 0.1 to 5 microinches (preferably 0.4 to 1 microninches).
Further scope of applicability of the present invention will become apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.
REFERENCES:
patent: 5378657 (1995-01-01), Lin
Chou Shu-Chin
Huang Ya-Ru
Lin Yu-Yu
Yeh Shinn-Horng
Collins D. M.
Industrial Technology Research Institute
Liauh W. Wayne
Picardat Kevin M.
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