BACKGROUND OF THE INVENTION
Integrated circuit chips are generally attached to leadframes which provide a way to make an electrical connection to a printed wiring board. The chip is coupled from its bonding pads to the lead fingers of the leadframe by way of gold wires, and the chip along with the inner portion of the lead frame are encapsulated in a package for environmental protection. The leads which remain external to the plastic encapsulation are then soldered to a printed wiring board surface, typically using a solder paste.
The leadframe is formed of highly electrically conductive material, such as copper, copper alloys, or alloy 42 by stamping or etching a metal blank into a plurality of leads, and an area where the chip is mounted. Attachment of the chip to the leadframe, wirebonding and soldering require a particular quality of leadframe surface. Most often the surface to be bonded must be free of oxides or other contaminants, and amenable to metallic interaction with other components, such as gold wire, or solder. For this to be attainable, the surface finish of the leadframe finish plays and an important role.
Stamped or etched lead frames are typically plated with a layer of nickel to cover the bare metal, and to serve as a barrier against copper diffusion, as well as to protect the plating bath from contaminants.
Various approaches for treating the bonding surfaces have been employed. Silver plating of the entire leadframe has been largely abandoned because silver migration between external leads resulted in short circuits. Spot silver plating the internal lead fingers provided a bondable surface for gold wires, and the external leads were coated with solder, either by plating or by solder dipping. This multistep process has added expense. Further, delamination of molding compound from lead frames with spot silver plating on the bond fingers and on the chip paddle has been identified as the cause of a failure which frequently occurs during solder reflow of the package to the printed wiring board.
More recently, the entire lead frame surface has been plated with palladium or palladium alloys over nickel containing layers. The nickel acts as a barrier against copper diffusion, as well as protects the plating baths from contaminants. The palladium plated finish provides a bondable surface. A leadframe plating technology which has been in high volume production for a number of years includes the following layers; a nickel strike, a nickel/palladium flash, a thick nickel plate and a palladium layer. A nickel strike over a base copper lead frame is provided to cover the copper and to protect the plating bath from contamination. The nickel/palladium flash serves to inhibit galvanic corrosion by a palladium/copper couple and the thick nickel plate inhibits diffusion of copper during thermal excursions encountered in the assembly of the integrated circuit packages. The thickness of each of the layers is tightly specified to assure that its intended purpose is accomplished.
The final, surface layer of palladium provides a bondable surface. It is well known that palladium is readily soluble in solder and the surface layer will be sacrificed during solder reflow. Palladium is specified in sufficient thickness to protect the underlying nickel from oxidation during assembly of the integrated circuit package so that solderability will not be compromised. Typically palladium plating thickness is 3 to 10 microinches over the entire surface of the leadframe and is applied by flood type electroplating.
However, there is a persistent need to improve and simplify current leadframes and plating procedures, while retaining all the desirable characteristics of palladium plated surfaces.
SUMMARY OF THE INVENTION
The primary object of the present invention is to provide a leadframe having a palladium surface finish with all the desirable characteristics of said finish, but without wasting precious metal as routinely required in prior systems. It is a feature of this invention that the portion of the leadframe which is external to the plastic package and which comes into contact with solder comprises palladium, in a minimum of 3 microinches thickness, and the internal portion of the unit has a lower palladium plating thickness. Said internal plating thickness comprising palladium is a minimum of 1 microinch; this level which has been shown to be sufficient for thermosonic bonding to gold wires. The external plating thickness is unchanged from present technology and therefore, does not cause any disruption in the user board assembly process.
It is also a feature of the present invention that said leadframe presents a single surface composition to the encapsulating molding compounds, and that surface provides good adhesion to molding compounds.
It is further a discovery of the present invention that because the leadframe finish comprises only a palladium plated surface, variations in thickness at the transition area constitute no reliability or cosmetic problems. Such variations typically occur with spot plating as a result of plating solution bleed at the masked interface. This feature permits relaxed specifications for edge control of the plated spot.
