Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Die bond
Reexamination Certificate
1998-04-24
2003-11-11
Potter, Roy (Department: 2822)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Die bond
C257S783000, C257S676000
Reexamination Certificate
active
06646354
ABSTRACT:
FIELD OF THE INVENTION
This invention relates to an adhesive composition and methods for using an adhesive composition in die packaging applications.
BACKGROUND OF THE INVENTION
In semiconductor manufacture, a single semiconductor die (or chip) is typically mounted within a sealed package. In general, a typical package protects the die from damage (e.g., breakage, physical abuse, etc.) and from contaminants in the surrounding environment (e.g., moisture, dust particles, etc.). In addition, the package provides a lead system for connecting electrical devices of the die to a printed circuit board or other external circuitry.
Each die has a lower surface (also referred to as the back of the die) that is devoid of circuitry, and an upper surface (also referred to as the face of the die) having integrated circuitry constructed thereon. The integrated circuitry is electrically accessible via die wire bonding pads which may be arranged in a variety of configurations on the face or edges of the die.
Typically, an initial component in the packaging process is a leadframe. The leadframe is a metal frame which supports several dice for packaging and provides the leads for a final semiconductor package. A typical leadframe strip is adapted to mount several semiconductor dice.
In one a conventional packaging process, each die is attached to a mounting paddle of the leadframe utilizing an adhesive layer. The adhesive layer is typically formed from an adhesive composition including an epoxy, an acrylic, a silicone or a polyimide material located between the bottom surface of the die and an upper surface of the mounting paddle. Also, during the packaging process, bond pads formed on the die are electrically connected to the leads of the leadframe using bond wires. Following the application of a protective layer, for example polyimide, to the face of the die, it and a portion of the leadframe to which it is adhered is encapsulated in a polymeric material. Similarly, all other die-leadframe assemblies on a leadframe strip are encapsulated resulting in encapsulated interconnected packages, i.e., interconnected by way of the leadframe. After encapsulation, a trim and form operation separates the interconnected packages. Leads of each package are also shaped into a desired configuration.
Attempts have been made to increase circuit densities and to improve packaging technologies. For example, some high density circuits, such as 16 mega bit memory devices, use lead-on-chip (LOC) packaging technology. A known lead-on-chip (LOC) semiconductor package is described in U.S. Pat. No. 4,862,245 (Pashby et al.). In general, a LOC die package is formed with a leadframe that does not include a mounting paddle for mounting the die. In the LOC die package, lead fingers of the leadframe are not only for electrical attachment to the bond pads of the die via bond wires but also adhere to the face of the die and support the die during processing. Prior to encapsulation, the die, in effect, is mounted to the lower surface of lead fingers. This configuration provides improved heat transfer from the die and shortens the length of the bond wires. In addition, this configuration enhances the reliability of the package seal.
One method of attaching and wire bonding a LOC die to an LOC leadframe is known as area wire bonding or A-wire bonding. When A-wire bonding is automated, the method typically includes attaching a double-side adhesive tape to the bottom surface of the leadframe fingers. Typically, the double-sided adhesive tape includes a thermoset adhesive on two opposing surfaces of the tape. The face of the die is then attached to a tape surface opposite the leadframe fingers. Generally, attaching the face of the die to the tape surface is accomplished in the presence of heat and pressure. The die-tape-leadframe fingers construction is then heated in an oven to cure the thermoset adhesive. After the adhesive is cured, the leadframe is transferred to a wirebonder machine that connects the bond wires to the bond pads on the die and to the lead fingers of the lead frame. Holes through the adhesive tape allow the bond wires to attach to the bond pads on the die.
In general, an A-wire process is relatively expensive and complicated. For example, the double-sided adhesive tape used in the process is an expensive component because the tape must have an accurate configuration and must be precisely attached to the leadframe fingers for each die to be attached. This is a delicate process and requires precise indexing of the die and precise alignment of the adhesive tape with the die and the leadframe fingers. Moreover, any irregularities (such as buckles, gaps, wrinkles, etc.) in the adhesive tape during attachment to the leadframe fingers tends to cause voids and adhesion problems during attachment of the die, which may ultimately result in the production of a defective product.
In another packaging method, a lead-under-chip (LUC) die may be attached to leadfingers of a leadframe. For example, a tape adhesive on the back of the die or on the leadfingers, along with positioning the die on the lead fingers, results in attachment of the die to the leadframe.
Additionally, because the tape requires an accurate configuration primarily based on the die configuration, many tape configurations (which can vary in tape width, hole patterns, etc.) are required to be stocked by semiconductor manufacturers. This also adds to the expense and complexity of the process. Furthermore, when hole patterns are punched out, a large amount of tape is wasted in the process.
Yet another packaging method includes attaching a semiconductor die to a leadframe as described in U.S. Pat. No. 5,286,679 (Farnworth et al.). The method includes forming a patterned layer of a thermoplastic or thermoset adhesive to one surface of a semiconductor wafer. Individual dice are then singulated from the wafer. During packaging, each adhesive coated die is attached to lead fingers of a leadframe by heating the adhesive layer and pressing the lead fingers, adhesive and die together. This heating process is sufficient to cure the adhesive layer formed from a thermoplastic material. However, when the adhesive layer is formed from a thermosetting material, a separate heating step is required for curing.
Thus, whether adhesive tapes, thermoplastic adhesives or thermoset adhesives are used, heat, typically at a temperature of about 300° C. or more, is generally supplied to the leadframe-adhesive-die interface during packaging which adds cost, time and equipment to the packaging process.
SUMMARY OF THE INVENTION
In view of the foregoing, there is a need in the semiconductor art for an improved adhesive and methods for using such an adhesive in packaging applications. Accordingly, the present invention is directed to a method for forming a pattern of adhesive on a wafer, a method of attaching a semiconductor die to a leadframe, a method of making a semiconductor package, a semiconductor package and an adhesive composition suitable for use in packaging applications.
One aspect of the present invention provides a method for applying an adhesive to a wafer. Preferably, the method includes the steps of providing a wafer having a surface; and applying an instant setting adhesive composition on the surface of the wafer in a configuration wherein a plurality of portions of the surface have the instant setting adhesive composition applied thereon, and further wherein one or more zones of the surfacet are essentially free of the instant setting adhesive composition.
The method may also include the step of singulating the wafer to form at least one die having the instant setting adhesive composition on at least a portion thereof. The zones may include singulation streets or regions having exposed bond pads. Additionally, the method may further include the step of applying an adhesion promoter to the surface of the wafer prior to applying the instant setting adhesive composition.
Another aspect of the present invention provides a method for applying an adhesive to a wafer. Preferably,
Cobbley Chad A.
Jiang Tongbi
Schrock Edward A.
Mueting Raasch & Gebhardt, P.A.
Potter Roy
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