Active solid-state devices (e.g. – transistors – solid-state diode – Combined with electrical contact or lead – Die bond
Reexamination Certificate
1999-01-11
2003-04-01
Jackson, Jerome (Department: 2827)
Active solid-state devices (e.g., transistors, solid-state diode
Combined with electrical contact or lead
Die bond
C257S784000, C174S259000
Reexamination Certificate
active
06541872
ABSTRACT:
TECHNICAL FIELD
This invention relates to semiconductor manufacturing and packaging. Particularly, it relates to a method for attaching a semiconductor die directly to an organic substrate such as a printed circuit board.
BACKGROUND OF THE INVENTION
In the final stages of semiconductor manufacturing, a semiconductor “chip” or die is typically enclosed within a sealed package. The primary purpose of the semiconductor package is to provide a lead system for electrically and mechanically connecting the circuits on the die to a supporting structure such as a printed circuit board (PCB). Without the lead system, electrical connections to the die are made difficult by the fragile structure of the die face. The package also provides physical and environmental (e.g., moisture, chemical) protection and serves to dissipate heat from the die.
The conventional semiconductor packaging process starts by securing the die to a mounting paddle of a metal lead frame with a suitable adhesive. Electrical connections between bond pads on the face of the die and connections on the leads are then made using fine bond wires. A protective coating may be applied to portions of the die, bonding wires, and lead frame. The package is then encapsulated in a plastic or ceramic material from which the leads extend outwardly therefrom. The package may be trimmed and the leads formed to achieve the desired configuration.
A variation of conventional packaging is known as lead-on-chip (LOC) packaging. LOC differs in that the LOC lead frame has no mounting paddle. The leads of the lead frame attach directly to the face of the die and support the die during the encapsulation process. LOC results in improved heat transfer and shorter bond wire length.
While both of these packaging methods have proven reliable, drawbacks exist. First, the encapsulation process adds cost to the finished semiconductor package. In addition, the equipment necessary for encapsulation is highly specialized and expensive. Finally, an encapsulated die is substantially larger and heavier than the die in its unpackaged state. As demand for smaller, more powerful electronic devices grows, semiconductor manufacturers are constantly seeking to increase semiconductor population within a given volume. Accordingly, the size of the semiconductor package becomes a significant concern.
To overcome these problems, alternatives to standard packaging have emerged. One such alternative is to eliminate the encapsulant and metal lead system altogether and attach the die directly to a PCB substrate. The resulting “chip-sized package” (CSP) may, in turn, may be attached through various means to other components including other printed circuit boards. By eliminating the die package and metal lead system, the die has a significantly smaller footprint (and volume). Thus, denser mounting may be achieved.
Bare die attachment to a PCB substrate generally involves first mounting the die to a die attach area on the substrate. The bond pads on the die face may then be wire bonded to connection points on the substrate using gold or aluminum wire. Or, as an alternative to wire bonding, the die may have a series of solder bumps on its face, which, when placed face down, contact connection points on the substrate. Heat or ultrasonic energy may be used to secure the solder bumps to the substrate. Since this process (often referred to as “flip chip bonding”) requires specialized equipment, wire bonding remains the predominant and economically preferred method of die interconnection.
Typically, an encapsulant is applied to the bond wire area to protect the bond wires and their connections. However, this encapsulant is typically a liquid material or “glob-top” applied locally and, thus, its application is not as complex or as costly as conventional encapsulation. Likewise, glob-top provides negligible volumetric increase to the die and substrate.
Since the CSP package has no metal lead system, an alternative method of external electrical and mechanical connection must be provided. The package may, for example, include a fine-pitch “ball grid array” or BGA. A BGA is an array of solder bumps or balls on a side of the PCB opposite the die attach area. Each ball is electrically connected through a conductive trace in the substrate to a wire bond connection point which, in turn, is wire bonded to the die. To mount the BGA package, the solder bumps contact conductive points on the receiving component and heat is applied to reflow the bumps. Other connection methods such as a “pin grid array” or PGA may be used. A PGA has a series of pins extending outwardly from the substrate rather than solder bumps. The pins are mechanically received in apertures on the receiving component. Accordingly, with CSP applications, the substrate itself must incorporate the lead system for electrical connection to the die.
While CSP reduces the bulky footprint common with conventional die packaging, attaching dice directly to PCBs introduces problems. One area of particular concern is the adhesive used to attach the die. The adhesive must physically secure the die and firmly retain it during all subsequent manufacturing operations (e.g., wire-bonding, glob-top curing, soldering). Generally speaking, die attach adhesives fall into one of two categories: tape and paste. In LOC packaging, adhesive tape or film is sometimes used to secure the die to the metal lead frame. This tape is typically a thermoplastic material such as polyimide film requiring high temperature processing. Often, lamination of LOC tape requires temperatures ranging from 325-400 deg C. While the lead frame and other components involved in conventional packaging are capable of this high thermal processing, organic substrates are not. Specifically, the substrate may severely outgas and degrade at temperatures well below 325 deg C. For this reason, paste or resin adhesives having substantially lower processing temperatures have been developed for use with organic substrates. While satisfactory in addressing the thermal processing issue, paste adhesives have inherent drawbacks.
For example, due to the viscous properties of the paste, it tends to “bleed” outwardly from where it is applied. In some instances, the paste may migrate to the wire bond area (or other non-solder masked area). When this occurs, the package is typically rejected. Careful manufacturing control is thus necessary to prevent paste bleed.
Another problem associated with the viscous properties of paste adhesives is bond line thickness and bond area coverage. Without maintaining an even paste thickness, the die may seat in a non-parallel orientation relative to the substrate. When this occurs, damage to the edge of the die and/or the substrate may occur.
A related problem caused by reduced bond line thickness concerns the globtop top or over-mold material. Such materials may contain filler particles that can contact and damage the die face. Increased bond line thickness has been found to reduce this occurrence. However, as discussed above, bond line thickness is difficult to control with paste. Simply adding more paste generally results in increased paste displacement rather than greater bond line thickness.
Yet another problem related to the viscous characteristics of the paste is voiding. Due to the consistency of the paste and the inclusion therein of solvent diluents, voids may form during paste dispensing. These voids increase outgassing during subsequent thermal processing. Outgassing may adversely affect wire bond effectiveness and glob-top adhesion.
Still yet another drawback to paste adhesives is the limitations inherent in dispensing the paste. Specifically, paste is limited by filler size and distribution to accomplish certain flow characteristics necessary for dispensing. Because of the method in which paste is dispensed, the rheological properties of the paste must fall within certain defined limits. Particularly, the filler material, size, distribution, and percentage within the paste is critical to provide effective flow of the adhesive. Accordingly, filler
Jiang Tongbi
Schrock Edward A.
Cruz Lourdes
Jackson Jerome
Micro)n Technology, Inc.
Schwegman Lundberg Woessner & Kluth P.A.
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