Long wire IC package fabrication method

Details Long wire IC package fabrication method Long wire IC package fabrication method

2000-05-05

2001-12-04

Nelms, David (Department: 2818)

Semiconductor device manufacturing: process

Packaging or treatment of packaged semiconductor

C438S015000

Reexamination Certificate

active

06326235

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Filed of the Invention
The present invention relates generally to the packaging of electronic components. More particularly, the present invention relates to a method of fabricating an integrated circuit package.
2. Description of the Related Art
Trends in modern integrated circuitry are generally towards smaller size and higher density. This has resulted in the requirement to fit a large number of bond pads (I/O pads) in a relatively small area of the integrated circuit, sometimes called the semiconductor chip.
To route electrical signals to and from the integrated circuit, the bond pads of the integrated circuit were electrically connected to traces on a substrate. However, since the traces were generally larger in size and pitch, i.e., spacing, than the bond pads, the number of traces which could be formed directly adjacent to the integrated circuit was limited.
One technique for overcoming the size and pitch limitations of the traces was to form the traces at a significant distance from the integrated circuit. However, this caused the length of the bond wires extending between the bond pads of the integrated circuit and the corresponding traces to be relatively long. Long bond wires were particularly problematic during the subsequent plastic encapsulation transfer molding process where the integrated circuit, the bond wires, and the substrate were transfer molded into a plastic encapsulant to form the finished integrated circuit package.
More particularly, during the plastic encapsulation transfer molding process, the plastic encapsulant was caused to flow around the integrated circuit, the bond wires, and substrate. This flow of plastic encapsulant against the bond wires caused: (1) the bond wires to become disconnected from the bond pads of the integrated circuit or from the traces; (2) caused the bond wires to break; and/or (3) caused the bond wires to move and short one another. This phenomena is called “wire sweep”.
In the event a plastic encapsulation injection molding process was used, wire sweep was even more problematic. Generally, the plastic encapsulant was injected around the integrated circuit, the bond wires and the substrate at a higher pressure in a plastic encapsulation injection molding process than in a plastic encapsulation transfer molding process. Due to this higher pressure, more force was exerted against the bond wires, which were susceptible to wire sweep.
To avoid wire sweep, the bond wires were supported by an intermediate section between the bond pads of the integrated circuit and the traces.
Rostoker, U.S. Pat. No. 5,753,970, which is herein incorporated by reference in its entirety, teaches a lead support structure which supported a bond wire at a point along its length such that the bond wire was mechanically constrained and could not short to adjacent bond wires (see lead support structures 700
a,
700
b,
700
c
of FIGS. 9
a,
9
b,
9
c,
respectively, of Rostoker). However, providing and attaching such a lead support structure to the substrate was relatively labor-intensive and complex and thus significantly added to the cost of the integrated circuit package.
Typically, each trace was aligned with the specific bond pad on the integrated circuit to which the trace was to be connected. The bond pads on the integrated circuit were connected to the traces in the same order that the bond pads were position on the integrated circuit. However, in certain instances, it was desirable to allow for cross-over connections between the bond pads and the traces. Generally, a cross-over connection is a connection between a bond pad and a trace which is not directly aligned with the bond pad.
Gow, 3rd et al., U.S. Pat. No. 5,168,368, which is herein incorporated by reference in its entirety, teaches a bridge which was bonded to a first set of fingers of a lead frame. The bridge was located between the integrated circuit and a second set of fingers. To form a cross-over connection, a bond wire extending between a bond pad and a finger of the second set of fingers was intermediately bonded to the bridge above the first set of fingers. In this manner, the bond wire was crossed over fingers of the first set of fingers. However, providing and attaching such a bridge was relatively labor-intensive and complex and thus significantly added to the cost of the integrated circuit package.
Thus, in both Rostoker and Gow, 3rd et al., an intermediate bonding structure was provided between the bond pads and traces (fingers) to reduce the unsupported length of the bond wires and thus avoid wire sweep. However, the intermediate bonding structures had to be fabricated separately and attached to the package during assembly which, as set forth above, was relatively labor-intensive and complex and thus significantly added to the cost of integrated circuit package.
SUMMARY OF THE INVENTION
In accordance with the present invention, a plastic encapsulated integrated circuit package having a large number of traces is presented. To allow the required large number of traces to be formed, the traces are spaced a significant distance from the integrated circuit. Bond pads of the integrated circuit are electrically connected to corresponding traces by corresponding long wires. Of importance, the long wires are not susceptible to wire sweep during the plastic encapsulation transfer molding process used to form the package body.
The long wires are intermediately bonded to intermediate bonding pads between the integrated circuit and the traces. Of importance, the distance between the bond pads of the integrated circuit and the intermediate bonding pads is sufficiently small such that looped portions of the long wires, which extend between the bond pads of the integrated circuit and the intermediate bonding pads, are not susceptible to wire sweep.
Of further importance, surface runner portions of the long wires, which extend between the intermediate bonding pads and the traces, extend along and contact the upper surface of the substrate and so are also not susceptible to wire sweep. More particularly, during the plastic encapsulation transfer molding process, the plastic encapsulant flows over the surface runner portions instead of around the surface runner portions. This, in turn, causes the plastic encapsulant to exert much less force on the surface runner portions than if the surface runner portions were suspended above the upper surface of the substrate.
Since the surface runner portions have much less force exerted on them during the plastic encapsulation transfer molding process, the surface runner portions can be formed of a greater length than conventional suspended bond wires.
Recall that in the prior art, the unsupported sections of the bond wires extending between the bond pads and the traces were suspended above the substrate. During the plastic encapsulation transfer molding process, the plastic encapsulant flowed around these unsupported sections of bond wires, which were susceptible to wire sweep.
For this reason, each unsupported section of bond wire had to be no greater than the maximum allowable wire bond length. Thus, even using an intermediate bonding structure of the prior art, the allowable distance between the bond pads and the traces was equal to approximate twice the maximum allowable wire bond length. However, the distance between the bond pads and the traces needs to be greater than twice the maximum allowable wire bond length in certain instances, for example, when a large number of traces are required.
To increase the distance between the bond pads and the traces, additional intermediate bonding structures were provided. However, these additional intermediate bonding structures had to be fabricated separately and attached to the package during assembly which further added to the cost of the integrated circuit package.
Advantageously, by essentially eliminating the susceptibility of the surface runner portions to wire sweep, the distance between the bond pads and the traces can be much greater than twice the maximum allowable wir

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