Encapsulated circuit using vented mold

Details Encapsulated circuit using vented mold Encapsulated circuit using vented mold

1999-07-12

2001-11-20

Ortiz, Angela (Department: 1732)

Plastic and nonmetallic article shaping or treating: processes

Mechanical shaping or molding to form or reform shaped article

To produce composite, plural part or multilayered article

C425S812000, C257S787000

Reexamination Certificate

active

06319450

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to apparatus and methods for encapsulating integrated circuits.
DESCRIPTION OF THE RELATED ART
Many techniques have been used to package integrated circuits using transfer molding equipment. These techniques include attaching an integrated circuit to a lead frame and wire bonds connecting the integrated circuit to conductive leads on the lead frame. The integrated circuit and lead frame are then placed into a mold, and an encapsulant (which may be, for example, a resin or thermoplastic molding compound) is injected into the mold cavity. Electrical components of the integrated circuit, including portions of the conductive leads, are encapsulated in the resin or molding compound. The encapsulated package is cured, deflashed and external conductive leads are prepared (trimmed, formed, coated, etc.) as required for the specific integrated circuit design. The finished product is an integrated circuit which has been encapsulated in a plastic body with electrically conductive leads extending from the plastic body.
Ideally, a molding process would provide plastic encapsulation which totally surrounds the integrated circuit and associated components with no void spaces or defects in the encapsulation. Molding processes and apparatus in the prior art have often failed to achieve this result.
U.S. Pat. No. 5,293,065 to Chan is incorporated by reference herein in its entirety. Chan describes a lead frame for an integrated electrical circuit. A plurality of elongated conductive leads extend in side by side relationship for attachment with the integrated electrical circuit. Chan's method requires a specialize lead frame. An opening includes a first slot through one portion of the lead frame, for injecting molding compound through the lead frame after the lead frame and attached integrated electrical circuit have been positioned in a mold cavity. Another opening includes a second slot through a portion of the lead frame opposite from the first slot, for air to escape from the mold cavity during injection of resin into the cavity. The second slot has a larger cross-sectional flow area as compared to the cross-sectional flow area of the first slot.
Other techniques have been developed for providing better air venting during encapsulation of integrated circuits. These techniques are better understood by reference to FIG.
1
.
FIG.
1
. shows a conventional air vent
20
in a corner of a mold
10
. The mold
10
has a main chamber
30
that is sized for overmolding an integrated circuit (not shown in FIG.
1
). As the mold
10
fills with the encapsulant, air exits the mold through the vent
20
. The air enters the vent
20
through the inside end
21
of the vent, and exits the vent
20
through the outside end
22
of the vent. The inside end
21
and outside end
22
have the same width.
In many cases, the air vent size is too small. Such undersized vents are easily blocked by mold flashes, which are difficult to remove during mold cleaning. According to a first conventional technique for improving venting, the air vents may be deepened (increased in height) or widened to increase the cross-sectional area of the vent. Unfortunately, deepening the air vent results in severe mold bleeding outside the mold cavity, which makes removing of unwanted plastic (flash) from the final product very difficult. Also, the size of the vent is limited by the pitch of the leads on the leadframe; the vent cannot be so large as to overlap one of the leads.
According to a second technique, the injection pressure of the encapsulant may be increased while filling the mold. Increasing injection pressure has an effect similar to deepening the air vent. Further, the bonding wires in the integrated circuit were observed to form short circuits due to the high filling pressure.
According to a third technique, the air vents may be cleaned frequently to avoid clogging by the mold compound. Frequent air vent cleaning improves venting, but is impractical, because each cleaning process takes up to about an hour to perform. This translates into loss of production capacity and increased operating costs.
An improved method for venting a mold is desired.
SUMMARY OF THE INVENTION
The present invention is a mold, and a method for forming the mold. At least one vent hole is formed in the mold. The vent hole is positioned to allow egress of air from the mold. The vent hole has an inside end and an outside end. The vent hole has a cross section that increases in area from the inside end to the outside end.
Additional aspects of the invention include a method for encapsulating an integrated circuit, and a circuit formed by the method. A mold is provided having at least one vent hole that allows egress of air. The vent has an inside end and an outside end. The vent has a cross section that increases in area from the inside end to the outside end. The integrated circuit is placed within the mold. A material to be molded is injected into the mold to encapsulate the integrated circuit.


REFERENCES:
patent: 5137479 (1992-08-01), Ohikata et al.
patent: 5293065 (1994-03-01), Chan
patent: 5665296 (1997-09-01), Jain et al.
patent: 5961912 (1999-10-01), Huang et al.
patent: 6007317 (1999-12-01), Mess

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