Integrated circuit chip mold seal

Details Integrated circuit chip mold seal Integrated circuit chip mold seal

2000-10-10

2001-10-16

Davis, Robert (Department: 1722)

Plastic article or earthenware shaping or treating: apparatus

Distinct means to feed, support or manipulate preform stock...

Female mold type means

C425S129100, C425S544000, C425SDIG004

Reexamination Certificate

active

06302673

ABSTRACT:

TECHNICAL FIELD OF THE INVENTION
The present invention generally relates to integrated circuit chip manufacturing and, more particularly, to an integrated circuit chip mold seal.
BACKGROUND OF THE INVENTION
Integrated circuit chip packages are typically formed by mounting an integrated circuit chip on a lead frame and coupling these two elements to form a package. The package may be encapsulated by, for example, injection molding the empty spaces of the internal area of the lead frame.
Different techniques have been used to prevent the mold material from leaving the internal area of a lead frame and moving to the external area during the encapsulation process. One approach has been the use of metal dambars. A lead frame may be a stamped metal structure including individual leads having internal, intermediate and external portions. The metal dambars constitute portions of the lead frame structure which join the intermediate portions of the individual leads.
According to a typical encapsulation process, a mold cavity is formed by sandwiching the lead frame between plates. The metal dambars provide a boundary to prevent the mold material from traveling to the external area of the lead frame and among the external portions of the leads. After the encapsulation process, the metal dambars are removed to electrically isolate the leads from one another. This may be accomplished by a precision stamping tool which mechanically cuts the dambars from between each individual lead. This is a time consuming and expensive process. Also, over time, lead frames are being made smaller and smaller and pin count is increasing. The lead frames are incorporating greater numbers of more narrow leads and the distance between leads is being reduced. Thus, greater precision is required to remove the dambars.
An alternative to using dambars is to fill the spaces between leads with an organic tape or paste. An organic material in liquid form may be dispensed along the intermediate portions of the leads to fill the spaces between the leads. The material may then be hardened by polymerization. This may be accomplished, for example, by ultraviolet radiation. One of the drawbacks to this process is that the application of the material is tedious and time consuming, and uniformity in application is virtually impossible. After the encapsulation process, the organic material may be left in place because it is a dielectric. However, typical materials used in this approach are generally not compatible with the service environment of the electronic device incorporating the package. For example, temperature cycling, vibration, exposure to gases, condensation and the like can cause the material to deteriorate. This can result in leakage and poor electrical isolation among the leads. Therefore, it is often desirable to remove the material. As with metal dambars, the removal process is expensive and time consuming, and requires precision instruments.
Another alternative to metal dambars is the application of a thermoplastic tape or a plastic preform to the lead frame. One or more sheets of tape or a plastic preform may be pressed against the leads while being heated. Heating causes the tape or preform to melt onto and between the leads. This approach has disadvantages similar to those described above in connection with using organic tape.
Another alternative to metal dambars is the use of two plastic film “linings” for the upper and lower halves of the mold cavity. The two films line the mold cavity and are drawn by a vacuum so that they are contoured to a surface of the cavity. At an intermediate temperature during the encapsulation process, the film flows into the spaces between leads to create a seal about the perimeter of the lead frame. After the encapsulation material is injected, the film may be stripped away and advanced to line and seal a mold cavity of a subsequent lead frame. This process is relatively costly, however, and requires special tools and equipment.
Other problems, shortcomings and disadvantages of known integrated circuit chip package encapsulation techniques and the elimination of metal dambars will be readily apparent to those having ordinary skill in the relevant art.
SUMMARY OF THE INVENTION
It is an object of the present invention to overcome the problems associated with known techniques for encapsulating integrated circuit chip packages.
It is another object of the present invention to provide an inexpensive and easy-to-use device for preventing fluid flow among the leads of a lead frame and between a mold cavity and its exterior during a process of encapsulating an integrated circuit chip package.
It is another object of the present invention to provide a simple encapsulation gasket which may be used with several different types of lead frames.
It is another object of the present invention to provide a system for encapsulating an integrated circuit chip package. The system includes a simple dam which may be easily removed after the encapsulation process.
It is another object of the present invention to provide a method of encapsulating an integrated circuit chip package which will save time and money over known encapsulation processes.
To accomplish these and other objects of the present invention, and in accordance with a first embodiment of the present invention, a gasket is provided for use in encapsulating an integrated circuit chip package. The package includes a lead frame having at least one lead. The gasket includes at least one preformed strip having at least one groove formed in a surface thereof. The at least one groove corresponds to the at least one lead.
When the gasket is in a non-compressed state, the at least one groove may have a depth slightly greater than a thickness of the at least one lead and a width slightly greater than a width of the at least one lead. When the gasket is in a compressed state, the at least one groove may have a depth and a width substantially equal to a thickness and a width, respectively, of the at least one lead. The lead frame may include a plurality of leads. When the strip is coupled to the lead frame and is in a compressed state, the strip may prevent a fluid from flowing between at least two of the plurality of leads.
According to an alternative aspect of the first embodiment, the strip is in the form of a continuous straight strip which may be cut into lengths equal to the lengths of sides of the lead frame. According to another alternative aspect, the gasket has a shape and dimensions corresponding to a shape and dimensions of the lead frame.
According to a second embodiment of the present invention, a system is provided for encapsulating an integrated circuit chip package having a lead frame having at least one lead. The system includes a first plate and a second plate spaced from the first plate. The lead frame is adapted to be sealingly disposed between the first and second plates to form a mold cavity. A gasket is provided which is adapted to be coupled to the lead frame to form a portion of a boundary of the mold cavity. The gasket includes a strip having a least one groove for receiving the at least one lead therein.
According to a third embodiment of the present invention, a method of encapsulating an integrated circuit chip package is provided. The package has a lead frame with at least one lead. According to the method, a gasket is preformed. The gasket is coupled to the lead frame. The lead frame and gasket are positioned between two plates to form a mold cavity. An encapsulation material is injected into the mold cavity. The method may also include the step of compressing the gasket such that the gasket material sealingly fills the space between at least two leads and the cross-sectional shape of the groove is substantially the same as the cross-sectional shape of the at least one lead.
Other features, aspects and advantages of the present invention will be understood by those having ordinary skill in the relevant art by reference to the detailed description in connection with the appropriate figures.


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