Plastic encapsulated IC package and method of designing same

Details Plastic encapsulated IC package and method of designing same Plastic encapsulated IC package and method of designing same

1998-02-11

2001-02-13

Niebling, John F. (Department: 2812)

Semiconductor device manufacturing: process

With measuring or testing

Packaging or treatment of packaged semiconductor

C438S112000, C438S106000, C438S127000

Reexamination Certificate

active

06187601

ABSTRACT:

FIELD OF THE INVENTION
The present invention relates to a design methodology for the plastic encapsulation of power integrated circuit (IC) packages, including the selection of plastic encapsulation and lead frame materials, and the determination of the radius of the fillet feature at the bonding interface.
BACKGROUND OF THE INVENTION
As illustrated in
FIGS. 1A
,
1
B and
2
, a plastic encapsulated power IC package includes a silicon die
10
, die attachment
20
, passivation
30
, wire interconnects
40
, a lead frame
50
, and plastic molding compound
60
. This package is often used in automotive electronics and typically operates in a severe thermal environment.
The thermal expansion mismatch of different materials produces thermal stresses along the adhesion interface between the materials during the cooling process and during low temperature thermal cycling. In traditional plastic encapsulated IC packages, there is a sharp, angular edge at the interface between the molding compound
60
and the lead frame
50
. The sharp corner of the edge causes a stress concentration which, in turn, causes crack initiation at the interface between the molding compound
60
and the lead frame
50
. Such cracks in the molding compound comprise a significant reliability issue for any power IC operated in such a harsh, thermally-cycled environment.
SUMMARY OF THE INVENTION
An objective of the present invention is to develop a methodology for designing the encapsulation of the power IC packages to include a circular fillet feature. The methodology covers: (1) the selection of the material system, including the molding compound material for encapsulation and the lead frame material, based on the stress singularity characterization in the finite element analysis; and (2) the determination of the radius of the fillet.
In carrying out the above object and other objects, features, and advantages of the present invention, a method is provided for designing an IC package having a plastic encapsulated portion and a lead frame portion that are bonded together at a bonding interface. The method includes the step of selecting material candidates for each of the plastic encapsulated portion and the lead frame portion. The method also includes the step of determining a peeling stress and a shear stress at the bonding interface for a plurality of combinations of material candidates. Finally, the method includes the step of selecting a design material combination from the plurality of combinations based on the peeling stress and the shear stress.
In further carrying out the above objects and other objects, features, and advantages of the present invention, a method is also provided for adding a circular fillet feature to the IC package. The method includes the step of determining a singularity point at the bonding interface having the highest stress. The method also includes the step of adding the circular fillet feature at the singularity point having a radius sufficient to reduce the stress.


REFERENCES:
patent: 3560813 (1971-02-01), Phy
patent: 4468411 (1984-08-01), Sloan et al.
patent: 4866506 (1989-09-01), Nambu et al.
patent: 5182630 (1993-01-01), Omi et al.
patent: 5313102 (1994-05-01), Lim et al.

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