Method of producing lead frame having uneven surfaces

Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Making plural separate devices

Reexamination Certificate

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Details

C438S123000, C438S112000

Reexamination Certificate

active

06197615

ABSTRACT:

BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a lead frame and production method thereof, used for manufacturing semiconductor device packages. More particularly, the present invention relates to a lead frame employed for manufacturing packages of semiconductor devices including integrated circuit (IC) devices, where the bonding strength between the lead frame and the molding compound for encapsulating the package is improved.
2. Description of the Related Art
In general, the lead frame employed for producing packages of semiconductor devices is manufactured and supplied as a strip consisting of a plurality of lead frames. The lead frame strip includes carrier rails and guide holes for supporting and holding each unit of the lead frame. Each unit of the lead frame includes a plurality of inner leads integrated with a plurality of outer leads, tie bars, and a die pad. The die pad is formed at the center of the lead frame and has an area where a semiconductor device is mounted.
In a plastic semiconductor device package, a plastic molding compound such as EMC (epoxy molding compound) forms a package body and encapsulates the device and electrical interconnections. A problem with such packages is that debonding between the lead frame and the molding compound causes failure of the package. Debonding occurs as a result of moisture penetration into the molding compound. In effect, moisture in the molding compound vaporizes during curing and test processes, which are performed under high temperature conditions, resulting in debonding of the molding compound from the lead frame.
Debonding failure becomes more serious as the semiconductor device packages are downsized. To solve the problems related to debonding failure, many efforts to improve the bonding strength between the lead frame and the molding compound have been made and include for example, the formation of dimples on the die pad of the lead frame, and the development of new molding compounds.
In an attempt to preserve bonding between the molding compound and the lead frame even under conditions of high temperature and high humidity, conventional etched lead frames are formed with dimples on the lower surface of the die pad. Also, conventional stamped frames are formed with slots or punched out areas on the die pad. The formation and the structure of dimples or slots on the die pad of lead frames is disclosed for example, in U.S. Pat. No. 4,942,452, U.S. Pat. No. 5,459,103 and U.S. Pat. No. 4,910,577.
The cost of etching dimples on the lower surface of the die pad of an etched lead frame is two to three times the cost of stamped lead frames. Moreover, when the lead frame is formed with dimples on the lower surface of the die pad, the debonding problem merely shifts from the die pad to the inner leads.
In the case of a stamped lead frame, since it is difficult to form uniform dimples, slots are usually formed. However, when slots are formed, it is difficult to attach the semiconductor device onto the die pad while bypassing the slots, and a complicated apparatus is required to attach the device. Punching the bottom surface of die pad to mimic the formation of dimples is less effective than forming the dimples themselves.
Accordingly, a need exists for a lead frame that can be mass produced and prevents debonding, but that is free from the disadvantages of conventional lead frames.
SUMMARY OF THE INVENTION
In one aspect, the present invention provides a lead frame including a lead frame body and tie bars integrally coupled to the lead frame body. A die pad is coupled to the tie bars and has an upper surface for mounting a semiconductor device thereon and a lower opposing surface. The surfaces of the inner leads of the lead frame, the tie bars, and the die pad are substantially formed with dimples.
The term “substantially formed” or “substantially provided on” as used herein means that the lead frame is formed with, or the produced lead frame has, dimples provided on most of its entire surface, preferably on at least 60% of its entire surface, more preferably on at least 80%, even more preferably on at least 90%. Most preferably, only a portion of the upper surface of the terminal ends of the inner leads is free from dimples. As used herein, the term “entire surface” means the entire opposing upper and lower planer surfaces of the lead frame including the lead frame body, tie bars, die pad and leads.
The present invention further provides at least one semiconductor device having a plurality of bonding pads, with the device being bonded to the upper surface of the lead frame. A plurality of electrical connections are provided between the bonding pads and the respective terminal ends of the inner leads of the lead frame. An encapsulant covers the lead frame, the semiconductor device and the electrical connections.
In another aspect, the present invention provides a method for producing a lead frame, including the steps of (i) preparing a lead frame material; (ii) modifying one or more surfaces of the lead frame material to produce an uneven surface; (iii) leveling the modified lead frame material; and (iv) stamping the lead frame material to produce a desired lead frame shape.


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