1. Field of Invention
The present invention relates to a lead frame structure. More particularly, the present invention relates to a lead frame structure having a cooling surface that can enhance the cooling capacity of a semiconductor package.
2. Description of Related Art
As semiconductor manufacturing continues to make progress, the level of integration of integrated circuit (IC) devices and hence computational speed will increase. When the devices in a silicon chip is in operation, a great amount of heat is generated. If the integrated circuit rises above a certain permissible range of temperature, some of the integrated circuit devices within the chip may produce errors. When the devices are seriously overheated, they may even be permanently damaged. Hence, how to lower the working temperature of (IC) devices becomes an important manufacturing consideration. A metallic lead frame inside a package is in direct contact with a silicon chip. Because metal is a good thermal conductor, most packages have a portion of the metallic lead frame exposed so that cooling rate can be increased.
FIG. 1 is a schematic, cross-sectional diagram showing a conventional semiconductor package with a lead frame inside. As shown in FIG. 1, the lead frame 16 inside the semiconductor package 10 includes a die pad 12 and a plurality of leads 14. Each lead 14 can be divided into an inner lead portion 18 and an outer lead portion 20. The die pad 12 is formed on a surface at a different height level from the inner lead portion 18. After molding, the inner lead portion 18 of the lead frame 16 is enclosed by packaging material 30 while the backside of the die pad 12 is exposed. The method of forming a semiconductor package involves the step of attaching a silicon chip 22 to a die pad 12. A wire-bonding operation is next carried out to connect from the bonding pads on the chip 22 to the leads 14 on the lead frame 16 using metallic wires 26. The silicon chip and the lead frame 16 are placed inside a set of mold such that the bottom surface of the die pad 12 is in direct contact with a bottom mold piece. Molding material 30 is injected into the mold so that only the outer lead portions 68 and the bottom surface 28 of the die pad are exposed. Finally, the leads are trimmed and formed to create a complete package.
When molding material 30 is injected into the mold, the mold and the die pad 12 may not form a perfect seal. Hence, some of the molding material 30 may leak through crevices between the mold and the bottom surface 28 of the die pad 12 and then spread over the entire bottom surface 28. Because the molding material 30 is a poor conductor of heat, cooling area will be greatly reduced and hence less heat will be carried away from the silicon chip 22. To return the cooling rate to the intended level, molding material on the die pad surface 28 has to be removed by polishing. However, this will increase the processing time and incur additional production cost.