Structure and Production Processes of Plastic Packages

The plastic package structure and assembly flow of semiconductor devices are shown in Fig. 2. The semiconductor chip is mounted on a tab lead frame, and the chip bonding pad is connected by a gold wire to the frame. The assembled frame is placed on a molding die, and then transfer molded. After that the outer leads are trimmed, formed, and metal finished (e.g. solder dipping or plating). The final products are tested electrically and then shipped to the buyers. 

Gold wire Gold/Silver plating Lead frame 

As shown in Table 1, the component mataials have different thermal expansion coefficients, and careful choice of materials is necessary for minimizing internal stress of the package, based upon its structure analysis. 

For plastic encapsulation materials, thermosetting resin is dominant in semiconductor applications. Thermoplastic polymer generally has a high viscosity which can potentially cause a wire sweep problem. Furthermore, it softens or even melts at higho temperatures. On the other hand, thermosetting resins have a lower melt viscosity before curing and higher heat resistance in the cured state. 

There are various ways to encapsulate such as potting, casting by liquid resin com-potmds, and transfer molding by powder, with transfer molding being the most popular method to encapsulate ICs or LSIs. The transfer molding process is expbined in Fig. 3. 

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