Semiconductor Packaging:  Molding/Plastic Encapsulation    

Molding is the process of encapsulating the device in plastic material. Transfer molding is one of the most widely used molding processes in the semiconductor industry because of its capability to mold small parts with complex features. In this process, the molding compound is first preheated prior to its loading into the molding chamber.

After pre-heating, the molding compound is forced by a hydraulic plunger into the pot where it reaches melting temperature and becomes fluid. The plunger then continues to force the fluid molding compound into the runners of the mold chase. These runners serve as canals where the fluid molding compound travels until it reaches the cavities, which contain the leadframes for encapsulation. 

In conventional flow chambers, the cavities are filled up in a 'christmas tree' fashion, i.e., cavities that are nearer the cull get filled up first. The highest filling velocity is experienced by the first cavity.  However, the filling velocity decreases as the first cavity is filled. 

Subsequent cavities are filled with increasing velocities until the last cavity, which ends up with the second highest filling velocity, next only to the first cavity.  As such, the first and last cavities are most prone to wiresweeping and die paddle shift.

Figure 1.  Examples of Mold Chases

Figure 3.  Example of an Automold System