Vacuum effect on the void formation of the molded underfill process in flip chip packaging

• Experiments were conducted to visualize the formation of voids in MUF.
• Vacuum condition can eliminate the enclosed air pocket after equilibrium.
• The required vacuum depends on the surface tension of the working fluid.
• A methodology was proposed to calculate the required vacuum quality for MUF.

The flip chip packaging technology uses solder bumps for the electrical connection between the chip and substrate. A two-step procedure is usually used for the flip chip packaging. In the process, underfill material is filled into the gap between the chip and substrate by the capillary force. After that, epoxy molding compound (EMC) is used to overmold the entire assembly in a mold cavity through the process of the transfer molding. On the other hand, the molded underfill (MUF) process performs the packaging at a single step. In the MUF process, the EMC material fills the mold cavity with preplaced flip chip assembly, and finishes the underfill and overmold at the same time. In this study, experiments were performed to study the mechanism of the void formation in MUF. The effect of vacuum on the void formation was investigated. It was found that high quality of vacuum was necessary in order to eliminate the air pocket enclosed during the filling process. For different materials, the required vacuum quality may be different and it is well below the perfect vacuum.