Viscosity underfill time

Figure 2.6 Viscosity versus underfill time using controUed-stress rheometer.

Figure 2.7. Viscosity vs underfill time using controlled-stress rheometer. (Source Loctite.)...

The size of the die is a further consideration in the time it takes for the underfill adhesive to traverse the distance and completely fill the gap. A plot of the square of the distance as a function of time produces a straight line as expected from Eq. (2.9) (Fig. 2.7). For a given die size, increased viscosity of the underfill material also increases the time to fill the gap. For high production rates, the fill time must be as short as possible. With increasing sizes of IC chips, this has presented... 

On the other hand, if the underfill material is too soft, it could result in first level interconnect failure. A less stiff underfill could also help reduce board level assembly warpage, which in turn helps the second-level interconnects.Thus, the stiffness of the underfill selected should be optimized such that it does not lead to delamination or first-level interconnect failures. In addition to stiffness, CTE, and Tg, there are several other factors that dictate the underfill material selected, such as thixotropy, viscosity, shrinkage, curing temperature, and curing time. 

---