Constrained collapse solder joint formation for wafer-level-chip-scale packages to achieve reliability improvement

Wafer-level-chip-scale-packages (WLCSP) are rapidly proving to be the package of choice for portable electronics applications. National Semiconductor's micro SMD package family has been a front-runner in the development of this package type. These packages have a proven reliability in the lower pin-count range (up to 36 I/O) when used in conjunction with standard surface mount assembly (SMT). However, extending this technology to higher pin-counts is a significant challenge. Since the preferred assembly method is to not employ an additional underfill step after reflow soldering, the options available to enhance the package reliability are limited. Several options including a pre-applied epoxy layer that will flow and form the underfill layer during solder reflow are under investigation as a potential solution. This approach has constraints in terms of compatibility with flux type used and the reflow profile used. Another approach involves creating a non-reflowable underfill layer. This is the approach described in this paper, and it has proven to work with all commercial assembly processes and to all extents and purposes is transparent to the surface mount assembly method used. This approach is based on the creation of an epoxy layer in either a film or paste layer form that acts as a layer surrounding and partially submerging the solder bumps. This layer achieves two results that directly impact the reliability of the WLCSP assembly. The primary advantage is the increase in solder joint height achieved, which improves the fatigue life when subjected to thermal excursions. The other major advantage is that with the solder bump being constrained from collapsing completely, the angle of wetting formed on the die side is increased, resulting in a more 'cylindrical' or barrel-shaped joint rather than a shape like a truncated sphere. Finite element modeling (P. Borgesen et al., IEEE Trans. on Comp., Hybrids, and Manuf. Tech., vol. 16, no. 3, pp. 272-283) has also borne out that a higher wetting angle results in higher reliability. There also appears to be an interaction between the pre-applied underfill layer thickness and the reliability of the outermost solder joint, which itself depends on the bump matrix size. A higher stand-off may not necessarily translate to a higher reliability due to this interaction effect.