Predicting the Reliability of Package-on-Package Interconnections Using Computational Modeling Software
Authors: Paul T. Vianco, Ph.D., Michael K. Neilsen, Jerome A. Rejent, J. Mark Grazier, and Alice C. Kilgo Company: Sandia National Laboratories Date Published: 1/31/2014
Abstract: Package-on-package (PoP) technology is gaining increased attention within military, space, and satellite electronics communities. In the associated applications, long-term reliability is a critical requirement of soldered interconnections. Accelerated aging tests were undertaken to assess the thermal mechanical fatigue (TMF) performance of PoP assemblies, using temperature cycling (-55/125C, 15 min hold, 0 – 7500 cycles). The top solder joints were Sn-Ag-Cu (SAC305); the bottom joints were either 100% SAC305 or a mixed alloy combination resulting from Sn-Pb paste and SAC305 balls. The PoP components were constructed with three underfill conditions: (a) no underfill; (b) underfill, in the bottom gap only; or (c) underfill, in both the top and bottom gaps. The experimental segment of this study indicated that the use of PoP components in high-reliability applications would require that a “system-level” understanding be developed for PoP assemblies due to the synergistic effects of materials, configuration, and environment on the TMF of the solder joints. Computational modeling provides an effective tool for understanding PoP performance in order to insert it into highreliability applications. The model predictions clearly illustrated the significant impact that package materials, especially the choice and placement of underfill, have on TMF. The findings described in this report reiterate the premise that to optimize the fidelity of computational model predictions, it is necessary to appreciate, not only the low-cycle fatigue behavior of the solder alloy, but also the significant effects caused by the molding compound and underfill on the long-term reliability of the interconnections.