TEMPERATURE CYCLING OF Pb-FREE AND MIXED SOLDER INTERCONNECTIONS USED ON A PACKAGE-ON-PACKAGE TEST VEHICLE
Authors: P. T. Vianco, J. M. Grazier, J. A. Rejent, and A.C. Kilgo; F. Verdi and C. Meola Company: Sandia National Laboratories; ACI Technologies, Inc. Date Published: 10/24/2010
Abstract: A study was undertaken to determine the long-term temperature cycling (-55°C/125°C) reliability of package-on-package (PoP) components for military, space, and satellite applications. The degradation mode was thermal mechanical fatigue (TMF). 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, bottom gap only; or (c) underfill, both the top and bottom gaps. When underfill was absent, failure occurred in the bottom solder joints. The mixed alloy solder joints exhibited a slightly longer fatigue life than did the 100% SAC305 solder joints without underfill. In the case of 100% SAC305 bottom solder joints, the presence of the underfill in only the bottom gap caused a somewhat ambiguous change to the solder joint reliability. Failure occurred preferentially in the top solder joints. When underfill was placed in both gaps, there was a definitive loss of fatigue life and the failure sites returned to the bottom solder joints. The PoP components having mixed alloy Sn-Pb/SAC305 solder joints exhibited a substantial loss of reliability with the addition of underfill to the bottom gap or to both gaps. The failures occurred exclusively in the bottom solder joints. This study indicated that the use of PoP components in military and other high reliability applications will require that a careful analysis be made of the effects of underfill materials on the TMF of the solder joints.