Solder joints can experience microstructural evolution at relatively low temperatures, even room temperature, due to their reduced solidus points. Because the processes that underlie these microstructural changes are thermally activated, the evolution is reproducible and, moreover, predictable with respect to the cumulative time and temperature history of the solder joint. Two such metrics in 63Sn-37Pb/Cu (Sn-Pb, wt.%) solder joints are the Cu-Sn intermetallic compound (IMC) layer thickness, which measures solely the cumulative temperature-time history, and the Pb-rich phase particle size, which determines both the cumulative temperature-time history and fatigue cycling history. These properties were used to examine the lifetimes of Sn-Pb solder interconnections on four printed wiring assemblies (PWAs). Two PWAs had been in the field for 17 year prior to evaluation. The other two PWAs were fielded and then subsequently exposed to an accelerated aging environmental test. A quantitative analysis was performed to measure IMC layer thickness at the Sn-Pb/Cu interface and the size distribution of the Pb-rich phase particles in the Sn-Pb solder microstructure. The IMC layer growth thickness data showed that relatively little growth had occurred within all four PWAs. This finding indicated that the PWAs that were exposed to the accelerated aging conditions had, in fact, not experienced those conditions. Measurements of the Pb-rich phase size substantiated the IMC data. The Pb-rich phase size data, when compared between various locations in the solder joints, were consistent with the occurrence of a limited amount of thermal mechanical fatigue (TMF) caused by the encapsulant and z-axis expansion behavior of the circuit board. This study showed that internal PWAs may be precluded from exposure to the outside temperature environments due to the intervening structures.