A previously developed laser spallation technique is adapted to measure in situ the tensile strengths of geometrically heterogeneous interfaces, in multilayer freestanding chip-scale packages that were baked for specific temperatures and times. The test procedure involved quantification of the stress waves inside the packages using interferometry, and subsequent stress field quantification, including that at the failed interface, using a wave mechanics simulation. The technique is generally applicable and can be used to test any type of freestanding package. In this work, it is demonstrated on freestanding 0.5mm-pitch MicroStar BGA(TM) packages. The ball grid array joint strengths for Pb-free solders on bare Cu pads were measured to be 942 +/- 91, 703 +/- 69, 666 +/- 70, 441 +/- 42, and 392 +/- 45MPa for samples that were thermally aged for 3, 10, 20, 40, and 80days, respectively. An important result of our study is that the critical laser energy for causing joint failure was found to be approximately proportional to the peak tensile stress at the joint. This validates the discussion earlier where no stress quantification was carried out, but the interfaces were characterized only in terms of the critical laser energies and the deteriorations in the material microstructure in the solder joint region caused by thermal aging were related to the reductions in the measured critical laser energies. This powerful result shows that in the future it may suffice to use the critical laser energy for material selection and quality control during manufacturing.