Thermal cycling accelerated life testing is an established technique for thermo-mechanical evaluation and qualification of electronic packages. Finite element life predictions for thermal cycling configurations are challenging due to several reasons including the complicated temperature/time dependent constitutive relations and failure criteria needed for solders, encapsulants and their interfaces; aging/evolving material behavior for the packaging materials (e.g. solders); difficulties in modeling plating finishes; the complicated geometries of typical electronic assemblies; etc. In addition, in-situ measurements of stresses and strains in assemblies subjected to temperature cycling are difficult because of the extreme environmental conditions and the fact that the primary materials/interfaces of interest (e.g. solder joints, die device surface, wire bonds, etc.) are embedded within the assembly (not at the surface). For these reasons, little is known about the evolution of the stresses, strains, and deformations occurring within sophisticated electronic packaging geometries during thermal cycling. In this work, we have used test chips containing piezoresistive stress sensors to characterize the in-situ die surface stress during long-term thermal cycling of electronic packaging assemblies. Using (111) silicon test chips, the complete three-dimensional stress state (all 6 stress components) was measured at each rosette site by monitoring the resistance changes occurring in the sensors. The packaging configuration studied in this work was flip chip on laminate where 5 × 5 mm perimeter bumped die were assembled on FR-406 substrates. Three different thermal cycling temperature profiles were considered. In each case, the die stresses were initially measured at room temperature after packaging. The packaged assemblies were then subjected to thermal cycling and measurements were made either incrementally or continuously during the environmental exposures. In the incremental measurements, the packages were removed from the chamber after various durations of thermal cycling (e.g. 250, 500, 750, 1000 cycles, etc.), and the sensor resistances were measured at room temperature. In the continuous measurements, the sensor resistances at critical locations on the die device surface (e.g. die center and die corners) were recorded continuously during the thermal cycling exposure. From the resistance data, the stresses at each site were calculated and plotted versus time. The experimental observations show cycle-to-cycle evolution in the stress magnitudes due to material aging effects, stress relaxation and creep phenomena, and development of interfacial damage.
Continuous nonenzymatic cross-replication of DNA strands with in situ activated DNA oligonucleotides