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No-flow underfill materials reduce assembly processing steps and can potentially be used in fine-pitch flip chip on organic board assemblies. Such no-flow underfills, when filled with nano-scale fillers, can significantly enhance the solder bump reliability, if the underfills do not prematurely delaminate or crack. Therefore, it is necessary to understand the risk of underfill delamination during assembly and during further thermal excursions. In this paper, the interface between silicon nitride (SiN) passivation and a nano-filled underfill (NFU) material is characterized under monotonic as well as thermo-mechanical fatigue loading, and fracture parameters have been obtained from such experimental characterization. The passivation-underfill interfacial delamination propagation under monotonic loading has been studied through a fixtureless residual stress induced decohesion (RSID) test. The propagation of interfacial delamination under thermo-mechanical fatigue loading has been studied using sandwiched assemblies and a model for delamination propagation has been developed. The characterization results obtained from this work can be used to assess the delamination propagation in flip-chip assemblies. Though the methods presented in this paper have been applied to nano-filled, no-flow underfill materials, their application is not limited to such materials or material interfaces.

Chip Scale Packages (CSP) are ideal intermediates between Direct Chip Attach (DCA) and Ball Grid Array (BGA) technologies in terms of both size and cost. Depending upon the application, chip scale packages are either underfilled for better solder joint reliability or are attached with a heat sink to keep the operating temperature of the chip under control. In many applications, as discussed in this paper, both an underfill and a heat sink are required. Quite expectedly the addition of two more materials, heat sink and adhesive, in the board level assembly results in fresh reliability concerns. In particular, the requirements on the underfill material and the heat sink attach adhesive are more rigorous and needless to say, a proper understanding of process and material issues is needed to make such a choice. The inelastic strains experienced by the solder joint (related to the underfill) and the peeling stresses at the heat sink attach adhesive interfaces (related to the thermal adhesive) are used as metric for comparing the number of material choices that are available. Based on the results, it is shown that it is important to choose materials that are thermo-mechanically matched with the rest of the system.

Accelerated thermal cycling tests are used to ascertain the reliability of solder interconnects in electronics assemblies. These tests typically last a few months and therefore, are highly resource intensive. Thermal shock tests on the other hand are faster but have been found to be ineffective in accelerating thermal cycling failures for eutectic tin lead solder. In this paper, thermal shock testing is proposed as an alternative to conventional thermal cycling testing for lead-free solder interconnects using Sn, Ag and Cu (SAC) solder. Results from the thermal shock and thermal cycling testing of Ball-Grid-Array (BGA) components are presented. Failure analyses of the solder joints reveal the failure mode for thermal shock in comparison to thermal cycling testing. Numerical modeling results for the thermal cycling and thermal shock testing for lead free and eutectic lead solder are then presented and discussed. The simulation results agree with the experiments and theory is proposed to account for the similar trends between thermal cycling and thermal shock testing for lead free solder.

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