Comorbid diabetes or hypercholesterolemia may indicate a smaller probability of major or moderate PTA improvement for patients with ISSNHL.

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High stresses in semiconductor die and other packaging elements can be developed in electronic assemblies subjected to extremely low ambient temperatures leading to reliability concerns. In this work, we have characterized and modeled the silicon die stresses occurring in flip chip assemblies at low temperatures. Stress measurements have been made at temperatures down to −180°C using test chips incorporating piezoresistive sensor rosettes. The obtained stress measurement data have been correlated with the predictions of nonlinear finite element models. A microtester has been used to characterize the stress-strain behavior of the solders and encapsulants from −180 to +150°C to aid in this modeling effort.

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Microelectronic encapsulants exhibit evolving properties that change significantly with environmental exposures such as isothermal aging and thermal cycling. Such aging effects are exacerbated at higher temperatures typical of thermal cycling qualification tests for harsh environment electronic packaging. In this work, measurements of material behavior changes occurring in flip chip underfill encapsulants exposed to isothermal aging have been performed. A novel method has been developed to fabricate freestanding underfill uniaxial test specimens so that they accurately reflect the encapsulant layer present in flip chip assemblies. Using the developed specimen preparation procedure, isothermal aging effects have been characterized at several elevated temperatures (+ 80, +100, + 125, and +150 °C). Samples have been aged at the four temperatures for periods up to 6 months. Stress-strain and creep tests have been performed on non-aged and aged samples, and the changes in mechanical behavior have been recorded for the various aging temperatures and durations of isothermal exposure. Empirical models have been developed to predict the evolution of the material properties (modulus, strength) and the creep strain rate as a function of temperature, aging time, and aging temperature. The evaluated underfill illustrated softening behavior at temperatures exceeding 100 °C, although the documented Tg ranged from 130–150 °C. The obtained results showed an obvious enhancement of the underfill mechanical properties as a function of the aging temperature and aging time. Both the effective elastic modulus (initial slope) and ultimate tensile strength (highest stress before failure) increase monotonically with the amount of isothermal aging or aging temperature, regardless of whether the aging temperature is below, at, or above the Tg of the material. From the creep results, it was seen that at a given time, the creep strains were much lower for the aged samples relative to the non-aged samples. Thermal aging has a significant effect on the secondary creep rate, which decreases with both the aging temperature and the aging time. Up to a 100X reduction in the creep rate was observed, and the major changes occurred during the first 50 days of the isothermal aging.

In this work, the effects of underfill cure temperature and JEDEC MSL preconditioning on underfill mechanical and strength properties, as well as flip chip assembly reliability have been explored. Baseline stress-strain curves, mechanical properties, and interfacial shear strengths of a capillary underfill were recorded for curing at 150 °C and 165 °C (30 minutes). In addition, the changes in the mechanical and strength properties resulting from MSL3 and MSL2 moisture preconditioning were evaluated. The MSL preconditioning of the underfill samples included the JEDEC specified humidity and temperature exposures, plus three simulated reflows at 245 °C or 260 °C. Thermal cycling life tests from −55 to 125 °C were also conducted on daisy chain flip chip assemblies incorporating the same underfill. The test matrix for the reliability testing included both 150 °C and 165 °C curing profiles, and two levels of precondition (none and MSL3). Finally, the failure mechanisms in the flip chip assemblies were studied using CSAM, x-ray and SEM analyses. The results clearly indicate the advantages of the higher curing temperature including improved mechanical properties, superior thermal cycling fatigue life, and enhanced resistance to detrimental effects from moisture exposure and solder reflow.

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