It is generally found that grinding transverse to the tensile stress direction in flexure bars subjected to four-point bending results in a lower strength compared to grinding in the longitudinal direction. In the present study, standard flexure specimens made from a reaction-bonded and a sintered reaction-bonded silicon nitride (RBSN and SRBSN) were surface ground under three different conditions in both longitudinal and transverse directions to assess the effect of grinding direction on strength. Four-point flexure tests were performed on the specimens and Weibull parameters were calculated. The results showed that while the strength was not affected by the grinding condition when grinding was performed in the longitudinal direction, the strength of the samples ground in the transverse direction was reduced as the material removal rate was increased by a factor of 30. This result was confirmed by fractography, which showed that almost all the fracture initiation sites in the longitudinally ground samples were associated with near-surface microstructural features, whereas in the transverse ground samples fracture initiated from damage introduced by grinding. The strength reduction by grinding in the transverse direction was found to be material dependent, and was larger for SRBSN than for RBSN.

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Failures in electronic packages under thermal fatigue usually result from cracking in solder joints due to creep/fatigue crack growth. Understanding the stress/strain behavior of such solder joints is the first step in characterizing their fracture behavior. A specimen has been developed to determine the stress/strain hysteresis response of 90Pb/10Sn solder joints under cyclic thermal loadings. Simple and special techniques have been developed to fabricate solder joints with relatively high melting points, such as 90Pb/10Sn. Four high-temperature strain gages are mounted on the specimen to measure mechanical strains which provide the basis for determining the shear stress and strain in the solder. A special Wheatstone bridge has been designed to improve the specimen sensitivity, e.g., 20 με/MPa in the test. Shear stresses in the solder as low as 0.5 MPa can be resolved accurately. The specimen was subjected to thermal cycling between 40°C to 140°C, with 10°C/min ramp rate and 10 minute hold times, in a thermal chamber developed in-house. Excellent experimental results have been obtained for 90Pb/10Sn solder joints in that detailed characteristics of stress relaxation and strain creep as a function of temperature were captured. The specimen developed is not only suitable for use in studying the constitutive response of soft solder alloys and other joining materials, such as adhesives, but can serve as a thermal fatigue specimen to study the fracture behavior.

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