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The thermal cyclic shear stress/strain hysteresis response and associated steady-state creep parameters of 97Sn-3Cu solder joints have been determined using a beam specimen previously developed by Pao et al. (1992a). The solder joint was subjected to a 40-minute thermal cycling from 40°C to 140°C. A constitutive equation based on elastic and steady-state creep deformation for the solder has been formulated and implemented in a finite element program, ABAQUS, to model the experiment. The results show that the constitutive equation based on one single creep mechanism cannot fully account for the deformation during cooling, as opposed to the case of 90Pb-10Sn where the entire cyclic deformation can be well modeled by a similar constitutive equation (Pao et al., 1992c). This suggests that another creep mechanism is dominant for lower stresses and higher temperature. The thermal fatigue results show that the failure mechanism of 97Sn-3Cu joints is similar to that of 90Pb-10Sn joints, but the number of cycles to failure of 97Sn-Cu solder joints is at least 5 times longer than 90Pb-10Sn solder joints. This indicates the potential application of 97Sn-3Cu in place of 90Pb-10Sn solder.

Leadless chip resistors (LCR) made by two different manufacturers and surface mounted on glass/epoxy printed circuit board (PCB) were subjected to thermal cycling between −55°C to 125°C in order to induce thermal fatigue failure/damage. The test units were subjected to a maximum of 250 thermal cycles. Solder joints in both types of LCRs were examined in scanning electron microscope and a relative comparison of the extent of fatigue damage is presented. The failure mechanism is associated with cracking in the eutectic compostion Sn/Pb solder initiated at the stress concentration sites. A nonlinear, time-dependent finite element modeling analysis has been performed to determine critical stress concentration sites in the solder joint. Key parameters leading to the initiation of solder damage are identified, and recommendations are made to improve the design in terms of solder configuration such as the radius of corner of the alumina substrate and the standoff height.

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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