Tensile and creep tests were conducted to characterize the deformation behavior of four dilute SnBi alloys: SnBi0.5 at. pct, SnBi1.5 at. pct, SnBi3 at. pct, and SnBi6 at. pct, the last two being supersaturated solid solutions at room temperature. The test temperatures were Ϫ20 ЊC, 23 ЊC, 90 ЊC, and 150 ЊC, and the strain rates ranged from approximately 10 Ϫ8 to 10 Ϫ1 1/s. In the tensile tests, all the alloys showed strain-hardening behavior up to room temperature. At higher temperatures, only the higher-Bi-content alloys exhibited strain softening. The deformation behavior of the alloys can be divided into two stress regimes, and the change from the low-stress regime to the high-stress regime occurred at around 6 ϫ 10 Ϫ4 Ͻ /E Ͻ 7.5 ϫ 10 Ϫ4 . The results suggest that, at the low-stress regime, the rate-controlling deformation mechanism changes from dislocation climb to viscous glide with the increasing Bi content of the alloy. At the high-stress regime, the activation energy of deformation is about equal in all the alloys (ϳ60 kJ/mol) and the stress exponents are high (10 Ͻ n Ͻ 12.5). Unlike in the other alloys, bismuth precipitated at room temperature from the solution-annealed and quenched SnBi6 at. pct alloy by the discontinuous mechanism. This strongly affects the mechanical properties and makes the alloy brittle at lower test temperatures. A comparison of the deformation behavior of the dilute SnBi alloys to that of the eutectic SnBi alloy suggests that the deformation of eutectic structure is controlled by the Sn-rich phase containing the equilibrium amount of dissolved Bi.

This study demonstrates the application of three-point and four-point bending tests for evaluating the reliability of chip scale packages under curvature loads. A three-point bend test is conducted on 0.5-mm-pitch chip-scale packages (CSPs) mounted on FR4 (Flame Retardant) substrates. This test is simulated by using the finite element method and the results are calibrated experimentally to formulate a reliability model. A three-point bend scheme is an ideal choice for generating reliability models because multiple packages can be tested under multiple loads in a single test. This reliability model can be used to predict the durability of the packages in the real product under any printed wiring board (PWB) curvature loading conditions. A four-point bending simulation is also demonstrated on the test substrate. Four-point bending test is an ideal method for testing a larger sample size of packages under a particular predefined stress level. This paper describes the bending simulation and testing on packages in a generic sense. Due to the confidentiality of the test results, the package constructional details, material properties, and the actual test data have not been presented here.

Based on the developed simple and physically meaningful analytical (‘mathematical’) stress model, we evaluate some major parameters (amplitude, frequency, maximum acceleration, stresses and strains) of the response of a ‘flexible-and-heavy’ square simply supported printed circuit board (PCB) to an impact drop load applied to its support contour. The analysis is restricted to the first mode of vibrations and is carried out in application to the PCB design employed in an advanced accelerated test setup (test vehicle). This setup is aimed at the assessment of the performance, in accelerated test conditions on the board level, of packaging materials (and, first of all, BGA solder joint interconnections) subjected to dynamic (drop or shock) loading. It is anticipated that heavy masses could be mounted on the PCB to accelerate its dynamic response to an impact load. These masses are expected to be small in size, so that while changing the total mass of the board and generating significant inertia forces, they do not affect the board's flexural rigidity or its stiffness with respect to the in-plane loading. The PCB's contour is considered non-deformable, which is indeed the case in many practical situations. This circumstance, if the drop height and/or the induced inertia forces are significant, leads to elevated in-plane (‘membrane’) stresses in the PCB and, as a result of that, to the nonlinear response of the board to the impact load: the relationship between the magnitude of the load (determined by the initial impact velocity) and the induced PCB deflections becomes geometrically nonlinear, with a rigid cubic characteristic of the restoring force. The carried out numerical example, although reflects the characteristics of the PCB and loading conditions in an actual experimental setup, is merely an illustration of the general concept and is intended to demonstrate the abilities of the suggested method. Predictions based on this method agree well with the finite element analysis (FEA) data. The model can be helpful in understanding the physics of the addressed problem. The obtained results can be easily generalized, if necessary, for PCBs of different aspect ratios and with other boundary conditions, for different distributions of the added masses, etc, and applied, with adequate modifications, to PCBs in actual use conditions as well. The model can be used, along with FEA simulations, in the analysis, structural (‘physical’) design and accelerated testing of electronic systems of the type in question, and particularly of 'flexible-and-heavy' PCBs, both in accelerated tests and in actual operation conditions.

This study investigates the effect of quasi-static bending loads (strain rate=0.05/s) on the durability of 0.5 mm pitch Chip Scale Package (CSP) interconnects when assembled on FR4 substrates. The substrates have rows of CSPs and are subjected to three-point bending loads. Overstress curvature limits are experimentally determined and used to identify limits for zero-to-max cyclic bending loads. The test configuration is simulated using finite element modeling (FEM) and the total strain accumulated in the solder joints is estimated. Using the FEM model, a calibration curve is constructed to relate the cyclic curvature range in the substrate to the cyclic strain range in the critical solder joint. Bending moments along the substrate are estimated from the forces applied at the center of the board during the fatigue test. Strains measured on the substrate surface and the bending displacements measured at the center are used to estimate curvatures at different locations along the substrate. Using the calibration curve, the total strains in the solder joint are obtained for the applied loading. A strain-range fatigue damage model proposed by Coffin and Manson, is used to predict the cycles to failure for the applied loading. Predicted durability is compared to experimental measurements. Concave substrate curvature is found to be more damaging than convex curvature, for interconnect fatigue. Finite element simulations are repeated for life-cycle loading to predict acceleration factors. Using the acceleration factors, the product durability is estimated for life-cycle environments.

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