Zhengfang Qian

Investigation of electronic packaging materials by using a 6-axis mini thermo-mechanical tester

Most existing mechanical testers are at most bi-axial and in general too large for microelectronic materials and structures. Existing mini testers are primarily single axis without any active specimen alignment monitoring and adjustment capability. Fundamental investigation needs to be conducted for packaging materials in terms of deformation and fracture processes, constitutive laws, and failure quantities. In this paper, a unique 6-axis sub-micron thermo-mechanical fatigue tester is described, including some calibration work for both load cell and machine stiffness. For the first time, an active specimen alignment monitoring and adjustment was demonstrated on a single lap shear sample, assisted by a high resolution laser moire measurement system. This paper also presents results for two types of polymer films, one lead-free solder alloy, and some other conventional materials. In particular, deformation and failure mechanisms for two polymer films have been discussed.

Processing mechanics for flip-chip assemblies

Abstract In this paper, a non-linear finite element framework has been implemented to simulate the sequential build-up of a flip-chip package. A generalized deformation model with element removal and addition is used to activate and deactivate the underfill material layer to simulate flip-chip package fabrication. Using process models, one can determine the warpage stresses at any intermediate stage in the process. In addition, topological change is also considered in order to model the sequential steps during the flip-chip assembly. Geometric and material nonlinearity which includes the creep behavior of underfill and solder balls, and temperature-dependent material properties are considered. Different stress-free temperatures for different elements in the same model are used to simulate practical manufacturing process-induced thermal residual stress field in the chip assembly. This approach (the processing model established in this paper) is in contrast to the non-processing model employed by many researchers, which is shown to yield overly conservative and sometimes erroneous results, leading to non-optimal design solutions. From the finite element analysis, it is found that the strains and deflections obtained from the non-processing model are generally smaller than those obtained from the processing model due to the negligence of the bonding process-induced residual strains and warpage. Furthermore, the fatigue life for the outmost solder ball predicted by the processing model is much shorter than that predicted by the non processing model based on the Coffin–Manson equation. On the other hand, in order to prove the soundness of the framework established in this paper, the test results obtained by using the laser moireinterferometry technique are compared with the results achieved from the proposed numerical analysis vehicle. It is shown that the deformation values of the flip-chip package predicted from the finite element analysis are in a good agreement with those obtained from the test.

Visco-elastic-plastic properties and constitutive modeling of underfills

The thermo-mechanical testing of HYSOL FP4526 underfill, a filled polymer, is reported, including the details of sample preparation and test procedures. Both strain rate and temperature dependence are found for the Youngs modulus of the underfill. The constitutive framework proposed for solder alloys (Qian and Liu, 1997) is, for the first time, applied successfully to the thermo-mechanical properties of HYSOL FP4526 in this paper. Excellent agreement between model predictions and experimental data is achieved.

Fatigue life prediction of flip-chips in terms of nonlinear behavior of solder and underfill

The nonlinear properties of eutectic solder and underfill FP4526 have been reported and incorporated into ABAQUS material model library for the fatigue life prediction of advanced electronic packages. The methods of unified constitutive modeling, separated constitutive modeling, and fatigue life prediction have been described and compared in detail. The nonlinear constitutive modeling of solder alloys and underfills plays an important role in the life prediction in terms of the correct calculation of inelastic strain range and inelastic strain energy density of hysteresis loop under various accelerated test conditions. Combined with unified constitutive modeling, strain-based approach of life prediction is recommended in this paper. The analyses performed in this report present valuable insights for the design of advanced flip-chip packages.

Scale effect on packaging materials

The scale effect on the typical packaging materials, underfill HYSOL(R) FP4526, FP4511 and eutectic solder alloy 63Sn37Pb is investigated in this paper. By using specially designed thin strip specimens and a computer controlled 6-axis mini fatigue tester, a series of reliable and consistent test data are obtained. It shows that the specimen scale has no significant effect on the test results within the specimen thickness investigated in this research.

Resolving displacement field of solder ball in flip-chip package by both phase shifting moire interferometry and FEM modeling

In this paper, phase shifting moire interferometry was used to resolve the deformation field of solder balls in a flip-chip package under thermal loading condition. A nanoscale deformation field of the outmost solder ball was obtained by using the proposed phase shifting technique associated with the corresponding image processing software. In addition, a nonlinear finite element technique, in which the viscoelastic material properties of underfill and the viscoplastic material properties of solder balls were considered was also adapted to simulate the global displacement field - the whole cross-section of the flip-chip package and the local displacement field - the whole cross-section of one solder ball in the flip-chip package. By comparing the predicted deformation values of the flip-chip package obtained from the finite element analysis with the test data obtained from the laser moire interferometry technique, good agreement is obtained. In particular, the nanoscale displacement contours of the solder ball both in x and y directions obtained from the phase shifting technique show much more similar distribution patterns compared with those modeled by the finite element method.

Investigation of nonlinear behaviors of packaging materials and its application to a flip-chip package

The creep behavior of a flip-chip package under thermal load is studied using finite element models and high density laser moire interferometry. FEA accuracy strongly depends on a reliable input material database, and thus a series of tests were first carried out using a 6-axis mini fatigue tester for eutectic 63Sn37Pb solder alloy and FP4526 underfill. From the test data, six FEA models are used to simulate creep under elastic, elastic-plastic, viscoelastic or viscoplastic behavior for both solder balls and underfill. The results show that solder and underfill nonlinear material behavior do not have a major effect on flip-chip warpage, but do affect the stress. In all visco FEA models, Von Mises stresses at the corner or center of the outermost solder ball greatly decrease. However, Von Mises stresses predicted by elastic-plastic and elastic models remain unchanged during the temperature holding time and are much higher than those from visco FEA models. Although solder ball stresses predicted by visco models have no major difference, this is not true for underfill. By comparing FEA results, it is suggested that the strain rate-dependent model is used to describe underfill creep behavior. However, the inelastic equivalent strain which is usually used as the fatigue life prediction parameter also shows big differences between FEA models, and thus a suitable model must be carefully chosen for accurate fatigue life prediction. Furthermore, flip-chip package creep is measured by real-time moire interferometry. The FEA model package deformation values are compared with the laser moire interferometry data and are in good agreement.

Thermo-mechanical creep of two solder alloys

Thermo-mechanical creep behaviors of a new lead free solder alloy 80Sn10In9.5Bi0.5Ag and eutectic solder alloy 63Sn37Pb are investigated in this paper. By using specially designed thin strip specimens and a computer controlled 6-axis mini fatigue tester, a series of reliable and consistent creep data are obtained. The 80Sn10In9.5Bi0.5Ag solder alloy shows very attractive creep characteristics and may have potential applications in electronics packaging technology. On the other hand, a new unified viscoplastic constitutive model proposed by Qian and Liu (1997b) is introduced to predict the creep properties of eutectic solder alloy. An excellent agreement between experimental data and model predictions is achieved. It is also observed that the creep rupture time will be significantly overpredicted if only power law regime is considered.

Creep behavior of a flip-chip package by both FEM modeling and real time moire interferometry

In this paper, the creep behavior of a flip-chip package under a thermal load was investigated by using nonlinear finite element technique coupled with high density laser moire interferometry. The real-time moire interferometry technique was used to monitor and measure the time-dependent deformation of flip-chip packages during the test, while the finite element method was adapted to analyze the variation of stresses at edges and corners of interfaces with time by considering the viscoelastic properties of the underfill and the viscoplastic behavior of the solder balls. The results show that the creep behavior of the underfill and the solder balls does not have significant effect on the warpage of the flip-chip under the considered thermal load due to their constrained small volume. The variation of the time-dependent deformation in the flip-chip package caused by the creep behavior of the underfill and the solder balls is in the submicron scale. The maximum steady state U-displacement is only reduced by up to 6.7% compared with the maximum initial state U-displacement. Likewise, the maximum steady state V-displacement is merely reduced by up to 10% compared with the maximum initial state V-displacement. The creep behavior slightly weakens the warpage situation of the flip-chip package. However, the modeling results show that the localized stresses at corners and edges of interfaces greatly decrease due to the consideration of viscoelastic properties of the underfill and the viscoplastic properties of the solder balls and thereby effectively prevents interfaces from cracking. In addition, the predicted deformation values of the flip-chip package obtained from the finite element analysis were compared with the test data obtained from the laser moire interferometry technique. It is shown that the deformation values of the flip-chip package predicted from the finite element analysis are in a fair agreement with those obtained from the test.

Processing mechanics for flip-chip assembly

In this paper, a non-linear finite element framework was established for processing mechanics modeling of flip-chip packaging assemblies and relevant layered manufacturing. In particular, topological change was considered in order to model the sequential steps during flip-chip assembly. Geometric and material nonlinearity which includes the viscoelastic property of underfill and the creep behavior of solder ball, temperature-dependent material properties were considered. Different stress-free temperatures for different elements in the same model were used to simulate practical manufacturing process-induced thermal residual stress field in the chip assembly. As comparison, two FEM models of flip- chip package considered, associated with different processing schemes, were analyzed. From the finite element analysis, it is found that the stresses and deflections obtained from non-processing model are generally smaller than those obtained from the processing model due to the negligence of the bonding process-induced residual stresses and warpage. The values of the stresses at the given point obtained from the processing model are about 20 percent higher than that obtained from the non-processing model. The deflection values at the given points obtained rom the processing model are usually 25 percent higher than those obtained form the non-processing model. Therefore, a bigger error may be caused by using non-processing model in the analysis of process-induced residual stress field and warpage in the packaging assemblies. It is also noted that the viscoelastic property of the underfill considered in the flip-chip only cause 10 percent change of the stresses and deflections at given points at most. It is shown that the effect of viscoelastic behavior on process-induced stresses during the flip-chip assemblies can be negligible. The creep behavior of the solder balls has stronger effect on the normal stress and the peeling stress of the solder ball at the considered point. The steady state peeling stress is about 20 percent lower than the initial state peeling stress, while the steady state normal stress in x direction is only half the initial state normal stress in x direction.