Lap shear test for solder materials: Local stress states and their effect on deformation and damage
Graphical abstract
Introduction
The analysis of shear, creep and thermal fatigue behavior of solder alloys is often done via lap-shear tests. This allows to determine material properties under shear dominated loads [[1], [2], [3], [4]]. There are no specific standards for lap-shear samples with respect to materials testing. Therefore, several sample designs with variation in shape and dimension are common in material testing of solders. The lap-shear test characterizes the shear- and interfacial strength of solder joints [[5], [6], [7], [8], [9]]. A great number of studies exist that illustrate the strength and elastic deformation of bonded lap-shear joints through simulation and experiments [4,[10], [11], [12], [13], [14]], focusing on the structural behavior of the components but not on determining material properties of the joints. Experimental studies of Zimprich et al. [13] showed that measured properties depend on the sample design. The solder dimensions such as thickness t and length l play an important role for the loading condition [[13], [14], [15]].
Video-extensometer (VE) strain measurements are often used for in-situ studies of lap-shear experiments [9,12,16]. The VE measures displacements of patterns on the sample surface and displacements are subsequently used to calculated local strains of the sample. There are no reports how the sample dimensions affect the strains at the boundary and if such boundary effects are critical for the calculated shear moduli.
Lap-shear joints have been analyzed using linear and non-linear Finite Element Methods (FEM) [10,11,[16], [17], [18], [19]] and also by deriving closed form solutions [2]. Studies focusing on lap-shear joints treat the solder elastic and consider the lap-shear sample under plane-strain condition. Cognard et al. [11] studied the stress distribution in solder joints under the assumption of a linear elastic adhesive, where significant peel-off effects at the boundary areas of the adhesive were demonstrated. The mechanics of bonding layers was studied and analytical solutions for the peel-off and shear stresses were derived by Abdelhadi et al. [20].
This work provides an analysis of the local stress state in the solder under the assumption of rate dependent material behavior. The stress state and stress distribution in both samples under variation of geometric dimensions are analyzed through numerical simulation for small deformations. Furthermore, the large strain deformation and damage behavior of both samples was investigated through in-situ experiments. Shear moduli determined from lap-shear experiments reveal significant deviation from its objective values. The root cause of this systematic error has, to the knowledge of the authors, not been investigated in literature. We propose a method to obtain corrected shear moduli for different sample designs. The well documented SAC305 solder was used to allow for quantitative comparison with literature data.
Section snippets
Numerical model
The mechanics of the lap-shear test is studied using FEM modeling for small deformations. In Fig. 1 the sample geometries of the FEM models and their boundary conditions are illustrated. Design A represents the commonly used standard sample geometry, while Design B is an improved design. Previous studies have used similar sample designs [21,22], yet a quantitative comparison for different sample designs in terms of stress states and deformation behavior is missing in literature. Both
Results and discussion
Stress distributions from numerical simulation of both designs are compared and the differences among designs A and B are discussed. In-situ measurements are used to compare the damage behavior at large deformations of both sample designs. The deviation of representative- and VE strain measurements are analyzed numerically and a strain correction method is presented. The strain correction is applied on experimental data and comparisons with measurements from literature are discussed.
Conclusion
This work provides insight into the mechanics of the lap-shear experiment. The lap-shear experiment is widely used, yet a detailed study of the local strain distribution and local stress state was missing. A correction method is presented to obtain more quantitative results from experimental results. The well-known Sn-3.0Ag-0.5Cu solder was studied numerically and through experiments to illustrate the underestimation of shear moduli. A common lap-shear designs (A) and an alternative one (B) are
CRediT authorship contribution statement
Georg Siroky:Conceptualization, Methodology, Formal analysis, Writing - original draft.Julien Magnien:Resources, Writing - review & editing.David Melinc:Resources.Ernst Kozeschnik:Writing - review & editing, Supervision.Dietmar Kieslinger:Project administration, Funding acquisition.Elke Kraker:Writing - review & editing, Project administration.Werner Ecker:Writing - review & editing, Supervision.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgment
Financial support by the Austrian Federal Government (in particular from Bundesministerium für Verkehr, Innovation und Technologie) represented by Österreichische Forschungsförderungsgesellschaft mbH, Austria within the framework of the “24. Ausschreibung Produktion der Zukunft, nationale Projekte” Programme (project number: 864808 project name: SOLARIS) is gratefully acknowledged.
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