Lap shear test for solder materials: Local stress states and their effect on deformation and damage

https://doi.org/10.1016/j.microrel.2020.113655Get rights and content

Highlights

  • Numerical study of local stresses and stress state in lap shear test

  • Experimental investigation of deformation and damage

  • Numerical investigation of strain measures in lap-shear test

  • Correction method to obtain objective shear moduli

Abstract

This work presents a detailed analysis of stress states and different strain measures of two lap-shear sample designs. The stress- and strain distribution are discussed for both sample designs through numerical simulation. Additionally in-situ measurements of lap-shear experiments are performed to compare the large strain and damage behavior of the different sample designs. Despite the use of in-situ video-extensometer strain measurements, a deviation among elastic moduli from tensile- and lap-shear experiments is observed. Finite element analysis reveals that inhomogeneous strains due to boundary effects are the reason for the observed moduli deviation. A strain correction method is presented to correct video-extensometer measurements. The parameters of the correction function were obtained from numerical simulation and are provided for several sample dimensions. To illustrate the effectiveness of the proposed strain correction method, the correction function is applied to experimental lap-shear test data. The corrected shear moduli compare well with shear moduli calculated from tensile tests reported in literature.

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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