Ken-ichi Ohguchi

Evaluation of Time-Independent and Time-Dependent Strains of Lead-Free Solder by Stepped Ramp Loading Test

This paper proposes a method to estimate basic material constants in an elasto-plastic-creep constitutive model for lead-free solders by conducting only a tensile test. The test employs a stepped ramp wave loading, which repeats instantaneous strain and strain maintenance. Time-independent strains are obtained from the stress-strain relations at the instantaneously strained parts, while the time-dependent strains are obtained from the stress-time relations during the strain maintaining periods. Based on the obtained time-independent and time-dependent strains, the values of the material constants in the elasto-plastic-creep model proposed by the authors are determined. Simulations of the viscoplastic deformations of a Sn―3.0Ag―0.5Cu solder alloy are also conducted to verify the validity of the proposed method.

Experimental Observation of Correlation Between Creep and Uniaxial Ratchetting of Sn∕37Pb and Sn∕3Ag∕0.5Cu Solder Alloys

Time-dependent deformations such as creep and ratchetting of solder alloys are significant deformation phenomena that need to be understood to ensure the safety and reliability of solder joints in electronic packaging. There is much research on creep deformation of solder alloys, but ratchetting deformation, especially the correlation between creep and ratchetting deformation of solder alloys has not been investigated. This paper discusses the correlation between creep and uniaxial ratchetting deformation to establish the differences in the time-dependent deformation of lead-free and lead-containing solder alloys. Uniaxial ratchetting tests were conducted by cyclic tension-compression tests or cyclic tension-unloading tests at several ratios of the maximum to minimum stresses. Additional creep tests following the uniaxial ratchetting were also conducted to observe the effect of the uniaxial ratchetting on creep deformation. An empirical method to select an optimal lead-free solder alloy is discussed by defining a uniaxial ratchetting strain rate. The additional creep tests also show that the uniaxial ratchetting deformation has a strong correlation to the creep deformation and that the correlation is different for lead-free and lead-containing solder alloys.

Evaluation of Creep Deformation by Indentation Test with a Constant Depth - Applicability for Inhomogeneous Materials

In design of the electronic device, FEM analyses considering the creep deformation of solder joints in-situ should be conducted to evaluate the strength reliability. The indentation test is one of effective method to evaluate the creep deformation in microscopic region. However, the result obtained by the indentation test does not coincide with that obtained by the tensile creep test. To solve the problem, the method to determine the suitable area for the indentation test had also been proposed by using the numerical test. To apply the proposed method for the actual indentation test, the homogeneity of specimen should be considered. In this paper, the analyses of the proposed indentation tests were conducted by using the homogeneous and inhomogeneous specimen models of Sn-3.0Ag-0.5Cu. Three types of the deformation behavior were given for the initial phase and the indenter was pushed into in the specimen at the three locations. As a result, it was found that there was not difference in the distribution of the principal stress caused by variation in the indent location. However, the proposed method can successfully determine the reference area except for the case when the deformation behaviors of the constituent phases have a large difference.

Estimation of Plastic and Creep Strain Development of SAC Solder Under Cyclic Loading by Using Stepped Ramp Waves

The fatigue tests under both loading conditions of the Fast-Slow and the Slow-Fast were conducted to discuss validity of an inelastic strain analysis method which was proposed previously. The development behaviors of plastic and creep strains during the tests were analyzed by employing the stepped ramp wave (SW) loading. Using the analysis results, the difference of the development behaviors of plastic and creep strains between the Fast-Slow condition and the Slow-Fast condition was clarified. Then, the fatigue failure surfaces obtained from the tests were observed by using SEM to clarify the difference of the fracture surface between the Fast-Slow condition and the Slow-Fast condition. By correlating the aspect of fracture surface with the development behavior of plastic and creep strain, the validity of the proposed inelastic strain analysis method was discussed.

Biaxial Ratchetting Deformation of Solders Considering Halt Conditions

Recently, Halt (Highly accelerated limit test) is widely employed for evaluation of reliability of electronic products. Halt condition is quite severe. The tested products are subjected to mechanical impacts, thermal shock, and vibration at same time. However, there has not been a reasonable and accurate evaluation method for Halt yet. To construct an accurate evaluation method of Halt, basic deformation mechanism of parts of the electronic products should be clarified from both experimental and theoretical points of view. In this paper, focusing on solder joints of circuit boards of electronic products, ratchetting deformation, especially, biaxial ratchetting deformation of solder joints is revealed from both experimentally and theoretically. The authors have already conducted biaxial ratchetting test combining axial and torsional cyclic loading using a tubular specimen of Type 304 stainless steel. However, as for solders, it is difficult to make tubular specimen. Since size of the solder joints is micron, a small size joint specimen of copper tube and solder is employed in this paper. First, to confirm the quality of the joint specimen such as boundary between copper and solder, both the tensile and cyclic loading tests are conducted at several temperatures using Sn-3Ag-0.5Cu. The basic characteristic of tensile and fatigue failure is obtained from these tests. After the confirmation of the accuracy of the joint specimen, biaxial ratchetting tests are conducted superposing the tensile load on cyclic torsion. The biaxial ratchetting tests are conducted using a biaxial loading testing machine developed for the joint specimens of solder and copper.

Evaluation of the Stress Distribution by the Stepped Load Indentation Test with Constant Depth

For accurate evaluation of the reliability of electronic package, FEM analyses considering the creep deformation of solder joint in-situ should be conducted. It is well known that the indentation creep test has an advantage to evaluate the creep deformation in microscopic region although there are the problems. Authors already modified the indentation creep test and proposed the method to estimate the steady state creep deformation by the modified test. For lead free solders generally used for the solder joint, the transient creep deformation should be measured by the indentation test in addition to the steady state creep deformation. The transient creep strain occurs in the indentation process. Therefore, it needs to separate the strain into the elastic-plastic strain and the creep strain. In this paper, the method to obtain the stress-strain relation using the indentation test is proposed. New indentation test used the stepped load was proposed to obtained the stress variation during the indenter was pushed and conducted by the numerical test. The suitable measurement point to obtain the relationship between the stress and the strain was identified. The stress variation estimated by proposed area well coincides with the equivalent stress variation of the nodal solution. Therefore, if the method to obtain the strain variation during the indentation process by the indentation in future is developped, it may be possible to estimate the stress-strain curve expressed the uniaxial deformation in the microscopic region.

A Method to Evaluate Creep Properties of Solder Alloys Using Micro Indentation

An indentation creep test is one of the effective methods to directly evaluate the creep deformation of solder joint. However, the indentation test does not give the same creep properties as those obtained by the uniaxial creep tests using a bulk specimen. In this paper, the authors proposes an indentation test conducted under the constant depth to determine a suitable reference area, which leads to the same creep characteristics as those obtained by the uniaxial creep test. A series of numerical micro indentation tests under a constant depth were conducted to construct a method to determine a new reference area without creep data obtained from the uniaxial creep test using bulk specimens. The numerical tests were conducted using finite element method (FEM). The numerical tests showed that the distribution of the principal stress plays an important role to determine the reference area of the indentation tests. Finally, it was found that the reference area obtained considering the distribution of the principal stress gives almost the same creep characteristic as those obtained by the uniaxial creep using bulk specimens.

Viscoplastic Constitutive Model to Divide Inelastic Strain into Time-Independent and Time-Dependent Strains

This paper proposes a viscoplastic constitutive model for solder alloys. The model is a type of so-called elasto-plastic-creep model in which the inelastic strain is divided into the time-independent plastic strain and the time-dependent creep strain. Especially in the proposed model, the creep strain is characteristically divided into the transient part and the steady-state part. An experimental method which we termed “stepped ramp wave (SW) loading test” is also shown. The SW test can quantify both the plastic strain and the creep strain generated under a loading. The parameters used in the proposed model can be estimated systematically during the quantification process. In addition, the development behaviours of the plastic and creep strains can be clarified by the quantification, and they are useful for constructing the model for each strain with high precision.

Development of solver system for FEM analysis of solder joints

As shown in Figure 1, soldered joints of electronic packages are used to join materials of differing coefficients of linear expansion. Because of this, thermal stress occurs in the soldered joint when there is a change in the peripheral temperature or board temperature. Thermal stress in soldered joints is accompanied by inelastic deformation, and when this is repeated the soldered joint may experience low-cycle fatigue failure. Because of this, there is a requirement in designs for electronic packages, for there to be both an allowance for repeated inelastic deformation of the soldered joints and also to give these joints adequate fatigue resistance. In order to achieve such designs, deformation analysis of the joints must be carried out to a high degree of precision using the finite element method (FEM). To make this possible, it is essential to carry out FEM analysis using a mathematical model (i.e. a constitutive model) capable of accurately describing inelastic deformation of soldered joints. However, the constitutive model supplied as the default in the generally used FEM analytical software may not be capable of such accurate description. This tendency is particularly marked currently when the use of lead-free solder has become unavoidable and there is thus a need for a constitutive model with high descriptive accuracy for use in FEM analysis. For the FEM analysis, the use of general FEM software capable of the glitch-free preparation of an FE model from CAD data is a prerequisite. This has now brought about the requirement for research in the use of an independently constructed lead-free soldered joint inelastic constitutive model in the general FEM analysis software. In this article, there is a description of the development process of a solver for the analysis of the deformation of soldered joints, citing as an example a solver for the general finite element analysis software, in whose development the author was involved.

Bending Strength of Cast Materials Reinforced with Hard Particles

Particle reinforcement via the insertion of hard particles is a promising process in materials reinforcing. Particle-reinforced spheroidal graphite martensitic cast iron (SGMC), in which mixed particles of cermet and cemented carbide are dispersed, was achieved by an insertion process. A four-point bending strength test was applied to evaluate the particle composite material. An evaporative pattern process was used on the bending-test specimen to form a composite layer in the central part. Using a combination of three sizes of cermet particles and two sizes of cemented-carbide particles, the bending strength was found to increase with each small-particle combination. The Weibull coefficient m of the four-point bending strength of the particle-reinforced composite material (PRCM) ranged from 4 to 13, and m was large in the specimen with large bending strength.