Constraining Effects of Lead-Free Solder Joints During Stress Relaxation

Abstract

Reliability and quality control of microelectronics depend on a detailed understanding of the complex thermomechanical properties of miniaturized lead-free solder joints. With the continuous reduction in size of modern electronic devices, including also the size of the solder joints themselves, mechanical constraint effects may become of importance for the reliability of the joints. In the present study stress relaxation tests in tensile mode were performed on model solder joints consisting of eutectic Sn-3.5Ag solder between Cu substrates. The gap size of the joints was varied between 750 lm and 150 lm in order to investigate the variation of the mechanical properties as a function of the gap size. As it turned out, stress relaxation was dramatically reduced when the solder gaps became smaller due to constraint effects already well known from earlier measurements of the tensile strength. By employing a traditional creep model, the stress exponents and the activation energies were derived and compared with available data in the literature. The consequences of these constraint effects for the case of thermomechanical fatigue are discussed.

Evaluation of Thin Copper Wire and Lead-Free Solder Joint Strength by Pullout Tests and Wire Surface Observation

“…The value of the tensile interface strength is a little high in comparison with the other results (σ f = 50 to 90 MPa) obtained for similar lead-free solders and copper [9,10], and that of the shear interface strength is lower than the previous result (τ f = 40 MPa) [11]. The constraint of plastic deformation at the bottom surface of the copper wire [12,13], the size and shape of the specimens, and the data dispersion (which will be described below) may be the reason for these differences. In order to investigate the effect of data dispersion on the evaluation of the interface strength, two parallel lines passing through the points most distant from the straight line given by Equation (2) are considered, as shown in Figure 12.…”

Influence of IMCs volume ratio in micro-scale solder joints on their mechanical integrity

“…A well accepted equation about dimensional constraint is Orowan's theory, expressed as following, where σ F is the fracture strength, σ UTS is the ultimate tensile strength, d is the diameter, and t is the thickness of the joint. [18][19][20] Growing IMCs layer weakens the ductility of solder joints which also results in a remarkable dimensional constraint of solder joints for the plastic deformation part shrinks. t in this equation is usually taken as size of the whole joint in larger solder joints in which IMCs takes much less proportion.…”

A new creep–fatigue life model of lead-free solder joint

“…With the development of electronic device, the size of solder joints becomes smaller and smaller, which made the solder joint reliability more challenged, for the size has a significant effect on its property [1][2][3]. Generally, solder joint failure is attributed to the effect of fatigue-creep interaction [4].…”