Strain rate effects on solder joint failure behavior
Tan, Kok Ee
Date of Issue2015
School of Mechanical and Aerospace Engineering
The effects of strain rate on bulk tensile stress strain properties of solder have been investigated by many researchers, however the effects of strain rate on stress strain properties of solder joint requires further study. Another aspect which is important for solder joint failure study is the effects of mixed mode loading on solder joint failure behavior and mixed mode fracture toughness measurement and analysis. In this PhD thesis, the strain rate dependent mechanical properties and stress strain behavior of 95.5Sn3.8Ag0.7Cu (SAC387) lead-free solder are investigated for a range of strain rates. The strain rate dependent elastic modulus, yield stress properties and stress strain constitutive model of the solder material are characterized using tensile tests. Tensile tests on dog-bone shaped bulk solder specimens were conducted using a non-contact video extensometer system. Iso-strain rate uni-axial tensile tests were conducted over the strain rates of 0.001, 0.01, 0.1 and 1 (s-1) at 25ºC. For all the tests conducted, higher strain rate is well correlated with higher elastic modulus and yield stress of the solder. Nanoindentation tests were conducted from slow to intermediate strain rates of 0.001, 0.01, 0.1, 1 and 10 (s-1) by using the continuous stiffness measurement (CSM) technique. The strain rate dependent yield stress results from nanoindentation test are expressed in a Cowper-Symonds model where the dynamic flow stress can be estimated over a wide range of strain rates. Strain rate sensitivity effects on the plastic yield stress behavior of SAC387 solder from nanoindentation and tensile test were investigated. The stress strain constitutive model behavior of 95.5Sn3.8Ag0.7Cu solder was investigated further under compression over strain rates ranging from 0.022 s-1 to 9.266 s-1 and under tension over strain rates ranging from 0.001 s-1 to 1.0 s-1. A new Ramberg-Osgood model is developed to describe the stress strain curve model at one particular strain rate. Modifications are done on the original Ramberg-Osgood model in order to form a strain rate dependent model that can describe stress strain curves over all range of strain rates. The model expressions are able to capture the strain rate dependence of the yield and work hardening parameters within the range of the test conditions reported in this thesis. A modified Ramberg-Osgood model consisting of ten constants is derived for strain rate dependent expressions by using linear regression curve fitting. Fracture behavior of solder joints were investigated for soldered tensile test specimens and for a complex combined loading complex mixed mode (CMM) test fixture developed for tensile and shear combined load tests over an intermediate strain rate range from 0.001s-1 to 0.1s-1. The fracture behavior of solder joints subjected to pure tensile, pure shear or varying combination of mixed-mode (tensile and shear) loading combinations were investigated in detail. The observed failure modes vary from brittle intermetallic (IMC) layer failure to ductile bulk solder shear failure. Under mixed-mode loading, a complex combination of IMC and solder failure mechanism was observed. The CMM tests were investigated with cracks fabricated in the solder joint and the fracture behavior of solder joint subject to mode mixity is reported. A fracture mechanics based failure assessment curve (FAC) criteria approach is proposed using interfacial fracture mechanics theory. Finite element analysis results provided mixed mode stress intensity factors for interface cracks used to calculate the fracture toughness parameters.
DRNTU::Engineering::Mechanical engineering::Mechanics and dynamics