Study of the Effect of Mechanical Cycling on the Constitutive Behavior of Lead-Free Solders
Type of DegreePhD Dissertation
Restriction TypeAuburn University Users
MetadataShow full item record
Solder joints in electronic assemblies are often subjected to cyclic (positive/negative) mechanical strains and stresses. Such exposures can occur in variable temperature application environments or during accelerated life thermal cycling tests used for qualification. Lead free solder materials are widely used in the electronic packaging industry due to environmental concerns. However, experimental testing and microstructural characterization have revealed that Sn-Ag-Cu (SAC) lead free solders exhibit evolving properties that change significantly with isothermal aging and cyclic loading. This dissertation addresses those changes in the microstructure and properties of lead free solders by conducting four different projects. In the first project, the effects of isothermal mechanical cycling on damage accumulation in lead free SAC305, SAC_Q and Innolot bulk solder alloys have been explored. Uniaxial samples of SAC305, SAC_Q and Innolot were prepared and subjected to various durations of prior mechanical cycling. The cycled samples were then subsequently subjected to stress-strain or creep testing. The cycling induced changes in several mechanical properties (elastic modulus, yield strength, ultimate strength, and creep strain rate) have been characterized and quantified with the amount of prior cycling. Moreover, these evolutions observed in the three lead free alloys were correlated with the observed changes in their microstructures that occurred during the cycling. In the second project, the effects of elevated temperature isothermal mechanical cycling on damage accumulation in lead free SAC305 and SAC_Q bulk solder were explored. Uniaxial samples of SAC305 and SAC_Q were prepared and subjected to various durations of prior mechanical cycling at 100 °C. The cycled samples were then subsequently subjected to stress-strain or creep testing. The cycling induced reduction of several mechanical properties (elastic modulus, yield strength, ultimate strength, and creep strain rate) have been characterized and quantified with the amount of prior cycling. The results obtained were then compared with the results obtained with room temperature cycling to better understand the effect of elevated temperature mechanical cycling. Moreover, these evolutions observed in the two lead free alloys were correlated with the observed changes in their microstructures that occurred during the cycling and compared with the changes in the microstructure obtained during room temperature cycling. In the third part of this dissertation, the effect of strain range on the damage evolution of mechanically cycled SAC305 solder material was investigated. The samples were cycled at various strain ranges and the corresponding hysteresis loop areas were evaluated. The data was then used to generate plots of initial loop area vs strain range which allowed the estimation of initial plastic work for any given strain range. Samples were then cycled at room temperature and elevated temperature such that the initial plastic work remained the same. This was done for four different initial plastic work. Such a study would allow us to deduce the changes brought about by cycling with any particular initial plastic work when cycled at either 25 °C or 100 °C. This evolution in the properties that occur during mechanical cycling would then allow us to estimate and quantify the damage accumulated due to mechanical cycling at any particular strain range. These properties could then be incorporated into a constitutive model (such as Anand Model) that could then provide a method which would allow the modelling of the damage accumulation within SAC305 lead free alloys. In the next part of this dissertation, the aging induced evolutions of the cyclic stress-strain behavior of SAC305 solder material was investigated. The specimens were aged at 125 °C for different durations (0, 1, 2, 5, 10, and 30 days) and the evolution of the hysteresis loop area, peak load, and plastic strain range were characterized. Later, the effects of prior aging on damage accumulation occurring in SAC305 was investigated while subjected to mechanical cycling (fatigue testing). Regions of interest near the center of the sample were marked using a nanoindentation system. Samples were then subjected to aging at 125 °C. After aging, the samples were then subjected to mechanical cycling. After various durations of cycling (e.g. 0, 10, 25, 50, 75, 100, 200, 300 cycles), the fixed were examined using scanning electron microscopy (SEM) Using the recorded images, the microstructural evolutions in the fixed regions were observed, and the effects of the initial aging on the results were determined. In the following project, the mechanical cyclic induced evolutions in mechanical properties of SAC 305 and SAC_Q individual solder balls were investigated. The test assemblies in this study were (3X3) BGA packages composed of total nine 0.75 mm diameter lead free solder balls which were formulated by reflowing solder spheres soldered onto 0.55 mm diameter Cu pads on FR4 coupons. The solder balls were cycled using an Instron Micromechanical tester along with a newly designed fixture which facilitates the tester to cycle a solder ball individually. Nanoindentation tests were performed on the specimens to study the evolution in mechanical properties (e.g. elastic modulus, hardness and creep strain rate) of the solder balls as a function of duration of cycling. A finite element model was also used to develop to estimate the plastic work accumulated in these solder balls when subjected to shear cycling. In the last chapter of this dissertation, 3X3 BGA sandwiched SAC305 solder joints soldered on to Cu pads on FR4 coupons were mechanically cycled in a shear direction. This was done using the MT-210 microtester machine coupled with newly designed fixtures to firmly grip the specimens. The load-displacement curves obtained were then studied. These sandwiched specimens were also polished using a novel technique that allowed the extraction of these samples after polishing. After polishing, some of the samples were aged at 125 °C for 10 days. The aged and non-aged samples were then subjected to shear mechanical cyclic loads at either room temperature or elevated temperature. The changes in microstructure of the solder joints with mechanical cycling were then recorded using a cross-polarized optical microscope. Any recrystallization effects brought about by mechanical shear cycling (both at 25 °C or 100 °C) or by aging were also investigated. Finally, a finite element model was developed to estimate the plastic work accumulated during the mechanical cycling and validated with experimental studies.