Characterisation of constitutive behaviour of SnAg, SnAgCu and SnPb solder in flip chip joints

The microstructural and creep behavior of bulk 63SnPb37 and the Pb-free solder alloy Sn3.9Ag0.6Cu are reported and compared. The Sn3.9Ag0.6Cu alloy showed much lower absolute creep rates than 63SnPb37. The size and distribution of the intermetallic compound (IMC) coarsened with increasing creep temperature. A number of coarsened precipitates of Cu 6 Sn 5 segregate around β-Sn grain boundaries. After creep at 80°C and 115°C. the β-Sn particles in the Sn3.9Ag0.6Cu alloy are strongly aligned at approximately 45°to the uniaxial tension, parallel to the maximum shear-stress planes. The powerlaw-defined stress exponent significantly increases with increasing stress in both the 63Sn37Pb and Sn3.9Ag0.6Cu alloys; therefore, the Dorn model is unsuitable for these materials over large stress and temperature ranges. Both sets of experimental data were successfully fit with the present power-law stress-dependent energy-barrier model and the Garofalo model. However, the application of the present power-law stress-dependent energy model resulted in a significantly lower estimated variance as compared to the Garofalo model.

Section: Metallography Of the 63sn37pb Eutecticmentioning

confidence: 94%

Section: Metallography Of Sn39ag06cumentioning

confidence: 99%

Tensile creep and microstructural characterization of bulk Sn3.9Ag0.6Cu lead-free solder

Xiao

Armstrong

2005

“…13 Studies conducted on Sn-3.5%Ag bulk materials reported stress exponents between 8 and 18, [19][20][21][22] which are significantly higher than stress exponents in pure bulk Sn where n ranges between 4.5 and 9. Some studies on Sn-rich alloys also reported stress exponent ranges varying with the stress values, with n between 4 and 6 at low stresses, [23][24][25][26][27] and between 6 and 16 at higher stresses. 20,22,23,25 Despite the considerable range of reported stress exponents, dislocation climb is often cited as the predominant mechanism controlling creep deformation.…”

Section: Resultsmentioning

confidence: 99%

Constraining Effects of Lead-Free Solder Joints During Stress Relaxation

Zimprich

Saeed

Weiß

et al. 2008

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.

“…Current constitutive relations for solder typically predict inelastic rates of deformation that vary with the stress to a power of 6 or more. [48][49][50][51][52][53][54][55] However, simple calculations (''Appendix'') show how this cannot lead to work during the hightemperature dwell that varies faster than the DNP. Only a linear dependence of the inelastic strain rate on the stress is compatible with a variation of the life span with DNP…”

Section: Constitutive Relationsmentioning

confidence: 99%

A Mechanistic Thermal Fatigue Model for SnAgCu Solder Joints

Børgesen

Wentlent

Hamasha

et al. 2018

The present work offers both a complete, quantitative model and a conservative acceleration factor expression for the life span of SnAgCu solder joints in thermal cycling. A broad range of thermal cycling experiments, conducted over many years, has revealed a series of systematic trends that are not compatible with common damage functions or constitutive relations. Complementary mechanical testing and systematic studies of the evolution of the microstructure and damage have led to a fundamental understanding of the progression of thermal fatigue and failure. A special experiment was developed to allow the effective deconstruction of conventional thermal cycling experiments and the finalization of our model. According to this model, the evolution of damage and failure in thermal cycling is controlled by a continuous recrystallization process which is dominated by the coalescence and rotation of dislocation cell structures continuously added to during the hightemperature dwell. The dominance of this dynamic recrystallization contribution is not consistent with the common assumption of a correlation between the number of cycles to failure and the total work done on the solder joint in question in each cycle. It is, however, consistent with an apparent dependence on the work done during the high-temperature dwell. Importantly, the onset of this recrystallization is delayed by pinning on the Ag 3 Sn precipitates until these have coarsened sufficiently, leading to a model with two terms where one tends to dominate in service and the other in accelerated thermal cycling tests. Accumulation of damage under realistic service conditions with varying dwell temperatures and times is also addressed.