Deformation characteristics and microstructural evolution of SnAgCu solder joints

The reliability of chip scale package (CSP) components against mechanical shocks has been studied by employing statistical, fractographic, and microstructural research methods. The components having high tin (Sn0.2Ag0.4Cu) solder bumps were reflow soldered with the Sn3.8Ag0.7Cu (wt.%) solder paste on Ni(P)|Au-and organic solderability preservative (OSP)-coated multilayer printed wiring boards (PWBs), and the assemblies were subjected to the standard drop test procedure. The statistically significant difference in the reliability performance was observed: the components soldered on Cu|OSP were more reliable than those soldered on Ni(P)|Au. Solder interconnections on the Cu|OSP boards failed at the component side, where cracks propagated through the (Cu,Ni) 6 Sn 5 reaction layer, whereas interconnections on the Ni(P)|Au boards failed at the PWB side exhibiting the brittle fracture known also as "black pad." In the first failure mode, which is not normally observed in thermally cycled assemblies, cracks propagate along the intermetallic layers due to the strong strain-rate hardening of the solder interconnections in drop tests. Owing to strain-rate hardening, the stresses in the solder interconnections increase very rapidly in the corner regions of the interconnections above the fracture strength of the ternary (Cu,Ni) 6 Sn 5 phase leading to intermetallic fracture. In addition, because of strain-rate hardening, the recrystallization of the as-soldered microstructure is hindered, and therefore the network of grain boundaries is not available in the bulk solder for cracks to propagate, as occurs during thermal cycling. In the black pad failure mode, cracks nucleate and propagate in the porous NiSnP layer between the columnar two-phase (Ni 3 P ϩ Sn) layer and the (Cu,Ni) 6 Sn 5 intermetallic layer. The fact that the Ni(P)|Au interconnections fail at the PWB side, even though higher stresses are generated on the component side, underlines the brittle nature of the reaction layer.

Section: Resultsmentioning

confidence: 99%

Failure mechanisms of lead-free chip scale package interconnections under fast mechanical loading

Mattila¹,

Kivilahti²

2005

“…The near-eutectic SnAgCu solder was modeled by Anand's constitutive model with the parameters provided by Reinikainen et al 16,17 All other materials were modeled as isotropic except the PWB and the component interposer, which were orthotropic. The details of the component board are presented in the section 'Experimental Verification'.…”

Section: Finite Element Analysismentioning

confidence: 99%

Multiscale Simulation of Microstructural Changes in Solder Interconnections During Thermal Cycling

Mattila

Kivilahti

2009

It has been observed that, under cyclic thermal loading conditions, the as-solidified microstructures of solder interconnections composed of a few large colonies are transformed into more or less equi-axed grains by recrystallization. These recrystallized microstructures provide continuous networks of grain boundaries through solder interconnections, and, consequently, they offer favorable paths for cracks to propagate intergranularly. In this work a quantitative multiscale method, combining the Monte Carlo (MC) and finite element (FE) methods, was developed for the first time to predict the recrystallization and grain growth in solder interconnections. This method can be used to establish a lifetime prediction model that is based on these microstructural changes. The inhomogeneous nucleation model was employed in the MC model, while the non-uniform stored energy distribution was scaled from the FE model results. The assumption that, during thermal cycling 5% of the work in the interconnections is stored as energy, gives results that are in agreement with experimental observations. The method predicts well the incubation period and the growth rate of the recrystallization as well as the expansion of the recrystallized region.

“…This is due to the increased strength of tin-rich solder alloys by strain-rate harden-ing. 11,12,[17][18][19] Therefore, good mechanical properties of intermetallic compounds are especially important, when interconnections are subjected to mechanical shocks.…”

Section: Introductionmentioning

confidence: 99%

Reliability of lead-free interconnections under consecutive thermal and mechanical loadings

Mattila¹,

Kivilahti²

2006

To simulate more realistically the effects of strains and stresses on the reliability of portable electronic products, lead-free test assemblies were thermally cycled (−45°C/+125°C, 15-min. dwell time, 750 cycles) or isothermally annealed (125°C, 500 h) before the standard drop test. The average number of drops to failure increased when the thermal cycling was performed before the drop test (1,500 G deceleration, 0.5 ms half-sine pulse). However, the difference was not statistically significant due to the large dispersion in the number of drops to failure of the assemblies drop tested after the thermal cycling. On the other hand, the average number of drops to failure decreased significantly when the isothermal annealing was carried out before the drop test. The failure analysis revealed four different failure modes: (1) cracking of the reaction layers on either side of the interconnections, (2) cracking of the bulk solder, (3) mixed mode of component-side intermetallic and bulk solder cracking, and (4) voidassisted cracking of the component-side Cu 3 Sn layer. The assemblies that were not thermally cycled or annealed exhibited only type (1) failure mode. The interconnections that were thermally cycled before the drop test failed by mode (2) or mode (3). The drop test reliability of the thermally cycled interconnections was found to depend on the extent of recrystallization generated during the thermal cycling. This also explains the observed wide dispersion in the number of drops to failure. On the other hand, the test boards that were isothermally annealed before the drop testing failed by mode (4).