Size effect on shear performance of microscale ball grid array structure Cu/Sn-3.0Ag-0.5Cu/Cu solder joints under current stressing

Minor Ag inducedshear performance alternation in BGA structure Cu/SnBi/Cu solder joints under electric current stressing

The shear performance and fracture behavior of microscale ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints with increasing electric current density (from 1.0 × 103 to 6.0 × 103 A/cm2) at various test temperatures (25 °C, 55 °C, 85 °C, 115 °C, 145 °C, and 175 °C) were investigated systematically. Shear strength increases initially, then decreases with increasing current density at a test temperature of no more than 85 °C; the enhancement effect of current stressing on shear strength decreases and finally diminishes with increasing test temperatures. These changes are mainly due to the counteraction of the athermal effect of current stressing and Joule heating. After decoupling and quantifying the contribution of the athermal effect to the shear strength of solder joints, the results show that the influence of the athermal effect presents a transition from an enhancement state to a deterioration state with increasing current density, and the critical current density for the transition decreases with increasing test temperatures. Joule heating is always in a deterioration state on the shear strength of solder joints, which gradually becomes the dominant factor with increasing test temperatures and current density. In addition, the fracture location changes from the solder matrix to the interface between the solder matrix and the intermetallic compound (IMC) layer (the solder/IMC layer interface) with increasing current density, showing a ductile-to-brittle transition. The interfacial fracture is triggered by current crowding at the groove of the IMC layer and driven by mismatch strain at the solder/IMC layer interface, and the critical current density for the occurrence of interfacial fracture decreases with increasing test temperatures.

The effects of manufacturing variations on the reliability of solder joints between a ceramic ball grid array (BGA) package and a printed wiring board (PWB) are investigated. Two cases are studied, namely, with and without spacers between the BGA package and the PWB to maintain the solder joint height. Manufacturing variations considered include changes in solder volume, joint height, and pad size. To evaluate the effect of manufacturing variations on reliability, every possible solder joint profile is first derived. The maximum strain is calculated next. Finally, the fatigue life is predicted. The calculations show that these manufacturing variations change the joint profile, and subsequently affect the fatigue life. Since the package is heavy, the use of spacers is necessary to control the solder joint height for reliable connections, and to maintain a large gap for cleaning. The solder joints formed with the use of spacers, may have convex, cylindrical or concave profiles. The concave solder joints are preferred, since they have long fatigue lives and are less sensitive to the manufacturing variations. For the convex solder joints, their fatigue lives are strongly affected by the joint height variation caused by package warpage and by the combined effects of solder volume and pad size.

Geometry effect on mechanical performance and fracture behavior of micro-scale ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints

In this study, the ball grid array (BGA) structure microscale lead-free Cu/Sn-3.0Ag-0.5Cu/Cu (Cu/SAC305/Cu) joints with different heights (i.e., 100 and 200 μm) were used to investigate the shear deformation and fracture behavior after soaking treatment with liquid nitrogen for different time. The experimental results indicated that the ultimate strength of the joint under shear loading generally decreases slightly with the increase in time of liquid nitrogen soaking, while it increases obviously with the decrease in joint height. Moreover, the fracture locations of all tested solder joints are inside the solder matrix. At the same time, it can be deduced from the stress- strain curve and fracture morphology that the fracture behavior remains ductile.

Coupling effect between electromigration and joule heating on the failure of ball grid array in 3D integrated circuit technology

Low cycle fatigue performance of ball grid array structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints

The solder joint's volume and interfacial reaction during the soldering significantly affect its solidification behavior and microstructure feature; further, the microstructure of the solder matrix and the intermetallic compounds (IMC) have a direct impact on the performance and reliability of the solder joint. In this study, the effects of the solder volume and interfacial reaction on the undercooling behavior and solidification microstructure of ball grid array (BGA) structure Sn-3.0Ag-0.5Cu/Cu (SAC/Cu) single-sided and Cu/Sn-3.0Ag-0.5Cu/Cu (Cu/SAC/Cu) double-sided joints with different solder diameters (0.76, 0.50 and 0.30 mm) were investigated by reflow soldering process using a differential scanning calorimeter (DSC). DSC reflow results show that the undercooling of both SAC/Cu single-sided and Cu/SAC/Cu double-sided joints decreases with the increase of solder ball diameter. However, there is no big difference in undercooling value between SAC/Cu and Cu/SAC/Cu joints with the same joint size (or solder ball size). Microstructural analysis shows that the primary solidification phase of 0.30-mm-diameter double-sided solder joints is Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn, instead of Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> in the solder joints with diameter of 0.50 and 0.76 mm, which leads to decrease of the undercooling of 0.30 mm solder joints to a value only a little higher than the undercooling of 0.50 mm solder joints. The undercooling of solder joints is influenced by several factors, mainly including the type of substrate material (UBM), solder dimension and primary solidification phase.

The dissolution of the under bump metallization (UBM) into the molten solder is inevitable during the soldering process. However, overly dissolved UBM can lead to the excessive growth of interfacial intermetallic compound (IMC) and change the solder's composition as well as result in the formation of some bulk IMC in the solder matrix; consequently, mechanical properties of the solder interconnects can be deteriorated seriously. In this study, the dissolution behavior of Cu UBM in single-sided ball grid array (BGA) structure Sn-3.0Ag-0.5Cu/Cu joints, which were assembled by differential scanning calorimetry (DSC) incorporated into liquid isothermal reflow process, were studied systematically. Results show that, at a relatively low reflow temperature of 217°C, which is the melting onset temperature of Sn-3.0Ag-0.5Cu solder, the dissolution rate of Cu UBM is very slow and its consumption is mainly used for the growth of interfacial IMC. Clearly, a slight increase of temperature from 217°C to 218°C can significantly increase the degree of Cu UBM consumption during isothermal reflow process, the proportion of the Cu UBM dissolved into the solder matrix in the total consumption of Cu UBM also increases dramatically from 15.5% to 71.0%; as the isothermal temperature is increased to 230°C, the proportion just increases to 72.6% due to an almost complete saturation of Cu content in the solder matrix.

Enhanced size effects on shear performance and fracture behavior of BGA structure micro-scale Cu/Sn–3.0Ag–0.5Cu/Cu joints at low temperatures

A plastic strain energy density methodology is proposed to evaluate the initiation and propagation of fatigue crack in lead-free solder joints. The relationships among the plastic strain, plastic strain energy, continuum damage mechanics(CDM) and fatigue life are clarified. Crack growth correlation constants for micro-scale ball grid array(BGA) structure solder joints(with standoff height h in the range 100 to 500 μm and a pad diameter 480 μm) are determined by a combination of experimental estimation and numerical calculation. The results show that the cycle numbers of crack initiation and propagation have power function relationship with the plastic strain energy density generated in each fatigue cycle. Crack propagation rate is affected by stress triaxiality, which is dependent on loading modes, i.e., stress triaxiality increases dramatically with decreasing h under tensile load because of the mechanical constraint effect arising from interfaces and package structure, while under shear load the standoff height has very limited effect on stress triaxiality. Furthermore, crack growth correlation constants identified in solder joints with h=100 μm can be well used to predict fatigue life of solder joints with different geometries, indicating that the fatigue life prediction method proposed in this study can effectively prevent the influence of plastic strain energy concentration caused by structural and volume factors on the prediction of fatigue life of BGA structure solder joints.

The solder joint's volume and interfacial reaction during the soldering significantly affect its solidification behavior and microstructure feature; further, the microstructure of the solder matrix and the intermetallic compounds (IMC) have a direct impact on the performance and reliability of the solder joint. In this study, the effects of the solder volume and interfacial reaction on the undercooling behavior and solidification microstructure of ball grid array (BGA) structure Sn-3.0Ag-0.5Cu/Cu (SAC/Cu) single-sided and Cu/Sn-3.0Ag-0.5Cu/Cu (Cu/SAC/Cu) double-sided joints with different solder diameters (0.76, 0.50 and 0.30 mm) were investigated by reflow soldering process using a differential scanning calorimeter (DSC). DSC reflow results show that the undercooling of both SAC/Cu single-sided and Cu/SAC/Cu double-sided joints decreases with the increase of solder ball diameter. However, there is no big difference in undercooling value between SAC/Cu and Cu/SAC/Cu joints with the same joint size (or solder ball size). Microstructural analysis shows that the primary solidification phase of 0.30-mm-diameter double-sided solder joints is Ag <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> Sn, instead of Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sub> Sn <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">5</sub> in the solder joints with diameter of 0.50 and 0.76 mm, which leads to decrease of the undercooling of 0.30 mm solder joints to a value only a little higher than the undercooling of 0.50 mm solder joints. The undercooling of solder joints is influenced by several factors, mainly including the type of substrate material (UBM), solder dimension and primary solidification phase.

Linear elastic fracture mechanics (LEFM) approach was used for studying the influence of the interfacial IMC layer thickness and solder mask layer thickness on fracture behavior of ball grid array (BGA) structure solder interconnects using finite element numerical simulation. The stress intensity factors (SIFs, KI and KII) at the crack tip of a predefined face-crack in the IMC layer of BGA joints were analyzed. The simulation results show that, increasing the thickness of the IMC layer results in high values of SIFs at the crack tip, so the crack may be most likely to propagate in the BGA structure interconnect with a thicker IMC layer. Moreover, for the BGA structure solder interconnect having a predefined crack with a certain distance to the IMC/solder interface, the kink angle of crack propagation increases with the increasing thickness of the IMC layer. Furthermore, a thicker solder mask layer leads to the decrease in both values of the maximum Von Mises stress and maximum tensile stress in the solder matrix, as well as a lower Von Mises stress in the IMC layer of the BGA joint, and this means that the increase in thickness of solder mask layer will improve the mechanical reliability of BGA structure interconnects.

Dimension of solder interconnects (or joints) and pitches has been continuously scaling down, resulting in inhomogeneous microstructure and severe electromigration (EM) effect in solder interconnects. In this study, the interaction effect between electromigration and microstructure evolution in ball grid array (BGA) structure Cu/Sn-58Bi/Cu solder interconnects under a direct current density of 1.5×108 A/m2 is studied by cellular automaton (CA) modeling embedded with finite element (FE) simulation. Results show that, compared with configuration or geometry of BGA interconnects, the influence of inhomogeneous eutectic phases on the distribution of current density is more obvious. The current density in the Sn-rich phase is much higher than that in the Bi-rich phase. Bi atoms in Sn-rich phase are more prone to migrate to the anode first, rather than migrating directly along the interface between Bi-rich phase and Sn-rich phase. Consequently, the damage in the Sn-rich phase, rather than at phase interfaces or in the Bi-rich phase, could usually be observed in experiments. By employing the criterion of EM induced atomic flux of Bi in FE analysis under CA rules, simulation results of Bi-rich phase segregation are consistent with the experimental observation under current stressing.

The research on lead-free solders has become the focus in electronic industry, conventional lead-free solder technology may not meet the increasingly severe service environment, so an attractive and potentially method to enhance the performance of a lead-free solder is to introduce reinforcements into a conventional alloy. This study employed polyhedral oligomeric silsesquioxanes (POSS) nano-particles as reinforcements for Sn3.5Ag0.5Cu (SAC) solder matrix. It was aimed at investigating the effect of POSS on the microstructure evolution and recrystallization behavior of ball grid array (BGA) solder joints under thermal fatigue. 3wt% POSS were incorporated into Sn3.5Ag0.5Cu (SAC) solder matrix by mechanical ball milling followed by reflow process, and found to be homogeneously dispersed into the solder matrix by EDX analysis. Then, two kinds of different components BGA assemblies were subjected to thermal cycling, and the surface morphology change and orientation evolution of solder joints before and after the thermal shock was respectively characterized by using SEM and EBSD. Experimental results revealed that the microstructure of solder matrix could be markedly refined with the addition of POSS, because of the reinforcements retarded the element diffusion and intermetallic compound aggregation. Vickers hardness has obviously improved, composite solder joint shows a better mechanical properties. After the thermal fatigue test of 928 cycles, at the interfaces extrusion deformation produced, the recrystallization accompanied with initiation and propagation of cracks in the BGA of SAC305 solder joint. But after the addition of nanoparticles, composite solder resulted in less grain boundaries in the same time, and it can prevent the appearance of recrystallization in solder.