Effects of surface finishes and loading speeds on shear strength of Sn–3.0Ag–0.5Cu solder joints

doi:10.1016/j.mee.2011.03.148

Microelectronic Engineering

Volume 89, January 2012, Pages 55-57

https://doi.org/10.1016/j.mee.2011.03.148 Get rights and content

Abstract

The effect of electroless nickel immersion gold (ENIG) and organic solderability preservative (OSP) surface finishes on the shear strength of Sn–3.0Ag–0.5Cu solder joints were investigated under the various loading speeds of 0.2–1000 mm/s. Maximum shear force increased with increasing shear speed, while ductility and toughness decreased, while OSP finishes show lower shear strength rather than ENIG finishes. Overall, OSP finishes showed ductile to pad lift modes transition while ENIG finishes showed ductile to brittle modes transition with increasing shear speed. Therefore, strong interfacial adhesion of ENIG finishes rather than OSP finishes seems to be related to stronger bonding strength and also higher toughness of ENIG finishes.

Graphical abstract

Highlights

► Effects of surface finishes and loading speeds on the shear strength of Sn–3.0Ag–0.5Cu solder were investigated. ► Shear strength increases and toughness decreases with increasing shear speed. ► ENIG show higher shear strength rather than OSP. ► ENIG showed ductile to brittle fracture modes with increasing shear speed. ► OSP showed ductile to pad lift fracture modes transition with increasing shear speed.

Introduction

Recently, advanced electronic package substrates with Pb-free solder joint for mobile applications have been used very fast and widely due to its higher performance and smaller form factors [1], [2], [3], [4], [5], [6]. In particular, the solder joint has several important roles such as interconnect, thermal emission, and protection from external loadings [7]. Because their size and weight are very small, they are often dropped while using which can cause mechanical failure of devices as well as electrical malfunction in printed circuit board (PCB) components [8]. The chip package mounted on the PCB is considerably affected by the surface finishes materials or technology of copper pad. Currently, organic solderability preservative (OSP) and electroless nickel immersion gold (ENIG) processing as environment-friendly surface finishes process are widely used [9], [10].

Many studies have been conducted for evaluation of the solder joint reliability with respect to the solder alloy types and surface finishes [7], [8], [11], [12], [13], [14], [15], [16], [17], [18], [19], [20], [21], [22], [23]. Currently, the most popular testing method to evaluate the mechanical reliability of the solder ball joint is the ball shear test due to its simple method and convenient implementation. In our knowledge, there have been little enough understandings on the mechanical failure mechanisms with respect to surface finishes and also loading speeds. Therefore, we investigated the effects of surface finishes and shear speeds on the mechanical bonding strength and failure mechanisms of Sn–3.0Ag–0.5Cu solder joints.

Section snippets

Experimental procedure

The solder ball used in this study had a composition of Sn–3.0Ag–0.5Cu with a diameter of 450 μm and a height of 390 μm. The substrate was a solder mask defined-type FR4 laminate with solder pad opening diameter of 300 μm. The pads comprised of surface finishes of the OSP and ENIG layers, respectively, over an underlying 20 μm-thick Cu pad. The Sn–3.0Ag–0.5Cu solder balls were bonded to the FR4 substrate in a reflow process employing rosin mildly activated flux in reflow machine with a maximum

Results and discussion

Fig. 1b and c shows the cross-sectional BSE images of interfaces between the Sn–3.0Ag–0.5Cu solder and Cu layer treated by ENIG and OSP surface finishes, respectively. During reflow processing, (Cu,Ni)₆Sn₅ IMC formed at bonding interface of ENIG as shown in Fig. 1b. It is thought that Cu atoms in Sn–3.0Ag–0.5Cu solder migrated to the solder/Ni(P) interface, which causes the formation of the Ni-containing Cu₆Sn₅ IMC, that is, (Cu,Ni)₆Sn₅. It was known that the (Cu,Ni)₆Sn₅ IMC is formed at the

Conclusions

Effects of surface finishes and loading speeds on the shear strength and failure modes of Sn–3.0Ag–0.5Cu solder bump were investigated. Overall, shear strength increases and toughness significantly decreases with increasing shear speed, while ENIG finishes show higher shear strength rather than OSP finishes. Overall, ENIG finishes showed ductile to brittle modes transition while OSP finishes showed ductile to pad lift modes transition with increasing shear speed. Therefore, weaker interfacial

Acknowledgement

This work is supported by a grant of National Platform Technology Development Program from Ministry of Knowledge Economy, Korea.

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Since the restriction of hazardous substances (RoHS) directive, lead-free soldering has been widely broad adopted. In many applications, lead-free alloys have been substituted for conventional SnPb (Tin-Lead) solder. Among lead-free alloys, SAC305(Sn-3.0Ag-0.5Cu) has gained the highest acceptance as a solder alloy. The fatigue performance of solder joints has become an essential reliability assessment criterion due to the transition to more reliable lead-free alloys from highly predictable lead-based alloys. This study examines the shear fatigue characteristics of sandwich test vehicles for SnPb and SAC305 at different temperatures using both Organic Solderability Preservative (OSP) and Electroless Nickel-Immersion Silver (ENIG) surface finishes. The fatigue experiments were performed using an Instron micromechanical tester at a constant strain rate, and microstructural analysis was carried out using Scanning Electron Microscopy (SEM) to identify the Intermetallic Compound (IMC) morphology and failure mode. The stress-strain (hysteresis) loops of SnPb and SAC305 were measured, and the fatigue life of the specimens was estimated using the strain-life equation at various temperatures. SAC305 was observed to have a better fatigue life than SnPb, particularly at higher strain levels or testing temperatures. The OSP surface finish demonstrated superior fatigue properties compared to the ENIG surface finish. Additionally, elevated testing temperatures were found to accelerate fatigue failure in solder joints. The Arrhenius model was utilized to develop a general empirical model that predicts the fatigue life of SAC305 and SnPb solder alloys with OSP and ENIG surface finishes as a function of the strain level and testing temperature.
Microstructural and shear strength properties of GNSs-reinforced Sn-1.0Ag-0.5Cu (SAC105) composite solder interconnects on plain Cu and ENIAg surface finish
2021, Journal of Materials Research and Technology
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Whereas, the restriction of this diffusion process by the nickel-phosphorous layer (Ni–P) of the ENIAg substrate and the subsequent formation of a relatively strong metallurgical bond comprising the Ni3Sn4 IMC at the solder/ENIAg interface explain the superior shear strengths of the SAC105-xGNS/ENIAg composite solder joints over the SAC105-xGNS/Cu counterparts. This can be further rationalized by the investigation of Kim et al. [48], where the exceptional interfacial bonding of the SAC solder on the Electroless Nickel Immersion Gold (ENIG) surface finish substrate was presented to justify the improved shear strength exhibited by the developed solder joints. Having demonstrated the superior shear strengths in both sample grades, the SAC105-0.01GNS/Cu and SAC105-0.01GNS/ENIAg composite solder joints were analysed using the field emission electron microscope (Fig. 10) to provide thorough insight into the strengthening effect of the ENIAg surface finish and the causative failure mechanism for the fractured composite solder joints.
In this study, the combined effect of GNSs (graphene nanosheets) and ENIAg (Electroless Nickel Immersion Silver) surface finish on the formation of intermetallic compounds (IMCs) and shear strength of the Sn-1.0Ag-0.5Cu (SAC105) solder system was studied. Both plain and composite solder systems (SAC105-xGNS; x = 0, 0.01, 0.05 and 0.1 wt%) were successfully prepared using the powder metallurgy technique and thereafter soldered on the plain Cu and ENIAg surface finish substrates. From the microstructural analysis, the Cu₆Sn₅ IMC was observed at the solder/substrate interface of the SAC105-xGNS/Cu solder joints. Moreover, the Ni₃Sn₄ and (Cu,Ni)₆Sn₅ IMC phases were observed at the solder/substrate interface of the SAC105-xGNS/ENIAg counterparts. The GNSs and ENIAg surface finish provided huge barrier for Sn and Cu atoms diffusion required for IMC formation. The interfacial IMC layer thickness decreased with increasing addition of GNSs for both sample grades. The SAC105-xGNS/ENIAg demonstrated lower IMC thicknesses that ranged between 2.98 and 2.53 μm relative to the 5.23–3.35 μm exhibited by the SAC105-xGNS/Cu. In general, the strengthening potential of the GNSs was well marked in both sample grades, with the SAC105-0.01GNS/Cu and SAC105-0.01GNS/ENIAg demonstrating the highest shear strengths of 11.2 MPa and 12.1 MPa, respectively.
Ductile fracture of solder-Cu interface and inverse identification of its interfacial model parameters
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Experimental test is quite an important means to investigate the failure of solder joint. Many works (Tian et al., 2011; Kim et al., 2012; Chong et al., 2006; Shnawah et al., 2012) have been devoted to the study of the fracture failure of Pb-free solder joints. Nevertheless, as being well known, the size of solder joint is quite small and its final configuration will change during its manufacture and service.
Interfacial failure is a crucial issue in the study of reliability in electronic packaging. The failure mechanism is complicated since ductile failure of the solders is generally accompanied with brittle fracture of the intermetallic compounds(IMCs). Besides, it is also greatly influenced by loading conditions. In this paper, fractures of SAC305-Cu interfaces under mode I and mode II loading conditions have been experimentally studied. Their interfacial micro-structures before and after failures have been analyzed using SEM and EDS. At the same time, numerical simulations of interfacial failure have also been performed using an improved cohesive zone model. Finally, the failure mechanisms for both mode I and mode II fractures were disclosed. In addition, though the cohesive zone models(CZM) were widely used to investigate the interfacial failure, their parameter identification is still an important concern, especially when considering both the accuracy and efficiency in the analysis. In our work, a parameter inverse analysis method has been developed, based on the Kalman filter algorithm and the response surface interpolation technique. This interpolation method can hugely improve the computational efficiency, but it will also introduce a large numerical error in the present cases, in which the interfacial failure is accompanied by large plastic deformation of solder, or the parameter searching ranges are set over-large. Aiming at this issue, a range-reduced inverse analysis scheme was further developed in our work. The developed method and scheme were validated robust and accurate through verifications of both pseudo and real experimental data. The cohesive zone model parameters acquired can be effective to simulate the interfacial failure of Cu and Pb-free solder. The whole inverse analysis is automatically achieved and can be easily applied to other complicated interfacial failure problems.
Geometry effect on mechanical performance and fracture behavior of micro-scale ball grid array structure Cu/Sn-3.0Ag-0.5Cu/Cu solder joints
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Due to the coefficient of thermal expansion (CTE) mismatch among the chips, solder joints and substrates, BGA solder joints are mainly subjected to shear stresses in service [2]. Consequently, solder ball shear tests have been widely adopted to evaluate the solder ball attachment strength in BGA joint packages [3–8]. Although ball shear tests can be used to evaluate the attachment strength quickly and conveniently, it cannot reflect the size and geometry effects of BGA solder joints accurately, where there are two attachment interfaces.
Solder joint integrity has long been recognized as a key issue affecting the reliability of integrated circuit packages. In this study, both experimental and finite element simulation methods were used to characterize the mechanical performance and fracture behavior of micro-scale ball grid array (BGA) structure Cu/Sn–3.0Ag–0.5Cu/Cu solder joints with different standoff heights (h, varying from 500 to 100 μm) and constant pad diameter (d, d = 480 μm) and contact angle under shear loading. With decreasing h (or the ratio of h/d), results show that the stiffness of BGA solder joints clearly increases with decreasing coefficient of stress state and torque. The stress triaxiality reflects the mechanical constraint effect on the mechanical strength of the solder joints and it is dependent on the loading mode and increases dramatically with decreasing h under tensile loading, while the change of h has very limited influence on the stress triaxiality under shear loading. Moreover, when h is decreased, the concentration of stress and plastic strain energy along the interface of solder and pad decreases, and the fracture location of BGA solder joints changes from near the interface to the middle of the solder. Both geometry and microstructure greatly affect the shear behavior of joints, the average shear strength shows a parabolic trend with decreasing standoff height. Furthermore, the brittle fracture of BGA solder joints after long-time isothermal aging was investigated. Results obtained show that, under the same shear force, the stress intensity factors, K_I and K_II, and the strain energy release rate, G_I, at the Sn–3.0Ag–0.5Cu/Cu₆Sn₅ interface and in the Cu₆Sn₅ layer obviously decrease with decreasing h, hence brittle fracture is more prone to occur in the joint with a large standoff height.
Effects of metallic nanoparticle doped flux on the interfacial intermetallic compounds between lead-free solder ball and copper substrate
2014, Materials Characterization
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Restrictions on the use of lead (Pb) based solder alloys in microelectronic assemblies because of the inherent toxicity of Pb pave the path to develop various tin based solder alloys. Among the lead-free solders, the ternary tin based tin–silver–copper (SAC) solder alloys are popular at present to replace the eutectic Sn–Pb solder due to their relatively low melting temperature (~ 217 °C–220 °C) compared to other Pb-free Sn based solder alloys, enhanced strength, improved creep and thermal fatigue characteristics and good compatibility with device components [1,2]. Intermetallic compounds (IMCs) formed between solder and substrate play a vital role in determining the long term reliability of microelectronic packages.
Lead free solders currently in use are prone to develop thick interfacial intermetallic compound layers with rough morphology which are detrimental to the long term solder joint reliability. A novel method has been developed to control the morphology and growth of intermetallic compound layers between lead-free Sn–3.0Ag–0.5Cu solder ball and copper substrate by doping a water soluble flux with metallic nanoparticles. Four types of metallic nanoparticles (nickel, cobalt, molybdenum and titanium) were used to investigate their effects on the wetting behavior and interfacial microstructural evaluations after reflow. Nanoparticles were dispersed manually with a water soluble flux and the resulting nanoparticle doped flux was placed on copper substrate. Lead-free Sn–3.0Ag–0.5Cu solder balls of diameter 0.45 mm were placed on top of the flux and were reflowed at a peak temperature of 240 °C for 45 s. Angle of contact, wetting area and interfacial microstructure were studied by optical microscopy, field emission scanning electron microscopy and energy-dispersive X-ray spectroscopy. It was observed that the angle of contact increased and wetting area decreased with the addition of cobalt, molybdenum and titanium nanoparticles to flux. On the other hand, wettability improved with the addition of nickel nanoparticles. Cross-sectional micrographs revealed that both nickel and cobalt nanoparticle doping transformed the morphology of Cu₆Sn₅ from a typical scallop type to a planer one and reduced the intermetallic compound thickness under optimum condition. These effects were suggested to be related to in-situ interfacial alloying at the interface during reflow. The minimum amount of nanoparticles required to produce the planer morphology was found to be 0.1 wt.% for both nickel and cobalt. Molybdenum and titanium nanoparticles neither appear to undergo alloying during reflow nor have any influence at the solder/substrate interfacial reaction. Thus, doping of flux with appropriate metallic nanoparticles can be successfully used to control the morphology and growth of intermetallic compound layers at the solder/substrate interface which is expected to lead to better reliability of electronic devices.
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Lead-free Sn solder bumps have already been used, but tin whiskers become the big problem for the ultrafine bumps with decreasing pitch. So Sn-based alloys with minor alloying elements such as Ag, Cu, Au, In and Ni should be used instead in the electronic industry since the alloys not only overcome the problem of tin whiskers, but also provide better mechanical properties [1–5]. There are several methods to form bumps including stencil printing, evaporation, and electroplating [6–8].
A novel method, multilayer electroplating, was proposed to prepare alloy bumps. It consists to electroplating different structural elements of alloys sequentially and then forming uniform alloy through reflowing. The formation of eutectic SnAg alloy bumps was taken as an example to verify the applicability of the method in this study.
Near-eutectic SnAg solder bumps were formed successfully through multilayer electroplating. After reflowing for 1 min at 250 °C, SnAg alloy with homogeneous structure and composition was obtained from multilayer structure. Fine Ag₃Sn particles with uniform distribution were precipitated inside the solder and Cu₆Sn₅ intermetallic compounds (IMCs) in scallop-like shape formed at the SnAg solder/Cu interface. As reflow time increasing, the morphology of Ag₃Sn remained unchanged but the thickness of Cu₆Sn₅ layer increased. The characteristic of multilayer electroplated SnAg solder bumps was similar to that of traditional alloy, which indicated that it was a suitable method to form SnAg alloy solder bumps.

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Effects of surface finishes and loading speeds on shear strength of Sn–3.0Ag–0.5Cu solder joints

Abstract

Graphical abstract

Highlights

Introduction

Section snippets

Experimental procedure

Results and discussion

Conclusions

Acknowledgement

J. Master. Sci.

Kor. J. Mater. Res.

J. Kor. Inst. Met. Mater.

Microelectron. Eng.

Microelectron. Eng.

Kor. J. Met. Mater.

Kor. J. Met. Mater.

Kor. J. Met. Mater.

Met. Mater. Int.

Met. Mater. Int.

Met. Mater. Int.

J. Microelectron. Packag. Soc.