Interface Growth and Void Formation in Sn/Cu and Sn0.7Cu/Cu Systems

In this work, the effects of electroplated Cu (EP Cu) and Cu addition (0.7%) in Sn solder on the intermetallic compounds (IMCs) growth and void formation were clarified by comparison with solder joints comprising of high purity Cu (HP Cu) substrate and pure Sn solder. After aging processes, a new IMC, Cu3Sn, was formed at the interface, in addition to Cu6Sn5 formed in the as-soldered joints. The EP Cu and Cu addition (0.7%) both had limited effects on the total IMCs thickness. However, the effects varied on the growth behaviors of different IMCs. Comparing to the void-free interface between Sn and HP Cu, a large number of voids were observed at the Cu3Sn/Cu interface in Sn/EP Cu joints. The formation of these voids may be induced by the impurities and fine grain, which were introduced during the electroplating process. The addition of Cu suppressed the inter-diffusion of Cu and Sn at the interface. Consequently, the growth of the Cu3Sn layer and formation of voids were suppressed.

FCCSP IMC Growth under Reliability Stress follow Automotive Criteria

The kirkendall void had been a well-known issue for long term reliability of semiconductor interconnects, while even the KVs existing at the interfaces of Cu & Sn, it may still be able to pass the condition of un-bias long term reliability testing, especially for 2,000 cycles of temperature cycling test and 2,000hrs of high temperature storage. A large numbers of KVs was observed after 200cycles of temperature cycling test at the intermetallic Cu3Sn layer which locate between the intermetallic Cu6Sn5 & Cu layers. These kinds of voids will growth proportional with the aging time at initial stage, but slowing down attribute to the barrier layer of Cu3Sn & Cu interfaces. This paper compare various IMC thickness as a function of stress test, the Cu3Sn & Cu6Sn5 do affected seriously by heat, but Ni3Sn4 is not affected by heat or moisture.

Improvement in Thermomechanical Reliability of Low Cost Sn-Based BGA Interconnects by Cr Addition

The exemption of Pb-bearing automobile electronics in the End of Life Vehicle (ELV) directive has recently expired, bring an urgent need to find Pb-free alloys that can maintain good performance under high-temperature and vibration conditions for automobile application. In this study, a new lead-free solder, Sn-0.7Cu-0.2Cr (wt.%) alloy, was developed. To evaluate the thermomechanical reliability of the new solder alloy in automobile electronics, a thermal shock test was performed. The results show that the presence of Cr in solder inhibits the growth of interfacial Cu3Sn layer and the formation of Kirkendall voids, which effectively improves the joint reliability under intense thermal shock condition compared with the commercial SAC305 and SC07 solders. Specifically, the shear strength of the Sn-0.7Cu-0.2Cr/Cu solder joints was higher by 23% and 44% than that of SAC305 and SC07 solder joints after 2000 cycles of thermal shock at 1 m/s shear speed.

Cu6Sn5 and Cu3Sn Intermetallics Study in the Sn-40Pb/Cu System during Long-term Aging

Replacing Sn-Pb solder with lead-free solder is a great challenge in the electronics industry. The presented lead-free solder is Sn based and forms two intermetallic species upon reaction with the Cu substrate, namely Cu6Sn5 and Cu3Sn. The growth of Cu6Sn5 and Cu3Sn intermetallics have been investigated with respect to Sn-40Pb/Cu solderjoints. The joints were aged under long-term thermal exposure using single shear lap joints and the intermetallics were observed using scanning electron microscopy. As-soldered solder joints exhibit a single Cu6Sn5 phase, however after aging a Cu3Sn layer below the Cu6Sn5 is observed to manifest. The Cu6Sn5 layer develops with a scalloped morphology, whereas the Cu3Sn layer always develops an undulating planar shape in phase with the Cu6Sn5. The Cu6Sn5 layer begins to transform from a scalloped- to a planar-shape as aging progresses in order to minimize the interfacial energy. The intermetallic layers exhibit a linear dependence on the square root ofaging time, which corresponds to diffusion-controlled growth. The activation energy for the growth of the Cu6Sn5, intermetallic layer has been determined to be 56.16 kJ/mol.

Effect of Ni Addition on the Formation and Growth of Intermetallic Compound at Eutectic SnBi/Cu Interface

The reactions between Cu and the eutectic SnBi (Sn58wt.%Bi) solder alloy with and without 1wt.%Ni addition were investigated in this paper. After as-reflowed process, the IMCs formed in the Sn58wt.%Bi/Cu and Sn58wt.%Bi1wt.%Ni/Cu joints were Cu6Sn5 and (CuNi)6Sn5, respectively. During aging process, the thickness of the IMC layers formed at each solder/Cu joint increased, and a new layer Cu3Sn formed adjacent to the Cu substrate. It was found that 1wt.%Ni addition in Sn58wt.%Bi solder alloy could slightly enhance the growth rate of the total IMC layer, but effectively reduce the growth rate of Cu3Sn layer during aging process. The growth behavior of IMC layer for each joint followed the diffusion-controlled mechanism during aging.

Interfacial Reactions Between Electrodeposited Sn-Cu, Sn-Ag-Cu Solders and Cu, Ni Substrates

Interfacial reactions between two near eutectic Pb-free solders, Sn-1.1Cu and Sn-3.6Ag-1.8Cu, and two common base materials, Cu and Ni, were studied by characterizing the formation and growth of intermetallic compounds (IMCs) during reflowing and aging using SEM, XRD, and TEM. A continuous bilayer of thin, uniform Cu3Sn and thick, nonuniform Cu6Sn5 was formed at the interfaces of both Sn-1.1Cu/Cu and Sn-3.6Ag-1.8Cu/Cu, with the Cu3Sn layer adjacent to the Cu substrate. This is attributed to the diffusion of Cu from the Cu substrate to the solder to first form Cu6Sn5, then Cu3Sn. The inclusion of Ag in the Sn-3.6Ag-1.8Cu solder film inhibited the diffusion of Cu and, therefore, the growth of Cu3Sn. For both Sn-1.1Cu/Ni and Sn-3.6Ag-1.8Cu/Ni, an intermetallic film of (Ni,Cu)3Sn4 was formed at the interface, and the film had three distinct morphologies: a continuous planar layer at the Ni interface, followed by long, thin needles and large, polygonal crystals. The layers and the crystals were thinner at the Sn-3.6Ag-1.8Cu/Ni interface, indicating that the addition of Ag slowed down the growth of the (Ni, Cu)3Sn4 films. At the Sn-3.6Ag-1.8Cu/Ni interface, Ag3Sn particles were also observed and they coarsened with aging time. No separate Ag particles were observed.

IMC Growth Mechanisms of SAC305 Lead-Free Solder on Different Surface Finishes and Their Effects on Solder Joint Reliability of FCBGA Packages

Intermetallic compound (IMC) growth behavior of lead-free solder plays an important role in ball grid array (BGA) solder joint reliability for flip chip BGA (FCBGA) packaging applications. The growth mechanism of IMC is reported based on a diffusion model. Thermal treatment such as accelerated thermal cycling (ATC) and isothermal aging exposure also contribute to the growth rate and morphology of lead-free solder IMC. Among the lead-free solder alloys, Sn-3.0wt.%Ag-0.5wt.%Cu (SAC305) solder is a promising substitute for Sn-Pb because of its good mechanical properties and wettability with current surface finishes. After thermal exposure, BGA solder joint reliability is degraded due to IMC formation and growth. In this study, two different thermal treatments, ATC and isothermal aging, and two different pad surface finishes, solder on pad (SOP) and electroless Ni immersion gold (ENIG), are considered in terms of IMC growth rate and mechanical solder joint reliability. An SOP finished interface forms a thin ε-phase Cu3Sn layer and a scallop-like η-phase Cu6Sn5 layer. In contrast, the ENIG finished interface forms a thick (Cu,Ni)6Sn5 IMC layer and prevents overall IMC growth. Different surface finished test vehicles are evaluated in an ATC test in a 0°C to 100°C temperature range and the Ni diffusion layer shows a longer solder joint fatigue lifetime than the nondiffusion barrier interface based on the micro cross-section and dye penetration analysis results. In an isothermal aging test at 100°C and 150°C, the aging temperature and time are valid factors to decide mechanical shock reliability. Interfacial fractures are found in the 100°C aged test vehicle due to easier crack propagation at the interface between the thin Cu3Sn layer and the scallop-like Cu6Sn5 layer based on SEM microstructure analysis results. Finally, this investigation proposes how to improve solder joint reliability and prevent interfacial fracture for SAC305 lead-free application.

Enhanced Growth Kinetics of Intermetallic Compounds between Bi-Containing Sn-3.5Ag Solders and Cu Substrate during Aging

The effect of adding Bi to a eutectic Sn-3.5Ag solder alloy on the growth kinetics of the intermetallic compounds (IMCs) in solder/Cu joints was examined at the aging temperatures of 130°C, 150°C and 180°C. At 150°C and 180°C, the growth rate of the Cu6Sn5 layer was significantly enhanced, but that of the Cu3Sn layer was rather reduced with increasing Bi content up to 12 wt.%. At 130°C, however, both the η and ε layers appeared to grow faster as the Bi content in the solder was increased to 12 wt.%. These results suggest that the accumulation of Bi ahead of the Cu6Sn5 layers can affect not only the interfacial reaction barrier but also the local thermodynamics at the interface between the Cu6Sn5 layer and the solder.