A brief introduction of the historical background of grain boundary engineering for structural and functional polycrystalline materials is presented herewith. It has been emphasized that the accumulation of fundamental knowledge about the structure and properties of grain boundaries and interfaces has been extensively done by many researchers during the past one century. A new approach in terms of the concept of grain boundary and interface engineering is discussed for the design and development of high performance materials with desirable bulk properties. Recent advancements based on these concepts clearly demonstrate the high potential and general applicability of grain boundary engineering for various kinds of structural and functional materials. Future prospects of the grain boundary and interface engineering have been outlined, hoping that a new dimension will emerge pertaining to the discovery of new materials and the generation of a new property originating from the presence of grain boundaries and interfaces in advanced polycrystalline materials.

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Grain boundary engineering: historical perspective and future prospects

The interfacial reactions between Sn-based solders and two common substrate materials, Cu and Ni, are the focuses of this paper. The reactions between Sn-based solders and Cu have been studied for several decades, and currently there are still many un-resolved issues. The reactions between Sn-based solders and Ni are equally challenging. Recent studies further pointed out that Cu and Ni interacted strongly when they were both present in the same solder joint. While this cross-interaction introduces complications, it offers opportunities for designing better solder joints. In this study, the Ni effect on the reactions between solders and Cu is discussed first. The presence of Ni can in fact reduce the growth rate of Cu3Sn. Excessive Cu3Sn growth can lead to the formation of Kirkendall voids, which is a leading factor responsible for poor drop test performance. The Cu effect on the reactions between solders and Ni is then covered in detail. The knowledge gained from the Cu and Ni effects is applied to explain the recently discovered intermetallic massive spalling, a process that can severely weaken a solder joint. It is pointed out that the massive spalling was caused by the shifting of the equilibrium phase as more and more Cu was extracted out of the solder by the growing intermetallic. Lastly, the problems and opportunities brought on by the cross-interaction of Cu and Ni across a solder joint is presented.

Interfacial reaction issues for lead-free electronic solders

By doping an organic compound functioning as an electron donor (hereinafter referred to as donor molecules) into an organic compound layer contacting a cathode, donor levels can be formed between respective LUMO (lowest unoccupied molecular orbital) levels between the cathode and the organic compound layer, and therefore electrons can be injected from the cathode, and transmission of the injected electrons can be performed with good efficiency. Further, there are no problems such as excessive energy loss, deterioration of the organic compound layer itself, and the like accompanying electron movement, and therefore an increase in the electron injecting characteristics and a decrease in the driver voltage can both be achieved without depending on the work function of the cathode material.

Light Emitting Device

A review on thermal cycling and drop impact reliability of SAC solder joint in portable electronic products

The study of mechanical properties of Sn–Ag–Cu lead-free solders with different Ag contents and Ni doping under different strain rates and temperatures

The thermal fatigue properties of Sn-xAg-0.5Cu (x=1, 2, 3, and 4 in mass%) flip-chip interconnects were investigated to study the effect of silver content on thermal fatigue endurance. The solder joints with lower silver context (x=1 and 2) had a greater failure rate compared to those with higher silver content (x=3 and 4) in thermal fatigue testing. Cracks developed in the solders near the solder/chip interface for all joints tested. This crack propagation may be mainly governed by the nature of the solders themselves because the strain-concentrated area was similar for tested alloys independent of the silver content. From the microstructural observation, the fracture was a mixed mode, transgranular and intergranular, independent of the silver content. Higher silver content alloys (x=3 and 4) had finer Sn grains before thermal cycling according to the dispersion of the Ag3Sn intermetallic compound, and even after the cycling, they suppressed microstructural coarsening, which degrades the fatigue resistance. The fatigue endurance of the solder joints was strongly correlated to the silver content, and solder joints with higher silver content had better fatigue resistance.

Effect of silver content on thermal fatigue life of Sn-xAg-0.5Cu flip-chip interconnects

The mechanical shear fatigue test has been performed to study the effect of silver content on the fatigue properties of Sn-xAg-0.5Cu (x=1, 2, 3, and 4) for flip-chip interconnections. The strength of the solder alloy increases with increasing silver content, preventing shear plastic deformation of the solder bump. The flip-chip joints made using higher silver content solder, such as 3%Ag and 4%Ag, exhibit longer fatigue life for all conditions. The fatigue ductility of the solder decreases with an increase in the silver content. The fatigue endurance of 1%Ag solder is superior to other solders over the plastic strain range of 3%, even though the strength of the solder is the lowest in the solders tested. Based on this study, the 3Ag solder may exhibit good fatigue performance for all conditions, and the 1Ag solder is optimum for severe strain conditions.

Effect of silver content on the shear fatigue properties of Sn-Ag-Cu flip-chip interconnects

The formation of fine Sn grains in a Sn-1.2 mass%Ag-0.5 mass%Cu-0.05 mass%Ni solder due to thermal strain was investigated from the viewpoint of recrystallization. After thermal fatigue, small general grains recrystallized at the strain concentrated location in Sn-1.2Ag-0.5Cu-0.05Ni. Through isothermal annealing, however, grains, which had near ‹110› orientation at a chip-substrate direction before isothermal annealing, coarsened preferentially. Hence, not isothermal annealing but thermal strain was a driving force for recrystallization. Both grain growth after recrystallization and coarsening of recrystallized grains in Sn-1.2Ag-0.5Cu-0.05Ni were slower than those in Sn-1.2 mass%Ag-0.5 mass%Cu, which suppressed crack initiation and increased fatigue life of Sn-1.2Ag-0.5Cu-0.05Ni.

Recrystallization of Sn Grains due to Thermal Strain in Sn-1.2Ag-0.5Cu-0.05Ni Solder

There is growing interest in Cu wire bonding for LSI interconnection due to cost savings and better electrical and mechanical properties. Cu bonding wires, in general, are severely limited in their use compared to Au wires; such as wire oxidation, lower bondability, forming gas of N 2 +5%H 2 , and lower reliability. It is difficult for conventional bare Cu wires to achieve the target of LSI application. A surface-enhanced Cu wire (EX1) has been developed. It is a Pd-coated Cu wire and has many advantages compared to bare Cu wires. Stitch strength was much better under fresh conditions and maintained without any deterioration after being stored in air for a prolonged period of time. EX1 had a lifetime of over 90 days in air, although it was 7days for the bare Cu wire. Spherical balls were formed with pure N 2 (hydrogen-free), whereas the bare Cu produced off-center balls. Cost-effective and secure gas, pure N 2 was only available for EX1. The reliability for Cu wire bonding under conditions of high humidity was investigated in pressure cooker test (PCT). The lifetime for EX1 and the bare Cu was over 800h and 250h, respectively. Humidity reliability was significantly greater for EX1. Continuous cracking was formed at the bond interface for the bare Cu wire, although there was no cracking for EX1. Corrosion-induced deterioration would be the root cause of failure for bare Cu wires in PCT. EX1 improves the bond reliability by controlling diffusion at the bond interface. The excellent performance of Pd-coated Cu wire, EX1 is comparable with Au wires and suitable for LSI packaging.

Surface-enhanced copper bonding wire for LSI

There is provided a bonding wire for semiconductor, capable of ensuring a favorable wedge bondability even when bonded to a palladium-plated lead frame, superior in oxidation resistivity and having a core wire of copper or a copper alloy. This bonding wire comprises: a core wire of copper or a copper alloy; a coating layer containing palladium and having a thickness of 10 to 200 nm; and an alloy layer formed on a surface of the coating layer, such alloy layer containing a noble metal and palladium and having a thickness of 1 to 80 nm. The aforementioned noble metal is either silver or metal, and a concentration of such noble metal in the alloy layer is not less than 10% and not more than 75% by volume.

Shinichi Terashima

Papers

Effect of silver content on thermal fatigue life of Sn-xAg-0.5Cu flip-chip interconnects

Effect of silver content on the shear fatigue properties of Sn-Ag-Cu flip-chip interconnects

Recrystallization of Sn Grains due to Thermal Strain in Sn-1.2Ag-0.5Cu-0.05Ni Solder

Surface-enhanced copper bonding wire for LSI

Bonding wire for semiconductor