Shinichi Terashima

Surface-enhanced copper bonding wire for LSI

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 N2+5%H2, 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 N2 (hydrogen-free), whereas the bare Cu produced off-center balls. Cost-effective and secure gas, pure N2 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.

Effect of crystallographic anisotropy of β-tin grains on thermal fatigue properties of Sn–1Ag–0˙5Cu and Sn–3Ag–0˙5Cu lead free solder interconnects

Abstract An effect of the crystallographic anisotropy of β-tin grains on thermal fatigue properties of Sn–1Ag– 0˙5Cu and Sn–3Ag–0˙5Cu lead free solder interconnects were discussed. From an orientation imaging microscopic observation, three types of microstructures (single crystal-like, fine grain type and large grain type) were observed in both solders. The single crystal-like microstructure disappeared and the large grain type occurred by further fatigue due to recrystallisation. Because single crystal-like microstructure had the {100} plane approximately parallel to strain concentrated areas, recrystallisation could be retarded if the slip systems of {100}<011> or {100}<010> operate and an amount of thermal strain decreases because these slip systems have the larger critical resolved shear stress due to an anisotropic nature of β-tin. One of the reasons Sn–3Ag–0˙5Cu had longer thermal fatigue life than Sn–1Ag–0˙5Cu can be the number of the single crystal-like or the fine grain type microstructures in Sn–3Ag–0˙5Cu were larger.

Thermal fatigue properties and grain boundary character distribution in Sn–x Ag–0˙5Cu (x = 1, 1˙2 and 3) lead free solder interconnects

Abstract Thermal fatigue properties of Sn–x Ag–0˙5Cu (x = 1, 1˙2 and 3, mass%) lead free solder interconnects were discussed from the viewpoints of both morphology and grain boundary character distribution. 3Ag showed the longer thermal fatigue life than 1Ag and 1˙2Ag. Both cracks, which were initiated by thermal strain, and grain boundary damage due to grain boundary sliding degraded the thermal fatigue lives. From a microstructural observation using orientation imaging microscopy, recrystallisation of tin grains was observed, and 3Ag suppressed coarsening of tin grains after further thermal fatigue as compared with 1Ag and 1˙2Ag. Moreover, 3Ag showed a larger amount of coincidence site lattice boundaries than 1Ag and 1˙2Ag. It is suggested that in 3Ag not only smaller tin grains but also larger amount of coincidence site lattice boundaries suppressed crack propagation and grain boundary sliding.

Significant reduction of wire sweep using Ni plating to realise ultra fine pitch wire bonding

Significant reduction of the wire sweep in molding is proposed because the wire sweep is considered to be the major problem to realise wire bonding with ultra fine pitches of under 30 micrometers. In the present proposal, Ni was plated for several micrometers before molding on bonded Au wires. Ni plating was carried out by means of electroless plating for several minutes in the aqueous solution kept at 358 K containing Ni and P. The wire sweep ratio for Ni plated wire (total diameter was 21 micrometers) was almost half of that for Au wire with the diameter of 15 micrometers except Ni plate, and was slightly smaller than that for Au wire with the diameter of 25 micrometers except Ni plate even the total diameter was smaller. It is considered that wire sweep suppression by this technique was due to the enhancement of both elastic and plastic properties.

Fatigue life enhancement of low silver content Sn-Ag-Cu flip-chip interconnects by Ni addition

The mechanical shear fatigue test has been performed to investigate the fatigue properties of Sn-1.2 mass%Ag-0.5 mass%Cu-0.05 mass%Ni for flip chip interconnections. The low cycle fatigue endurance of the alloy is almost equivalent to that of Sn-3 mass%Ag-0.5 mass%Cu alloy, even though the fatigue endurance of Sn-1 mass%Ag-0.5 mass%Cu alloy was poorer than that of the 3 mass%Ag alloy. Adding Ni to low silver content Sn-Ag-Cu alloy leaded to fine microstructure, and the microstructure had a network structure of Ag3Sn intermetallic compound together with fine Cu-Sn intermetallic compound. The microstructure resulted in high strain hardening exponents, which leaded to good low cycle fatigue endurance of the alloy.

Isothermal shear fatigue life of Sn-xAg-0.5Cu flip chip interconnects

The mechanical shear fatigue test has been performed to look for the effect of silver content on the fatigue properties of Sn-xAg-0.5Cu (x = 1, 2, 3 and 4) flip chip interconnections. As the strength of the solder alloy increases with increasing silver content, the increase in silver content results in preventing a shear plastic deformation of solder bump. Therefore, the flip chip joints made using higher silver content solder such as 3 and 4Ag exhibit longer fatigue life, if the same levels of displacement is applied. The fatigue ductility of the solder decreases with increasing the silver content. Therefore, the fatigue endurance of the 1Ag solder itself is superior to other solders in high plastic strain regime, even though the strength of the solder is the lowest in the solders tested. Based on this study, the 3Ag solder might exhibit good fatigue performance for all condition, and the 1Ag solder is optimum for severe strain condition.Copyright

Effect of thermomechanical fatigue on drop impact properties of Sn-Ag-Cu lead-free solder joints

Drop impact property after the solder joints were subjected to thermomechanical fatigue stresses were investigated. The solder compositions were SAC105 (Sn-1Ag-0.5Cu), SAC305 (Sn-3Ag-0.5Cu), and LF210N (Sn-2Ag-1Cu-0.05Ni) and the temperature range of the thermomechanical fatigue was from −45 degree of centigrade to +125 degree of centigrade. After 200 cycles of thermomechanical treatment, the drop property of SAC105 and SAC305 is almost the same low level whereas the drop property of LF210N keeps as-reflowed level. Microstructural observation revealed the difference of the fracture mode in these three solder compositions.