Yoshiharu Kariya

Tin Pest in Lead-free Solders

Considers the effect of low temperatures on the characteristics of lead-free alloys based upon tin, which is the most popular basis for the new alloys.

Influence of Cyclic Strain-Hardening Exponent on Fatigue Ductility Exponent for a Sn-Ag-Cu Micro-Solder Joint

The fatigue ductility exponent in the Coffin–Manson law for a Sn-Ag-Cu micro-solder joint was investigated in terms of the cyclic strain-hardening property and the inelastic strain energy in fracture for isothermal fatigue. The fatigue ductility exponent was found to increase with temperature and holding time under strain at high temperature. This exponent is closely related to the cyclic strain-hardening exponent, which displays the opposite behavior in that it decreases with increasing temperature and with coarsening of intermetallic compound particles while holding under strain at high temperature. This result differs from the creep damage mechanism (grain boundary fracture), which is a primary reason for the significant reduction in fatigue life for all strain ranges for large-size specimens.

Effect of joint size on low-cycle fatigue properties of Sn-Ag-Cu solder joint

The influence of the joint size on low cycle fatigue characteristic of Sn-Ag-Cu has been investigated by a miniature joint specimen fabricated using micro solder balls. The influence of the size on crack initiation life is not remarkable, while the life which reaches complete failure reduces greatly when the ball size is decreased less than 150?m. The reduction in life is due to that the crack propagation stage does not appear and complete failure occurs simultaneously with the crack initiation in the ball size less than 150µm. The failure in the smallest size is induced by formation of subgrain boundary formation that occurs in whole area of the joint.

Mechanical properties of lead-free solder alloys evaluated by miniature size specimen

Tensile and fatigue testing using miniature specimen was proposed to compare the mechanical behaviors of Sn-3.0%Ag-0.5%Cu alloy in small and large volumes respectively. It was found that the tensile strength data of the miniature specimen was not similar with clear inferiority to that of the large volume under all test conditions. The differences in their mechanical properties are attributed to dendrite morphology and intermetallic compounds size. Thus, the mechanical behavior and the solidification microstructure strongly depend on specimen volume, which suggests that small size be used to measure mechanical behaviors of solder alloys.

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

Low-Cycle Fatigue Life and Fatigue Crack Propagation of Sintered Ag Nanoparticles

The low-cycle fatigue life and fatigue crack propagation behavior of sintered silver nanoparticles were investigated using miniature specimens sintered at two different temperatures. The fatigue crack initiation life and fatigue crack propagation rate of sintered Ag nanoparticles were extremely sensitive to changes in the range of inelastic energy density and the cyclic J integral, exhibiting brittle characteristics, in contrast to tin-based lead-free solder alloys. With increasing sintering temperature, the fatigue crack propagation rate decreased. On the other hand, the effect of sintering temperature on the fatigue crack initiation life differed depending on the use of either a smooth specimen (low-cycle fatigue test) or notched specimen (fatigue crack propagation test). For the notched specimens, the probability of grain boundaries around the notch decreased due to increased sintering temperature. Therefore, the fatigue crack initiation life was increased with an increase in sintering temperature in the fatigue crack propagation test. In the smooth specimen, however, the fatigue life decreased with an increase in sintering temperature, as the elastic modulus of the specimen increased with increasing sintering temperature. In the low-cycle fatigue test, the specimen sintered with high internal stress started to develop crack initiation early, causing a decrease in the crack initiation life.

Effect of Ni Addition on Bending Properties of Sn-Ag-Cu Lead-Free Solder Joints

Effect of Ni addition on the bending properties of Sn-Ag-Cu lead-free solder joints was investigated. Three point bending tests were conducted with a 324pin flip chip specimen at 10 Hz frequency and with 3mm substrate displacement. The test temperatures were 298 K and 398 K. At each temperature, Sn-l.2Ag-0.5Cu with a small amount of Ni addition, LF35 showed higher performance. The reason LF35 showed higher bending performance was discussed based on a mechanical fatigue test using a micro bulk specimen. Ni addition caused a fine subgrain of j3 -Sn, hence the crack propagation mechanism is changed, resulting in the improved fatigue performance of LF35.

Low cycle fatigue properties of solder alloys evaluated by micro bulk specimen

Micro-bulk fatigue testing was developed to investigate the fatigue lives and damage mechanisms of Sn-3.0Ag-0.5Cu and Sn-37Pb solder alloys. The fatigue lives of micro-bulk solder obeyed Manson-Coffin’s empirical law, and the fatigue ductility exponents were about 0.55 for both Sn-Ag-Cu and Sn-Pb alloys. The fatigue life of Sn-3.0Ag-0.5Cu alloy was 10 times longer than that of Sn-37Pb alloy under symmetrical wave profile, although fatigue resistance of Sn-3.0Ag-0.5Cu alloy was not so superior under asymmetrical wave condition. The fatigue crack was developed from extrusion and intrusion of slip band in Sn-3.0Ag-0.5Cu alloy, while the crack was observed at colony boundary in Sn-37Pb alloy. The difference in damage mechanism may affect the sensitivity of fatigue life to reversibility of loading profile.Copyright

Effect of Temperature and Strain Rate on Low-Cycle Fatigue Life of Bi-Sn Eutectic Alloy

In this study, the effects of temperature and strain rate on low cycle fatigue life of Bi-Sn eutectic alloys have been studied. The fatigue life improves with the increasing of temperature and the decreasing of strain rate. This is a reverse phenomenon from characteristics found in general metals. As temperature increases and strain rate decreases, grin boundary sliding becomes the dominant deformation mechanism and the fatigue ductility coefficient increases, resulting in an improvement of fatigue life. To the extent of this study, dependence on temperature and strain rate can be expressed by Manson-Coffin’s law modified using Z-parameters.Copyright