The Microstructure and Low Cycle Fatigue of Sn-Ag-xSb Lead-Free Solder Joints

• Main Authors: Cheng-Shyan Lee, 李政賢
• Other Authors: Hwa-Teng Lee
• Format: Others
• Language: zh-TW
• Published: 2003
• Online Access: http://ndltd.ncl.edu.tw/handle/06903952361823730750

碩士 === 國立成功大學 === 機械工程學系碩博士班 === 91 === The goal of this research is to evaluate the effect of Sb additions (0~10 wt%) on the melting point and microstructure of Sn-Ag solder. The reliability of low cycle fatigue of the solder joint is evaluated by the thermal storage test for 150˚C. The materials tested are Sn-3.5Ag with 1.73, 3.85, 5.12 and 10.05 wt% Sb additions respectively. Single-lap specimens were used to simulate real solder joints. Solder balls of 1.7mm in diameter were prepared in the lab, and re-flowed between two pure Cu substrates. The 150˚C thermal storage test is conducted after soldering, and the storage time is 225 and 625 hours respectively. Experimental result shows melting points of the Sn-Ag-xSb solder are increased with greater Sb additions. The melting points are 221.3˚C(Sn-3.5Ag) and 228.2˚C(adding 10.05%) respectively, and the solid-liquid regions also expand as the content of Sb increases. Microstructures of the Sn-Ag solder with different Sb additions are similar and can be characterized as consisting of β-Sn dendrite and interdenritic eutectic network. Sb atoms are solved in the β-Sn when adding 1.73 wt% Sb into the Sn-Ag solder, and SbSn compounds of several μm in size are formed in the β-Sn when adding 3.85 and 5.12 wt% Sb. As Sb addition reaches 10.08 wt%, cubic SbSn compounds of 20~30 μm in length form in the solder. Shear strength of the as-soldered solder joints is increased with greater Sb additions. The shear strengths are 89.3N(Sn-3.5Ag), 114.2N(1.73%), 129.0N(3.85%), 140.5N(5.12%) and 183.2N(10.05%). In the condition with constant ±0.025mm displacement, fatigue life of the as-soldered joint is approximately increases with greater Sb additions. The reason is the plastic strain of the solder joint is decreases with greater Sb additions. The lesser plastic strain the better fatigue life and the rate of load-drop is increased with greater Sb additions. After 150˚C thermal storage, fatigue life will improve because of the softening of solder joints. While the creaks that propagate along the interface due to the increased thickness of the Intermetallic Compound (IMC) greatly reduce the fatigue life. Therefore, the fatigue life after thermal storage is influenced by two factors. Fatigue cracks initiate at the location between the IMC layers and the neck of hourglass-shaped specimens. The fracture mode transits from solder fracture mode to mixture mode then to IMC fracture mode with increasing Sb additions and longer storage time.