【作者】 高原；

【导师】 郭福；

【作者基本信息】 北京工业大学 ， 材料加工工程， 2007， 硕士

【摘要】 众所周知,含铅钎料(主要是Sn-37 Pb)以良好的物理化学性能,充足的供应及较低廉的价格,长期以来含铅钎料在表面封装的机械和电气连接中一直发挥着重要的作用。在过去的几年中,由于铅和铅制品的毒性对人的身体和环境造成了危害,国际许多大型电子产品生产商和科研机构都越来越关注于Sn-Pb钎料替代品的研究。同时,钎料作为电子产品机械和电器连接中最为薄弱的环节,其高力学性能和服役可靠性的需求也日趋迫切。目前应用比较广泛的无铅钎料都是以Sn-Ag基二元或三元合金钎料,其中,Sn-3.5Ag和Sn-3.0Ag-0.5Cu钎料合金被人们视为最有可能替代传统Sn-Pb钎料的无铅钎料。许多研究还表明,稀土元素可以细化晶粒,向Sn-3.0Ag-0.5Cu钎料合金中添加微量的稀土可以提高其性能。因此,本课题选择共晶Sn-3.5Ag、Sn-3.0Ag-0.5Cu以及Sn-3.0Ag-0.5Cu-0.1RE三种无铅合金钎料作为本次研究的对象。本课题是对Sn-3.5Ag、Sn-3.0Ag-0.5Cu以及Sn-3.0Ag-0.5Cu-0.1RE三种无铅合金钎料的钎焊接头进行热疲劳试验和残余机械性能的试验。热疲劳试验采用模拟实际工况条件下的温度曲线—-40℃至125℃范围的温度曲线,极值温度停留10min,升温和降温时间为5min,对接头进行0、100、250、500的热疲劳循环周期后,观察热疲劳后试样的显微组织和剪切强度变化,从而对Sn-Ag系钎料合金的服役可靠性进行比较和评价。结果显示,经历不同周期的热疲劳试验后,三种钎料的钎焊接头的显微组织变化及晶界裂纹形成和形貌明显不同。Sn-3.5Ag钎料钎焊接头出现金属间化合物与钎料基体分离的现象,裂纹沿45°角向钎料接头内部扩展;Sn-3.0Ag-0.5Cu钎料钎焊接头裂纹扩展路径与界面处金属间化合物层平行;Sn-3.0Ag-0.5Cu-0.1RE钎料钎焊接头在经历热疲劳周期后,裂纹在钎料与界面处的金属间化合物层处产生,并远离金属间化合物Cu6Sn5层沿10°角向钎料基体内部扩展。同时,对热疲劳后的接头试样进行拉剪实验,三种试样的剪切强度存在微小差别,拉剪强度都随着热疲劳周期的增加而降低,但在经历相同热疲劳周期后,Sn-3.0Ag-0.5Cu钎焊接头的剪切强度高于另外两种钎料的试样。Sn-3.5Ag钎焊接头的拉剪强度下降速率最快,且其剪切强度最低。Sn-3.0Ag-0.5Cu-0.1RE接头的拉剪强度也随着热疲劳周期的增加而下降,但是其下降速率是三种钎料合金中最慢的。此外,三种材料经历相同热疲劳周期后的断口形貌特征也各不相同。Sn-3.5Ag钎料接头的断口形貌主要呈现出韧性断裂特征;Sn-3.0Ag-0.5Cu钎焊接头断口呈现出脆性和韧性混合特征;Sn-3.0Ag-0.5Cu-0.1RE接头的断口特征也呈现脆性和韧性混合特征,但脆性断裂区域随着热疲劳周期的增加明显多于韧性断裂区域。更多还原

【Abstract】 As everybody knows, due to the good properties, sufficient supply and low prices, solders, especially Sn-37Pb, have been playing important roles in both mechanical and also electrical connection in surface mount technology. In the past a few years, more and more investigations on the substitutes for Sn-Pb solders have been made by many large electronic manufactures and research institutes world wide due to environmental and economic concerns. Solder joints are still the weakest link in the architecture of the electronic packaging. Therefore, new types of lead-free solders with higher mechanical properties and good reliability performance are extremely demanding.At present, most of the lead-free solders are Sn-Ag based binary or ternary alloys, among which Sn-3.5Ag and Sn-3.0Ag-0.5Cu solder alloys have been considered as the most promising Pb-free substitutes for the Sn-Pb solders. Many researches also showed that rare earth elements can refine the microstructure, so adding rare earth to the Sn-3.0Ag-0.5Cu based solders could improve the properties of the solder joints. Therefore, the eutectic Sn-3.5Ag, Sn-3.0Ag-0.5Cu, and Sn-3.0Ag-0.5Cu-0.1RE solders were selected in the current research as the testing alloys.In this research, thermomechanical fatigue (TMF) and consequent mechanical tests were performed on solder joints made with Sn-3.5Ag, Sn-3.0Ag-0.5Cu and Sn-3.0Ag-0.5Cu-0.1RE lead-free alloys. The temperature profile used in the experiment simulated the real service condition, i.e., from -40oC to 125oC with a dwell time of 10 min and a ramp time of 5 min. After 0, 100, 250, and 500 cycles, surface damage factures were observed and residual shear strengths of the solder joints were evaluated.The results have shown that the morphology of the microstructure and the crack were different after different cycles of thermomechanical fatigue test. For the Sn-3.5Ag solder joints, the crack initiated near the interfacial IMC and propagated into the bulk solder along a 45o line. Crack initiated near the interfacial IMC and propagated parallel along the interfacial IMC of the Sn-3.0Ag-0.5Cu solder joints. In the Sn-3.0Ag-0.5Cu-0.1RE solder joints, crack also initiated near the interfacial IMC and propagated away from the interfacial IMC along approximately a 10°angle.Residual shear strengths of Sn-3.5Ag, Sn-3.0Ag-0.5Cu, and Sn-3.0Ag-0.5Cu-0.1RE solder joints after TMF have shown that the differences of the shear strengths among the three kinds solder joints were not very significant, but the shear strength of Sn-3.0Ag-0.5Cu was better than others. The shear strength of all solder joints decreased with TMF. Sn-3.5Ag solder joints had the fastest decreasing rate and lowest shear strength. The shear strength of Sn-3.0Ag-0.5Cu-0.1RE solder joints decreased with the increase of the TMF cycles and the decreasing rate is the slowest.Features of fracture surfaces of thermomechanically-fatigued Sn-3.5Ag, Sn-3.0Ag-0.5Cu, and Sn-3.0Ag-0.5Cu-0.1RE solder joints were observed and compared. Sn-3.5Ag solder joints exhibited primarily ductile fractured areas. Sn-3.0Ag-0.5Cu solder joints exhibited a mixture of both ductile and brittle fracture surface. Sn-3.0Ag-0.5Cu-0.1RE solder joints contained significantly more brittle areas in the fracture surface with increasing number of TMF cycles.更多还原