【作者】 孙鹏；

【导师】 刘建影；

【作者基本信息】 上海大学 ， 机械制造及其自动化， 2008， 博士

【摘要】 随着欧盟RoHS指令案和我国《电子信息产品污染防治管理办法》的实施，电子产品的无铅化进程已全面展开。和传统的Sn-Pb焊料体系相比，尽管对无铅焊料合金、焊接和使用过程中焊点的界面反应及其可靠性的认识已取得较大进展，但对焊点界面的演化行为和可靠性尚缺乏系统、深入的研究，因此对无铅焊点的界面组织与焊料成分、界面耦合效应、温度循环可靠性以及低周疲劳的研究，将促进我国电子封装的无铅化进程。本文通过对Sn-Ag基、Sn-Zn基和自主研发的Sn-0．4Co-0．7Cu等多种无铅焊点的研究，取得如下成果：共晶Sn-3．5Ag和Sn-4．0Ag-0．5Cu与化学镀Ni（P）浸金镀层（ENIG）间的界面反应研究表明，Sn-3．5Ag焊点的界面产物为Ni₃Sn₄，Sn-4．0Ag-0．5Cu焊点中的为（Ni,Cu）₃Sn₄和（Cu,Ni）₆Sn₅两种三元金属间化合物（IMC）：时效处理后Sn-3．5Ag对Ni（P）镀层的消耗较大，形成了较明显的富磷层，并可见“游离”Ni（P）层的Ni₃Sn₄ IMC颗粒，而Sn-4．0Ag-0．5Cu焊料对Ni（P）镀层的消耗较少，三元IMC和Ni（P）层结合良好。Sn-0．4Co-0．7Cu和ENIG镀层的回流焊界面反应与Sn-4．0Ag-0．5Cu和Sn-0．7Cu的比较研究显示，Sn-0．4Co-0．7Cu焊点的母相中生成了两种亚稳态的三元块状大颗粒的（Co,Cu）Sn₂和类似于Sn-0．7Cu焊点母相中Cu₆Sn₅的、颗粒尺寸较小的（Cu,Co）₆Sn₅ IMC；三种焊料的界面产物均为（Cu,Ni）₆Sn₅，其IMC层的厚度差别不大。基于Flip Chip和BGA器件焊点结构，研究了Ni／Sn-3．5Ag-3．0Bi／Cu和Ni／Sn-8．0Zn-3．0Bi/Cu三明治结构焊点的液态界面反应。在Ni／Sn-3．5Ag-3．0Bi/Cu焊点中，Ni层与焊料的界面形成了（Cu,Ni）₆Sn₅ IMC，生长速率为3．80×10^-10cm²／s：在Ni／Sn-8．0Zn-3．0Bi／Cu焊点中，焊料与Ni层的界面生成了由Sn、Ni、Cu、Zn四种元素组成的、难以确定结构的IMC，其生长速率为2．93×10^-12cm²／s；两种焊点中焊料和Cu层的界面产物分别为Cu₆Sn₅和Zn₅Cu₈，生长速率分别为1．44×10^-10cm²／s和1．36×10^-10cm²／s；240℃下液态焊点中Ni和Cu两界面间Cu原子的浓度梯度导致在焊接过程中Cu原子自Cu层向Ni层一侧扩散，Cu原子在Sn-3．5Ag-3．0Bi焊点中的扩散系数大约为1．1×10^-5cm²／s。定量研究了回流焊接过程中三明治焊点的界面耦合效应对界面IMC形成的影响。Sn-3．8Ag-0．7Cu焊膏组装的无铅塑封BGA器件的温度循环可靠性研究表明，无铅塑封BGA器件的焊点失效机理为热机械循环导致的焊点疲劳失效，失效裂纹主要集中在器件一侧的焊点内；经历-55℃～100℃温度循环的塑封BGA焊点Weibull特征寿命为5415周；0℃～100℃温度循环下的焊点Weibull特征寿命为14094周，超过预期。界面（Cu,Ni）₆Sn₅ IMC的生长受扩散机制控制，-55℃～100℃温度循环试验中塑封BGA器件Sn-Ag-Cu焊点和印刷电路板Ni（P）镀层一侧的界面IMC的生长速率为3．43×10^-15cm²／s；0℃～100℃测试条件下的生长速率为2．30×10^-16cm²／s；研究了Sn-8．0Zn-3．0Bi焊点在位移加载条件下的等温低周疲劳行为并藉FEM分析了焊点中的应力应变状态。在±40μm位移加载条件下的Sn-8．0Zn-3．0Bi焊点的低周疲劳寿命略高于Sn-37Pb焊点，在±60μm位移加载条件下的寿命反而低于Sn-37Pb；疲劳裂纹主要在焊点与界面连接的角部起裂，失效模式其主要集中于焊点中部直径较小区域的贯穿型和沿焊点界面的扩展型裂纹。基于3D FEM的动态硬化模型，推导获得了单剪切结构Sn-8．0Zn-3．0Bi焊点的Coffin-Manson方程，N_f=0．0294（△γ）^-2.833，模拟结果与实验结果吻合。更多还原

【Abstract】 Two key technologies used by the electronics industry are chip technology and packaging technology. Solder plays a crucial role in both of them. During the last decade, there has been a strong worldwide environmental movement towards lead-free electronic products.The interfacial reaction of Sn-3.5Ag and Sn-4.0Ag-0.5Cu solders on Electroless Nickel, Immersion Gold （ENIG） metallization after high temperature storage （HTS） testing was investigated from a metallurgical point of view. It was noticed that only Ni₃Sn₄ IMCs were found in the Sn-Ag system, while two kinds of interfacial products, （Ni,Cu）₃Sn₄ and （Cu,Ni）₆Sn₅ existed in the Sn-Ag-Cu system. During soldering and aging, Ni from the electroless Ni（P） was consumed to form IMCs, P atoms were accumulated and formed the P rich Ni（P） layer. In Sn-3.5 Ag solder joint, more Ni atoms were consumed comparing with Sn-4.0Ag-0.5Cu solder joint and one dark layer between Ni₃Sn₄ and Ni（P） could be observed. The interfacial layer between the Sn-Ag-Cu solder and electroless Ni（P） coating showed better thermal stability than eutectic Sn-Ag solder since no spalling was observed.The interfacial reactions between the eutectic Sn-0.4Co-0.7Cu alloy and ENIG metallization was investigated after reflow soldering. Common Sn-4.0Ag-0.5Cu and eutectic Sn-0.7Cu solders were used as references. Two types of IMCs were found in the solder matrix of the Sn-0.4Co-0.7Cu alloy, namely coarser （Co,Cu）Sn₂ and finer （Cu,Ni）₆Sn₅ particles, while only one ternary （Cu,Ni）₆Sn₅ interfacial compound was detected between the solder alloy and the ENIG coated substrate. It was noted that the type of the interfacial IMCs in all the solder joints was the same one-（Cu,Ni）₆Sn₅ and the thickness of the interfacial IMCs layer in the Sn-Co-Cu solder joint was also similar to that of Sn-Ag-Cu and Sn-Cu solder joints.Furthermore, the coupling effect in both Sn-3.5Ag-3.0Bi and Sn-8.0Zn-3.0Bi solder joints in sandwich structure was studied as a function of reflow time. The coupling effect between the ENIG metallization and the Cu substrate was confirmed since the type of IMCs on Ni（P） layer changed from Ni-Sn phase to Cu-Sn phase, apparently as a result of the diffusion of Cu atoms from the opposite Cu substrate. Furthermore, the growth rate constant of ternary （Cu,Ni）₆Sn₅ IMCs between Sn-Ag-Bi solder and Ni（P） substrate was 3.8×10^-10cm²/s. One complex alloy Sn-Ni-Cu-Zn was formed at the Sn-Zn-Bi/Ni（P） interface; however the growth of this complex alloy on the ENIG coated substrate was suppressed and it’s growth rate constant was 2.93×10^-12cm²/s. The growth rate constants of interfacial Cu₆Sn₅ in Sn-3.5Ag-3.0Bi and Cu₅Zn₈ in Sn-8.0Zn-3.0Bi were 1.44×10^-10cm²/s and 1.36×10^-10cm²/s, separately. The diffusion coefficient of Cu atom in the molten Sn-3.5Ag-3.0Bi solder at 240℃was calculated to be about 1.1×10^-5cm²/s.Fourth, PBGA package was assembled on the FR-4 PCB and the temperature cycling （TC） was carried out in a systematic manner for two different TC profiles in a single chamber Heraeus climate cabinet. The first TC profile ranged from-55℃to 100℃and the second one ranged between 0℃to 100℃. It was found that Ag₃Sn IMCs coarsen in the solder matrix and interfacial （Cu,Ni）₆Sn₅ IMCs layer growth during the temperature cycling. The growth rate constants of （Cu,Ni）₆Sn₅ for TC test ranging from -55℃to 100℃was 3.43×10^-15cm²/s and another one ranging from 0℃to 100℃was 2.30×10^-16cm²/s. The common failure mode of the solder joints analyzed in this work were cracks in the solder matrix. No fatigue cracks were found to propagate through the interfacial intermetallic layer for all the cases. The Weibull lifetime for TC test from -55℃to 100℃was 5415 cycles the second one ranged from 0℃to 100℃was 14094 cycles, both of them were qualified.The low cycle fatigue behavior of Sn-8Zn-3Bi solder joint, which is one promising lead-free candidate for low melting temperature soldering, was investigated using single lap shear samples. For the test loading condition using±40μm amplitude, the average lifetime of Sn-8Zn-3Bi was longer than that of Sn-37Pb, while in the±60μm displacement loading case, the average lifetime of Sn-8Zn-3Bi was shorter than that of Sn-37Pb solder joint. The fatigue cracks originated from the corner and prorogated through the solder joints in most cases. The 2D Finite element （FE） modeling work could described the strain and stress distribution in agreement with the experimental observation well, but the modeled stiffness of PCB was too low and allowed PCB to bend too much. 3D FE simulation based on the dynamic hardening model was performed and the Coffin-Manson equation was given based on results from experiment and simulation both:N_f= 0.0294 （△γ）^-2.833更多还原