【作者】 刘威；

【导师】 王春青；

【作者基本信息】 哈尔滨工业大学 ， 材料加工工程， 2008， 博士

【摘要】 UBM (Under Bump Metallization)及各种焊盘表面的Au镀层会与Sn发生反应生成脆性的Au-Sn化合物。随着接头尺寸减小,Au在钎焊接头中的含量会大比例增加。此外,在MEMS (Micro Electromechanical System)、光电子、微传感器件及微波器件中,Au有时被直接用作焊接层。Au-Sn化合物的生长和形貌将对接头的性能和可靠性产生决定性影响,而激光加热的特点决定用该方法得到接头内部Au-Sn化合物的生长和形貌十分特殊。本文采用分别镀有不同厚度(0.1、0.5、0.9和4.0微米)Au层的焊盘,直径为120微米的Sn-3.5Ag-0.75Cu、Sn-2.0Ag-0.75Cu-3.0Bi和Sn-37Pb钎料球,研究激光重熔微小接头中Au含量对钎料合金与焊盘界面处Au-Sn化合物生长和演变的影响。结果表明,在老化过程中接头中的针状或层片状的AuSn4化合物均会发生粗化,并在界面处生成层状化合物,AuSn4化合物层厚度随着接头中Au含量的增加而增长。此外,还研究了焊盘表面镀有4.0微米厚Au层接头在老化过程中界面处AuSn4化合物反应动力学。结果表明,钎料合金成分对Au-Sn化合物的生长和演变速度影响巨大。Sn-3.5Ag-0.75Cu、Sn-2.0Ag-0.75Cu-3.0Bi和Sn-37Pb接头中AuSn4化合物的生长激活能分别为153.0、20.1和77.8kJ/mol。对激光重熔微小接头在老化过程中出现塌陷现象的机理进行研究,发现钎料合金中的Sn向微小接头边缘处富Au的Au-Sn化合物和剩余的Au层扩散,并发生反应,导致接头边缘处钎料合金中的Sn大量被消耗是塌陷现象产生的直接原因。随着Au层厚度的增长和Au在接头中所占比例的增加,Au-Sn反应会持续进行下去,塌陷现象将变得更为严重。而焊盘表面镀有较薄(0.1微米)Au层的接头,在重熔时Au即已经全部溶解进入接头,钎料与底层的Ni进行连接。在老化过程中,仅有Au-Sn化合物向Ni层的沉积,而没有Sn大量被消耗,不出现塌陷现象。在Sn基合金中添加不同含量的Pb、Bi、Sb、Ag、Cu和Zn合金元素,研究合金元素对激光重熔接头界面处Au-Sn化合物和相生长和演变的影响,发现Pb和Bi元素会促进Au-Sn化合物的生长,增加Pb和Bi元素含量,Au-Sn化合物的生长速度随之提高;Sb元素会促进Au-Sn化合物的生长,但如果增加较多的Sb元素,其对Au-Sn化合物生长的促进效果反而有所减弱;Ag元素会对Au-Sn化合物的生长具有抑制作用,但随着Ag含量的增多,其对Au-Sn化合物生长抑制的效果并没有发生叠加。Cu和Zn元素的作用机制与以上元素不同,Cu元素会与Sn和Au元素发生作用生成Sn-Cu-Au三元相取代界面处的AuSn4化合物;Zn元素则会与Au作用生成Zn-Au和Zn-Au-Sn相。增加Cu和Zn元素的含量对Au-Sn化合物生长的抑制效果更为显著。以八面体簇模型,采用相对论DV-Xα法计算合金元素对Au与Sn相互作用的影响,从原子轨道相互作用强弱变化的角度来研究不同接头中Au-Sn化合物生长情况的差异,并提出了合金元素对Au-Sn化合物演变影响的原子轨道相互作用机理。计算结果与试验的结论互相印证。更多还原

【Abstract】 Au surface finishes of UBM (Under Bump Metallization) and on pads will react with Sn to form brittle Au-Sn IMCs (Intermetallic Compounds). With miniaturization of joints, the content of Au in the solder joints will be increased tremendously. In addition, Au may be used as soldered layers sometimes in MEMS (Micro Electro-mechanical System), micro-sensor, photoelectron and microwave components. Growth and morphology of Au-Sn IMCs will decide the property and reliability of solder joints. Formation and growth of IMCs will be affected by reflow processes. In the laser reflowed solder joints, growth and morphology of Au-Sn IMCs will be very special, which are caused by the characteristics of laser heating method, such as high energy input and short heating time.In this thesis, pads with different thickness (0.1, 0.5, 0.9 and 4.0 microns) of Au layers, and solder balls (Sn-3.5Ag-0.75Cu, Sn-2.0Ag-0.75Cu-3.0Bi and Sn-37Pb) with the diameter of 120 microns were utilized to fabricate laser reflowed micro-solder joints. The specimens were used to study the effect of Au content in the solder joints on growth and evolution of Au-Sn IMCs at the interface of the solders and pads. The results indicated that needle-like and plate-like AuSn4 IMCs in the laser reflowed solder joints became thicker during the aging process. Eventually, a AuSn4 IMCs layer was formed at the interface of the solders and pads. With the increase of Au content in the solder joints, thickness of the AuSn4 IMCs layer was increased. In addition, the reaction kinetics of AuSn4 IMCs during aging process was also investigated, the IMCs were at the interface of the solders and pads with 4.0 microns thickness of Au. It was found that composition of solder alloys affected the evolution speed of AuSn4 IMCs greatly. The activation energy of AuSn4 IMCs’growth in the Sn-3.5Ag-0.75Cu, Sn-2.0Ag-0.75Cu-3.0Bi and Sn-37Pb solder joints was 153.0, 20.1 and 77.8 kJ/mol, respectively.When the laser reflowed micro-solder joints were aged at high temperature, a sagging phenomenon was observed. The mechanism of the sagging phenomenon’s emergence was studied. It was found that the Sn in the solder alloys diffused into the Au-rich Au-Sn IMCs and Au layer at the edges of micro-solder joints, and reacted with them. The diffusion and reaction process consumed lots of Sn in the solder alloys and caused emergence of the sagging phenomenon. With the increase of Au content and thickness of Au layers in the solder joints, the reaction of Au and Sn continued, and the sagging phenomenon became more serious than that in the solder joints with thin Au layers on pads. However, in the solder joints with thinner (such as 0.1 micron) Au layers on pads, the Au was dissolved into the solders during the reflow process, and the solders reacted with the Ni beneath the Au layer. In the subsequent aging process, redistribution of Au-Sn IMCs onto the Ni layer occurred. Sn in the solder was not consumed much, therefore, no sagging phenomenon happens in the solder joints.Different content of Pb, Bi, Sb, Ag, Cu and Zn was added into the Sn-bearing solder alloys to study the effect of alloying element on growth and evolution of Au-Sn IMCs and phases in the laser reflowed solder joints. It was found that Pb and Bi accelerated growth of Au-Sn IMCs. With the increase of Pb and Bi’s content in the solder alloys, growth speed of Au-Sn IMCs was also increased; Sb also accelerated growth of Au-Sn IMCs, however, if the content of Sb exceed certain value, the acceleration effect of Sb on growth of Au-Sn IMCs was weakened; Ag restrained growth of Au-Sn IMCs, however, with the increase of Ag’s content in the solder alloys, the restraining effect of Ag on growth of Au-Sn IMCs was not enhanced. The acting mechanism of Cu and Zn on growth of Au-Sn IMCs was different from that of Pb, Bi, Sb and Ag. Cu reacted with Sn and Au to form Sn-Cu-Au ternary phases, the AuSn4 IMCs at the interface of the solder alloys and Au foils were replaced by the phases; Zn reacted with Au to form Zn-Au and Zn-Au-Sn phases. If the content of Cu and Zn in the Sn-bearing solder alloys was increased, the restraining effects of them on growth of Au-Sn IMCs were getting more remarkable.Octahedron clusters and the DV-Xαmethod were adopted to calculate the effect of alloying element on the interaction of Au and Sn. The effect of alloying element on Au-Sn IMCs’growth was explained from the view of the change of orbital interaction between atoms. Finally, the mechanism of orbital interaction between atoms was brought forward, which described the effect of alloying elements on growth of Au-Sn IMCs. Results of the calculation and conclusion from the experiments confirmed with each other.更多还原