网络首发时间: 2019-03-18 17:25
稀有金属 2020,44(06),603-608 DOI:10.13373/j.cnki.cjrm.xy18110025
3D封装用Cu/Sn/Cu焊点的组织与剪切性能研究
魏纯纯 陈明和 孙磊 武永
南京航空航天大学机电学院
摘 要:
研究了250℃温度,不同键合时间对Cu/Sn/Cu焊点的界面金属间化合物(intermetallic compound,IMC)生长行为及剪切性能的影响。结果表明:键合时间由30 min增加到120 min,Cu/Sn/Cu焊点界面IMC的厚度逐渐变厚。当键合时间为30 min时,焊点界面IMC厚度为12. 8μm。随着时间增加到120 min,焊点中的液相Sn消耗殆尽,形成了全IMC。在Cu/Sn/Cu焊点中,发现焊点两端界面IMC呈现非对称生长。键合时间为30 min时,焊点界面Cu3 Sn的厚度分别为1. 73μm(冷端)和0. 95μm(热端),冷端IMC的增长速率明显高于热端,主要原因归结于温度梯度。通过对焊点界面IMC进行电子背散射衍射(EBSD)分析,发现存在的大块晶粒组织为Cu6 Sn5 ,而Cu3 Sn的晶粒相对较小。根据横截面方向(transverse direction,TD)反极图显示,Cu6 Sn5 的晶粒取向多平行于[001]与[111]之间。此外,随着键合时间的增加,焊点的剪切强度不断增加,当键合时间为120 min时,焊点的剪切强度由8. 5 MPa增加到18. 6 MPa,焊点的断裂模式由初始的混合断裂逐渐转变为脆性断裂。
关键词:
Cu/Sn/Cu焊点 ;界面金属间化合物 ;剪切强度 ;断裂模式 ;
中图分类号: TG407
作者简介: 魏纯纯(1993-),女,江苏常州人,硕士研究生,研究方向:电子封装互连材料,E-mail:1157164014@qq.com; *陈明和,教授,电话:13951809276,E-mail:meemhchen@nuaa.edu.cn;
收稿日期: 2018-11-25
基金: 国家自然科学基金项目(51805256); 江苏省研究生科研与实践创新计划项目(KYCX18_0318); 南京航空航天大学博士学位论文创新与创优基金项目(BCXJ18-06)资助;
Microstructure and Shear Property of Cu/Sn/Cu Solder Joints for 3D Packaging
Wei Chunchun Chen Minghe Sun Lei Wu Yong
College of Mechanical & Electrical Engineering,Nanjing University of Aeronautics and Astronautics
Abstract:
The effects of different bonding times on the growth behavior and shear properties of intermetallic compounds(IMCs)at Cu/Sn/Cu solder joints were investigated. The results showed that the bonding time increased from 30 min to 120 min,and the thickness of the IMC at the Cu/Sn/Cu solder joint interface gradually became thicker. When the bonding time was 30 min,the solder joint interface IMC thickness was 12.8 μm. As time increased to 120 min,the liquid phase Sn in the solder joint was depleted,forming a full IMC. In the Cu/Sn/Cu solder joints,it was found that the interface IMC exhibited asymmetric growth at both ends of the solder joint.The thickness of Cu3 Sn at the solder joint interface was 1.73 μm(cold end)and 0.95 μm(hot end),respectively. The growth rate of IMC at the cold end was significantly higher than that at the hot end,which was mainly due to the temperature gradient. By performing electron backscattered diffraction(EBSD)analysis on the solder joint IMC,it was found that the large grain structure existed as Cu6 Sn5 ,while the Cu3 Sn crystal grains were relatively small. According to the transverse direction(TD)inverse pole figure,the grain orientation of Cu6 Sn5 was more parallel between[001]and[111]. In addition,as the bonding time increased,the shear strength of the solder joint increased continuously. When the bonding time was 120 min,the shear strength of the solder joint increased from 8.5 MPa to 18.6 MPa,and the fracture mode of the solder joint gradually changed from the initial mixed fracture to brittle fracture.
Keyword:
Cu/Sn/Cu solder joint; IMC; shear strength; fracture mode;
Received: 2018-11-25
近年来,随着电子技术的快速发展,电子产品逐渐向微型化、髙集成度、低消耗的方向发展
[1 ]
。方型扁平式封装技术、球状引脚栅格阵列封装技术、芯片级封装等传统的封装方法由于自身的局限性已无法满足日益提高的互连密度。在这种情况下,可以实现芯片堆叠、充分利用立体空间的三维封装成为一种极具前景的封装技术
[2 ,3 ,4 ]
。在三维封装技术中,芯片叠层键合技术是实现三维封装的关键技术之一
[5 ,6 ]
。目前,应用于芯片叠层技术主要包括:粘接键合、热压键合、表面活化键合、超声波键合以及瞬时液相键合
[7 ]
。其中,瞬时液相键合技术由于可以实现低温键合高温服役的效果越来越受到业界的关注。目前,应用于瞬时液相键合的互连材料体系主要有Cu-Sn,Cu-In,Cu-Ga,AgSn,Au-Sn,Au-In,Ni-Sn
[8 ,9 ,10 ,11 ,12 ,13 ,14 ]
。
本文以Cu/Sn/Cu焊点为研究对象,采用瞬时液相键合技术,研究不同的键合时间对Cu/Sn/Cu焊点界面金属间化合物(IMC)的生长和剪切强度影响。
1实验
实验材料为纯Sn焊膏。使用线切割机将Cu片切割成尺寸为5 mm×10 mm×0.4 mm,随后对Cu片打磨、抛光后放入超声波清洗仪中清洗。将Sn焊膏均匀的涂覆在Cu基板表面,组成Cu/Sn/Cu结构,如图1所示。随后将其置于精密石墨恒温电热板进行键合,键合温度为250℃,键合压力为1.5N,键合时间为30,45,60,90,120 min。试验结束后对样品进行镶嵌、打磨、抛光,使用5%HNO3 +95%C2 H5 OH(体积分数)的溶液腐蚀以便观察焊点的组织形貌。
采用JSM-6360型扫描电镜(SEM)对焊点的界面IMC组织形貌进行观察并采用能谱仪(EDS)分析相成分。为了更好的研究界面IMC厚度的变化,使用Image-J软件对界面IMC的厚度进行测量。焊点的剪切强度测试在UTM5000型电子万能拉伸机上进行。选用5 mm×50 mm×0.4 mm的Cu片进行搭接如图2所示,以0.1 mm·min-1 拉伸速率拉伸,采用SEM观察断口的微观组织形貌。
图1 Cu-Sn-Cu TLP键合示意图
Fig.1 TLP bonding diagram of Cu-Sn-Cu
图2 焊点剪切试验示意图
Fig.2 Schematic diagram of solder joint shear test
2结果与讨论
2.1界面IMC生长形貌
图3为不同键合时间下Cu/Sn/Cu焊点界面的SEM图像。可以看出,键合时间为30 min时,Sn与Cu之间生成了一层类似扇贝状的金属间化合物,经EDS检测,如图4所示,该金属间化合物分别为Cu6 Sn5 和Cu3 Sn。随着键合时间增加,两端的Cu6 Sn5 化合物逐渐相连,液态的Sn被隔离成孤岛状。继续增加键合时间,液态的Sn进一步消耗,孤岛状逐渐变小。当键合时间达到120 min时,液态Sn全部消耗,形成了全金属间化合物。此外,从图3中可以明显发现,焊点两端界面IMC的生长呈现不对称性,且远离加热板的冷端界面IMC的生长速率显著大于热端。分析原因,主要由于采用表面接触式加热,导致焊点受热不均产生的温度梯度。温度梯度驱使Cu原子从热端向冷端迁移,热端Cu原子浓度降低,Cu6 Sn5 生长受到抑制,反之冷端界面金属间化合物的生长速率得到提高
[15 ]
。此外,由于冷端的Cu6 Sn5 较厚,穿过Cu6 Sn5 层的Cu原子的继续溶解受到限制,滞留在Cu/Cu6 Sn5 的Cu原子与Cu6 Sn5 继续反应得到Cu3 Sn,因此,冷端的Cu3Sn较厚。而热端Cu原子迁出过快,Cu3 Sn生长较为缓慢,厚度较薄
[16 ]
。
图5为焊点界面IMC厚度与键合时间的关系图,可以发现,随着键合时间的增加,焊点界面IMC的厚度逐渐增加。当键合时间为30 min时,焊点界面Cu6 Sn5 的厚度分别为11.08μm(冷端)和2.98μm(热端),Cu3 Sn的厚度为1.73μm(冷端)和0.95μm(热端)。当键合时间为45 min时,焊点界面Cu6 Sn5 的厚度分别为14.1μm(冷端)和4.3μm(热端),Cu3 Sn的厚度为1.82μm(冷端)和1.12μm(热端)。随着键合时间增加到120 min,已经生成全金属间化合物,此时焊点界面Cu3 Sn的厚度分别为3.1μm(冷端)和2.5μm(热端)。
图3 250℃不同键合时间下Cu/Sn/Cu焊点的界面组织
Fig.3 SEM images of Cu/Sn/Cu solder joints at 250℃for different bonding time
(a)30 min;(b)45 min;(c)60 min;(d)90 min;(e)120 min
图4 Cu/Sn/Cu焊点EDS分析
Fig.4 EDS analysis of each phase of Cu/Sn/Cu solder joints
(a)Point 1 and(b)Point 2 in Fig.3
2.2 IMC晶粒取向
图6为键合10 h焊点界面组织的SEM图像,通过EDS面扫描分析可以看出,焊点中Cu和Sn分布比较均匀,但在焊点中出现许多孔洞,分析原因,Cu6 Sn5 中的孔洞可能在钎焊过程中部分助焊剂未能及时挥发所引起的,而Cu3 Sn内部则是Cu和Sn的不平衡扩散所导致的柯肯达尔孔洞。图7为焊点IMC的电子背散射衍射(EBSD)分析结果。从图7(a)中的衍射带对比度可以看出,Cu6 Sn5 为大块的晶粒组织,而Cu3 Sn的晶粒相对较小,晶粒宽度为1.5~3.0μm的柱状晶。横截面方向(TD)反极图显示,Cu3 Sn无织构特征,而Cu6 Sn5 相的晶粒取向多平行于[001]与[111]之间。
图5 不同键合时间下焊点的界面层厚度
Fig.5 IMC thickness of solder joints during different bonding times
图6 250℃键合10 h焊点界面组织
Fig.6 SEM images of solder joint at 250℃for 10 h
图7 焊点的EBSD分析结果
Fig.7 EBSD analysis of solder joint
(a)Diffraction zone contrast;(b)IPF of solder joint in TD di-rection
2.3焊点的剪切强度
图8为250℃下不同键合时间焊点的剪切强度。结果表明,随着键合时间的增加,焊点的剪切强度逐渐增加。当键合时间为30 min,焊点的剪切强度为8.5 MPa,当键合时间为120 min时,焊点的剪切强度已增加至18.6 MPa。
图8 不同键合时间下Cu/Sn/Cu焊点的剪切强度
Fig.8 Shear strength of Cu/Sn/Cu solder joints during differ-ent bonding times
图9为不同键合时间焊点的断口组织形貌,可以发现键合30 min,断裂出现在基体与界面IMC层的交界处,属于混合断裂
[17 ]
。随着键合时间的增加,断口逐渐向脆性断裂转变,通过对比焊点界面IMC层的生长形貌,发现由于键合30 min时,焊点主要由IMC和Sn构成,随着键合时间的增加,焊点的组织转变为Cu3 Sn/Cu6 Sn5 ,而Cu3 Sn/Cu6 Sn5 的增加可以提高焊点的剪切强度
[18 ]
。
3结论
1.随着键合时间的增加,界面IMC的厚度逐渐增加,当键合时间为30 min时,焊点界面生成了一层金属间化合物,当键合时间增加到120min时,焊点中的液相Sn已消耗殆尽,形成了全IMC。
2.在Cu/Sn/Cu焊点中,发现焊点两端界面IMC的生成呈现非对称生长,且冷端IMC的增长速率明显高于热端。
3.EBSD分析结果表明Cu6 Sn5 为大块的晶粒组织,而Cu3 Sn的晶粒相对较小。且Cu6 Sn5 的晶粒取向多平行于[001]与[111]之间。
4.随着键合时间的增加,焊点的剪切强度逐渐增大。对断口形貌进行分析,焊点的断裂模式由混合断裂逐渐向脆性断裂转变。
图9 250℃不同键合时间下Cu/Sn/Cu焊点的断口形貌
Fig.9 Shear fracture morphologies of Cu/Sn/Cu solder joints at 250℃for different bonding time
(a)30 min;(b)45 min;(c)60 min;(d)90 min;(e)120 min
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