Studies of electroless nickel under bump metallurgy-solder interfacial reactions and their effects on flip chip solder joint reliability
Studies of electroless nickel under bump metallurgy-solder interfacial reactions and their effects on flip chip solder joint reliability
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DOI：
10.1007/s11664-002-0109-4
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发表时间：
2002-05-01
影响因子：
2.1
通讯作者：
Cho, CL
中科院分区：
文献类型：
--
作者：
Jeon, YD;Paik, KW;Cho, CL
在含Sn焊料凸点的Al焊盘上制备了Ni-P凸点下冶金（UBM）层。通过控制络合剂和镀液的pH值，优化了化学镀镍层中磷的含量。研究了化学镀Ni UBM/钎料界面的界面反应。焊料回流过程中在界面处形成的金属间化合物（IMC）主要是Ni 3Sn 4，Ni-Sn IMC与化学镀镍层之间的Ni-Sn反应也形成了富P的Ni层。根据焊料材料和回流温度，在小于10分钟的焊料回流中形成1至4微米的Ni 3Sn 4 IMC和1800-5000埃的富P Ni层。发现富磷镍层中含有镍、磷和少量的锡（约7at.%）。进一步的截面透射电子显微镜（TEM）分析证实，富P Ni层的组成为75原子%。镍，20原子% P和5at.%通过X射线能谱仪（EDS）分析了镀层中的Sn，通过观察晶粒尺寸分析了富P Ni层中发生的相变。Kirkendall空隙也被发现在Ni 3Sn 4 IMC中，就在富P的Ni层上，经过广泛的焊料回流。Kirkendall空隙被认为是脆性断裂的主要原因;建议通过优化适当的工艺条件来限制富P Ni层的生长。Ni-Sn IMC和富P Ni层的生长动力学遵循三个步骤：在焊料回流的第一个1分钟期间的快速初始生长，随后是减少的生长步骤，最后是扩散控制的生长。在扩散控制生长过程中，膜厚与时间呈线性关系（1/2）。进行倒装芯片凸块剪切测试以测量IMC和富P Ni层对凸块粘附特性的影响。大多数故障发生在焊料和Ni 3Sn 4 IMC。Ni-Sn IMC的脆性特性和Kirkendall空洞在化学镀镍UBM-Sn焊料系统中导致脆性凸块失效，这导致凸块粘附强度降低。
The electroless-deposited Ni-P under bump metallurgy (UBM) layer was fabricated on Al pads for Sn containing solder bumps. The amount of P in the electroless Ni film was optimized by controlling complexing agents and the pH of plating solution. The interfacial reaction at the electroless Ni UBM/solder interface was investigated in this study. The intermetallic compound (IMC) formed at the interface during solder reflowing was mainly Ni3Sn4, and a P-rich Ni layer was also formed as a by-product of Ni-Sn reaction between the Ni-Sn IMC and the electroless Ni layer. One to four microns of Ni3Sn4 IMC and a 1800-5000 Angstrom of P-rich Ni layer were formed in less than 10 min of solder reflowing depending on solder materials and reflow temperatures. It was found that the P-rich Ni layer contains Ni, P, and a small amount of Sn (similar to7 at.%). Further cross-sectional transmission electron microscopy (TEM) analysis confirmed that the composition of the P-rich Ni layer was 75 at.% Ni, 20at.%P, and 5at.%Sn by energy-dispersive x-ray spectroscopy (EDS) and the phase transformation occurred in the P-rich Ni layer by observing grain size. Kirkendall voids were also found in the Ni3Sn4 IMC, just above the P-rich Ni layer after extensive solder reflow. The Kirkendall voids are considered a primary cause of the brittle fracture; restriction of the growth of of the P-rich Ni layer by optimizing proper processing conditions is recommended. The growth kinetics of Ni-Sn IMC and P-rich Ni layer follows three steps: a rapid initial growth during the first 1 min of solder reflow, followed by a reduced growth step, and finally a diffusion-controlled growth. During the diffusion-controlled growth, there was a linear dependence between the layer thickness and time(1/2). Flip chip bump shear testing was performed to measure the effects of the IMC and the P-rich Ni layers on bump adhesion property. Most failures occurred in the solder and at the Ni3Sn4 IMC. The brittle characteristics of the Ni-Sn IMC and the Kirkendall voids at the electroless Ni UBM-Sn containing solder system cause brittle bump failure, which results in a decreased bump adhesion strength.