Correlation Between Whisker Initiation and Compressive Stress in Electrodeposited Tin–Copper Coating on Copper Leadframes
Correlation Between Whisker Initiation and Compressive Stress in Electrodeposited Tin–Copper Coating on Copper Leadframes
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铜引线框架上电镀锡铜涂层中晶须引发与压缩应力之间的相关性
DOI：
10.1109/tepm.2010.2045384
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发表时间：
2010
影响因子：
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通讯作者：
A. Nishimura
中科院分区：
文献类型：
--
作者：
Takahiko Kato;H. Akahoshi;Masato Nakamura;T. Terasaki;T. Iwasaki;T. Hashimoto;A. Nishimura
为了评价涂层应力对IC封装引线晶须萌生的贡献，采用有限元分析方法研究了涂层中的应力分布。采用两种不同的引线框架材料，即铜-铁(以下简称CUFE，对应CDA编号C19400)和铜-铬(CuCR，对应CDA编号C18045)，采用相同的锡-铜涂层组成两种不同的引线框架样品，研究了涂层表面晶须的萌生行为。两种样品的晶须萌生趋势与涂层明显不同。也就是说，在室温下长期储存后，铜铬样品上的涂层没有观察到晶须的萌生，而铜铁样品上的涂层形成了长的晶须(最大长度超过200μm)。对引线框架样品的有限元计算表明，涂层具有双向应力梯度，即一个向表面的梯度，另一个向引线框架基材的梯度。这也表明了两个样品中的应力分布有所不同。涂层晶界(GBs)上的法向应力向CuFE样品表面的梯度大于CuCR样品。这一结果表明，由于锡原子沿GB的通量与应力梯度成正比，所以CuFE样品涂层中沿GB的锡原子通量比CuCR样品上的大。这与上述晶须在样品中的引发行为相吻合。因此，我们得出结论，在CUFE样品中，晶须要么从紧邻GB顶部的表面颗粒开始，要么从位于同一GB两侧的表面颗粒开始。为了证实这一结论，研究了TiN扩散位置和晶须形成位置之间的相关性。原子扩散的分子动力学模拟表明，在垂直于GB的方向施加压应力时，TiN的主要扩散位是GB。研究了晶须根与涂层微观结构的关系，结果表明，晶须根位于涂层中GB交叉点的顶部。这些结果表明，晶须的萌生位置与主要的锡扩散位置相关，每个晶须要么从紧邻GB顶部的表面颗粒开始，要么从位于同一GB两侧的表面颗粒开始。
To evaluate the contribution of coating stress to whisker initiation from IC package leads, the stress distribution in the coating was investigated by finite-element analysis (FEA). Two different leadframe samples, which were composed of the same tin-copper coating on two different copper-leadframe materials, namely, copper-iron (hereafter, CUFE; corresponding to CDA number C19400) and copper-chromium (CUCR; CDA number C18045), were used to examine the whisker-initiation behavior on the coating surfaces. The two samples showed significantly different tendencies of whisker initiation from the coating. That is, after long-term storage at room temperature, no whisker initiation was observed on the coating on the CUCR sample, whereas long whiskers (with a maximum length of more than 200 μm) were formed from the coating on the CUFE sample. The FEA calculation on the leadframe samples revealed that the coatings had a two-directional stress gradient, namely, one gradient toward the surface and another toward the base leadframe material. It also indicated a difference between the stress distributions in the two samples. The gradient of normal stress on the coating's grain boundaries (GBs), toward the surface of the CUFE sample, was found to be larger than that in the CUCR sample. This result implies that the tin-atom flux along a GB in the coating on the CUFE sample was larger than that on the CUCR sample because the atom flux along the GB was proportional to the stress gradient. It agrees with the above-mentioned whisker-initiation behaviors in the samples. We thus conclude that in the CUFE sample, a whisker initiates either from a surface grain immediately on top of a GB or from surface grains located on both sides of the same GB. To confirm this conclusion, the correlation between the tin-diffusion sites and whisker formation sites was investigated. Simulation of atom diffusion by molecular dynamics indicated that the dominant tin-diffusion site is a GB when compressive stress is applied in the direction normal to the GB. Investigation of the correlation between the whisker roots and coating microstructures of the CUFE sample showed that the whisker roots were located on top of GB intersections in the coating. These results indicate that whisker-initiation sites are correlated with dominant tin-diffusion sites and that each whisker initiates either from a surface grain located immediately on top of a GB or from surface grains located on both sides of the same GB.