nanoDMA III Dynamic Testing Technique for Mechanical Characterization of Pb-Free Solder Bumps

随着微电子制造商从设备包中减少或消除PB的努力，对不同无PB焊料合金的机械性能的表征已成为主要行业挑战。在处理和应用过程中，焊料连接在不同时间尺度上经历的压力和温度变化将显着影响材料的微观结构。

考虑到微电路上的焊料撞击的体积很小，可以使用纳米构造技术直接测量焊料的机械响应，该技术可以对蠕变行为，模量和硬度进行定量测量，作为对温度。

纳米尖锐蠕变测试

常规（准静态）压痕测试涉及施加载荷以将凹痕迫使凹痕进入样品表面。施加的负载保持恒定一段时间，然后撤回。材料的硬度和模量可以用探针形状，凹痕渗透深度，施加力和卸载刚度的数据来估计。

力和位移连续测量以创建力与位移曲线。曲线在初始卸载点处的斜率产生接触刚度。因此，单个准静态测试在测试的最大穿透深度下产生了硬度和模量的单一测量。

动态压痕测试涉及将相对较小的正弦振荡叠加到准静态负载曲线上。在整个测试过程中，使用负载振幅，相位滞后和位移幅度的值连续计算接触刚度。由于始终确定刚度，因此也可以连续测量硬度和模量。

Most materials show some degree of creep while the quasi-static indenter load is kept stable. However, it is challenging task to characterize the creep behavior. Displacement error from thermal drift, while generally small at short durations, becomes prohibitively large after several minutes or hours. The Hysitron nanoDMA® III reference creep testing technique can tackle this problem by applying a dynamic force all through the test, enabling continuous measurement of contact stiffness. Schematic representation of a nanoDMA III creep test is illustrated in Figure 1.

图1。Nanodma III蠕变测试的示意图。

At first, the estimation of the modulus of the material (the reference modulus) is performed when error from thermal drift is insignificant. With the knowledge of the modulus, contact stiffness is continuously measured by keeping the quasi-static load as constant. With these data, the contact area and, therefore, contact depth and hardness can be computed without any reliance on the quasi-static displacement measurement, making background thermal drift immaterial. In this manner, creep tests can be reliably performed as long as several hours.

Mechanical Characterization of Pb-Free Solder Bumps

在铜支柱上制备了3％Ag和97％SN组成的焊料颠簸，并在铜支柱上构成了Si晶圆。该结构的光学显微照片如图2所示。焊接约为20µm的焊盖在顶部。AHysitron TI 950 TriboIndenter^®coupled with nanoDMA III and xSol High Temperature Stage was applied t measure creep effects at 25, 50, 100, 150, and 175°C, utilizing a diamond Berkovich probe.

Figure 2.硅，铜，焊料结构的光学显微照片。(Sample provided by Jürgen Grafe, FHG-IZM-ASSID, Dresden, Germany, and Kong-Boon Yeap, FHG-IFZP, Dresden, Germany.)

Before taking each measurement, the topography of the sample surface imaged using the instrument's in-situ SPM imaging capability to position each test accurately on the top of the dome, where the surface was almost flat and at right angles to the probe (Figure 3). For each test, the force was ramped rapidly to peak force and then held constant for 1000s and the stiffness was continuously measured by a 220Hz oscillation in the mean time. As the load was held, there was an increase in the contact stiffness with the increasing penetration depth of the probe.

Figure 3.在测试前用圆圈指示的目标测试位置进行焊点的地形SPM图像。

The quasi-static force for each test was selected such that the initial depth of the indent would be roughly 500nm, and the dynamic force was chosen to yield a displacement amplitude of roughly 1nm. The target depth was selected as a balance of two considerations.

对于非常小的缩进，测量精度可能会受到表面粗糙度的影响，但是如果缩进太大，则焊料凸起周围周围的自由边缘可能会产生影响。

图4。在每个温度下的深度与时间。

图4和5说明了随着时间的推移，接触深度和硬度的连续变化，而图6将模量描述为温度的函数。在25°C和150°C的温度范围之间的蠕变曲线中未观察到差异，但是在175°C时很容易明显差异。

图5。硬度与在每个温度下的时间。

图6。在每个温度下测量的模量。

结论

The creep rates were much higher at 175°C, and the hardness deteriorated by 70% over the course of the 1000 second hold. The change in creep behavior indicates a transition in the deformation mechanism between 150°C and 175°C, confirming that the mechanical properties of the solder bumps decay rapidly above 150°C.

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