Determination method of internal stress of coating

弯曲阴极法

这种方法使用较早，操作也比较简单。其基本形式是：采用一块长而窄的金属薄片作阴极，背向阳极的一面绝缘。电镀时一端用夹具固定，另一端可以自由活动。电镀后，镀层中产生的内应力迫使阴极薄片朝向阳极（张应力）或背向阳极（压应力）弯曲。用读数显微镜或光学投影法可测量阴极的形变。

镀层的内应力可通过电镀后阴极的形变（弯曲阴极的弯曲半径R 或弯曲度X，或阴极下端偏移最Z'）按下列公式分别计算

式中：

S———镀层内应力（Pa）；

E———基体材料弹性模量（Pa）；

t ———阴极基体厚度（mm）；

d———镀层厚度（mm）；

R———阴极弯曲的曲率半径（mm）；

Z'———阴极下端偏移量（mm）；

Z———阴极的弯曲度（mm）；

L ———阴极的长度（mm）。

刚性平带法

刚性平带法是用塑料框架夹住一块金属薄片，在它的一面电镀。或者把两块金属片叠合在一起，夹在塑料框架内，两面电镀。电镀后将薄片从框架中取出。由于夹持力消失，薄片就自由地弯曲到平衡状态。通过仪器测量薄片的曲率半径，即可计算出镀层内应力：

式中：

S———镀层内应力（Pa）；

E———基体材料弹性模量（Pa）；

t———基体厚度（mm）；

d———镀层厚度（mm）；

R———阴极弯曲的曲率半径（mm）。

螺旋收缩仪法

螺旋收缩仪法是把不锈钢片制作成螺旋管，并使它的一端固定，然后进行单面电镀（内表面涂以绝缘漆）。由于镀层的应力使螺旋管产生扭曲，借助于自由端齿轮变速装置使指针偏转。According to指针偏转的角度计算镀层的应力

式中：

S———镀层平均内应力（Pa）；

D———指针偏转角度（°）；

p———螺旋管螺距（mm）；

t———螺旋管材料厚度（mm）；

d———镀层厚度（mm）；

K———螺旋管偏转常数〔（N·m/℃）〕，它是指针转动一度所需的扭曲力矩。

使用本方法时要用市售螺旋应力仪，并且在每次测量之前，按仪器使用说明，测定螺旋管的K值。

应力仪法

This method uses a circular metal plate as the cathode, which is pressed against a container containing the plating solution. The circular metal sheet is made of copper or stainless steel with a thickness of 0.25~0.6mm and a diameter of 100mm. A capillary with the measuring solution is attached to the top of the disc or to the side of the container. When the side of the wafer contacting the plating solution is electroplated, the stress generated by the coating makes the wafer bend (tensile stress makes the wafer sink, and compressive stress makes the wafer bulge), resulting in a change in the volume of the container. This causes the liquid level in the capillary to rise or fall, thereby measuring the nature and magnitude of the stress. The measurement accuracy of the strain gauge is the same as that of the spiral shrinkage gauge. Disc cathodes do not need to be insulated on one side, and can be measured while the electrolyte is stirred.

The internal stress can be calculated as follows:

In the formula:

S - internal stress (Pa);

r——the radius of the plated surface of the wafer cathode (mm);

Ha, Hb——respectively, capillary readings after electroplating and before electroplating (mm);

t——thickness of cathode material (mm);

d——coating thickness (mm);

k——wafer constant (m³/N).

In order to obtain the disk constant, the loaded disk can be immersed in a solution with known density, and at two different depths (L1, L2), the corresponding liquid level heights (Ha, Hb) in the capillary can be measured. Calculate the disc constant K:

In the formula)——wafer constant (m³/N);

L1, L2——the depth of the disc immersed in the solution (mm);

Ha, Hb——corresponding to (%, (&) capillary liquid level (mm);

ρ——solution density (g/mm3).

 electromagnetic assay

Electromagnetic assay also belongs to the bent cathode method. The difference is that when the sheet cathode is bent, the electromagnet installed on the upper part of the cathode can continuously exert a force to prevent it from bending. The magnitude of this force can be measured by means of the current flowing through the electromagnet, and the internal stress of the coating can be calculated accordingly. . For details, see the instrument manual.

Resistance strain gauge measurement method

This method measures the internal stress of the coating based on the principle that the resistance value changes due to the expansion and contraction of the resistance wire.

Paste the strain gauge made of resistive material to the measured part on the back of the electroplated surface of the sample. During electroplating, the internal stress generated by the coating causes a slight change in the resistance value of the strain gauge, and the change value can be measured by a resistance strain gauge.

length change method

In this method, the cathode sheet is electroplated on both sides. After electroplating, the test piece does not bend, but the length changes. When the coating produces tensile stress, the test piece shortens; when the coating produces compressive stress, the test piece elongates. The internal stress of the coating can be determined according to the change of the length of the test piece.