Foreword Nickel plating is one of the most important plating types in the electroplating industry and has been widely used in machinery, electronics, aerospace, aerospace, defense and other fields. In particular, in recent years, the application of nickel plating in high-tech fields such as special processing and micro/nano manufacturing has made it more widely used. In the electroplating process, due to a certain deformation of the crystal of the metal or infiltration of a different phase, a certain internal stress is generated, which is particularly evident in the nickel plating process. The internal stress of the plating layer can be divided into two categories: one is a stress that causes the coating itself to expand in volume, which is called compressive stress; the other is a stress that causes the coating volume to shrink, which is called tensile stress. During the electroplating process, excessive tensile stress causes the coating to crack; and excessive compressive stress causes the coating to foam, resulting in failure of the coating. In micro/nano fabrication, unbalanced tensile or compressive stress can easily cause deformation of the machined part, thereby failing the manufacturing process. Therefore, an in-depth understanding of the causes of stress in the coating, the characterization of internal stress and its regulation methods have important practical significance for electroplating production. In this paper, the causes of internal stress in the nickel-plated layer are reviewed. The measurement method of internal stress in the nickel-plated layer is introduced, and various factors affecting the internal stress of the nickel-plated layer are analyzed. 1 Reasons for the formation of internal stress in the nickel plating layer The stress in the plating layer is generated during the formation of metal by metal ions. In order to avoid or control the adverse effects of internal stress on the coating, the cause of the formation of internal stress has been studied very early. However, due to the complexity of this issue, there is still no perfect unified theoretical explanation. To sum up, there are several theoretical explanations for the causes of stress in the nickel plating layer: (1) Hydrogen Permeation Theory If hydrogen is precipitated during metal electrodeposition, it will exist in the form of hydrogen atoms or hydrides. Hydrogen has good diffusivity in nickel and can quickly escape to form hydrogen molecules. The escape of hydrogen causes the coating to shrink to form tensile stress. This theory can better explain that the nickel plating layer tends to have a large tensile stress when the cathode is subjected to a large amount of hydrogen evolution. (2) Crystal agglomeration theory The crystal nuclei generated during electrodeposition are agglomerated with each other in order to lower the surface energy during growth, thereby generating tensile stress. Before the nucleation of the crystal nucleus, it is compressed by the force applied by the substrate, and the surface tension during the growth of the nucleus also has a contraction effect, so that the plating layer exhibits compressive stress. This theory can explain the effect of the surface orientation of the matrix material on the internal stress of the nickel plating layer. (3) inclusion theory It is very difficult to obtain a pure plating layer free from impurities during electrodeposition, and impurities are deposited or intercalated with the plating metal on the grain boundaries, which causes the metal lattice around the impurities to be twisted, thereby generating internal stress. This theory can better explain the effect of additives in the bath on internal stress. (4) Theory of excess energy Metallic nickel has a high activation overpotential, which releases a large amount of energy in the form of heat when nickel is deposited on the surface of the substrate. The heat of the coating atoms causes the crystal to expand, and when it returns to the normal "cold atom" it creates internal stress. The evidence for this theory is that the greater the overpotential, the greater the tensile stress during electrodeposition. But this theory cannot explain compressive stress. (5) Dislocation theory Dislocations are line defects created by the relative sliding of atomic planes in a crystal lattice. Since the edges of the dislocations are oriented along the vacancies, tensile stress is generated. If the organic additive can be adsorbed in the vacancy, the formation of dislocations will be affected, and the internal stress will be lowered. In fact, the theory of dislocations is the basis of the various theories mentioned above. Several theories that generate internal stresses in the coating are actually interrelated and complementary to each other. For example, dislocations are high-energy regions in the sedimentary layer, so the theory of excess energy can be seen as a deduction of dislocation theory and coalescence theory. Sometimes, a theory can be used to explain the cause of internal stress; but in most cases, only one theoretical explanation is not perfect. 2 Method for measuring internal stress of nickel plating layer The measurement of the internal stress of the coating mainly uses a mechanical method. Commonly used mechanical methods include sheet bending cathode method, spiral contraction method, wafer deformation method and length variation method. These methods are based on discussing the relationship between stress and strain within the elastic limits of the material. 2.1 Sheet cathode bending method The sheet cathode bending method is a classical method of measuring internal stress. The basic form is: using a long and thin metal foil as the cathode, and one side facing away from the anode is insulated; when the plating is performed, one end is fixed by the clamp and the other end is free to move; after plating, the internal stress generated in the plating layer forces the cathode of the sheet to bend. The internal stress calculation formula is: Where: σ is the internal stress of the coating, Pa; E is the elastic modulus of the thin film cathode, Pa; t is the thickness of the thin film cathode, mm; R is the bending radius of the thin film cathode, mm; d is the thickness of the plating layer, mm. The main disadvantages of the sheet cathode bending method are: (1) the insulating layer on the back side of the sample tends to contaminate the plating solution and affect the rigidity of the sample. This effect is difficult to consider and correct in the calculation of internal stress; (2) Insulation is not The shedding of the cathode completely or during the plating process also changes the degree of bending of the cathode; (3) after the narrow strip sample is bent, the distance from the anode is changed, which changes the current distribution on the cathode. 2.2 Spiral contraction method The spiral contraction method is to make a stainless steel sheet into a spiral tube, and fix one end thereof, and then plate only on the outer surface of the spiral tube, and the inner surface is coated with an insulating varnish. When the coating generates internal stress, the spiral tube is twisted. The twisting force is transmitted to the dial by means of the core rod amplifying device, and the internal stress of the plating layer is calculated from the angle at which the dial pointer is deflected. Its calculation formula is: Where: σ is the internal stress of the coating, MPa; K is the deflection constant, (N·mm)/°; θ is the deflection angle of the pointer, °; p is the pitch of the spiral tube, mm; t is the wall thickness of the spiral tube, mm ;d is the thickness of the coating, mm. When using this measurement method, the K value of the spiral tube must be determined according to the instrument's instruction manual before each measurement. 2.3 wafer deformation method The basic device for the wafer deformation method is to use a circular metal piece as a cathode and press it on a container containing a plating solution. The circular metal piece is made of copper or stainless steel having a thickness of 0.25 to 0.60 mm and a diameter of 100 mm. Connect a capillary containing the measuring solution on top of the wafer or on the side of the container. When the wafer is plated on one side of the plating solution, the internal stress generated by the plating causes the wafer to bend, causing the volume of the container to change, thereby causing the liquid level in the capillary to rise or fall, whereby the capillary reading can calculate the inside of the coating. stress. Its calculation formula is: Where: σ is the internal stress of the coating, MPa; r is the radius of the plated surface of the wafer, mm; ha and hb are the capillary readings before and after electroplating, mm; K is the constant of the wafer, mm3·N-1; t is a circle The thickness of the cathode of the sheet, mm; d is the thickness of the coating, mm. The measurement accuracy of the wafer deformation method is comparable to that of the spiral contraction method. The wafer cathode does not require backside insulation and can be measured with stirring of the electrolyte. 2.4 length variation method The length variation method uses the thin film cathode to simultaneously electrodeposit on both sides, and although the sheet cathode is not bent, the length changes. When the plating layer generates tensile stress, the test piece is shortened; when the plating layer generates compressive stress, the test piece is elongated. The foil cathode is clamped to a special fixture, fixed at one end, free at one end, and equipped with a length sensor at the free end to measure the change in length. Its calculation formula is: Where: σ is the internal stress of the coating, Pa; E is the elastic modulus of the foil cathode, Pa; d is the thickness of the coating, mm; t is the thickness of the cathode of the foil, mm; ΔL and L are the variation and length of the cathode length of the foil, respectively , mm. In addition to the above four mechanical measurement methods, the internal stress of the plating layer can also be measured by an X-ray diffraction method, a resistance strain method, and an electromagnetic measurement method. The X-ray diffraction method determines the internal stress of the plating layer by the difference in the interplanar spacing or the distortion of the lattice in the presence or absence of an internal stress. The results of the X-ray diffraction test are relatively reliable, but are not suitable for the internal stress test of the film layer below 10 μm. The resistance strain method is a method of measuring the stress in the plating layer by using the principle that the resistance of the electric resistance wire expands and contracts to change the resistance value. The electromagnetic measurement method is also a curved cathode method, except that when the sheet cathode is bent, the electromagnet mounted on the upper portion of the cathode can continuously apply a force that prevents it from being bent. The magnitude of this force can be determined by means of the current flowing through the electromagnet and the internal stress of the coating is calculated accordingly. Rubber Grommet,Rubber Hose Grommet,Rubber Cable Grommet Neway Rubber Products Co., Ltd. , http://www.lxrubbers.com
Nickel plating inner stress and its measuring method