Analyze the control of the PCB bath plating solution
The main purpose of the control of the PCB plating bath solution is to keep all chemical components within the range specified by the process. Because the chemical and physical properties of the coating can only be ensured within the parameters specified in the process. There are many types of process methods used for control, including chemical analysis, physical tests and determination of the acid value of the solution, specific gravity or colorimetric determination of the solution. These processes are all to ensure the accuracy, consistency and stability of the parameters of the bath. The choice of control method depends on the type of layer.
For example, the parameters of the high-dispersion, bright, high-acid, low-copper plating bath are determined by adjusting or adjusting the break-down data provided by the chemical method. In addition to chemical analysis, the chemical copper precipitation solution must also have a pH acid ratio or ratio. Color measurement, etc. If the chemical composition is within the process range after analysis, it is necessary to pay great attention to the changes of other parameters and the surface state of the substrate to be plated, such as the temperature of the plating solution, the current density, the mounting method, and the effect of the substrate surface treatment state on the bath. In particular, it is necessary to control the inorganic impurity-zinc of the bright acid copper plating solution. If it exceeds the allowable process specification value, it will directly affect the surface state of the copper layer. The content of copper impurities in the plating tin-lead alloy bath must be strictly controlled. A certain amount will affect the wettability, solderability and protection of tin-lead alloy coatings.
Although the analytical method is reliable for bath control, it does not guarantee a good coating. Therefore, it is also necessary to rely on the plating test. In particular, in order to ensure that the plating layer has good electrical and mechanical properties, many electroplating baths have added organic additives to improve the structure and performance of the plating layer. These additives are difficult to be effective by chemical analysis methods, and are analyzed and compared using the plating method. It is an important supplementary means for controlling the chemical composition of the plating solution. The supplementary control includes measuring the additive content and adjusting, filtering, and purifying. These need to be carefully "observed" on the test plate of the Huo's plating bath, and then the distribution state of the sample plate is studied, analyzed, and inferred to improve or improve the process. Step purpose.
I. Electroplating test
The control principle of the plating bath should include the main chemical composition of the bath. In order to achieve correct judgment, advanced and reliable test equipment and analysis methods are needed. Some tank fluids also need auxiliary methods such as measuring their specific gravity and acid value (PH). In order to be able to directly observe the surface state of the coating, most manufacturers now use the process method of Huo's tank test. The specific test procedure is to incline the test plate by 37 ° with the same length as the long side, and the anode is placed vertically along the long side. The change in anode-to-cathode distance will be regularly spaced along the cathode, and the result will continue to change along the test plate. From the current distribution state of the test plate, it can be scientifically judged whether the current density used in the plating bath is within the range specified by the process. It is also possible to observe the direct influence of the additive content on the current density and the effect on the surface coating quality.
Second, the bending negative plate test method:
This method is adopted because it can cover a wide range, it exposes an angle, and because of its vertical shape, its upper and lower surfaces can adapt to the dielectric effect. This can test the advantages and disadvantages of the current range and dispersion ability.
3. Judgment and Inference:
Through the test method used above, the actual record of the test plate can be used to first determine the phenomenon that occurs in the low current area of the test plate during electroplating, and it can be judged that additional additives need to be added; and in the high current area, the plating layer There will be defects such as rough surface, blackening and irregular appearance, which indicates that the inorganic metal impurities contained in the bath directly affect the surface state of the coating. If the surface of the coating is pitted, it means that the surface tension should be reduced. The damaged plating layer often shows excessive additive and decomposition in the bath. Such phenomena fully explain the need to analyze and adjust in time so that the chemical composition of the bath solution conforms to the process parameters specified by the process. Excessive additives and decomposed organic matter must be treated, filtered and purified using activated carbon.
In short, although the computer is used for automatic control, tests must be carried out with the aid of auxiliary means to achieve double insurance. Therefore, the control methods commonly used in the past need to be used or further research and development of new test methods and instruments to make the PCB plating and coating process more complete.