2024-01-19
The physical parameters that need to be studied include anode type, anode-cathode spacing, current density, agitation, temperature, rectifier, and waveform.
Speaking of anode type, it is nothing but a soluble anode and an insoluble anode. Soluble anodes are usually made of phosphorous-containing copper spheres, which easily produce anode mud, pollute the plating solution, and affect its performance. Insoluble anodes, also known as inert anodes, are generally made of titanium mesh coated with a mixture of tantalum and zirconium oxides. Insoluble anodes have good stability, do not require anode maintenance, do not produce anode mud, and are suitable for both pulse and DC plating. However, the consumption of additives is relatively high.
The spacing between the cathode and the anode in the electroplating filling process of PCB manufacturing service is very important and differs in design for different types of equipment. However, it should be noted that no matter how it is designed, it should not violate Faraday's law.
There are many types of agitation, including mechanical oscillation, electric vibration, air vibration, air agitation, and jet flow (Educator).
For electroplating filling, jet flow design is generally preferred based on the configuration of traditional copper tanks. However, factors such as whether to use bottom spray or side spray, how to arrange spray pipes and air agitation pipes in the tank, the hourly flow rate of spray, the spacing between the spray pipe and the cathode, and whether the spray is in front of or behind the anode (for side spray) all need to be considered in designing the copper tank. In addition, the ideal way is to connect each spray tube to a flow meter in order to monitor the flow rate. Due to the large amount of jet flow, the solution is prone to heating up, so temperature control is also very important.
Low current density and low temperature can reduce the deposition rate of surface copper while providing enough Cu2+ and a brightener to the hole. Under these conditions, the filling capacity can be enhanced, but the plating efficiency is also reduced.
The rectifier is an important part of the electroplating process. Currently, research on electroplating filling is mostly limited to full-panel electroplating. If graphic electroplating filling is considered, the cathode area will become very small. At this time, the output accuracy of the rectifier is highly required.
The choice of rectifier output accuracy should be determined according to the product's lines and hole sizes. The thinner the lines and the smaller the holes, the higher the accuracy required for the rectifier. Generally, a rectifier with an output accuracy of within 5% is suitable. Choosing a rectifier with too high accuracy will increase equipment investment. The selection of output cable wiring for the rectifier should first be placed as close as possible to the plating tank to reduce the length of the output cable and the rise time of the pulse current. The selection of the cable cross-sectional area should be based on a current-carrying capacity of 2.5A/mm². If the cable cross-sectional area is too small, the cable length is too long, or the voltage drop of the circuit is too high, the current transmission may not reach the required production current value.
For tanks with a width greater than 1.6m, a double-sided power supply should be considered, and the lengths of the double-sided cables should be equal. This can ensure that the current error on both sides is controlled within a certain range. Each flyback pin of the plating tank should be connected to a rectifier on both sides, so that the current on both sides of the part can be adjusted separately.
Currently, there are two types of electroplating filling from the waveform point of view, pulse electroplating and direct current (DC) electroplating. Both of these electroplating filling methods have been studied by researchers. DC electroplating filling uses traditional rectifiers, which are easy to operate, but are helpless for thicker boards. Pulse electroplating filling uses PPR rectifiers, which are more complicated to operate but have stronger processing capabilities for thicker boards.
The impact of the substrate on electroplating filling cannot be ignored. Generally, there are factors such as dielectric layer material, hole shape, thick-to-diameter ratio, and chemical copper plating layer.
The dielectric layer material has an impact on filling. Non-glass-reinforced materials are easier to fill than glass-reinforced materials. It is worth noting that glass fiber protrusions in the hole have a negative effect on chemical copper plating. In this case, the difficulty in electroplating filling lies in improving the adhesion of the seed layer rather than the filling process itself.
In fact, electroplating filling on glass fiber-reinforced substrates has been applied in practical production.
Currently, both manufacturers and developers attach great importance to fill technology for holes of different shapes and sizes. The filling capacity is greatly influenced by the ratio of the thickness to the diameter of the hole. Relatively speaking, the DC system is more commonly used in commerce. In production, the size range of the holes will be narrower, generally with a diameter of 80µm~120µm and a depth of 40µm~80µm, and the thick-to-diameter ratio does not exceed 1:1.
The thickness, uniformity, and placement time of the chemical PCB copper plate layer all affect the filling performance. The filling effect is poor if the chemical copper plating layer is too thin or uneven. Generally, it is recommended to perform filling when the thickness of the chemical copper is >0.3µm. In addition, the oxidation of chemical copper also has a negative impact on the filling effect.
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