What are Copper Plating Intermediates?
Copper Plating Intermediates are chemical compounds employed in the electroplating of copper to help deposit copper on the substrate. These intermediates are key to regulating the properties of the copper layer — its thickness, smoothness, hardness and adhesion. Copper plating intermediates typically occur as solutions, salts or additives and are the basis for achieving quality and effective copper electroplating on electronics, PCBs, and decorative plating.
Optimization of Electroplating Process Parameters
The most important electroplating parameters including plating temperature, pH and current density directly influence the quality, rate of deposition, gloss, hardness and adhesion of the copper layer. In order to get the best copper plating intermediates, you need to be able to recognize how these values influence the electroplating result. Below is a detailed analysis of these parameters and their optimization strategies:
1. Effect of Plating Temperature
Temperature is a key factor in electroplating processes, with its effects mainly observed in deposition rate, plating quality, and stability.
Deposition Rate
The increase in temperature typically accelerates the copper deposition rateFor instance, in chemical copper plating process the deposition rate – from 35°C to 70°C – becomes exponentially higher at 20m/h with a lower solution stability of plating solution. The copper in the plating increases slightly, but not much, in another test when temperature changes from 40-45°C. But once the temperature increases from 45°C to 55°C, the copper in the plating spikes, peaking at 55°C.
Plating Quality
Although higher temperatures accelerate the deposition rate, excessive temperatures can deteriorate the plating quality. For example, at a temperature of more than 70°C, when deposition is rapid, but the reaction is faster, hydrogen evolution occurs and the surface roughens with more micropores and the result is a brownish loose structure. Organic additives are also more susceptible to decomposition under high temperatures that influence the corrosion and adhesion of the plated surface.
Plating Solution Stability
Excessively high temperatures can reduce the stability of the plating solution. For example, in the chemical copper plating process, when the temperature exceeds 70°C, although the deposition rate is fast, the vigorous reaction leads to hydrogen evolution, causing a significant decrease in the solution's lifespan and increased susceptibility to decomposition. In another study, when the temperature exceeds 75°C, the natural decomposition of the solution accelerates, and it is crucial to control the bath temperature within the required process range during operation.
Current Density and Limiting Current Density
The higher the plating solution temperature, the better solubility copper sulfate will be; thus preventing copper sulfate crystalization and increasing the limiting current density. But the temperature might also be high enough for additives to run out, and some intermediates decompose and no longer work.
Optimal Temperature Range
With deposition rate, plating quality, and solution stability being some of the factors to consider, the operating temperature is best kept to a lesser level. For instance, some reports recommend 30-40°C to run, while others say to set it between 40-60°C.
This effect of temperature on copper intermediates is complex. The range should in fact be calculated for individual electroplating process and materials, and temperatures need to be monitored carefully for electroplating success.
2. Effect of pH
The pH of copper plated solutions is a very important value since it directly determines the stability of copper ions, deposition rate, and physical and chemical integrity of the plating.
Electroplating Rate
In the chemical copper plating process, the plating rate generally increases with an increase in pH, reaching a maximum value. Beyond this point, further increases in pH will cause the plating rate to decrease. For example, under certain conditions, when the pH is 12.8, both the plating rate and decomposition time reach their optimum. However, some studies indicate that when the pH is too high, the plating rate actually decreases due to the decomposition of the solution.
Plating Quality
pH has a significant effect on plating quality. Higher pH values (such as above 10) lead to a reduction in the glossy area of the plating, making it prone to burning, and the anode area is more likely to produce Cu(OH)₂ deposits. Additionally, excessively high pH values cause rough crystallization of the plating, resulting in a dark red color and reduced dispersion capability. On the other hand, when the pH is lower, although it facilitates the movement of copper particles to the cathode, it can also lead to hydrogen gas evolution, which is detrimental to the co-deposition process.
Plating Solution Stability
High pH can lead to the natural decomposition of the plating solution, affecting its stability. Therefore, the pH of most chemical copper plating solutions is typically controlled around 12. Furthermore, when the pH is too low, the adsorption performance of brighteners decreases, leading to an increased consumption of brightener and a faster accumulation of organic impurities.
Optimal pH Range
Various researches have suggested various threshold pHs. The best pH is 8.3-9.0, for instance, in phosphoric acid copper plating. For citric acid-tartrate copper plating, the ideal pH is between 8.5 and 10. At certain specific conditions (eg, formaldehyde as a reducing agent), the correct pH can be 12.8.
In conclusion, the effect of pH on copper intermediates is multifaceted and needs to be optimized based on the specific process conditions and goals. Typically, a pH range between 8.5 and 10 yields good electroplating results and plating quality.
3. Effect of Current Density
Current density plays a critical role in the copper electroplating process, directly influencing the deposition rate, surface morphology, mechanical properties, and electrical performance of copper intermediates. Key effects of current density on copper intermediates are summarized below:
Deposition Rate and Uniformity
At low current densities, the transfer rate of copper ions is slow, leading to a low deposition rate and uneven plating. For instance, when the current density is below 0.25 A/dm², the bottom portion of a Through-Silicon Via (TSV) remains unsealed due to insufficient electrochemical promotion.
As current density increases, the deposition rate rises significantly. But outside a range, the plating layer becomes less consistent. For instance, if the density is already higher than 1.5 A/dm², the plating rates go up, but the uniformity of the copper layer drops precipitously.
Surface Morphology
The copper plating layer shows cauliflower growth at low current densities (0.8 A/dm²) and sags and dips.
And the higher the current density, the smoother the plating will become. At a current density of 2.4 A/dm², a flat and dense plating layer is formed. However, further increases in current density result in a rougher surface.
Mechanical Properties
Current density significantly affects the hardness and wear resistance of the copper plating layer. As current density increases, these properties improve, though the risk of corrosion may also rise.
Studies indicate that increasing current density reduces crystal grain size, thereby enhancing tensile strength.
Resistivity
Current density also impacts the resistivity of the copper plating layer. A dense and smooth plating structure facilitates carrier transport, resulting in lower resistivity. In contrast, a loose and rough structure impedes carrier transport, leading to higher resistivity.
Crystal Structure
At low current densities, the copper plating layer tends to exhibit a (220) preferential atomic orientation.
At high current densities, the plating layer shifts toward a (111) preferential atomic orientation.
Optimal Current Density Range
The ideal current density range is typically between 1-5 A/dm², with specific values adjusted based on plating requirements and workpiece geometry. For instance, in certain applications, a current density of 1.5 A/dm² has been shown to yield optimal experimental results.
Current density is one of the most important control parameters in copper electroplating. By optimizing current density, it is possible to increase deposition rates while maintaining plating quality and uniformity, achieving the desired mechanical and electrical properties.
Conclusion
By optimizing parameters such as temperature, pH, and current density in the electroplating process, precise control over the performance of copper plating intermediates can be achieved. Every parameter has its desired control range that should be tuned for electrolyte composition, machine setup and real-world process applications. If properly tuned these parameters can help make the copper layer of this product be of much higher quality, more durable and adapt to various industrial sectors.
Reference
- Li, T. C. et al. "Effect of copper electroplating process on electrical properties and uniformity of copper layer on silicon velvet surface." Materials Science 13.11(2023):977-983.
