At present, the preparation of PdCu wire in China mainly adopts electroplating, electroless plating, and direct plating technology. Electroplating and electroless plating technology is the earliest application technology. The preparation of palladium-plated copper (PdCu) wire by electroplating and electroless plating technology will produce harmful substances and pollute the environment [25]. Direct plating is a new Pd plating technology developed by researchers in recent years. It is simple to operate and does not produce harmful substances. It can be called “green Pd plating technology”.

Electroplating is an important surface treatment technology. It uses electrochemical methods to process the surface of metal and non-metal products, so that various metal coatings can be obtained on the surface [26] to improve the corrosion resistance and oxidation resistance of materials. The plating solution is generally composed of metal salt, reducing agent, buffer and complexing agent, stabilizer, brightener, etc. The process parameters of electroplating mainly include pH value, temperature, time, and current density. At present, there are two kinds of palladium-plating solutions: ammonium salt type and phosphate type [27]. The preparation of PdCu wire by electroplating technology is to plate a thicker Pd layer on a thicker (Φ > 0.15 mm) Cu wire, and then to obtain the PdCu wire of the required specifications after drawing the PdCu wire, intermediate heat treatment, PdCu wire finished drawing, PdCu wire final annealing, PdCu wire test or rewinding, and other steps. However, due to the different plastic deformation properties of Cu and Pd during the drawing process, it is difficult to draw fine (Φ < 0.016 mm) PdCu wire [28]. Therefore, in some ultra-narrow spacing microelectronic packaging, it is difficult to prepare the required specifications of fine PdCu wire by electroplating technology, and the preparation process will produce harmful substances and pollute the environment. In the process of drawing, there may also be problems such as palladium layer shedding and uneven thickness of the palladium layer. Figure 1 shows the electroplating process [28].

Electroless plating is a plating method in which metal ions in the plating solution are reduced to metal and deposited on the surface of parts by means of a suitable reducing agent without impressed current. Electroless palladium plating can deposit palladium membrane with uniform thickness on the surface of the complex matrix. It is the most widely used method to prepare palladium membrane because of its low cost, simple equipment, easy operation, good uniformity, and compactness [29].

Zhou et al. [30] developed a chemical method for the preparation of PdCu wire and applied for an invention patent. This method uses copper with a mass percentage greater than 99.95%, and then completes the preparation by melting and drawing, electroless palladium plating before annealing of the finished product, annealing, cleaning, rewinding and other steps. Compared with the traditional PdCu wire, the performance of the PdCu wire obtained by this preparation method has been improved, and the production process is safe, energy-saving, and simple. Zhang et al. [31] first studied electroless palladium plating on the surface of copper sheet, and then analyzed that due to the difference in the surface area of copper wire and copper sheet, the activation energy of palladium deposition on the surface of copper wire was small. By increasing the concentration of isopropanolamine to control the growth of the palladium particles, the formula composition and process suitable for electroless palladium plating on copper wire were obtained: PdCl₂ 2 g/ L, NaH₂PO₂·H₂O 12 g/L, 38% (mass fraction) HCl₄ mL/L, 28% (mass fraction) NH₃ H₂O 160 mL/L, Bismuth nitrate 55 mg/L, auxiliary complexing agent isopropanolamine 35 mL/L pH 9.8, at a temperature of 52 °C and a duration of 35 min. The surface of the coating obtained by the formula composition and process is uniform and fine, and there is no delamination and crack, as shown in Figure 2.

PdCu wire is prepared by electroless plating. Although the production process has low cost, good uniformity, and compactness of the obtained palladium film, ultimately, the chemical solution is used, and the waste liquid will be produced in the preparation process, which will cause certain pollution to the environment and bring pressure to the ecological environment. The following focuses on a “green palladium plating process”.

Direct palladium-plating technology, namely coating-palladium technology, is used to directly coat nano-palladium organic solution on the finished bonded copper wire, and deposit nano-palladium on the surface of copper wire by a heat-treatment process [28]. Therefore, the preparation of PdCu wire by direct-plating technology requires the preparation of PdCu wire blank line first, and then adjusting the coating process parameters, and finally obtaining the finished product.

Figure 3 shows the drawing process. The 6 N copper was drawn into 8 mm single-crystal copper rod in a single-crystal continuous casting furnace, and then the single-crystal copper rod was drawn into copper bonding wires of different specifications after rough drawing, medium drawing, fine drawing, and micro wire drawing [32].

Direct palladium-plating process [33]: (1) mold cleaning; (2) prepare the copper wire to be coated and wear the mold; (3) absorb the coating liquid with a medical needle tube without a rubber head; (4) according to the diameter of copper wire and the thickness of palladium layer, the speed of the bidirectional pump is set, and the bidirectional pump is opened; (5) when the speed of the pump reaches a predetermined value, start coating; (6) observe the coating color and adjust the temperature and speed appropriately. The coating schematic is shown in Figure 4.

The principle of direct palladium-plating technology [34]: Firstly, by adding various chemical substances, the nano ultrafine powder is uniformly dispersed and suspended in the plating solution, and the plating solution has high wettability and stability. The surfactant in the plating solution is coated around the nanoparticles, and the dispersion between the nanoparticles is realized by the steric hindrance. Then, the drawn fine copper bonding wire is immersed in a specially prepared solution containing nano-metal powder. By drying the heating device and optimizing the heating deposition process parameters, a smooth and flat coating can be formed on the surface of the copper bonding wire. The effect of drying is to volatilize or decompose the solvent and other organic substances in the solution, leaving only the pure coating to adhere to the surface of the copper wire. Under the action of heat treatment, the coating completes the sintering process and is accompanied by grain growth and interface diffusion, thus forming a surface nano-coating with high interfacial bonding strength and brightness. The schematic diagram and device of surface nano-coating are shown in Figure 5 and Figure 6, respectively [34].

直接电镀制备的PdCu线具有良好的化学稳定性，不开裂和剥落，粘接性能优异，涂层均匀，结构致密。与电镀和化学镀相比，直接电镀具有工艺简单、成本低（约为电镀和化学镀的11%）、生产过程中环保、无有害物质、涂层表面光滑均匀等优点[25，33]。从图7可以看出，直镀钯铜线的表面质量优于电镀和化学镀。所以， 一般来说， 直接电镀钯铜线的性能（断裂力， 伸长率， 等）会优于电镀和化学镀.

综上所述，采用直接镀钯技术可制备细铜键合丝，制备的细铜键合丝性能好，化学性能稳定，钯层均匀，密度好。直接镀钯技术不仅简单，而且在制备过程中不产生有害物质，不污染空气和土地，优于电镀和化学镀。