A process for plating nickel over silver for integrated circuit lead frames

Abstract

The application discloses a nickel-silver plating process for integrated circuit lead frames, which comprises the following steps in sequence: degreasing, activation, pre-plating of alkaline copper, plating of acid copper, plating of nickel, plating of silver and post-treatment. In the process, the pre-plating of alkaline copper fills the pits of the substrate, and the plating of acid copper controls the surface roughness of the copper layer to be Ra≤0.1 μm. The plating of nickel forms a double-layer gradient nickel barrier layer on the surface of the copper layer, which comprises a dense bottom layer of nickel and a low-stress surface layer of nickel. The plating of silver is carried out by first pre-plating a thin layer of silver on the entire surface, then selectively thickening the silver plating in the functional area, and then removing the thin silver layer in the non-functional area to expose the nickel barrier layer. The process realizes pinhole-free coverage of the nickel barrier layer at a relatively small thickness by the cooperation of the bottom copper flattening and the double-layer gradient nickel, effectively blocks the mutual diffusion of copper and silver, exposes the corrosion-resistant nickel layer in the non-functional area, eliminates the problems of copper accumulation and peeling caused by the oxidation of the copper layer, and simultaneously reduces the thickness of the silver layer, thereby reducing the consumption of precious metals.

Description

Technical Field

[0001] This application relates to the field of metal surface treatment technology, specifically to a nickel-silver plating process for integrated circuit lead frames. Background Technology

[0002] As a core structural component of chip packaging, the integrated circuit (IC) leadframe has become increasingly important in recent years with the rapid development of electronic technology. It not only provides mechanical support for the chip but also establishes a stable electrical connection between the chip and external pins. The leadframe substrate is typically made of copper or copper alloys, thanks to their excellent electrical and thermal conductivity, as well as their moderate cost advantage. Local silver plating in the functional area of ​​the leadframe (chip bonding area) can improve the bonding strength of the wires and enhance the electrical connection and heat dissipation during chip operation, thus finding widespread application in the field of electronic packaging.

[0003] In traditional processes, the mainstream method for silver plating the bonding area of ​​leadframe chips is to directly and locally plate silver on the copper layer surface, forming a layered structure where copper and silver are in direct contact. Furthermore, to suppress copper-silver interdiffusion, existing technologies also employ methods such as increasing the silver layer thickness or using multilayer structures like copper / nickel / copper / silver. Some "nickel-on-silver" processes are also used to some extent.

[0004] However, these existing processes have many drawbacks. In long-term use, the layered structure with direct copper / silver contact suffers from continuous oxidation and corrosion of the copper layer in humid air. This leads to the formation of a loose, basic copper carbonate layer on the exposed non-functional copper layer surface. This loose, corroded layer has poor adhesion to the substrate, causing corrosion at the exposed non-functional copper areas, resulting in copper buildup and plating peeling, severely impacting the long-term stability of the chip under humid and hot conditions. Simultaneously, copper and silver can easily interdiffused under certain conditions. Due to the different diffusion rates of copper and silver atoms, Kirkendal voids form at the interface, weakening the bonding between plating layers, reducing long-term durability, and causing increased contact resistance or even interface failure. Increasing the silver layer thickness leads to high consumption of precious metals and high costs; multilayer structures such as copper / nickel / copper / silver are complex to manufacture, and the non-functional copper layer still faces the risk of exposure. Existing "nickel-on-silver" processes generally suffer from problems such as: excessively thick nickel layers, which significantly increases thermal resistance and contact resistance; thin nickel layers are prone to pinholes, through which copper and silver atoms can diffuse, resulting in insufficient blocking effect; copper layers in non-functional areas still have the risk of exposure; and there is a lack of coordinated design for the flattening of the bottom copper surface and the densification of the nickel layer. Summary of the Invention

[0005] To address the technical problems in the prior art, this application provides a nickel-silver plating process for integrated circuit lead frames.

[0006] The nickel-silver plating process for integrated circuit lead frames provided in this application adopts the following technical solution:

[0007] A nickel-silver plating process for integrated circuit lead frames, using the lead frame substrate as the workpiece to be processed, includes the following steps:

[0008] S1. Degreasing: The lead frame substrate is degreased to remove surface grease contaminants, followed by water washing;

[0009] S2. Activation: Electrolytic activation is performed on the lead frame substrate to remove the surface oxide layer, followed by water washing;

[0010] S3. Pre-plating alkaline copper: Electroplating an alkaline copper layer on the surface of the lead frame substrate to fill the pits on the substrate surface.

[0011] S4. Acid copper plating: An acid copper layer is electroplated on the surface of the alkaline copper layer. The alkaline copper layer and the acid copper layer together constitute a copper layer. The surface roughness Ra of the copper layer is controlled to be ≤0.1μm.

[0012] S5. Nickel plating: A dense nickel barrier layer is formed by electroplating on the surface of the copper layer. The nickel barrier layer has a double-layer gradient structure, including a dense bottom nickel layer disposed close to the copper layer and a low-stress surface nickel layer disposed on the dense bottom nickel layer.

[0013] S6. Silver plating: First, a thin silver layer is pre-plated on the entire surface of the nickel barrier layer; then, the silver plating is selectively thickened in the functional area; subsequently, the pre-plated thin silver in the non-functional area is removed using a stripping solution, exposing the nickel barrier layer in the non-functional area; finally, a lead frame with a copper layer / nickel barrier layer / silver layer structure in the functional area and a copper / nickel barrier layer structure in the non-functional area is obtained.

[0014] S7. Post-processing: The lead frame is washed with water, treated with protective water, and dried in sequence.

[0015] In some embodiments, in step S6, the thickness of the pre-plated thin silver layer on the entire surface is 0.05 to 0.3 μm; the thickness of the thickened silver plating layer in the functional area is 1 to 10 μm.

[0016] In some embodiments, in step S1, the degreasing treatment is a cyclic treatment of alternating cathodic degreasing and anodic degreasing, with no less than 2 cycles and a degreasing current density of 2 to 5 ASD; in step S2, the electrolytic activation uses a dilute sulfuric acid solution with a mass concentration of 3 to 8% and an electrolytic activation time of 20 to 60 seconds.

[0017] In some embodiments, in step S3, the alkaline copper layer is formed by electroplating with an alkaline copper plating solution with potassium cyanide and cuprous cyanide as the main components, the anode is a copper block, the cathode is the lead frame substrate, and the thickness of the alkaline copper layer is 5 to 30 μm.

[0018] In some embodiments, in step S4, the acid copper layer is formed by electroplating with an acidic copper plating solution containing 180-250 g / L copper sulfate, 50-80 g / L sulfuric acid, and copper plating additives; the copper plating additives contain bis(3-sulfopropyl) disulfide, N,N-dimethylimidazoline thione, polyethylene glycol, and chloride ions, wherein the content of bis(3-sulfopropyl) disulfide is 5-20 mg / L, the content of N,N-dimethylimidazoline thione is 1-10 mg / L, the content of polyethylene glycol is 100-500 mg / L, and the content of chloride ions is 30-80 mg / L; the thickness of the acid copper layer is 40-150 μm.

[0019] In some embodiments, in step S5, the dense underlayer nickel is formed by electroplating with a nickel sulfate system containing 20-50 g / L nickel chloride at a low current density of 1-3 ASD, so that the dense underlayer nickel forms a fine-grained structure with small grains and dense grain boundaries, thereby inhibiting the mutual diffusion of copper and silver atoms along the grain boundaries; the low-stress surface layer nickel is formed by electroplating with a nickel sulfamate system. The total thickness of the nickel barrier layer is 0.5-2 μm, wherein the thickness of the dense underlayer nickel is 0.1-0.5 μm, and the thickness of the low-stress surface layer nickel is 0.3-1.5 μm.

[0020] In some embodiments, the formulation of the nickel sulfamate plating solution is as follows: nickel ions provided by nickel sulfamate at a concentration of 60-100 g / L, nickel chloride at a concentration of 5-15 g / L, and boric acid at a concentration of 35-45 g / L; the plating temperature is 50-60°C, and the current density is 3-15 ASD; the anode is a nickel bead, and the cathode is the lead frame substrate.

[0021] In some embodiments, in step S5, the low-stress nickel surface layer is electroplated using a stepped current process: in the initial plating stage, a dense fine-grained nucleus layer is formed with a current density of 1 to 3 ASD; in the growth stage, the grain orientation is controlled with a current density of 3 to 8 ASD; and in the sealing stage, the micropores are sealed with a current density of 0.5 to 1.5 ASD to eliminate pinhole defects.

[0022] In some embodiments, in step S5, the low-stress surface layer nickel is electroplated using a pulse electroplating process. The pulse electroplating includes a forward pulse and a reverse pulse. The reverse pulse is used to dissolve the sharp protrusions on the surface of the nickel barrier layer, thereby improving the surface smoothness of the nickel barrier layer and reducing the internal stress of the plating layer.

[0023] In some embodiments, in step S6, the selective thickening silver plating uses a mask or selective silver plating fixture to shield the non-functional areas, exposing the functional areas to the plating solution for selective thickening silver plating; both the overall pre-plating of thin silver and the selective thickening silver plating of the functional areas are electroplated using a mixed plating solution containing silver nitrate, potassium cyanide, potassium hydroxide, tartrate, and silver plating intermediates, with the anode being a silver plate and the cathode being a lead frame; the stripping solution is a silver stripping solution that selectively dissolves silver but does not corrode the nickel barrier layer; in step S7, the protective water treatment uses an anti-oxidation protective solution to treat the surface of the lead frame to prevent the nickel barrier layer in the non-functional areas from oxidizing during storage and subsequent packaging.

[0024] In summary, this application includes at least one of the following beneficial technical effects:

[0025] 1. By setting a double-layer gradient structure nickel barrier layer consisting of a dense bottom nickel layer and a low-stress surface nickel layer between the copper layer and the silver layer, and controlling the surface roughness of the copper layer to Ra≤0.1μm, the two work together to achieve a non-penetrating pinhole coverage with a thickness of 0.5~2μm, effectively blocking the mutual diffusion of copper and silver atoms along the grain boundaries, suppressing the formation of Kirkendal voids, and controlling the influence of the nickel barrier layer on thermal resistance and contact resistance within an acceptable range.

[0026] 2. By adopting a three-stage silver plating process of "pre-plating thin silver on the whole surface - selective thickening of silver plating in functional areas - stripping thin silver plating in non-functional areas to expose the nickel barrier layer", the non-functional areas are finally exposed with a dense and corrosion-resistant nickel barrier layer instead of a copper layer. This avoids the problem of copper accumulation and plating peeling caused by the oxidation of the copper layer in a humid and hot environment to form a loose oxide layer. After 600 hours of high temperature and high humidity aging test at 85℃ / 85%RH, it was verified that there was no copper accumulation or peeling in the non-functional areas.

[0027] 3. Since the nickel barrier layer blocks the interdiffusion of copper and silver, the silver layer in the functional area does not need to be thickened to resist diffusion. The thickness of the silver layer can be reduced from more than 3μm in the traditional process to 1-2μm. The consumption of precious metal silver is reduced to about 1 / 3 to 2 / 3 of that in the traditional copper / silver direct structure. Attached Figure Description

[0028] Figure 1 A schematic diagram of the process flow for nickel-silver plating of an integrated circuit lead frame provided in an embodiment of this application;

[0029] Figure 2 This is a schematic diagram of the layered structure of the IC lead frame after the process of this application;

[0030] Figure 3 for Figure 2 A magnified cross-sectional schematic diagram of region 3 of the nickel barrier layer.

[0031] Explanation of reference numerals in the attached figures: 1. Lead frame substrate; 11. Functional area; 12. Non-functional area; 2. Copper layer; 21. Alkaline copper layer; 22. Acid copper layer; 3. Nickel barrier layer; 31. Dense bottom nickel layer; 32. Low-stress top nickel layer; 4. Silver layer. Detailed Implementation

[0032] The technical solutions in the embodiments of the present invention will be further described in detail below with reference to the accompanying drawings. The described embodiments are only possible technical implementations of the present invention, but are not limited thereto. Other embodiments obtained by those skilled in the art in conjunction with the embodiments of the present invention without creative effort are also within the protection scope of the present invention.

[0033] This application primarily employs a nickel-silver plating process to treat the lead frame, achieving reliable blocking of copper-silver interdiffusion and eliminating copper buildup and plating peeling. A further detailed description of this application follows. All processes can be implemented on a conventional continuous electroplating automated production line. The production line is equipped with a pulse / DC adjustable current power supply, a plating solution filtration and circulation device, a temperature controller and heating system, an automatic water washing and conveying system, and a real-time pH and liquid level monitoring device to ensure the stability of plating solution parameters in each process.

[0034] Example 1

[0035] Please refer to Figures 1 to 3 The nickel-silver plating process for integrated circuit leadframes provided in this application uses a copper alloy leadframe substrate 1 as the workpiece to be processed. The process includes steps such as degreasing, activation, pre-plating with alkaline copper, plating with acid copper, nickel plating, silver plating, and post-treatment. These steps are performed sequentially. Through a series of treatments on the leadframe substrate 1, a layered leadframe structure is finally obtained, with functional area 11 consisting of a copper layer 2 / nickel barrier layer 3 / silver layer 4 and non-functional area 12 consisting of a copper layer 2 / nickel barrier layer 3. This achieves reliable blocking of copper-silver interdiffusion, elimination of copper buildup and plating peeling, and reduced consumption of the precious metal silver. This is because, through the synergistic effect of each step, a dense nickel barrier layer 3 is set between the copper layer 2 and the silver layer 4, effectively blocking the interdiffusion of copper and silver atoms. Simultaneously, the dense, corrosion-resistant nickel barrier layer 3 is exposed in the non-functional area 12, avoiding copper buildup and plating peeling caused by the oxidation of the copper layer 2. Furthermore, since the interdiffusion of copper and silver is blocked, the silver layer 4 in the functional area 11 can be thinned, reducing the consumption of precious metals.

[0036] Please refer to Figure 1Specifically, in step S1, the degreasing process employs a cyclical treatment alternating between cathodic and anodic degreasing. During cathodic degreasing, the lead frame substrate 1 is placed in an alkaline degreasing tank. Current is passed through the tank, making the lead frame substrate 1 the cathode, and the grease is removed under the mechanical stripping action of hydrogen bubbles. Anodic degreasing is also carried out in the degreasing tank, this time with the lead frame substrate 1 acting as the anode. Residual oil is removed through the synergistic removal of oxygen bubbles and electrochemical oxidation. The degreasing current density is controlled between 2 and 5 ASD, and the number of cycles is no less than 2. In this embodiment, the degreasing current density is 3 ASD, the cathodic degreasing time is 30 seconds, the anodic degreasing time is 15 seconds, and the cycle is repeated twice. This effectively eliminates the limitations of each method, ensuring the surface of the lead frame substrate 1 is clean. It is then thoroughly rinsed with deionized water. Alternatively, an ultrasonic degreasing tank can be used, which utilizes the cavitation effect of ultrasound to enhance the degreasing effect and can adapt to lead frame substrates 1 of different shapes.

[0037] During the degreasing process, cathodic degreasing uses hydrogen bubbles generated by electrolysis to mechanically peel off the grease from the surface of the lead frame substrate 1, causing it to fall off quickly. Anodic degreasing, on the other hand, utilizes oxygen bubbles and electrochemical oxidation to remove residual grease that the cathodic degreasing could not completely remove. These two processes alternate and cycle, complementing each other's limitations to ensure a highly clean surface on the lead frame substrate 1. This provides a grease-free substrate surface for subsequent electroplating steps, preventing poor adhesion of the plating layers due to grease residue.

[0038] S2. In the activation step, electrolytic activation uses a dilute sulfuric acid solution with a mass concentration of 3-8%. The lead frame substrate 1 is placed in the dilute sulfuric acid solution, and the surface oxide layer is removed by electrolysis. The electrolytic activation time is controlled between 20 and 60 seconds. In this embodiment, a 5% mass concentration dilute sulfuric acid is used, and the electrolytic activation time is 30 seconds, followed by water washing. The activation equipment can be an activation tank with electrolytic electrodes, or a spray activation device can be used to uniformly spray the dilute sulfuric acid solution onto the surface of the lead frame substrate 1 in the form of a spray to accelerate the activation speed.

[0039] During the activation step, dilute sulfuric acid solution dissolves and removes the oxide layer on the surface of the lead frame substrate 1 under electrolysis, exposing a fresh, active metal surface. The technical effect of this step is that the active surface after removing the oxide layer significantly improves the adhesion between the subsequent alkaline copper layer 21 and the lead frame substrate 1, avoiding delamination or poor adhesion between the plating layer and the substrate due to the presence of the oxide layer.

[0040] In step S3, the alkaline copper plating layer 21 is formed by electroplating with an alkaline copper plating solution mainly composed of potassium cyanide and cuprous cyanide. The electroplating equipment is generally an alkaline copper plating tank, with a copper block as the anode and a lead frame substrate 1 as the cathode. During the electroplating process, the thickness of the alkaline copper layer 21 is controlled to reach 5–30 μm. In this embodiment, the thickness of the alkaline copper layer 21 is approximately 15 μm, which is used to fill the pits on the substrate surface. The alkaline copper layer 21 has good adhesion to the copper alloy substrate 1, providing a uniform substrate for subsequent acid copper leveling. Other alkaline plating solutions containing copper ions can also be used, as long as they can ensure adhesion to the substrate and the effect of filling pits.

[0041] During the pre-plating of alkaline copper, copper ions in the alkaline copper plating solution are deposited on the surface of the lead frame substrate 1 under the action of an electric field, preferentially filling the micro-pits and defect areas on the substrate surface to form an alkaline copper layer 21. The technical effect of this step is that the alkaline copper layer 21 can effectively fill the micro-pits on the surface of the lead frame substrate 1, eliminate the micro-unevenness of the substrate surface, and provide a uniform and flat substrate for the subsequent electroplating of the acid copper layer 22, enabling the subsequent acid copper layer 22 to achieve further fine leveling on the flat substrate.

[0042] Please refer to Figure 2 In step S4, the acid copper plating step, the acid copper layer 22 is formed by electroplating with an acidic copper plating solution containing 180-250 g / L copper sulfate, 50-80 g / L sulfuric acid, and copper plating additives. The copper plating additives contain bis(3-sulfopropyl) disulfide (SPS), N,N-dimethylimidazoline thione (DMIB), polyethylene glycol (PEG), and chloride ions. The content of bis(3-sulfopropyl) disulfide is 5-20 mg / L, the content of N,N-dimethylimidazoline thione is 1-10 mg / L, the content of polyethylene glycol is 100-500 mg / L, and the content of chloride ions is 30-80 mg / L. In this embodiment, a mixed plating solution containing 220 g / L copper sulfate, 60 g / L sulfuric acid, and copper plating additives (SPS 12 mg / L, DMIB 5 mg / L, PEG 300 mg / L, and chloride ions 50 mg / L) is used. The electroplating equipment is an acid copper plating tank, equipped with an anode copper block and a cathode lead frame substrate 1. The plating solution is continuously filtered and circulated to maintain its cleanliness. By controlling the electroplating parameters, the thickness of the acid copper layer 22 is made to reach 40–150 μm. In this embodiment, the thickness of the acid copper layer 22 is approximately 80 μm. The alkaline copper layer 21 and the acid copper layer 22 together constitute the copper layer 2. The surface roughness Ra of the copper layer 2 is controlled to be ≤0.1 μm. In this embodiment, the surface roughness Ra of the copper layer 2 is measured to be 0.08 μm, which meets the requirement of Ra≤0.1 μm, laying the foundation for the uniform and pinhole-free coverage of the subsequent ultrathin nickel barrier layer 3. The additives in the plating solution can also be adjusted according to the actual situation, as long as the flatness and quality of the copper layer 2 are guaranteed.

[0043] During the acid copper plating process, copper ions in the acidic copper plating solution are deposited on the surface of the alkaline copper layer 21 under the action of an electric field. SPS in the copper plating additive acts as an accelerator to promote copper deposition in the recesses, while DMIB acts as an inhibitor to suppress copper deposition in the convex areas. PEG and chloride ions work synergistically as inhibitors to further regulate the uniformity of copper deposition. The three elements work together to achieve a finely smooth surface on the copper layer 2. The technical effect of this step is as follows: Figure 2 As shown, after steps S3 and S4 are completed, a copper layer 2 consisting of an alkaline copper layer 21 and an acid copper layer 22 is formed on the surface of the lead frame substrate 1. Its surface is highly flat (Ra≤0.1μm), laying a crucial foundation for the uniform, pinhole-free coverage of the subsequent ultra-thin nickel barrier layer 3. The flatter the surface of the copper layer 2, the less likely the subsequently electroplated nickel barrier layer 3 will develop pinhole defects at surface protrusions or pits, thus enabling the extremely thin nickel barrier layer 3 to achieve a reliable blocking effect.

[0044] Please refer to Figure 2 and Figure 3In step S5, the nickel barrier layer 3 has a double-layer gradient structure, including a dense underlayer nickel 31 near the copper layer 2 and a low-stress surface layer nickel 32 on top of the dense underlayer nickel 31. The dense underlayer nickel 31 is formed by electroplating with a nickel sulfate system containing 20-50 g / L nickel chloride at a low current density of 1-3 ASD. The electroplating equipment is a nickel plating bath. At low current density, the nucleation rate is high, the grains are fine, and the grain boundaries are dense, thus forming a fine-grained structure with fine grains and dense grain boundaries in the dense underlayer nickel 31, effectively suppressing the mutual diffusion of copper and silver atoms along the grain boundaries. In this embodiment, the dense underlayer nickel 31 uses a nickel sulfate system containing 30 g / L nickel chloride (higher than the 10 g / L of the nickel sulfamate surface layer system), and is electroplated with a low current density of 2 ASD to obtain a dense underlayer nickel 31 with a grain boundary density of approximately 0.3 μm. The low-stress nickel 32 surface layer is formed by electroplating with a nickel sulfamate system. The formulation of the nickel sulfamate system is as follows: nickel ions provided by nickel sulfamate 60-100 g / L, nickel chloride 5-15 g / L, and boric acid 35-45 g / L; the electroplating temperature is 50-60℃, and the current density is 3-15 ASD; the anode is a nickel bead, and the cathode is the lead frame substrate 1. In this embodiment, the nickel sulfamate system plating solution contains nickel ions provided by nickel sulfamate 80 g / L, nickel chloride 10 g / L, and boric acid 40 g / L, at a temperature of 55℃. The low-stress nickel 32 surface layer can be electroplated using a stepped current process. In the initial plating stage, a dense fine-grained nucleus layer is formed with a current density of 1-3 ASD; in the growth stage, the grain orientation is controlled with a current density of 3-8 ASD; and in the sealing stage, the micropores are sealed with a current density of 0.5-1.5 ASD to eliminate pinhole defects. In this embodiment, the low-stress nickel layer 32 is electroplated using a stepped current method (initial plating 1-3 ASD, growth 5-8 ASD, and sealing 1 ASD) to obtain a low-stress nickel layer 32 with a thickness of approximately 0.8 μm. Alternatively, a pulse electroplating process can be used, which includes forward pulses and reverse pulses. The reverse pulse is used to dissolve the sharp protrusions on the surface of the nickel barrier layer 3, thereby improving the surface smoothness of the nickel barrier layer 3 and reducing the internal stress of the plating layer. The total thickness of the nickel barrier layer 3 is controlled between 0.5 and 2 μm, wherein the thickness of the dense underlayer nickel 31 is 0.1-0.5 μm, and the thickness of the low-stress nickel layer 32 is 0.3-1.5 μm. In this embodiment, the total thickness of the nickel barrier layer 3 is approximately 1.1 μm. The nickel plating solution may also contain one or more of sodium chloride, sodium sulfate, sodium lauryl ether sulfate (LAES), polysarcosine, and sodium dodecyl sulfate (SDS) as a wetting agent or grain refiner to improve the nickel plating effect.

[0045] During the nickel plating process, such as Figure 3As shown, the nickel barrier layer 3 is electroplated onto the surface of the copper layer 2 in two steps to form a double-layer gradient structure. First, a dense bottom layer nickel 31 is electroplated at a low current density. Due to the high nucleation rate at low current density, the resulting dense bottom layer nickel 31 has fine grains and dense grain boundaries, which can effectively block the diffusion channels of copper and silver atoms along the grain boundaries, making it the core functional layer for inhibiting copper-silver interdiffusion. Then, a low-stress surface layer nickel 32 is electroplated on the dense bottom layer nickel 31. The nickel sulfamate plating solution has low internal stress, resulting in low internal stress in the low-stress surface layer nickel 32, which can prevent the nickel barrier layer 3 from cracking due to internal stress concentration and improve the interfacial bonding with the upper silver layer 4. The overall nickel barrier layer 3 is composed of the dense bottom layer nickel 31, which plays a blocking role, and the low-stress surface layer nickel 32, which plays a stabilizing and bonding role. This makes the extremely thin nickel barrier layer 3 free of pinholes and with low internal stress, thus balancing barrier performance and plating stability. The technical effect of this step is that the double-layer gradient structure enables the nickel barrier layer 3 to achieve reliable blocking without penetrating pinholes even when it is extremely thin at 0.5 to 2 μm, minimizing the impact of thermal resistance and contact resistance, while ensuring that the coating does not crack or peel off due to internal stress.

[0046] Please refer to Figure 2In step S6, the silver plating process employs a three-stage procedure: "pre-plating the entire surface—selective plating for thickening functional areas—removal of plating to expose nickel." First, a thin silver layer is pre-plated across the entire surface of the nickel barrier layer 3, with a thickness of 0.05–0.3 μm. Electroplating is performed using a mixed plating solution containing silver nitrate, potassium cyanide, potassium hydroxide, tartrate, and a silver plating intermediate. The anode is a silver plate, and the cathode is a lead frame. The silver plating intermediate is a commercially available bright / complexing intermediate for cyanide silver plating systems, used to refine silver grains and improve the brightness and uniformity of the plating layer. In this embodiment, a thin silver layer with a thickness of approximately 0.15 μm is pre-plated across the entire surface. Then, selective thickening silver plating is performed on functional area 11. This selective thickening silver plating involves using a mask or selective silver plating fixture to shield non-functional area 12, exposing functional area 11 to the plating solution for selective thickening silver plating. The same silver plating system is used for selective silver plating on functional area 11, and the thickened silver plating thickness of the silver layer 4 in functional area 11 is 1–10 μm, preferably 1–2 μm. In this embodiment, the silver layer 4 in functional area 11 is thickened to approximately 2 μm. Finally, a stripping solution is used to remove the pre-plated thin silver layer in non-functional region 12. Since the thickness of the silver layer 4 in functional region 11 (1–10 μm) is much greater than the thickness of the only pre-plated thin silver layer in non-functional region 12 (0.05–0.3 μm), during the overall immersion in the stripping solution, the thin silver layer in non-functional region 12 is completely removed, exposing the nickel barrier layer 3. The silver layer 4 in functional region 11 is only stripped to an extremely thin thickness comparable to the pre-plated thin silver layer, while retaining sufficient bonded silver layers. This achieves differentiated silver stripping between non-functional region 12 and functional region 11. The stripping solution selectively dissolves silver but is non-corrosive to the nickel barrier layer 3, and has a silver dissolution rate of not less than 0.5 μm. The silver stripping solution used in this embodiment is an acidic silver stripping solution containing 80-120 g / L thiourea and 20-40 g / L sodium m-nitrobenzenesulfonate, with the pH adjusted to 1-2 by sulfuric acid. The operating temperature is 25-40°C, and the stripping time is 30-90 s. The silver dissolution rate of this solution is not less than 0.5 μm / min, and the corrosion rate of the nickel barrier layer 3 under the above operating conditions does not exceed 2 nm / min. This ensures that the nickel barrier layer 3 is not damaged while removing the approximately 0.05-0.3 μm thin layer of pre-plated silver from the non-functional area 12. Alternatively, the silver stripping solution can be other selective silver stripping systems using thiourea as a complexing agent and sodium m-nitrobenzenesulfonate or o-nitrobenzenesulfonate as an oxidant, or a similar commercial silver stripping brightener that selectively dissolves silver and does not corrode the nickel barrier layer 3 can be used to expose the nickel barrier layer 3 in the non-functional area 12. The final result is a layered lead frame with functional area 11 consisting of copper layer 2 / nickel barrier layer 3 / silver layer 4 and non-functional area 12 consisting of copper layer 2 / nickel barrier layer 3.

[0047] During the silver plating process, such as Figure 2As shown, firstly, a thin silver pre-plated layer covers the entire surface of the nickel barrier layer 3, providing an adhesion base for the subsequent thickening of the silver plating in the functional area 11. Then, the non-functional area 12 is masked using a mask or selective silver plating fixture, exposing the functional area 11 to the plating solution. Selective thickening of the silver plating is performed only in the functional area 11, so that the silver layer 4 in the functional area 11 reaches the required thickness. Finally, the thin silver pre-plated layer in the non-functional area 12 is removed using a stripping solution. Since the stripping solution selectively dissolves silver and is not corrosive to the nickel barrier layer 3, only the thin silver layer in the non-functional area 12 is removed, exposing the intact nickel barrier layer 3. The technical effect of this step is that a three-layer structure of copper layer 2 / nickel barrier layer 3 / silver layer 4 is ultimately formed in the functional area 11, and a two-layer structure of copper layer 2 / nickel barrier layer 3 is formed in the non-functional area 12. The non-functional area 12 exposes a dense and corrosion-resistant nickel barrier layer 3 instead of a loose copper layer 2, eliminating the problems of copper accumulation and plating peeling caused by the oxidation of the copper layer 2.

[0048] S7. In the post-processing step, the lead frame is sequentially washed with water, treated with protective water, and dried. The water washing uses deionized water for thorough rinsing. The protective water treatment uses an anti-oxidation protective solution to treat the surface of the lead frame. The lead frame is immersed in the anti-oxidation protective solution bath, forming a protective film on the surfaces of the nickel barrier layer 3 and the silver layer 4 to prevent oxidation and discoloration of the nickel barrier layer 3 in the non-functional area 12 during storage and subsequent packaging. In this embodiment, the anti-oxidation protective solution is a water-based anti-discoloration treatment solution containing benzotriazole corrosion inhibitors. The immersion temperature is 25–40°C, and the time is 10–60 seconds, forming a nanoscale organic corrosion inhibitor protective film on the nickel and silver surfaces. Drying is then performed.

[0049] During the post-processing steps, water washing removes residual plating solution and stripping solution from the lead frame surface, preventing residual chemicals from corroding the plating layer. Anti-oxidation protection water treatment forms a thin protective film on the surfaces of the nickel barrier layer 3 and the silver layer 4, effectively preventing oxidation and discoloration of the nickel barrier layer 3 in non-functional area 12 and the silver layer 4 in functional area 11 during storage and subsequent packaging. Drying removes moisture from the lead frame surface, preventing water stains from affecting subsequent packaging quality. The technical effect of this step is to ensure that the lead frame maintains a good surface condition during subsequent storage and packaging, extending the product's shelf life.

[0050] The entire process is implemented as follows: Please refer to... Figure 1The nickel-silver plating process of this application comprises seven steps, S1 to S7. First, in step S1 (degreasing), the lead frame substrate 1 undergoes alternating cathodic and anodic degreasing to remove surface grease contaminants, followed by water rinsing. Then, in step S2 (activation), the lead frame substrate 1 is electrolytically activated to remove the surface oxide layer, followed by water rinsing. Next, in step S3 (alkaline copper pre-plating), an alkaline copper layer 21 is electroplated on the surface of the lead frame substrate 1 to fill surface pits. Finally, in step S4 (acid copper plating), an acid copper layer 22 is electroplated on the surface of the alkaline copper layer 21, with the alkaline copper layer 21 and acid copper layer 22 forming a cohesive layer. The copper layer 2 is formed by controlling its surface roughness Ra to be ≤0.1μm. Then, in the S5 nickel plating step, a dense base layer nickel 31 and a low-stress surface layer nickel 32 are sequentially electroplated onto the surface of the copper layer 2, forming a double-layer gradient nickel barrier layer 3. Next, in the S6 silver plating step, a thin silver layer is pre-plated across the entire surface, then selectively thickened silver plating is applied to the functional area 11. Subsequently, the pre-plated thin silver layer on the non-functional area 12 is removed, exposing the nickel barrier layer 3. Finally, in the S7 post-treatment step, water washing, anti-oxidation protective water treatment, and drying are performed sequentially. Please refer to [reference needed]. Figure 2 The final IC lead frame is obtained with functional area 11 being copper layer 2 / nickel barrier layer 3 / silver layer 4 and non-functional area 12 being copper layer 2 / nickel barrier layer 3. Among them, functional area 11 (chip bonding area) is located on the lead frame substrate 1 in the area where chip bonding is required, and non-functional area 12 is the other areas on the lead frame substrate 1 other than functional area 11.

[0051] The implementation principle of this embodiment is as follows: This nickel-silver plating process, through the rational design and synergistic effect of each step, solves the problems of copper-silver interdiffusion, copper accumulation and plating peeling, and high consumption of precious metals in existing processes. A dense nickel barrier layer 3 with a double-layer gradient structure is set between the copper layer 2 and the silver layer 4, effectively blocking the interdiffusion of copper and silver atoms, inhibiting the formation of Kirkendal voids, and improving the long-term durability of the plating. Through the process of "pre-plating silver—selective plating thickening—removing plating to expose nickel," the non-functional area 12 is finally exposed to the dense corrosion-resistant nickel barrier layer 3, eliminating the problems of copper accumulation and plating peeling caused by the oxidation of the copper layer 2. Simultaneously, the fine smoothing of the copper layer 2 surface provides a flat base for the uniform, pinhole-free coverage of the ultra-thin nickel barrier layer 3, enabling the nickel barrier layer 3 to achieve reliable blocking even at extremely thin thicknesses, minimizing the impact of thermal resistance and contact resistance. Moreover, because copper-silver interdiffusion is blocked, the silver layer 4 in the functional area 11 can be thinned, reducing the consumption of the precious metal silver. Each step of this process can be implemented on a conventional continuous electroplating production line. The process parameters are clear and highly controllable, making it easy to scale up production.

[0052] Example 2

[0053] Please refer to Figure 2 and Figure 3 (The cross-sectional layered structure in this embodiment is the same as that in Embodiment 1, and is still referred to) Figure 2 and Figure 3 (The layered structure and nickel barrier layer shown) differs from the previous embodiment in that: in step S5, the low-stress surface layer nickel 32 is plated using a pulse electroplating method, with a forward pulse current density of 8 ASD and a reverse pulse current density of 2 ASD, and the forward and reverse pulse duty cycles are optimized. The reverse pulse dissolves the tip protrusion, further improving the flatness of the nickel barrier layer 3 and further reducing the internal stress. The surface layer nickel 32 thickness is approximately 1.0 μm, and the total thickness of the nickel barrier layer 3 is approximately 1.3 μm. The remaining steps are the same as in Embodiment 1.

[0054] The implementation principle of this embodiment is as follows: A low-stress nickel 32 layer is plated using pulse electroplating, which allows for better control of the surface smoothness and internal stress of the nickel layer. The dissolving effect of the reverse pulse eliminates sharp protrusions on the nickel layer surface, making the nickel layer smoother, reducing pinhole defects, and further improving the blocking effect of the nickel barrier layer 3. Simultaneously, reducing internal stress prevents the nickel barrier layer 3 from cracking due to stress concentration, improving the stability and long-term durability of the plating. Testing showed that the pinhole rate of the nickel layer in this embodiment is almost zero (see performance test results), indicating that the pulse reverse dissolving process is superior to the stepped current process in eliminating penetrating pinholes.

[0055] Example 3

[0056] Please refer to Figure 2 and Figure 3 This embodiment is basically the same as Embodiment 1, except that: in step S5, the total thickness of the nickel barrier layer 3 is controlled to be approximately 0.6 μm (0.2 μm for the dense bottom layer nickel 31 and 0.4 μm for the low-stress surface layer nickel 32); in step S6, the silver layer 4 of the functional region 11 is thickened to approximately 1 μm. This embodiment further thins the nickel barrier layer 3 and the silver layer 4, further reducing the impact on thermoelectric performance and the consumption of precious metal silver while ensuring reliable barrier properties. The remaining steps are the same as in Embodiment 1.

[0057] The implementation principle of this embodiment is as follows: by further reducing the total thickness of the nickel barrier layer 3 to about 0.6 μm and the silver layer 4 to about 1 μm, under the synergistic effect of the double-layer gradient structure and the bottom copper leveling, it is still possible to ensure that the nickel barrier layer 3 has no penetrating pinholes and reliably blocks the interdiffusion of copper and silver. At the same time, the influence of thermal resistance and contact resistance is further reduced, and the consumption of precious metal silver is reduced to the minimum. According to the calculation, the silver consumption is only about 1 / 3 of that of the copper / silver direct structure (Comparative Example 1).

[0058] Comparative Example 1

[0059] A direct copper / silver structure process was adopted, meaning that after acid copper plating in step S4, nickel plating was skipped in step S5, and silver was directly plated in functional area 11 (silver layer 4 thickness 3μm). All other conditions were the same as in Example 1. In this comparative example, copper layer 2 and silver layer 4 were in direct contact, while copper layer 2 was exposed in non-functional area 12.

[0060] Comparative Example 2

[0061] A four-layer structure of copper / nickel / copper / silver is adopted, that is, after nickel plating in step S5, a layer of copper is plated, and then silver is plated (silver layer 4 is 3μm thick), and the remaining conditions are the same as in Example 1. The non-functional area 12 of this structure is still in the state of exposed copper layer 2.

[0062] Comparative Example 3

[0063] A single-layer nickel structure was adopted: the nickel barrier layer 3 was a single nickel sulfamate system constant current electroplating (non-double-layer gradient structure) with a thickness of 1.1 μm; and after acid copper plating in step S4, the roughness was not controlled to Ra≤0.1 μm (Ra is about 0.25 μm), while the other conditions were the same as in Example 1. This comparative example is used to verify the key role of the synergistic design of bottom copper leveling and double-layer gradient nickel.

[0064] Performance test results

[0065] The lead frames prepared in the above embodiments and comparative examples were subjected to the following performance tests: (1) high temperature and high humidity aging test (85℃ / 85% RH, 600h); (2) nickel layer pinhole rate (filter paper drop test); (3) bonding strength (wire pull force, tested according to MIL-STD-883 method 2011); (4) contact resistance (tested using the four-probe method); (5) silver consumption cost (100% of the comparative example 1). The test results are shown in the table below:

[0066] Example 1 2μm none Extremely low Extremely low excellent Slight increase is acceptable ≈67% Example 2 2μm none Extremely low Almost zero excellent Slight increase is acceptable ≈67% Example 3 1μm none Low Low good Slight increase is acceptable ≈33% Comparative Example 1 3μm There is obvious copper buildup high — benchmark Low 100% Comparative Example 2 3μm Occasionally middle Low good middle 100%+ Comparative Example 3 2μm Occasionally middle High (with pinhole penetration) good higher ≈67%

[0067] The test results show that after 600 hours of high temperature and high humidity aging, there was no copper accumulation or peeling in the non-functional area 12 of Examples 1-3 of the present invention. The void ratio of the copper / nickel interface in the functional area 11 was extremely low, indicating that the nickel barrier layer 3 effectively blocked the interdiffusion of copper and silver. Furthermore, the exposure of the nickel barrier layer 3 in the non-functional area 12 effectively eliminated the oxidation and corrosion problem of the copper layer 2.

[0068] Comparative Example 1 (copper / silver direct structure) showed significant copper accumulation and high interface void ratio after aging, verifying the deficiencies of the prior art. Comparative Example 3 (single-layer non-gradient nickel, bottom copper not leveled to Ra≤0.1μm) had high contact resistance due to the presence of pinholes in the thin nickel layer, resulting in insufficient blocking. This proves that the synergistic design of leveling the bottom copper to Ra≤0.1μm and the double-layer gradient dense nickel blocking layer 3 is the key to achieving reliable blocking without pinholes in the thin nickel layer. The synergy between the two produces technical effects that are difficult to predict in the prior art.

[0069] Furthermore, since the copper layer 2 does not directly contact the silver layer 4 and there is no need to thicken the silver layer 4 to resist diffusion, the silver layer 4 can be thinned to 1-2 μm, and the silver consumption cost is significantly reduced compared to Comparative Example 1. The silver consumption in Example 3 is only about 1 / 3 of that in Comparative Example 1.

[0070] The specific embodiments described above do not constitute a limitation on the scope of protection of this application. Any other corresponding changes and modifications made based on the technical concept of this application should be included within the scope of protection of this application.

Claims

1. A nickel-silver plating process for integrated circuit lead frames, characterized in that, Using the lead frame substrate (1) as the part to be processed, the following steps are performed in sequence: S1. Degreasing: The lead frame substrate (1) is degreased to remove surface grease contaminants, and then washed with water. S2, Activation: Electrolytic activation is performed on the lead frame substrate (1) to remove the surface oxide layer, followed by water washing; S3, Pre-plating alkaline copper: Electroplating an alkaline copper layer (21) on the surface of the lead frame substrate (1) to fill the pits on the substrate surface; S4, Acid copper plating: An acid copper layer (22) is electroplated on the surface of the alkaline copper layer (21). The alkaline copper layer (21) and the acid copper layer (22) together constitute a copper layer (2). The surface roughness Ra of the copper layer (2) is controlled to be ≤0.1μm. S5. Nickel plating: A dense nickel barrier layer (3) is formed by electroplating on the surface of the copper layer (2). The nickel barrier layer (3) is a double-layer gradient structure, including a dense bottom nickel layer (31) disposed near the copper layer (2) and a low-stress surface layer nickel layer (32) located on the dense bottom nickel layer (31). S6, Silver plating: First, a thin silver layer is pre-plated on the entire surface of the nickel barrier layer (3); then, selective thickening of silver plating is performed on the functional area (11); subsequently, the pre-plated thin silver layer on the non-functional area (12) is removed using a stripping solution, so that the non-functional area (12) exposes the nickel barrier layer (3); finally, a lead frame is obtained in which the functional area (11) is a layered structure of copper layer (2) / nickel barrier layer (3) / silver layer (4) and the non-functional area (12) is a layered structure of copper layer (2) / nickel barrier layer (3); S7. Post-processing: The lead frame is washed with water, treated with protective water, and dried in sequence.

2. The nickel-silver plating process according to claim 1, characterized in that, In step S6, the thickness of the pre-plated thin silver layer on the entire surface is 0.05 to 0.3 μm; the thickness of the thickened silver plating of the silver layer (4) in the functional area (11) is 1 to 10 μm.

3. The nickel-silver plating process according to claim 1, characterized in that, In step S1, the degreasing treatment is a cyclic treatment that alternates between cathodic degreasing and anodic degreasing, with no less than 2 cycles and a degreasing current density of 2 to 5 ASD; in step S2, the electrolytic activation uses a dilute sulfuric acid solution with a mass concentration of 3 to 8% and an electrolytic activation time of 20 to 60 seconds.

4. The nickel-silver plating process according to claim 1, characterized in that, In step S3, the alkaline copper layer (21) is formed by electroplating with an alkaline copper plating solution with potassium cyanide and cuprous cyanide as the main components. The anode is a copper block and the cathode is the lead frame substrate (1). The thickness of the alkaline copper layer (21) is 5 to 30 μm.

5. The nickel plating and silver plating process according to claim 1, characterized in that, In step S4, the acid copper layer (22) is formed by electroplating with an acidic copper plating solution containing 180-250 g / L copper sulfate, 50-80 g / L sulfuric acid and copper plating additives; the copper plating additives contain bis(3-sulfopropyl) disulfide, N,N-dimethylimidazoline thione, polyethylene glycol and chloride ions, wherein the content of bis(3-sulfopropyl) disulfide is 5-20 mg / L, the content of N,N-dimethylimidazoline thione is 1-10 mg / L, the content of polyethylene glycol is 100-500 mg / L, and the content of chloride ions is 30-80 mg / L; the thickness of the acid copper layer (22) is 40-150 μm.

6. The nickel-silver plating process according to claim 1, characterized in that, In step S5, the dense bottom nickel (31) is formed by electroplating with a nickel sulfate system plating solution containing 20-50 g / L of nickel chloride at a low current density of 1-3 ASD, so that the dense bottom nickel (31) forms a fine-grained structure with small grains and dense grain boundaries, thereby inhibiting the mutual diffusion of copper and silver atoms along the grain boundaries; the low-stress surface layer nickel (32) is formed by electroplating with a nickel aminosulfonate system plating solution.

7. The nickel-silver plating process according to claim 6, characterized in that, The formulation of the nickel sulfamate plating solution is as follows: nickel ions provided by nickel sulfamate 60-100 g / L, nickel chloride 5-15 g / L, boric acid 35-45 g / L; electroplating temperature 50-60℃, current density 3-15 ASD; anode is nickel beads, and cathode is the lead frame substrate (1).

8. The nickel-silver plating process according to claim 7, characterized in that, In step S5, the low-stress nickel surface layer (32) is electroplated using a stepped current process: in the initial plating stage, a dense fine grain nucleus layer is formed with a current density of 1 to 3 ASD; in the growth stage, the grain orientation is controlled with a current density of 3 to 8 ASD; and in the sealing stage, the micropores are sealed with a current density of 0.5 to 1.5 ASD to eliminate pinhole defects.

9. The nickel-silver plating process according to claim 7, characterized in that, In step S5, the low-stress surface layer nickel (32) is electroplated using a pulse electroplating process. The pulse electroplating includes a forward pulse and a reverse pulse. The reverse pulse is used to dissolve the sharp protrusions on the surface of the nickel barrier layer (3) to improve the surface smoothness of the nickel barrier layer (3) and reduce the internal stress of the plating layer.

10. The nickel plating and silvering process according to claim 1, characterized in that, In step S6, the selective thickening silver plating uses a mask or selective silver plating fixture to shield the non-functional area (12), exposing the functional area (11) to the plating solution for selective thickening silver plating; the whole-surface pre-plating thin silver and the selective thickening silver plating of the functional area (11) are both electroplated using a mixed plating solution containing silver nitrate, potassium cyanide, potassium hydroxide, tartrate and silver plating intermediates, with the anode being a silver plate and the cathode being a lead frame; the stripping solution is a silver stripping solution that selectively dissolves silver but does not corrode the nickel barrier layer (3), and has a silver dissolution rate of not less than 0.5 μm / min and a corrosion rate of not more than 2 nm / min on the nickel barrier layer (3); in step S7, the protective water treatment uses an anti-oxidation protective solution to treat the surface of the lead frame to prevent the nickel barrier layer (3) of the non-functional area (12) from oxidizing during storage and subsequent packaging.

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