A surface treatment method for an integrated circuit lead frame

Abstract

The application discloses a surface treatment method of an integrated circuit lead frame, which comprises the following steps: depositing a nickel-phosphorus alloy layer on a substrate surface by electroless plating; performing electrochemical dealloying treatment on the surface of the nickel-phosphorus alloy layer, taking the lead frame as an anode and placing it in an electrolyte containing sulfuric acid, so that nickel atoms on the surface are dissolved and phosphorus atoms are retained, and a phosphorus-rich nanoporous framework structure is formed on the surface of the nickel-phosphorus alloy layer; and electroplating a silver layer on the surface of the nanoporous framework structure, so that silver is deposited in the pores to form silver anchor nails, and a silver cover layer is formed on the surface. According to the method, the silver anchor nails and the phosphorus-rich framework form three-dimensional mechanical interlocking through the nanoporous framework structure, the interface bonding strength between the silver layer and the nickel-phosphorus alloy layer is enhanced, the amorphous structure at the bottom of the nickel-phosphorus alloy layer, which is not subjected to the dealloying treatment, eliminates grain boundary diffusion channels, and the diffusion of copper atoms to the silver layer is prevented, and the nanoporous framework structure provides anchoring sites for the silver anchor nails, so that the silver layer remains stable in high-temperature reflow soldering and temperature cycling.

Description

Technical Field

[0001] This invention relates to the field of semiconductor packaging technology, and more specifically to a surface treatment method for integrated circuit lead frames. Background Technology

[0002] Leadframes are one of the key structural components in integrated circuit packaging. With the continuous miniaturization and high performance trends of electronic devices, their performance has a crucial impact on the overall performance and reliability of integrated circuits. As semiconductor technology rapidly advances, higher requirements are placed on the conductivity, thermal conductivity, and bonding performance of leadframes with chip soldering materials and bonding wires. Copper and copper alloys, due to their excellent conductivity, thermal conductivity, and processing properties, have become the mainstream substrate for leadframes and are widely used in various integrated circuit packages.

[0003] Currently, to improve the performance of copper and copper alloy leadframes, surface treatment solutions mainly include silver plating and nickel-palladium-gold plating. Among these, silver plating is widely used due to silver's excellent conductivity, good solderability, and low cost. Traditional silver plating processes for leadframes typically involve first plating a nickel barrier layer on a copper substrate, followed by silver plating on the nickel layer. The nickel barrier layer prevents copper atoms from diffusing into the silver layer, thus avoiding discoloration and failure of the silver surface due to copper diffusion. However, traditional electroplated nickel layers are usually crystalline, and the grain boundaries provide rapid channels for copper atom diffusion. Under long-term high-temperature service conditions, copper atoms can diffuse along the nickel grain boundaries to the silver surface, leading to discoloration and decreased solderability. In addition, some solutions propose acid etching to roughen the nickel surface to increase mechanical adhesion to the silver layer. However, the roughened structure formed by acid etching is typically on the micrometer scale, making it difficult to achieve nanometer-level fine anchoring. Furthermore, the roughening process is difficult to control precisely, potentially leading to excessive corrosion of the nickel layer and loss of its barrier function. Another approach uses an amorphous nickel-phosphorus alloy layer to replace the crystalline nickel layer in order to eliminate grain boundary diffusion channels. However, the surface of the amorphous nickel-phosphorus layer is smooth and dense, and its bonding with the silver layer relies solely on physical adsorption at the planar interface. As a result, the bonding force is not as strong as that of the coarsened crystalline nickel layer.

[0004] In summary, the core contradiction facing existing technologies lies in the following: while roughening the crystalline nickel layer can improve its adhesion to the silver layer, the presence of grain boundaries provides a channel for copper atom diffusion; although the amorphous nickel-phosphorus alloy layer eliminates grain boundary diffusion channels, its smooth surface results in insufficient adhesion to the silver layer. Existing micron-level acid etching roughening methods cannot achieve nanoscale fine anchoring structures on the surface of amorphous nickel-phosphorus alloy layers, and the micron-level roughening pits may become the starting point for crack initiation during thermal cycling due to the stress concentration effect at the bottom of the pits, accelerating silver layer peeling. In the selective silver plating process of lead frames, after the silver layer in the non-functional areas is removed, the exposed nickel layer surface is prone to oxidation and may become a path for the electrochemical migration of silver ions under high temperature and high humidity environments, leading to short circuits between adjacent pins. Therefore, there is an urgent need for a surface treatment method that can construct a nanoscale anchoring structure on the surface of amorphous nickel-phosphorus alloy layers while maintaining the diffusion barrier function of the dense amorphous bottom layer. Summary of the Invention

[0005] The purpose of this invention is to provide a surface treatment method for integrated circuit lead frames. By constructing a nanoporous structure on the surface of an amorphous nickel-phosphorus alloy layer, and utilizing the anchoring effect of silver in the nanopores, a high-strength bond is achieved between the silver layer and the nickel-phosphorus alloy layer, while maintaining the dense amorphous structure at the bottom of the nickel-phosphorus alloy layer as an effective diffusion barrier layer.

[0006] The surface treatment method for an integrated circuit lead frame provided in this application adopts the following technical solution:

[0007] A surface treatment method for an integrated circuit lead frame includes the following steps:

[0008] A nickel-phosphorus alloy layer is deposited on the surface of a copper or copper alloy substrate by electroless plating. The phosphorus content in the nickel-phosphorus alloy layer is 10-14 wt%, and the nickel-phosphorus alloy layer has an amorphous structure.

[0009] The surface of the nickel-phosphorus alloy layer is subjected to electrochemical dealloying treatment. The lead frame is placed as the anode in an electrolyte containing 0.5-2 mol / L sulfuric acid, and an anode potential of 0.3-0.8V is applied. With a saturated Ag / AgCl electrode as a reference, the nickel atoms on the surface of the nickel-phosphorus alloy layer are dissolved while the phosphorus atoms are retained, forming a phosphorus-rich nanoporous framework structure on the surface of the nickel-phosphorus alloy layer. The pore size of the nanoporous framework structure is 5-50 nm and the pore depth is 10-100 nm. The bottom of the nickel-phosphorus alloy layer retains the amorphous structure that has not been dealloyed.

[0010] A silver layer is electroplated on the surface having the nanoporous framework structure, wherein silver is deposited in the channels of the nanoporous framework structure to form silver anchors, and a silver coating layer is formed on the surface of the nanoporous framework structure.

[0011] Compared with the prior art, the beneficial effects of the technical solution provided by the present invention include:

[0012] 1. A nanoporous framework structure is constructed in situ on the surface of an amorphous nickel-phosphorus alloy layer by electrochemical dealloying, and silver is deposited in the nanopores to form silver anchors. This transforms the physical adsorption bond between the silver layer and the nickel-phosphorus alloy layer from a traditional planar interface to a three-dimensional nanoscale mechanical interlocking bond, thereby improving the peel strength of the silver layer in the 90° peel test compared to the traditional planar interface bonded silver plating layer. At the same time, the gradient channel structure with a wider top and narrower bottom formed by the segmented potential mode makes the silver anchors wedge-shaped. When the silver layer is subjected to peel force, the wide top of the wedge-shaped silver anchors is locked by the pore walls of the phosphorus-rich framework, further enhancing the peel resistance of the interface.

[0013] 2. The bottom of the nickel-phosphorus alloy layer retains an amorphous structure that has not been dealloyed, serving as a diffusion barrier layer. This eliminates the rapid diffusion channels provided by grain boundaries for copper atoms in the crystalline nickel layer. Under high-temperature aging test conditions of 175℃ / 1000 hours, it can prevent copper atoms from diffusing from the copper or copper alloy substrate toward the silver layer. Combined with the phosphorus oxide passivation layer generated on the phosphorus-rich skeleton surface by low-temperature oxidation treatment, a nanoscale diffusion barrier is added between the silver anchor and the phosphorus-rich skeleton. This prevents residual nickel atoms in the skeleton from diffusing through the interface to the silver layer during high-temperature processes such as reflow soldering, thereby maintaining the purity of the silver layer surface and its welding performance.

[0014] 3. By performing atmospheric passivation treatment on the exposed nanoporous framework structure of the non-functional area, phosphine and water vapor are used to react with the phosphorus-rich framework surface to generate a phosphate-phosphorus oxide composite passivation film in situ, which blocks the opening of the nanopores and prevents external water vapor and corrosive media from penetrating along the nanopores to the bottom of the nickel-phosphorus alloy layer and the copper or copper alloy substrate. At the same time, it inhibits the electrochemical migration of silver ions from the functional area to the surface of the non-functional area under high temperature and high humidity environment, reducing the risk of short circuit failure between adjacent pins due to the growth of silver dendrites. Attached Figure Description

[0015] Figure 1 This is a schematic diagram of the overall cross-sectional structure of the integrated circuit lead frame provided in this application embodiment after surface treatment;

[0016] Figure 2 yes Figure 1 A locally magnified cross-sectional schematic diagram of the nanoporous framework structure and the dense amorphous structure formed by electrochemical dealloying of the nickel-phosphorus alloy layer.

[0017] Figure 3 yes Figure 1 A partially enlarged cross-sectional schematic diagram of a silver-phosphorus alloy transition layer formed at the interface between the silver anchor and the phosphorus-rich framework after the addition of sodium hypophosphite to the silver plating solution for silver plating.

[0018] Figure 4 yes Figure 1 A schematic diagram of the overall structure after selective silver plating and passivation of the functional and non-functional areas of the middle lead frame.

[0019] Figure 5 This is a schematic diagram of in-situ electrochemical impedance spectroscopy monitoring during the electrochemical dealloying process provided in the embodiments of this application, showing the curve of impedance modulus changing with dealloying time.

[0020] Explanation of reference numerals in the attached figures: 1. Copper or copper alloy substrate; 2. Nickel-phosphorus alloy layer; 21. Dense amorphous structure; 22. Nanoporous framework structure; 221. Open channel; 222. Deep channel; 223. Phosphorus-rich framework; 3. Silver layer; 31. Silver anchor; 32. Silver capping layer; 33. Silver-phosphorus alloy transition layer; 4. Phosphorus oxide passivation layer; 5. Composite passivation film; 6. Functional area; 61. Chip pad area; 62. Wire bonding area; 7. Non-functional area. Detailed Implementation

[0021] 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, and are not limited thereto. Other embodiments obtained by those skilled in the art based on the embodiments of the present invention without creative effort are all within the protection scope of the present invention.

[0022] This application mainly employs a method of constructing a nanoporous structure on a copper or copper alloy substrate and then plating it with silver, which achieves the effect of improving the interfacial bonding force between the silver layer and the nickel-phosphorus alloy layer and the encapsulation reliability. The technical solution of this application will be further described in detail below.

[0023] Please refer to Figures 1 to 4 The surface treatment method for integrated circuit lead frames provided in this application includes the following steps: substrate pretreatment, electroless plating of a nickel-phosphorus alloy layer 2 on the surface of a copper or copper alloy substrate 1, electrochemical dealloying treatment on the surface of the nickel-phosphorus alloy layer 2, and electroplating of a silver layer 3 on the surface of a nanoporous framework structure 22. In this method, the copper or copper alloy substrate 1 is first pretreated, then a nickel-phosphorus alloy layer 2 is deposited on the surface of the copper or copper alloy substrate 1 by electroless plating, then an electrochemical dealloying treatment is performed on the surface of the nickel-phosphorus alloy layer 2 to form a nanoporous framework structure 22, and finally a silver layer 3 is electroplated on the surface of the structure. This treatment method improves the interfacial bonding force between the silver layer 3 and the nickel-phosphorus alloy layer 2, and utilizes the dense amorphous structure 21 at the bottom of the nickel-phosphorus alloy layer 2 to prevent copper atoms from diffusing into the silver layer 3.

[0024] Substrate pretreatment steps:

[0025] Before electroless plating of the nickel-phosphorus alloy layer 2, the copper or copper alloy substrate 1 needs to be pretreated to ensure the adhesion between the nickel-phosphorus alloy layer 2 and the copper or copper alloy substrate 1. The pretreatment steps include: (1) Alkaline degreasing: Immerse the copper or copper alloy substrate 1 in an alkaline cleaning solution containing 30-50 g / L sodium hydroxide and 20-30 g / L sodium carbonate at 50-60°C for 3-5 minutes to remove grease and organic contaminants from the surface of the copper or copper alloy substrate 1, and then rinse it with deionized water; (2) Pickling activation: Immerse the degreased copper or copper alloy substrate 1 in a pickling solution containing 50-100 g / L sulfuric acid at room temperature for 30-60 seconds to remove the oxide film from the surface of the copper or copper alloy substrate 1, exposing a fresh copper surface, and then rinse it with deionized water; (3) Palladium activation: Immerse the pickled copper or copper alloy substrate 1 in an activation solution containing 0.1-0.5 g / L palladium chloride and 5-10 ml / L hydrochloric acid at room temperature for 30-60 seconds to adsorb palladium catalytic nuclei on the surface of the copper or copper alloy substrate 1, providing catalytic active sites for subsequent electroless plating, and then rinse it with deionized water.

[0026] Please refer to Figure 1 Specifically, the step of electroless plating the nickel-phosphorus alloy layer 2 involves a copper or copper alloy substrate 1 and an electroless plating solution. The electroless plating solution formula is as follows: nickel sulfate 20-30 g / L, sodium hypophosphite 20-30 g / L, lactic acid 15-25 ml / L (as a complexing agent), sodium acetate 10-15 g / L (as a buffer), and thiourea 0.5-1 mg / L (as a stabilizer). The copper or copper alloy substrate 1 typically has good electrical conductivity, thermal conductivity, and processing performance, and its shape is determined according to the actual lead frame design. The temperature of the electroless plating solution is controlled at 85-90℃, the pH value is controlled at 4.5-5.0, the plating time is 15-45 minutes, and the target thickness of the nickel-phosphorus alloy layer 2 is 1-5 μm. Through this electroless plating process, an amorphous nickel-phosphorus alloy layer 2 with a phosphorus content of 10-14 wt% can be deposited on the surface of the copper or copper alloy substrate 1. Maintaining the phosphorus content within the range of 10-14 wt% is crucial: when the phosphorus content is below 10 wt%, the nickel-phosphorus alloy layer 2 may exhibit a microcrystalline or nanocrystalline structure rather than a completely amorphous structure. The presence of grain boundaries provides channels for copper atom diffusion, weakening the diffusion barrier function. When the phosphorus content is above 14 wt%, the excessive phosphorus content in the nickel-phosphorus alloy layer 2 leads to a decrease in the hardness and corrosion resistance of the alloy layer. Furthermore, during the subsequent electrochemical dealloying process, the reduced proportion of soluble nickel atoms results in insufficient porosity in the formed nanoporous structure, affecting the anchoring effect of the silver anchor. By adjusting the molar ratio of sodium hypophosphite to nickel sulfate in the electroless plating solution (controlled between 1.0 and 1.5) and the pH value of the plating solution, the phosphorus content in the nickel-phosphorus alloy layer 2 can be precisely controlled.

[0027] In the aforementioned electroless plating step, the nickel-phosphorus alloy layer 2 exhibits an amorphous structure, lacking grain boundaries, thus eliminating the pathway for rapid diffusion of copper atoms along the grain boundaries. Compared to traditional crystalline electroplated nickel layers, the amorphous nickel-phosphorus alloy layer 2 can more effectively prevent copper atoms from diffusing from the copper or copper alloy substrate 1 towards the silver layer 3 under long-term high-temperature service conditions, thereby avoiding discoloration and failure of the silver layer 3 surface due to copper diffusion and ensuring the long-term solderability and conductivity of the lead frame.

[0028] Please refer to Figure 1 and Figure 2The electrochemical dealloying step requires placing the lead frame as the anode in an electrolyte containing sulfuric acid and applying an anodic potential. The electrolyte contains 0.5-2 mol / L sulfuric acid and 0.01-0.1 mol / L nickel sulfate, with the electrolyte temperature controlled at 20-30℃. The addition of nickel sulfate maintains the nickel ion concentration in the solution, preventing excessively rapid dealloying and uncontrolled pore structure due to low nickel ion concentration. According to the Nernst equation, the anodic dissolution potential is inversely proportional to the logarithm of the nickel ion concentration in the solution. When the nickel ion concentration is too low, the overpotential corresponding to the same applied anodic potential increases, leading to an excessively rapid dissolution rate of nickel atoms and resulting in a coarse and uneven pore structure. By adding 0.01-0.1 mol / L nickel sulfate, the nickel ion concentration in the solution can be maintained at a suitable level, allowing the dealloying process to proceed at a controllable rate. The application of an anodic potential causes nickel atoms on the surface of the nickel-phosphorus alloy layer 2 to preferentially dissolve while phosphorus atoms remain, forming a phosphorus-rich nanoporous framework structure 22 on the surface of the nickel-phosphorus alloy layer 2. After electrochemical dealloying, the phosphorus content of the phosphorus-rich framework 223 in the nanoporous framework structure 22 increases to 30-60 wt% (the original phosphorus content of the nickel-phosphorus alloy layer is 10-14 wt%), while the bottom of the nickel-phosphorus alloy layer 2 retains the undealloyed amorphous structure 21. Specifically, the electrochemical dealloying treatment can adopt a segmented potential variation mode. In the first stage, an anodic potential of 0.6-0.8V (with a saturated Ag / AgCl electrode as a reference) is applied for 3-10 seconds, forming open channels 221 with a pore size of 30-50 nm on the surface of the nickel-phosphorus alloy layer 2. At a higher anodic potential, the dissolution rate of nickel atoms is faster, and the number of nickel atoms dissolved per unit time is greater, resulting in larger pore sizes. In the second stage, an anodic potential of 0.3-0.5V (using a saturated Ag / AgCl electrode as a reference) is applied for 5-20 seconds to continue etching at the bottom of the open channel 221, forming deep channels 222 with a pore size of 5-20 nm. As the anodic potential decreases, the dissolution rate of nickel atoms decreases, and the pore size of the newly formed channels decreases accordingly. This ultimately forms a gradient nanoporous structure with pore size decreasing from the surface to the depth, resulting in the subsequently deposited silver anchors 31 exhibiting a wedge-shaped morphology with a wide surface and narrow root. When subjected to pull-out force, these wedge-shaped silver anchors 31, due to the constraint of the thicker top portion by the surrounding phosphorus-rich framework 223, produce a locking effect similar to an expansion bolt, significantly improving the peel strength of the silver layer 3.

[0029] In the aforementioned segmented potential-variable dealloying process, the higher anodic potential in the first stage drives the rapid dissolution of nickel atoms, forming large-diameter open channels 221 on the outermost layer of the nickel-phosphorus alloy layer 2, providing a spacious entrance channel for the subsequent entry of silver ions. The lower anodic potential in the second stage reduces the dissolution rate, forming small-diameter deep channels 222 at the bottom of the open channels 221, thus constituting a gradient channel structure that is wider at the top and narrower at the bottom. The technical effect of this gradient structure is that, on the one hand, the large pore size of the open channels 221 facilitates the smooth diffusion of silver ions into the depth of the channels during subsequent electroplating; on the other hand, the small pore size of the deep channels 222 narrows the root of the silver anchors 31, forming a wedge-shaped anchoring structure. When the silver layer 3 is subjected to peeling force, the wide top of the wedge-shaped silver anchors 31 is locked by the hole wall of the phosphorus-rich skeleton 223, producing a significant mechanical interlocking effect, thereby improving the interfacial bonding strength between the silver layer 3 and the nickel-phosphorus alloy layer 2.

[0030] Please refer to Figure 2 After electrochemical dealloying and before electroplating the silver layer 3, the nanoporous framework structure 22 can be subjected to low-temperature oxidation treatment. In an atmosphere containing 5%-21% oxygen by volume, the structure is treated at 150-250°C for 1-10 minutes, resulting in a 1-5 nm thick phosphorus oxide passivation layer 4 on the surface of the phosphorus-rich framework 223 within the nanoporous framework structure 22. The phosphorus-rich framework 223 in the nanoporous framework structure 22 contains a large amount of phosphorus, which readily reacts with oxygen in the 150-250°C temperature range to form phosphorus oxides (mainly P2O5 and phosphite compounds). These phosphorus oxides form a dense film on the surface of the phosphorus-rich framework 223, only 1-5 nm thick, which does not block the nanopores but prevents residual nickel atoms in the framework from diffusing towards the silver layer 3 during subsequent high-temperature processes (such as reflow soldering). Furthermore, the surface of phosphorus oxide possesses certain hydrophilicity and chemical activity, which is beneficial for the uniform nucleation and deposition of silver ions on the inner wall of the channels during subsequent silver electroplating, thereby improving the filling quality of the silver anchor 31. The oxygen volume fraction is controlled at 5%-21% (i.e., from oxygen-deficient atmosphere to air atmosphere), and the temperature is controlled at 150-250℃. This ensures both the generation rate and quality of phosphorus oxide while avoiding excessively high temperatures that could cause the amorphous nickel-phosphorus alloy layer 2 to undergo a crystallization transformation (the crystallization temperature of nickel-phosphorus alloys is typically above 300℃). The thickness of the phosphorus oxide passivation layer 4 under different temperature-time combinations is as follows: at 150℃ / 10 minutes, the passivation layer thickness is approximately 1-2 nm; at 200℃ / 5 minutes, the passivation layer thickness is approximately 2-3 nm; at 250℃ / 1 minute, the passivation layer thickness is approximately 1-2 nm; and at 250℃ / 5 minutes, the passivation layer thickness is approximately 3-5 nm. The thickness of the phosphorus oxide passivation layer 4 can be characterized by X-ray photoelectron spectroscopy (XPS) depth profiling or transmission electron microscopy (TEM) cross-sectional analysis.

[0031] The working process and technical effects of the phosphorus oxide passivation layer 4 are as follows: During the low-temperature oxidation process, phosphorus atoms on the surface of the phosphorus-rich framework 223 react with oxygen to generate a phosphorus oxide film mainly composed of P2O5 in situ. The phosphorus oxide passivation layer 4 has a dual function: First, as a diffusion barrier layer, the phosphorus oxide passivation layer 4 forms a dense barrier between the phosphorus-rich framework 223 and the subsequently deposited silver anchors 31, preventing residual nickel atoms in the phosphorus-rich framework 223 from diffusing through the interface towards the silver layer 3 during the high-temperature process, thereby ensuring the purity and surface quality of the silver layer 3; Second, the phosphorus oxide passivation layer 4 improves the wettability of the surface of the nanoporous framework structure 22, enabling silver ions to nucleate and deposit more uniformly on the inner wall of the pores during the subsequent electroplating process, avoiding local over-deposition or under-deposition, and improving the filling density and structural uniformity of the silver anchors 31.

[0032] Please refer to Figure 1 and Figure 3The silver plating step 3 comprises two stages. The silver plating solution uses a cyanide-free silver plating system with the following formula: silver nitrate 20-40 g / L, succinimidyl 50-80 g / L (as a complexing agent), potassium acetate 30-50 g / L (as a conductive salt), and brightener 0.5-2 ml / L. The plating solution temperature is controlled at 20-30℃, and the pH value is controlled at 8-9. In the first stage, silver plating is performed at a current density of 0.1-0.5 A / dm², allowing silver ions to diffuse into the pores of the nanoporous framework structure 22 and deposit to form silver anchors 31. This stage uses a relatively low current density to allow silver ions sufficient time to diffuse deep into the nanopores and nucleate and deposit on the inner walls of the pores. If a high current density is used from the beginning, silver ions will preferentially deposit rapidly on the surface of the nanoporous framework structure 22, quickly blocking the pore openings, preventing the pores from being effectively filled with silver, and reducing the anchoring effect of the silver anchors 31. In the second stage, silver plating is performed at a current density of 1-5 A / dm² to form a silver coating layer 32 on the surface of the nanoporous framework structure 22. Increasing the current density after the formation of the silver anchors 31 aims to rapidly deposit a continuous silver coating layer 32 on the surface of the nanoporous framework structure 22, thereby improving production efficiency. Furthermore, the first stage can also employ a pulsed reverse electroplating mode, including alternating forward deposition pulses and reverse dissolution pulses. The current density of the forward deposition pulse is 0.1-0.5 A / dm², with a pulse width of 50-200 ms; the current density of the reverse dissolution pulse is 0.05-0.2 A / dm², with a pulse width of 5-20 ms. The reverse dissolution pulse is used to dissolve excessive silver deposits at the pore openings due to edge effects, keeping the pore openings open during deposition and simultaneously releasing deposition stress within the silver anchors 31. During electrodeposition within nanopores, the current density at the pore opening is higher than that at the depth of the pore (edge ​​effect). This results in a faster silver deposition rate at the pore opening compared to the depth, easily creating a "bottleneck" or even completely blocking the pore opening. This leads to voids within the pore, reducing the effective anchoring area and strength of the silver anchor 31. By periodically applying reverse dissolution pulses, excess silver deposits at the pore opening can be preferentially dissolved, keeping the pore opening open throughout the deposition process. This allows silver ions to continuously diffuse into the depth of the pore and deposit, ultimately forming a dense, void-free silver anchor 31.

[0033] In the aforementioned pulsed reverse electroplating process, the forward deposition pulse causes silver ions to be reduced and deposited on the inner wall of the nanopores at the cathode, while the reverse dissolution pulse causes the silver preferentially deposited at the pore opening to be dissolved and removed by the anode. Because the silver deposit at the pore opening has a larger area exposed to the electrolyte and a more concentrated electric field, its dissolution rate is higher during the reverse pulse than that of the silver deposit deep within the pore, thus selectively removing the "bottleneck" deposit at the pore opening. Furthermore, the reverse dissolution pulse can release the internal stress accumulated in the silver anchor 31 during deposition due to crystallization within the confined space, reducing the risk of microcracks in the silver anchor 31 due to residual stress release during subsequent thermal cycling, thereby improving the structural integrity and long-term reliability of the silver anchor 31.

[0034] Please refer to Figure 3When the aforementioned low-temperature oxidation treatment step is not implemented, sodium hypophosphite with a concentration of 0.001-0.01 mol / L can be added to the silver plating solution during the first stage of silver plating. It should be noted that the low-temperature oxidation treatment step (generating the phosphorus oxide passivation layer 4) and the step of adding sodium hypophosphite to co-deposit the silver-phosphorus alloy transition layer 33 are two alternative solutions and should not be implemented simultaneously. This is because the phosphorus oxide passivation layer 4 will form a dense oxide barrier on the surface of the phosphorus-rich framework 223, hindering the formation of a metallurgical bond between the phosphorus atoms generated by the reduction of sodium hypophosphite and the phosphorus-rich framework 223, thus preventing the effective formation of the silver-phosphorus alloy transition layer 33. Therefore, in actual processes, one solution should be selected based on specific requirements. While silver ions are deposited within the pores of the nanoporous framework structure 22 to form silver anchors 31, sodium hypophosphite, under cathodic reduction, is co-deposited in situ at the interface between the silver anchors 31 and the phosphorus-rich framework 223 to form the silver-phosphorus alloy transition layer 33, with a thickness of 1-3 nm. The presence of the silver-phosphorus alloy transition layer 33 can be characterized by transmission electron microscopy (TEM) combined with energy-dispersive X-ray spectroscopy (EDS) line scanning: at the interface between the silver anchor 31 and the phosphorus-rich skeleton 223, EDS line scanning can observe an overlapping transition region of silver and phosphorus signals, and the width of this transition region corresponds to the thickness of the silver-phosphorus alloy transition layer 33. Sodium hypophosphite has good chemical compatibility in cyanide-free alkaline silver plating solution. Under pH 8-9 conditions, hypophosphite ions (H2PO2⁻) are stable and do not react adversely with silver complexes. The relationship between the thickness of the silver-phosphorus alloy transition layer 33 and the concentration of sodium hypophosphite is as follows: when the concentration of sodium hypophosphite is 0.001 mol / L, the transition layer thickness is approximately 1 nm; when the concentration is 0.005 mol / L, the transition layer thickness is approximately 2 nm; and when the concentration is 0.01 mol / L, the transition layer thickness is approximately 3 nm. During conventional silver plating, the interface between the silver anchor 31 and the phosphorus-rich skeleton 223 is a physical contact interface between pure silver and a phosphorus-rich amorphous alloy. The bonding force between the two mainly comes from mechanical interlocking and van der Waals forces. After undergoing multiple thermal cycles (such as temperature cycling tests from -65℃ to 150℃), due to the thermal expansion coefficient of silver (approximately 19.7 × 10⁻⁻⁻⁶), the bonding force between the two components is reduced. 6 / K) and phosphorus-rich skeleton 223 (thermal expansion coefficient approximately 12-13×10⁻) 6The difference in thermal expansion coefficients between the silver anchor 31 and the skeleton (H2PO2⁻) leads to repeated thermal stress at the interface, potentially causing fatigue loosening of the interface and ultimately reducing the peel strength of the silver layer 3. By adding a small amount of sodium hypophosphite to the silver plating bath, under cathodic reduction conditions, hypophosphite ions (H2PO2⁻) can be reduced simultaneously with silver deposition, releasing phosphorus atoms which then co-deposit with silver atoms to form a silver-phosphorus alloy transition layer 33. This silver-phosphorus alloy transition layer 33 has a composition between pure silver and the phosphorus-rich skeleton 223, acting as a compositional gradient transition, transforming the physical contact interface between the silver anchor 31 and the phosphorus-rich skeleton 223 into a metallurgical bonding interface. The bonding strength of the metallurgical bonding interface is much higher than that of the physical contact interface. Furthermore, during thermal cycling, the silver-phosphorus alloy transition layer 33, due to its intermediate composition characteristics, can partially buffer the thermal stress caused by the difference in thermal expansion coefficients between the two materials, significantly improving the fatigue resistance of the silver anchor 31. The concentration of sodium hypophosphite is controlled within a low range of 0.001-0.01 mol / L to ensure that the thickness of the silver-phosphorus alloy transition layer 33 is only 1-3 nm, which will not significantly affect the conductivity of the silver anchor 31 body. If the sodium hypophosphite concentration is too high, the silver-phosphorus alloy layer will be too thick. The presence of phosphorus will reduce the conductivity of silver, affecting the electrical performance of the lead frame. In addition, excessively high concentrations of sodium hypophosphite may interfere with the normal electrodeposition process of silver, leading to a decrease in the quality of the silver layer 3.

[0035] The working process and technical effects of the silver-phosphorus alloy transition layer 33 are as follows: During the first stage of silver plating, hypophosphite ions (H₂PO₂⁻) diffuse into the nanopores along with silver ions. Under the action of the cathode reduction potential, the hypophosphite ions are reduced at the interface at the silver deposition front, releasing phosphorus atoms. The phosphorus atoms and the simultaneously deposited silver atoms form the silver-phosphorus alloy transition layer 33. This transition layer 33 is located between the silver anchor 31 and the phosphorus-rich skeleton 223 (e.g., ...). Figure 3 (As shown in the dashed area), its composition gradually transitions from a high phosphorus content near the phosphorus-rich skeleton 223 to a low phosphorus content near the silver anchor 31, forming a continuous compositional gradient. This metallurgical bonding interface replaces the original purely physical contact interface, transforming the bond between the silver anchor 31 and the phosphorus-rich skeleton 223 from a weak bond relying on mechanical interlocking and van der Waals forces to a strong bond relying on interatomic metallic bonds. During thermal cycling, the silver-phosphorus alloy transition layer 33, acting as a buffer layer for composition and thermal expansion coefficient, can disperse the concentrated thermal stress at the interface to a larger volume range, inhibiting the initiation and propagation of interface fatigue cracks. This significantly improves the fatigue pull-out resistance of the silver anchor 31 under repeated temperature changes, extending the service life of the lead frame.

[0036] Please refer to Figure 4Following the electroplating of silver layer 3, the process includes selectively thickening the silver plating in the functional areas 6 of the lead frame to 2-10 μm. Functional areas 6 include chip pad areas 61 and wire bonding areas 62. The purpose of selectively thickening the silver plating in functional areas 6 is to obtain a sufficiently thick silver layer 3 in chip pad areas 61 and wire bonding areas 62 to meet the requirements for silver layer 3 thickness during chip soldering and wire bonding. The silver layer 3 in chip pad areas 61 needs to have sufficient thickness to ensure good wetting and reaction with the solder, and the silver layer 3 in wire bonding areas 62 needs to have sufficient thickness to ensure reliable connection of the bonding leads (gold or copper wires). Specifically, a mask is used to cover non-functional areas 7, exposing functional areas 6 (chip pad areas 61 and wire bonding areas 62). The exposed functional areas 6 are then thickened with silver, preventing the non-functional areas 7 from being plated during the thickening process. After the thickened silver plating is completed, the mask is removed. Then, the silver coating layer 32 of the non-functional region 7 is dissolved and removed using a chemical silver stripping solution, exposing the surface of the nanoporous framework structure 22 of the non-functional region 7. It should be noted that a thin silver coating layer 32 has already been formed in the non-functional region 7 in step five (electroplated silver layer 3). During the thickened silver plating process in step six, due to the protection of the mask, the non-functional region 7 will not be further plated. Therefore, only the thin silver coating layer 32 formed in step five remains on the non-functional region 7, which can be dissolved and removed by the chemical silver stripping solution. The chemical silver stripping solution is usually an alkaline solution containing an oxidant (such as hydrogen peroxide) and a complexing agent (such as ammonia), which can selectively dissolve silver without corroding the nickel-phosphorus alloy layer 2. During the chemical silver stripping process, the alkaline environment (pH 10-12) of the silver stripping solution does not cause significant corrosion to the phosphorus-rich framework 223 in the nanoporous framework structure 22. Therefore, the nanoporous framework structure 22 remains intact after silver stripping.

[0037] During the selective silver plating and stripping process described above, the chip pad area 61 and wire bonding area 62 of functional region 6 retain and thicken the silver capping layer 32. The silver anchors 31 penetrate deep into the channels of the nanoporous framework structure 22 to provide a firm anchor, ensuring that the thickened silver layer 3 will not peel off due to interface stress during subsequent chip soldering reflow soldering and wire bonding ultrasonic hot pressing processes. After the silver capping layer 32 is removed from the non-functional region 7, the exposed nanoporous framework structure 22 provides a high specific surface area reaction substrate for subsequent atmosphere passivation treatment, which is beneficial for forming a dense composite passivation film 5.

[0038] Please refer to Figure 4After removing the silver coating 32 from the non-functional region 7, the exposed nanoporous framework structure 22 of the non-functional region 7 can be subjected to atmosphere passivation treatment. The lead frame is placed in a mixed atmosphere containing 0.1%-5% phosphine and 1%-10% water vapor, with the balance being nitrogen, and treated at 100-200℃ for 5-30 minutes to generate a phosphate-phosphine oxide composite passivation film 5 in situ on the surface of the exposed nanoporous framework structure 22 of the non-functional region 7. Although phosphine (PH3) is a toxic gas, its volume fraction in this process is only 0.1%-5%, which is far lower than its spontaneous combustion concentration in air (approximately 1.8% volume fraction in pure oxygen, but there is no risk of spontaneous combustion in a nitrogen protective atmosphere), and the entire treatment process is carried out in a nitrogen protective atmosphere, so there is no risk of combustion or explosion. During process implementation, a sealed tubular furnace or atmosphere furnace should be used, equipped with a PH3 gas detection alarm device (alarm threshold set at 0.3 ppm) and an alkaline spray tail gas treatment system (using NaOH solution to absorb PH3 in the tail gas) to ensure operational safety and environmental compliance. The exposed nanoporous framework structure 22 in non-functional region 7 after silver stripping has a very large specific surface area. Without protective treatment, in high-temperature and high-humidity environments, water vapor and corrosive gases can easily penetrate along the nanopores to the bottom of the nickel-phosphorus alloy layer 2 and even the copper or copper alloy substrate 1, leading to corrosion and failure. Furthermore, the phosphorus-rich framework 223 on the surface of the exposed nanoporous framework structure 22 may slowly dissolve and release phosphate ions in a humid environment, altering the local electrochemical environment and promoting the electrochemical migration of silver ions from functional region 6 to non-functional region 7. Through atmosphere passivation treatment, utilizing the chemical activity of the phosphorus-rich framework 223 in the nanoporous framework structure 22, its surface reacts with phosphine and water vapor, generating a composite passivation film 5 of phosphate and phosphorus oxide in situ. The chemical reaction process of atmosphere passivation treatment is as follows: First, at a temperature of 100-200℃, water vapor reacts with phosphorus atoms on the surface of the phosphorus-rich framework 223, oxidizing phosphorus to phosphorus oxide (P2O5); subsequently, phosphine (PH3) is catalytically oxidized on the surface of the phosphorus-rich framework 223 (nickel atoms in the phosphorus-rich framework 223 act as catalysts), and the generated intermediate products further react with water vapor to form phosphoric acid (H3PO4) and polyphosphates; finally, phosphoric acid reacts with nickel atoms on the surface of the phosphorus-rich framework 223 to generate phosphate compounds such as nickel phosphate, which together with phosphorus oxides constitute the composite passivation film 5. The generated composite passivation film 5 has the following characteristics: First, the film layer is dense and can block the opening pores 221 of the nanopores, preventing the intrusion of external water vapor and corrosive media; second, phosphates have good chemical stability and corrosion resistance, and can remain intact for a long time in high temperature and high humidity environments; third, the composite passivation film 5 has a low surface energy and a certain degree of hydrophobicity, further reducing the adsorption and penetration of water vapor on the surface of the non-functional area 7.

[0039] The working process and technical effects of the composite passivation film 5 are as follows: During the atmospheric passivation process, phosphine (PH3) molecules diffuse into the opening channels 221 of the nanoporous framework structure 22, reacting chemically with phosphorus atoms and oxides on the surface of the phosphorus-rich framework 223 to generate polyphosphate oxides. Simultaneously, water vapor (H2O) reacts with the phosphorus oxides, converting some of the phosphorus oxides into phosphates. Finally, a phosphate-phosphorus oxide composite passivation film 5 is formed on the surface of the nanoporous framework structure 22 and the inner walls of the channels. This composite passivation film 5 seals the entrances of the opening channels 221, forming a physical barrier that prevents external corrosive media such as water vapor and chloride ions from penetrating along the nanopores to the dense amorphous structure 21 at the bottom of the nickel-phosphorus alloy layer 2 and the copper or copper alloy substrate 1, thereby preventing substrate corrosion. In addition, the high chemical stability and low surface energy of the composite passivation film 5 inhibit the electrochemical migration of silver ions from the functional region 6 along the surface of the non-functional region 7 under high temperature and high humidity conditions, avoiding short circuit failures caused by the growth of silver dendrites between adjacent pins, and significantly improving the long-term reliability of the packaged device under harsh conditions.

[0040] Please refer to Figure 5 During the electrochemical dealloying process, in-situ electrochemical impedance spectroscopy monitoring can be performed simultaneously to determine the dealloying endpoint. Specifically, during the application of the anodic potential, the application of the anodic potential is paused every 2-5 seconds, with each pause lasting 0.5-2 seconds. During each pause, a single-frequency AC perturbation signal with a frequency of 10 kHz and an amplitude of 5-10 mV is applied to the lead frame, and the electrochemical impedance modulus |Z| of the nickel-phosphorus alloy layer 2 is measured. The reason for choosing 10 kHz as the single-frequency measurement frequency is that at this frequency, the electrochemical impedance mainly reflects the resistive characteristics (rather than the capacitive or diffusion characteristics) of the nickel-phosphorus alloy layer 2, and its modulus |Z| has an approximately linear correspondence with the effective thickness of the dense amorphous structure 21, which facilitates real-time determination of the dealloying process. Compared with full-band scanning (such as 1 kHz-100 kHz), single-frequency measurement only requires 0.1-0.5 seconds, and the interference to the dealloying process is minimal. When the electrochemical impedance modulus |Z| decreases to 30%-60% of the initial impedance modulus |Z|0 at the start of dealloying, it is determined that the pore depth of the nanoporous framework structure 22 has reached the target depth, and the dealloying process is stopped. Figure 5 As shown, the horizontal axis represents the dealloying time (s), and the vertical axis represents the impedance modulus |Z| (Ω). The black dots on the curve represent EIS measurement points during each pause. The dealloying process is divided into a first stage (0.6-0.8V) and a second stage (0.3-0.5V), with the impedance modulus |Z| continuously decreasing over time. The shaded area (30%|Z|0~60%|Z|0) is the judgment interval. When the impedance modulus |Z| decreases to within this interval, the pore depth of the nanoporous framework structure 22 is determined to have reached the target depth, corresponding to time t. stopAt this point, the dealloying process should be stopped. Figure 5 The diagram on the right illustrates the change in the thickness ratio of the porous layer (nanoporous framework structure 22) to the dense layer (dense amorphous structure 21) in the nickel-phosphorus alloy layer 2 during the initial, intermediate, and final stages: In the initial stage, the nickel-phosphorus alloy layer 2 is entirely a dense structure; in the intermediate stage, the nanoporous framework structure 22 begins to form on the surface, and the thickness of the dense layer decreases; in the final stage, the thickness of the nanoporous framework structure 22 further increases, while the thickness of the dense amorphous structure 21 decreases accordingly, but still retains sufficient thickness to perform its diffusion-blocking function. During the electrochemical dealloying process, as nickel atoms on the surface of the nickel-phosphorus alloy layer 2 dissolve and the nanoporous framework structure 22 forms, the electrolyte gradually penetrates into the nanopores. The dense amorphous nickel-phosphorus alloy layer 2 exhibits high electrochemical impedance, while the impedance of the nanoporous layer decreases significantly due to electrolyte penetration. Therefore, the electrochemical impedance modulus |Z| of the entire nickel-phosphorus alloy layer 2 reflects the proportional relationship between the effective thickness of the dense amorphous portion (dense amorphous structure 21) and the depth of the nanoporous layer (nanoporous framework structure 22). As dealloying proceeds, the nanoporous layer gradually deepens, the effective thickness of the dense amorphous portion decreases accordingly, and the total impedance modulus |Z| continuously decreases. By pre-calibrating the correspondence between the decrease in impedance modulus |Z| and the pore depth of the nanoporous layer, the dealloying process can be determined in real time and non-destructively. The process can be precisely stopped when the nanoporous layer reaches the target depth, avoiding excessive dealloying that leads to insufficient thickness of the dense amorphous barrier layer (dense amorphous structure 21), and also avoiding insufficient dealloying that leads to poor anchoring effect of the nanoporous structure. Compared to traditional methods that determine the dealloying endpoint by controlling the time, the in-situ electrochemical impedance spectroscopy monitoring method can automatically compensate for changes in the dealloying rate caused by factors such as fluctuations in the initial thickness of the nickel-phosphorus alloy layer 2, batch differences in phosphorus content, and changes in electrolyte temperature. This significantly improves the pore depth consistency and process stability of the nanoporous framework structure 22. In actual production, the impedance modulus |Z| can be lowered to a preset threshold as a trigger signal to automatically stop the dealloying process, achieving closed-loop control.

[0041] The working process and technical effects of in-situ electrochemical impedance spectroscopy monitoring are as follows: During each pause cycle of the dealloying process, the potentiostat switches to AC impedance measurement mode, applies a small-amplitude (5-10mV) sinusoidal AC perturbation signal to the lead frame at a fixed frequency of 10kHz, measures the corresponding current response, and calculates the impedance modulus |Z|. Since the small-amplitude perturbation does not change the surface state of the nickel-phosphorus alloy layer 2, this measurement process is non-destructive. As dealloying proceeds, the depth of the nanoporous framework structure 22 gradually increases, the electrolyte penetration depth increases accordingly, and the effective thickness of the dense amorphous structure 21 decreases, resulting in a continuous decrease in the total impedance modulus |Z|. When |Z| decreases to 30%-60% of the initial value |Z|0, it indicates that the pore depth of the nanoporous framework structure 22 has reached the preset target depth. At this point, a stop signal is automatically triggered, terminating the application of the anodic potential. The technical advantages of this method are: it enables real-time, online monitoring of the dealloying process, precisely controlling the pore depth of the nanoporous framework structure 22, avoiding insufficient thickness of the dense amorphous structure 21 due to over-dealloying (loss of diffusion barrier function), and also avoiding insufficient depth of the nanoporous structure due to inadequate dealloying (poor anchoring effect of silver anchors 31). Compared with traditional timing control methods, this method can automatically compensate for process fluctuations, significantly improving product consistency and yield.

[0042] Please refer to Figure 4In the electrochemical dealloying process, different dealloying parameters can be applied to the chip pad area 61 and the lead bonding area 62 of the lead frame using a mask to form differentiated nanoporous structures. In this case, the electrochemical dealloying process also employs a segmented potential variation mode, but the potential and time parameters for each stage are independently defined for different functional regions, replacing the method of applying uniform parameters to the entire surface. For the chip pad area 61, the anode potential used in the first stage of the segmented potential variation mode is 0.7-0.8V for 5-10 seconds, and the anode potential used in the second stage is 0.4-0.5V for 10-20 seconds, forming a deep hole structure with a depth of 50-100nm. For the wire bonding area 62, the anode potential used in the first stage of the segmented potential variation mode is 0.6-0.7V for 3-5 seconds, and the anode potential used in the second stage is 0.3-0.4V for 5-10 seconds, forming a shallow hole structure with a depth of 10-30nm. The different functional areas 6 of the lead frame experience significantly different stress types and have different requirements for the surface quality of the silver layer 3 during the packaging process. During chip soldering, the solder (such as a tin-silver-copper alloy) melts in the chip pad area 61 during reflow soldering and reacts with the silver layer 3 to form an intermetallic compound. When the solder solidifies and shrinks, it generates significant shear stress at the interface between the silver layer 3 and the nickel-phosphorus alloy layer 2. Therefore, the chip pad area 61 requires a deeper nanoporous structure and longer silver anchors 31 to provide a larger anchoring area and stronger shear resistance. In contrast, the surface of the silver layer 3 in the wire bonding area 62 needs to be ultrasonically thermocompressed with the bonding wires (gold or copper wires) to form reliable solder joints. The bonding process places extremely high demands on the flatness and cleanliness of the silver layer 3 surface. If the nanoporous structure is too deep, the surface roughness of the silver capping layer 32 will increase accordingly, potentially affecting the contact area and bonding strength between the bonding wires and the silver layer 3. Therefore, the wire bonding area 62 employs a shallower nanoporous structure to maintain a certain anchoring effect while keeping the surface roughness of the silver capping layer 32 low, meeting the process requirements of wire bonding. The differentiated dealloying process is achieved by using photolithography or printing mask technology to first process the entire surface of the lead frame according to the dealloying parameters of the lead bonding area 62 (lower potential, shorter time). Then, the lead bonding area 62 is covered with a mask, and the chip pad area 61 is further treated with a higher potential and a longer time of dealloying, thereby realizing the differentiated nanoporous structure of different functional areas 6 on the same lead frame.

[0043] The working process and technical effects of the differentiated nanoporous structure are as follows: In the chip pad area 61, the deep hole structure (hole depth 50-100nm) gives the silver anchors 31 a greater anchoring depth and a larger sidewall contact area. During the chip soldering reflow process, under the shear stress generated by the solidification and shrinkage of the solder, the silver anchors 31, which are deep in the channel, resist shear deformation through their wedge-shaped structure and large-area sidewall friction, preventing the silver layer 3 from slipping or peeling off at the soldering interface. In the wire bonding area 62, the shallow hole structure (hole depth 10-30nm) provides a basic anchoring effect while keeping the surface roughness of the silver cover layer 32 at a low level (usually Ra<50nm), ensuring sufficient contact area between the bonding wires and the surface of the silver layer 3, meeting the strict requirements of ultrasonic hot-press bonding for interface flatness, and ensuring that the tensile strength and shear strength of the bonding joint meet the standards. Through this differentiated design, different functional areas 6 on the same leadframe can obtain optimal nanoporous structure parameters, taking into account both the high bonding strength requirements of chip welding and the high surface quality requirements of wire bonding, thereby improving the overall reliability and yield of the packaged device.

[0044] Experimental Example 1:

[0045] The following is a complete process flow example.

[0046] Step 1: Substrate Pretreatment. Take a 0.15mm thick C194 copper alloy lead frame strip as copper or copper alloy substrate 1, and perform the following steps in sequence: alkaline degreasing (40g / L NaOH + 25g / L Na2CO3, 55℃, 4 minutes), deionized water rinsing, acid pickling and activation (80g / L H2SO4, room temperature, 45 seconds), deionized water rinsing, palladium activation (0.3g / L PdCl2 + 8ml / L HCl, room temperature, 45 seconds), and deionized water rinsing.

[0047] Step 2: Electroless plating of a nickel-phosphorus alloy layer. The pretreated lead frame was immersed in an electroless plating solution (nickel sulfate 25 g / L, sodium hypophosphite 25 g / L, lactic acid 20 ml / L, sodium acetate 12 g / L, thiourea 0.8 mg / L). The plating solution temperature was 88℃, pH 4.8, and the plating time was 30 minutes, resulting in an amorphous nickel-phosphorus alloy layer 2 with a thickness of approximately 3 μm and a phosphorus content of approximately 12 wt%. X-ray diffraction (XRD) confirmed that the nickel-phosphorus alloy layer 2 was an amorphous structure (the diffraction pattern showed broadened diffuse scattering peaks, without sharp crystalline diffraction peaks).

[0048] Step 3: Electrochemical dealloying treatment. The lead frame coated with nickel-phosphorus alloy layer 2 was used as the anode, a platinum sheet as the cathode, and a saturated Ag / AgCl electrode as the reference electrode. It was placed in an electrolyte containing 1 mol / L H₂SO₄ and 0.05 mol / L NiSO₄ at a temperature of 25°C. A segmented potential-changing mode was used: a 0.7V anode potential was applied for 8 seconds in the first stage, and a 0.4V anode potential was applied for 15 seconds in the second stage. After treatment, the surface morphology was observed using field emission scanning electron microscopy (FE-SEM), confirming the formation of open channels 221 with a pore size of approximately 40 nm and deep channels 222 with a pore size of approximately 10 nm, with a total pore depth of approximately 70 nm. An approximately 2.93 μm thick, undealloyed amorphous structure 21 remained at the bottom of the nickel-phosphorus alloy layer 2.

[0049] Step 4: Low-temperature oxidation treatment. The dealloyed lead frame was placed in a tube furnace, and air (oxygen volume fraction 21%) was introduced. The furnace was treated at 200°C for 5 minutes to form a phosphorus oxide passivation layer 4 with a thickness of approximately 2-3 nm on the surface of the phosphorus-rich framework 223. The presence and thickness of the phosphorus oxide passivation layer 4 were confirmed by XPS depth profiling.

[0050] Step 5: Silver plating. A cyanide-free silver plating solution (silver nitrate 30g / L, succinimide 65g / L, potassium acetate 40g / L) was used at a temperature of 25℃ and a pH of 8.5. First stage: A pulsed reverse electroplating mode was employed. The forward deposition pulse current density was 0.3A / dm², with a pulse width of 100ms, and the reverse dissolution pulse current density was 0.1A / dm², with a pulse width of 10ms, for a total time of 5 minutes. This allowed silver ions to fill the nanopores, forming silver anchors 31. Second stage: Silver plating was performed using a direct current density of 3A / dm², with the time adjusted according to the target thickness.

[0051] Step Six: Selective Silver Plating Thickening and Silver Removal. Non-functional region 7 is covered with a photolithographic mask, exposing functional region 6 (chip pad area 61 and lead bonding area 62). The exposed functional region 6 is then silver-plated to a thickness of 5 μm using a current density of 3 A / dm². At this time, non-functional region 7 is protected by the mask and not thickened. After removing the mask, the lead frame is immersed in a chemical silver removal solution (30 g / L H₂O₂ + 50 ml / L NH₃·H₂O, pH 11, room temperature) for 2 minutes to dissolve and remove the silver coating layer 32 (i.e., the thin silver coating layer formed on the surface of non-functional region 7 in step five).

[0052] Step 7: Atmosphere passivation treatment. The silver-removed lead frame is placed in a sealed tube furnace, and a mixed gas containing 1% PH3 + 5% H2O + 94% N2 (volume fraction) is introduced. The furnace is treated at 150°C for 15 minutes, forming a phosphate-phosphorus oxide composite passivation film 5 on the surface of the exposed nanoporous framework structure 22 in the non-functional region 7. The exhaust gas is then absorbed and treated by a 10% NaOH solution spray tower.

[0053] Performance test results:

[0054] (1) Peel strength test (90° peel method): The peel strength of the silver layer 3 in functional area 6 is 8.5 N / cm, while the peel strength of the comparative sample using the traditional process (crystalline nickel layer + direct silver plating) is only 3.2 N / cm, which is about 166% higher.

[0055] (2) High-temperature aging test (175℃ / 1000 hours): After aging, no copper element signal was detected on the surface of silver layer 3 by XPS analysis (detection limit 0.1at%), and the surface of silver layer 3 remained bright and undiscolored. In contrast, copper element diffusion was detected on the surface of the silver layer of the control sample (crystalline nickel layer) after aging for 500 hours.

[0056] (3) Temperature cycling reliability test (-65℃ to 150℃, 1000 cycles): Silver layer 3 showed no peeling, blistering or cracking, and the peel strength retention rate was 92% (from 8.5 N / cm to 7.8 N / cm). The comparison sample showed local blistering after 500 cycles.

[0057] (4) High temperature and high humidity test (85℃ / 85%RH / 168 hours, 5V bias applied): No dendrite growth was observed in non-functional area 7, and the insulation resistance between adjacent pins remained at 10 ohms. 9 Ω or higher. The control sample (without atmospheric passivation treatment) developed a dendritic short circuit after 72 hours.

[0058] Experimental Example 2:

[0059] The difference between this experimental example and Experimental Example 1 is that step four (low-temperature oxidation treatment) is not performed. Instead, 0.005 mol / L sodium hypophosphite is added to the silver plating solution in the first stage of step five to co-deposit a silver-phosphorus alloy transition layer 33 with a thickness of about 2 nm at the interface between the silver anchor 31 and the phosphorus-rich skeleton 223. The remaining steps are the same as in Experimental Example 1.

[0060] TEM-EDS line scanning analysis confirmed that there is a silver-phosphorus transition region with a width of about 2 nm at the interface between the silver anchor 31 and the phosphorus-rich skeleton 223. The silver signal gradually decreases from the high value on the silver anchor 31 side, and the phosphorus signal gradually decreases from the high value on the phosphorus-rich skeleton 223 side. The two overlap in the transition region, confirming the existence of the silver-phosphorus alloy transition layer 33.

[0061] Performance test results:

[0062] (1) Peel strength test: The peel strength of the silver layer 3 in functional area 6 is 9.2 N / cm, which is slightly higher than that in Experiment 1 (8.5 N / cm), indicating that the bonding strength of the metallurgical bonding interface is better than that of the phosphorus oxide passivation layer scheme.

[0063] (2) Temperature cycling reliability test (-65℃ to 150℃, 2000 cycles): Silver layer 3 showed no peeling, blistering or cracking, and the peel strength retention rate was 95% (from 9.2 N / cm to 8.7 N / cm), which is better than the 92% retention rate of Experimental Example 1. This shows that the metallurgical bonding interface of the silver-phosphorus alloy transition layer 33 has better fatigue resistance in thermal cycling than the physical barrier interface of the phosphorus oxide passivation layer scheme.

[0064] (3) The remaining test results are similar to those of Experiment 1.

[0065] Comparative Example 1:

[0066] The traditional process involves electroplating a crystalline nickel layer (Watt nickel, 3μm thick) onto a C194 copper alloy substrate, followed by direct electroplating of a silver layer (5μm thick), without any roughening or dealloying treatment.

[0067] Performance test results:

[0068] (1) Peel strength: 3.2 N / cm.

[0069] (2) High temperature aging (175℃ / 500 hours): The surface of the silver layer turned yellowish-brown, and XPS detected that the copper content reached 5.3 at.

[0070] (3) Temperature cycling (-65℃ to 150℃, 500 times): local blistering of the silver layer and peel strength reduced to 1.5 N / cm.

[0071] Comparative Example 2:

[0072] An amorphous nickel-phosphorus alloy layer (phosphorus content 12wt%, thickness 3μm) was electrolessly plated on a C194 copper alloy substrate, and a silver layer (thickness 5μm) was directly electroplated without dealloying.

[0073] Performance test results:

[0074] (1) Peel strength: 4.1 N / cm (better than the crystalline nickel layer of Comparative Example 1, but much lower than 8.5 N / cm of Experimental Example 1).

[0075] (2) High temperature aging (175℃ / 1000 hours): The silver layer surface did not change color and no copper element was detected by XPS, confirming that the diffusion blocking function of the amorphous nickel-phosphorus alloy layer is effective.

[0076] (3) Temperature cycling (-65℃ to 150℃, 1000 times): After 800 cycles, the silver layer showed local peeling, and the peeling strength dropped to 2.3 N / cm, indicating that the planar interface bonding force between the smooth surface of the amorphous nickel-phosphorus alloy layer and the silver layer was insufficient to withstand long-term thermal cycling stress.

[0077] The test results of the above experimental examples and comparative examples show that: by constructing a nanoporous framework structure and forming silver anchors on the surface of the amorphous nickel-phosphorus alloy layer, the present invention increases the peel strength of the silver layer from 3.2-4.1 N / cm in the traditional process to 8.5-9.2 N / cm, an increase of about 110%-188%; at the same time, it maintains the diffusion blocking function at the bottom of the amorphous nickel-phosphorus alloy layer, and no copper diffusion occurs on the surface of the silver layer after high-temperature aging at 175℃ / 1000 hours; after 2000 temperature cycles, the peel strength retention rate of the silver layer reaches 95%, which is far superior to the traditional process.

[0078] This application addresses the problems of existing lead frame surface treatment technologies through a series of steps. An amorphous nickel-phosphorus alloy layer 2 is chemically plated onto a copper or copper alloy substrate 1, utilizing its amorphous structure to block copper atom diffusion. Electrochemical dealloying forms a nanoporous framework structure 22, providing anchoring sites for the silver anchors 31 and improving the bonding force between the silver layer 3 and the nickel-phosphorus alloy layer 2. A gradient nanoporous structure formed by a segmented potential variation mode makes the silver anchors 31 wedge-shaped, enhancing their anti-peeling performance. A phosphorus oxide passivation layer 4 generated by low-temperature oxidation treatment improves the reliability of diffusion blocking. A pulsed reverse electroplating mode solves the orifice sealing problem, achieving dense filling of the silver anchors 31. Atmosphere passivation treatment forms a composite passivation film 5 in the non-functional area 7, preventing corrosion and electrochemical migration. In-situ electrochemical impedance spectroscopy monitoring enables precise determination of the dealloying endpoint. A silver-phosphorus alloy transition layer 33 improves the fatigue resistance of the silver anchors 31. The differentiated nanoporous structure design of functional region 6 addresses the needs of different functional regions 6, improving encapsulation reliability and bonding quality. Compared with traditional processes, it significantly enhances the performance and reliability of the leadframe, making it suitable for mass production and applications.

[0079] 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 surface treatment method for an integrated circuit lead frame, characterized in that, Includes the following steps: A nickel-phosphorus alloy layer (2) is deposited on the surface of a copper or copper alloy substrate (1) by chemical plating. The phosphorus content in the nickel-phosphorus alloy layer (2) is 10-14 wt%, and the nickel-phosphorus alloy layer (2) has an amorphous structure. Electrochemical dealloying treatment is performed on the surface of the nickel-phosphorus alloy layer (2). The lead frame is placed as the anode in an electrolyte containing 0.5-2 mol / L sulfuric acid, and an anode potential of 0.3-0.8V is applied. With a saturated Ag / AgCl electrode as a reference, the nickel atoms on the surface of the nickel-phosphorus alloy layer (2) are dissolved while the phosphorus atoms are retained, forming a phosphorus-rich nanoporous framework structure (22) on the surface of the nickel-phosphorus alloy layer (2). The pore size of the nanoporous framework structure (22) is 5-50 nm and the pore depth is 10-100 nm. The bottom of the nickel-phosphorus alloy layer (2) retains an amorphous structure (21) that has not been dealloyed. A silver layer (3) is electroplated on the surface having the nanoporous framework structure (22), wherein silver is deposited in the pores of the nanoporous framework structure (22) to form silver anchors (31), and a silver capping layer (32) is formed on the surface of the nanoporous framework structure (22).

2. The surface treatment method for the integrated circuit lead frame according to claim 1, characterized in that, The electrochemical dealloying treatment employs a segmented potential variation mode, including: In the first stage, an anodic potential of 0.6-0.8V is applied for 3-10 seconds to form an open channel (221) with a pore size of 30-50nm on the surface of the nickel-phosphorus alloy layer (2). In the second stage, an anode potential of 0.3-0.5V is applied for 5-20 seconds, and the bottom of the opening channel (221) is etched to form a deep channel (222) with a diameter of 5-20nm. This results in a gradient nanoporous structure with pore size decreasing from the surface to the depth, causing the subsequently deposited silver anchors (31) to exhibit a wedge shape with a wide surface and a narrow root.

3. The surface treatment method for the integrated circuit lead frame according to claim 1, characterized in that, After the electrochemical dealloying treatment and before the electroplating of the silver layer (3), the process further includes a low-temperature oxidation treatment of the nanoporous framework structure (22): The phosphorus oxide passivation layer (4) is generated on the surface of the phosphorus-rich skeleton (223) in the nanoporous skeleton structure (22) at 150-250°C for 1-10 minutes in an atmosphere containing 5%-21% oxygen by volume. The thickness of the phosphorus oxide passivation layer (4) is 1-5 nm. The phosphorus oxide passivation layer (4) is used to block nickel atoms from diffusing toward the silver layer (3) and to improve the wettability and deposition uniformity of silver ions on the surface of the nanoporous framework structure (22).

4. The surface treatment method for the integrated circuit lead frame according to claim 1, characterized in that, The steps of electroplating the silver layer (3) include: In the first stage, silver plating is performed at a current density of 0.1-0.5A / dm², so that silver ions diffuse into the channels of the nanoporous framework structure (22) and deposit to form silver anchors (31). In the second stage, silver plating is performed at a current density of 1-5 A / dm² to form a silver coating layer (32) on the surface of the nanoporous framework structure (22).

5. The surface treatment method for the integrated circuit lead frame according to claim 4, characterized in that, The first stage employs a pulsed reverse electroplating mode, which includes alternately applied forward deposition pulses and reverse dissolution pulses; The current density of the forward deposition pulse is 0.1-0.5 A / dm², and the pulse width is 50-200 ms; the current density of the reverse dissolution pulse is 0.05-0.2 A / dm², and the pulse width is 5-20 ms. The reverse dissolution pulse is used to dissolve the silver deposits that have been excessively deposited at the well opening due to the edge effect, so as to keep the well opening open during the deposition process.

6. The surface treatment method for the integrated circuit lead frame according to claim 1, characterized in that, Following the electroplated silver layer (3), the following is also included: The non-functional area (7) is covered by a mask to expose the functional area (6) of the lead frame. The exposed functional area (6) is selectively thickened with silver plating to 2-10 μm. The functional area (6) includes a chip pad area (61) and a lead bonding area (62). After removing the mask, The silver coating (32) of the non-functional region (7) is dissolved and removed by chemical silver stripping solution, thereby exposing the surface of the nanoporous framework structure (22) of the non-functional region (7).

7. The surface treatment method for the integrated circuit lead frame according to claim 6, characterized in that, After the silver coating (32) of the non-functional region (7) is removed, the process further includes an atmosphere passivation treatment of the exposed nanoporous framework structure (22) of the non-functional region (7): The nonfunctional region (7) is treated at 100-200°C for 5-30 minutes in a mixed atmosphere containing 0.1%-5% phosphine and 1%-10% water vapor, with the balance being nitrogen, so that a phosphate-phosphorus oxide composite passivation film (5) is generated in situ on the surface of the exposed nanoporous framework structure (22). The composite passivation film (5) blocks the opening channels (221) of the nanoporous framework structure (22) to prevent external water vapor and corrosive media from intruding and to inhibit the electrochemical migration of silver ions from the functional region (6) to the non-functional region (7).

8. The surface treatment method for the integrated circuit lead frame according to claim 1, characterized in that, During the electrochemical dealloying process, in-situ electrochemical impedance spectroscopy monitoring is performed simultaneously to determine the dealloying endpoint, specifically including: During the application of the anode potential, the application of the anode potential is paused every 2-5 seconds, and each pause lasts for 0.5-2 seconds. During each pause, a single-frequency AC disturbance signal with a frequency of 10kHz and an amplitude of 5-10mV is applied to the lead frame, and the electrochemical impedance modulus of the nickel-phosphorus alloy layer (2) is measured. When the electrochemical impedance modulus drops to 30%-60% of the initial impedance modulus at the start of dealloying, it is determined that the pore depth of the nanoporous framework structure (22) has reached the target depth, and the dealloying process is stopped. The decrease in the electrochemical impedance modulus reflects the reduction in the effective thickness of the dense amorphous structure (21) in the nickel-phosphorus alloy layer (2) and the increase in the electrolyte penetration depth of the nanoporous framework structure (22).

9. The surface treatment method for the integrated circuit lead frame according to claim 4, characterized in that, In the first stage of silver plating, sodium hypophosphite with a concentration of 0.001-0.01 mol / L is also added to the silver plating solution; While silver ions are deposited in the pores of the nanoporous framework structure (22) to form silver anchors (31), sodium hypophosphite is co-deposited in situ at the interface between the silver anchors (31) and the phosphorus-rich framework (223) under cathodic reduction to form a silver-phosphorus alloy transition layer (33), the thickness of the silver-phosphorus alloy transition layer (33) is 1-3 nm. The silver-phosphorus alloy transition layer (33) forms a metallurgical bond between the silver anchor (31) and the phosphorus-rich skeleton (223), replacing the physical contact interface between pure silver and the phosphorus-rich skeleton (223), thereby improving the fatigue resistance of the silver anchor (31) during thermal cycling.

10. The surface treatment method for an integrated circuit lead frame according to claim 1, characterized in that, The electrochemical dealloying process adopts a segmented potential variation mode, and different dealloying parameters are applied to the chip pad area (61) and the lead bonding area (62) of the lead frame through a mask to form a differentiated nanoporous structure. Specifically, the process includes: first, processing the entire surface of the lead frame according to the dealloying parameters of the lead bonding area (62), then covering the lead bonding area (62) with a mask, and then continuing to process the chip pad area (61) according to the dealloying parameters of the chip pad area (61). For the chip pad area (61), the anode potential used in the first stage of the segmented potential variation mode is 0.7-0.8V and the time is 5-10 seconds, and the anode potential used in the second stage of the segmented potential variation mode is 0.4-0.5V and the time is 10-20 seconds, forming a deep hole structure with a hole depth of 50-100nm; For the wire bonding region (62), the anode potential used in the first stage of the segmented variable potential mode is 0.6-0.7V and the time is 3-5 seconds, and the anode potential used in the second stage of the segmented variable potential mode is 0.3-0.4V and the time is 5-10 seconds, forming a shallow hole structure with a hole depth of 10-30nm.

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