A copper pad laser texturing structure for inhibiting whisker growth of a tin plating layer and a processing method thereof

Abstract

The present application relates to a kind of copper pad laser texturing structure for inhibiting tin coating whisker growth, including copper pad, the pit formed by laser texturing treatment on the surface of copper pad;Pit is concentric circular array distribution, pit diameter is 6-10 μm, depth is 3-5 μm;After copper pad is completed laser texturing treatment, tin plating is carried out using tin plating method.The present application is carried out surface activation using dilute sulfuric acid solution on the copper pad after laser texturing treatment, washing, vacuum drying pretreatment, further through acidic tin sulfate electroplating solution is carried out tin plating treatment, control electroplating process parameter makes tin layer fill pit structure fully, and form the even tin plating layer with thickness of 10~15 μm on the surface of copper pad, obtain the tin plating copper pad structure capable of significantly inhibiting tin whisker growth, lead-free environmental protection, high reliability.

Description

Technical Field

[0001] This invention belongs to the technical field of electronic packaging and relates to a laser texturing structure and processing method for copper pads that inhibits the growth of tin plating whiskers. Background Technology

[0002] In the field of electronic packaging, copper pads are widely used in printed circuit boards (PCBs), semiconductor devices, electronic connectors, and other electronic components due to their excellent electrical conductivity, thermal conductivity, and mechanical properties. To improve the solderability and corrosion resistance of copper pads and extend the service life of electronic components, the industry commonly uses a process of tin plating on the surface of copper pads. This tin plating layer enables reliable connections between the copper pads and other components.

[0003] Traditional tin plating involves columnar crystals preferentially growing along a specific crystal orientation, resulting in a singular orientation. Internal stress easily accumulates and propagates along grain boundaries, forming localized stress concentration zones. This provides a continuous driving force for the nucleation and growth of tin whiskers, ultimately inducing whisker growth into slender, needle-like or filamentous whiskers. Tin whiskers typically range in diameter from a few micrometers to tens of micrometers and in length to hundreds of micrometers. Excessive growth can easily lead to short circuits between adjacent copper pads, causing electronic equipment malfunctions and, in severe cases, even equipment burnout. This poses significant safety hazards and economic losses to high-end electronics fields such as aerospace, automotive electronics, and precision instruments. Currently, methods to suppress whisker growth in tin plating mainly include adding alloying elements (primarily lead) and optimizing electroplating process parameters. However, adding lead does not comply with RoHS environmental directives, and optimizing the tin plating process only alleviates early whisker growth after plating; it does not prevent whisker growth caused by intermetallic compounds formed in the plating substrate during service. Summary of the Invention

[0004] The purpose of this invention is to provide a laser texturing structure and processing method for copper pads that suppresses the growth of tin-plated whiskers, which can solve the above-mentioned problems and obtain a tin-plated copper pad structure that can significantly suppress the growth of tin whiskers, is lead-free and environmentally friendly, and has high reliability.

[0005] According to the technical solution provided by the present invention: a copper pad laser texturing structure for inhibiting the growth of tin plating whiskers includes a copper pad, and the surface of the copper pad is formed by laser texturing to form pits; the pits are distributed in a concentric circle array, the pit diameter is 6-10μm, and the depth is 3-5μm; after the copper pad is laser texturing, it is tin-plated by electroplating.

[0006] A method for laser texturing copper pads to suppress the growth of tin plating whiskers, used to form a laser texturing structure for copper pads that suppresses the growth of tin plating whiskers, includes the following steps: Step S1, Pad pretreatment: The copper pads are degreased, washed with water, acid-washed, washed with water again, and dried in sequence; Step S2, Laser texturing: The pre-treated copper pads are textured using a picosecond pulsed laser to form concentric arrays of pits; the surface-treated copper pads are then ultrasonically cleaned and dried. Step S3, Tin plating: The surface-treated copper pads are sequentially activated, washed with water, and dried; the pits are filled by electroplating or chemical tin plating to form a uniform tin plating layer on the surface.

[0007] As a further improvement of the present invention, in step S1, an alkaline degreasing agent is used for degreasing, the degreasing temperature is 40-60℃, and the degreasing time is 6-10 min; the sample surface is ultrasonically cleaned with deionized water for water washing; dilute sulfuric acid solution is used for acid washing, the acid washing temperature is 30℃, and the acid washing time is 1-3 min; and hot air drying is used for drying, the temperature is 90℃, and the time is 8-12 min.

[0008] As a further improvement of the present invention, in step S2, the laser texturing process uses a picosecond pulsed laser with a wavelength of 1064nm, a pulse width of 8-15ps, an average power of 5-12W, a focusing lens focal length of 100-160mm, a focusing spot diameter of 6-8μm, and a processing method of single-point pulse dotting.

[0009] As a further improvement of the present invention, in step S2, the pits after laser texturing are ultrasonically cleaned with deionized water at a frequency of 50 kHz for 4 min; the cleaned pits are then vacuum dried at a temperature of 70°C and a vacuum of 0.09 MPa for 10 min.

[0010] As a further improvement of the present invention, in step S3, the pits after laser texturing are pretreated before tin plating. The surface activation is performed using a 5% volume fraction dilute sulfuric acid solution, the pickling temperature is 30°C, and the pickling time is 2 minutes. The activated pits are then cleaned with deionized water and vacuum dried.

[0011] As a further improvement of the present invention, in step S3, the pretreated pits are electroplated with a tin plating layer. The electroplating solution is an acidic tin sulfate electroplating solution, the electroplating temperature is 30°C, the current density is 0.7A / dm², and the electroplating time is 25min, so that the plating layer fills the pits, and the tin plating layer thickness on the surface of the pit structure is 10-15μm. The positive and progressive effects of this invention are as follows: 1. This invention constructs a concentric array of pits on the surface of copper pads through laser texturing, enabling control over the crystal morphology of the tin plating layer. During tin electroplating, the pit structure formed by laser texturing alters the nucleation and growth patterns of tin atoms during deposition, transforming the columnar crystal structure of the traditional tin plating layer into a uniform equiaxed crystal structure. The tin crystal structure is body-centered tetragonal, exhibiting significant anisotropy in mechanical properties.

[0012] 2. After laser texturing, the tin plating layer formed by electroplating the copper pads has an equiaxed crystal structure with uniform grain size and numerous grain boundaries. This effectively blocks stress transmission paths, disperses residual stress within the plating layer and stress generated during service, and eliminates the stress-driven conditions for whisker growth. Simultaneously, the more dense and uniform equiaxed crystal structure inhibits the rapid diffusion and migration of tin atoms along specific directions, blocking the transfer of tin atoms required for tin whisker growth, thus achieving long-term and significant inhibition of whisker growth in the tin plating layer. Attached Figure Description

[0013] Figure 1 This is a schematic diagram of a laser texturing structure for copper pads that suppresses the growth of tin-plated whiskers.

[0014] Figure 2 Comparison of tin plating crystallization before and after laser texturing of copper pads.

[0015] Figure 3 This is an observation diagram of whisker growth on the surface of the copper pad laser texturing treatment method for inhibiting whisker growth in Example 1 of the tin plating, after accelerated aging for 1000 hours in an environment with a temperature of 85°C and a relative humidity of 85%.

[0016] Figure 4 This is an observation diagram of whisker growth on the surface of the copper pad laser texturing treatment method for inhibiting whisker growth in Example 2, after accelerated aging for 1000 hours in an environment with a temperature of 85°C and a relative humidity of 85%.

[0017] Figure 5 This is an observation diagram of whisker growth on the surface of the copper pad laser texturing treatment method of Example 3, which uses the method to suppress whisker growth in tin plating, after being accelerated aged for 1000 hours in an environment with a temperature of 85°C and a relative humidity of 85%.

[0018] Figure 6 This is an observation diagram of whisker growth on the surface of the coating of Comparative Example 1 after accelerated aging in an environment of 85℃ and 85% relative humidity for 1000 hours.

[0019] Figure labeling: 1. Copper pad; 2. Recess; 3. Tin plating. Detailed Implementation

[0020] It should be noted that, unless otherwise specified, the embodiments and features described in the present invention can be combined with each other. The present invention will now be described in detail with reference to the accompanying drawings and embodiments.

[0021] To enable those skilled in the art to better understand the present invention, the technical solutions of the present invention will be clearly and completely described below with reference to the accompanying drawings of the embodiments. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort should fall within the scope of protection of the present invention.

[0022] It should be noted that the terms "first," "second," etc., in the specification, claims, and accompanying drawings of this invention are used to distinguish similar objects and are not necessarily used to describe a specific order or sequence. It should be understood that such data can be interchanged where appropriate for the embodiments of the invention described herein. Furthermore, terms such as "comprising" and "having" mean that in addition to those already listed in "comprising" and "having," other unlisted contents may also be included; for example, a process, method, system, product, or device may include a series of steps or units, not necessarily limited to those steps or units explicitly listed, but may include other steps or units not explicitly listed or inherent to these processes, methods, products, or devices.

[0023] Due to the angle of the drawing, some parts may not be drawn, but their positions and connections can be understood from the text descriptions.

[0024] like Figure 1 As shown, a laser texturing structure for copper pads to suppress the growth of tin-plated whiskers includes a copper pad 1, and pits 2 formed on the surface of the copper pad 1 by laser texturing. The pits 2 are distributed in a concentric circle array, with a diameter of 6-10 μm and a depth of 3-5 μm. After the copper pad 1 is laser texturized, it is tin-plated by electroplating.

[0025] The copper pad 1 is circular, with a diameter of either 0.6 mm or 0.76 mm. The spacing between adjacent recesses 2 is between 1 and 2 times the diameter of recess 2.

[0026] This invention relates to a laser texturing method for copper pads to suppress the growth of tin plating whiskers, comprising the following steps: Step S1, Pad pretreatment: Select copper pad 1 of a conventional printed circuit board, degrease it with an alkaline degreaser at 40-60℃ for 6-10 minutes, ultrasonically clean the surface with deionized water, then acid wash it with dilute sulfuric acid solution at 30℃ for 1-3 minutes, rinse it with deionized water, and dry it with hot air at 90℃ for 8-12 minutes. Step S2, Laser texturing: Using a picosecond pulsed laser with a wavelength of 1064nm, the pulse width is set to 8-15ps, the average power to 5-12W, the focal length of the focusing lens to 100-160mm, and the diameter of the focused spot to 6-8μm. Concentric arrays of pits 2 are processed on the surface of the copper pad 1 using a single-point pulse dotting method. The pits 2 have a diameter of 6-10μm and a depth of 3-5μm. After processing, the pads are ultrasonically cleaned with deionized water for 4 minutes using an ultrasonic cleaning device with an ultrasonic frequency of 50kHz. The laser-textured pads are then vacuum dried for 10 minutes at 70℃ and a vacuum degree of 0.09MPa. Step S3, Tin plating treatment: Use a 5% volume fraction dilute sulfuric acid solution to activate the solution at 30°C for 2 minutes, rinse with deionized water and vacuum dry; use an acidic tin sulfate electroplating solution to electroplat for 25 minutes at 30°C and a current density of 0.7A / dm², so that the tin layer fills the pits 2 on the surface of the pad, and the thickness of the tin plating layer 3 on the surface of the pad is 10-15μm.

[0027] Example 1 A method for laser texturing of copper pads to suppress the growth of tin plating whiskers includes the following steps: Step S1, Pad pretreatment: Select copper pad 1 of a conventional printed circuit board, degrease it at 50°C for 8 minutes with an alkaline degreaser, clean the surface with deionized water using ultrasonic cleaning, then pickle it with dilute sulfuric acid solution at 30°C for 2 minutes, rinse it with deionized water, and dry it with hot air at 90°C for 12 minutes. Step S2, Laser texturing: A picosecond pulsed laser with a wavelength of 1064nm was used, with a pulse width of 10ps, an average power of 8W, a focusing lens focal length of 130mm, and a focused spot diameter of 7μm. Concentric arrays of pits 2 were processed on the surface of the copper pad 1 using a single-point pulse dotting method. The pits 2 had a diameter of 8μm and a depth of 3μm. After processing, an ultrasonic cleaning device with an ultrasonic frequency of 50kHz was used to ultrasonically clean the pads with deionized water for 4 minutes. The laser-textured pads were then vacuum dried for 10 minutes at 70℃ and a vacuum degree of 0.09MPa. Step S3, Tin plating treatment: Use a 5% volume fraction dilute sulfuric acid solution to activate the solution at 30°C for 2 minutes, rinse with deionized water and vacuum dry; use an acidic tin sulfate electroplating solution to electroplat for 25 minutes at 30°C and a current density of 0.7A / dm², so that the tin layer fills the pits 2 on the surface of the pad, and the thickness of the tin plating layer 3 on the surface of the pad is 12μm.

[0028] like Figure 4 As shown, the coating processed in this embodiment was accelerated to aging at 85°C and 85% relative humidity for 1000 hours, and the growth of whiskers on the coating surface was observed. Small hill-like protrusions were formed on the coating surface, and the number of hills was slightly more than that of the tin coating in Example 1. No whisker growth was observed.

[0029] Example 2 A method for laser texturing of copper pads to suppress the growth of tin plating whiskers includes the following steps: Step S1, Pad pretreatment: Select copper pad 1 of a conventional printed circuit board, degrease it for 10 minutes at 40°C with an alkaline degreaser, clean the surface with deionized water using ultrasonic cleaning, pickle it with dilute sulfuric acid solution at 30°C for 1 minute, rinse it with deionized water, and dry it with hot air at 90°C for 8 minutes. Step S2, Laser texturing: A picosecond pulsed laser with a wavelength of 1064nm was used, with a pulse width of 8ps, an average power of 5W, a focusing lens focal length of 100mm, and a focused spot diameter of 6μm. The concentric circle array pad surface pits 2 were processed in a single-point pulse dotting method. The pits 2 had a diameter of 6μm and a depth of 4μm. After processing, an ultrasonic cleaning device with an ultrasonic frequency of 50kHz was used to ultrasonically clean the pads with deionized water for 4 minutes. The laser-textured pads were then vacuum dried for 10 minutes at 70℃ and a vacuum degree of 0.09MPa. Step S3, Tin plating treatment: Use a 5% volume fraction dilute sulfuric acid solution to activate the solution at 30°C for 2 minutes, rinse with deionized water and vacuum dry; use an acidic tin sulfate electroplating solution to electroplat for 25 minutes at 30°C and a current density of 0.7A / dm², so that the tin layer fills the pits 2 on the surface of the pad, and the thickness of the tin plating layer 3 on the surface of the pad is 15μm.

[0030] like Figure 5 As shown, the coating treated in this embodiment was accelerated to aging at 85°C and 85% relative humidity for 1000 hours, and the growth of whiskers on the coating surface was observed. Small hill-shaped protrusions were formed on the tin coating surface, and the number of hills was the smallest, indicating that the coating in Example 3 had the lowest stress during the heat preservation process and no whisker growth was observed.

[0031] Example 3 A method for laser texturing of copper pads to suppress the growth of tin plating whiskers includes the following steps: Step S1, Pad pretreatment: Select copper pad 1 of a conventional printed circuit board, degrease it at 60°C for 6 minutes with an alkaline degreaser, clean the surface with deionized water using ultrasonic cleaning, then pickle it with dilute sulfuric acid solution at 30°C for 3 minutes, rinse it with deionized water, and dry it with hot air at 90°C for 10 minutes. Step S2, Laser texturing: A picosecond pulsed laser with a wavelength of 1064nm was used, with a pulse width of 15ps, an average power of 12W, a focusing lens focal length of 160mm, and a focused spot diameter of 8μm. The concentric circle array pads were textured with single-point pulse dotting, resulting in pits 2 with a diameter of 10μm and a depth of 5μm. After processing, the pads were ultrasonically cleaned with deionized water for 4 minutes using an ultrasonic cleaning device with an ultrasonic frequency of 50kHz. The laser-textured pads were then vacuum-dried for 10 minutes at 70℃ and a vacuum degree of 0.09MPa. Step S3, Tin plating treatment: Use a 5% volume fraction dilute sulfuric acid solution to activate the solution at 30°C for 2 minutes, rinse with deionized water and vacuum dry; use an acidic tin sulfate electroplating solution to electroplat for 25 minutes at 30°C and a current density of 0.7A / dm², so that the tin layer fills the pits 2 on the surface of the pad, and the thickness of the tin plating layer 3 on the surface of the pad is 10μm.

[0032] The working process of this invention is as follows: like Figure 2 As shown, after the copper pad 1 enters the acidic tin sulfate plating solution, both the pit 2 and the surface of the copper pad 1 begin to crystallize simultaneously. The crystals at the bottom of the pit 2 grow vertically, while those on the sidewalls grow horizontally. Due to the sharp corners at the bottom and top of the pit 2, the current density is relatively high when energized, resulting in the fastest deposition rate. The sharp corners exhibit oblique growth, and the crystals at the bottom, sidewalls, and sharp corners collide and are compressed, resulting in a uniform equiaxed crystal structure in the subsequent crystal formation within the pit 2. As the plating layer thickens, the equiaxed crystals in the pit 2 extend and collide and are compressed with the columnar crystals on the surface of the copper pad 1, further contributing to the uniform equiaxed crystal structure in the subsequent crystal formation within the pit 2. When the plating crystals overflow from the pit 2, they interact with the columnar crystals already formed on the plane, interrupting the continued formation of the columnar crystals and causing the subsequent plating layer to become an equiaxed crystal structure. The equiaxed crystal structure features uniform grain size and numerous grain boundaries, effectively blocking stress transmission paths, dispersing residual stress within the plating layer and stress generated during service, and eliminating the stress-driven conditions for whisker growth. Meanwhile, the equiaxed crystal structure is more dense and uniform, which can suppress the rapid diffusion and migration of tin atoms along a specific direction and block the transfer of tin atoms required for the growth of tin whiskers, thereby achieving long-term and significant suppression of the growth of tin coating whiskers.

[0033] Comparative Example 1 A pure copper pad 1 of the same specifications as in Example 1 was selected. Without laser texturing, only the pretreatment, activation, and electroplating tin processes identical to those in Example 1 were used to prepare a conventional tin-plated copper pad 1 sample. Figure 6 As shown, the coating of Comparative Example 1 was subjected to accelerated aging at 85°C and 85% relative humidity for 1000 hours, resulting in the formation of fine whiskers and raised hillocks on the coating surface. Some whiskers reached a length of 20 μm, indicating a high risk of whisker growth.

[0034] The samples from Examples 1-3 and Comparative Example 1 were subjected to accelerated aging at 85°C and 85% relative humidity for 1000 hours, and the growth of tin whiskers was observed. Significant whisker growth was observed on the surface of the Comparative Example 1 sample, with some whiskers reaching lengths of up to 20 μm, posing a potential short-circuit failure hazard. The samples from Examples 1-3 showed partial protrusions on their surfaces, with no obvious long whisker formation, demonstrating a significant whisker growth inhibition effect.

Claims

1. A laser texturing structure for copper pads to suppress the growth of tin plating whiskers, characterized in that, The copper pad (1) includes a pit (2) formed by laser texturing on the surface of the copper pad (1); the pit (2) is distributed in a concentric circle array, with a diameter of 6-10 μm and a depth of 3-5 μm; after the laser texturing is completed, the copper pad (1) is tin-plated by electroplating.

2. A method for laser texturing copper pads to suppress the growth of tin plating whiskers, used to form a laser texturing structure for copper pads that suppresses the growth of tin plating whiskers, characterized in that, Includes the following steps: Step S1, Pad pretreatment: The copper pads (1) are degreased, washed with water, acid-washed, washed with water again, and dried in sequence; Step S2, Laser texturing: The pre-treated copper pads (1) are textured using a picosecond pulse laser to form concentric arrays of pits (2); the surface-treated copper pads (1) are then ultrasonically cleaned and dried. Step S3, Tin plating: The surface-treated copper pads (1) are subjected to surface activation, water washing and drying in sequence; The pit (2) is filled by electroplating or chemical tin plating to form a uniform tin plating layer on the surface.

3. The method for laser texturing of copper pads to suppress the growth of tin plating whiskers according to claim 2, characterized in that, In step S1, an alkaline degreasing agent is used for degreasing at a temperature of 40-60℃ for 6-10 minutes; the sample surface is ultrasonically cleaned with deionized water for washing; dilute sulfuric acid solution is used for acid washing at a temperature of 30℃ for 1-3 minutes; and hot air drying is used at a temperature of 90℃ for 8-12 minutes.

4. The method for laser texturing of copper pads to suppress the growth of tin plating whiskers according to claim 2, characterized in that, In step S2, the laser texturing process uses a picosecond pulsed laser with a wavelength of 1064nm, a pulse width of 8-15ps, an average power of 5-12W, a focusing lens focal length of 100-160mm, a focused spot diameter of 6-8μm, and a single-point pulse dotting method.

5. The method for laser texturing of copper pads to suppress the growth of tin plating whiskers according to claim 2, characterized in that, In step S2, the pits (2) after laser texturing are ultrasonically cleaned with deionized water at a frequency of 50 kHz for 4 min. The cleaned pits (2) are then vacuum dried at a temperature of 70 ℃ and a vacuum of 0.09 MPa for 10 min.

6. The method for laser texturing of copper pads to suppress the growth of tin plating whiskers according to claim 2, characterized in that, In step S3, the pits (2) after laser texturing are pretreated before tin plating. The surface activation is carried out with a 5% volume fraction of dilute sulfuric acid solution, the pickling temperature is 30℃, and the pickling time is 2min. The activated pits (2) are cleaned with deionized water and then vacuum dried.

7. The method for laser texturing of copper pads to suppress the growth of tin plating whiskers according to claim 2, characterized in that, In step S3, the pretreated pit (2) is electroplated with tin. The electroplating solution is acidic tin sulfate electroplating solution, the electroplating temperature is 30℃, the current density is 0.7A / dm², and the electroplating time is 25min, so that the plating layer fills the pit (2). The thickness of the tin plating layer on the surface of the pit (2) structure is 10-15μm.

Images