Method for selectively removing gold from high-density bonding pad

Abstract

The invention discloses a method for selectively removing gold from a high-density bonding pad. The method comprises the following steps: S1, carrying out soldering paste printing welding on the bonding pad on the surface of a substrate; s2, a composite shielding tool is fixedly placed on the surface of the substrate, and the composite shielding tool shields a non-gold-removing area on the bonding pad; s3, the substrate is heated, and the soldering flux is used for activating the bonding pad; s4, a red copper net is placed on the surface of the bonding pad, and soldering tin formed by melting the soldering paste is sucked through the red copper net; and S5, repeating the step S4 until no convex soldering tin exists on the surface of the bonding pad. Through the huge inner surface area and dense gaps of the high-mesh red copper mesh, a strong capillary acting force is generated, tin and copper instantly form an alloy compound on the surface of the high-activation red copper mesh, and a plurality of bonding pad molten soldering tin can be sucked at the same time, so that the problem of incomplete gold removal or bonding pad damage during selective gold removal of the bonding pad is solved, and the service life of the bonding pad is prolonged. The selective and efficient gold removal of the high-density bonding pad is realized, and the reliability and stability of a product are improved.

Description

Technical Field

[0001] This invention relates to the field of electronic packaging technology, and in particular to a method for selective gold removal from high-density pads. Background Technology

[0002] In the fields of microelectronic packaging and high-reliability soldering, pad surface treatment is crucial to interconnect quality. To improve the solderability and oxidation resistance of pads, most manufacturers in the industry use gold plating for electronic component pads. However, excessively thick gold layers can quickly form gold-tin intermetallic compounds (such as AuSn4) with the tin in the solder. These compounds tend to accumulate at the interface, and their brittleness and low strength lead to a significant decrease in the mechanical strength of the solder joint, causing early failure and severely affecting the reliability of the product package. Therefore, gold removal treatment of gold-plated pads before soldering has become a key pretreatment step in improving the long-term reliability of solder joints in high-reliability military or aerospace products.

[0003] Currently, the common solder pad gold removal processes in the industry are divided into four methods: chemical etching, machine polishing, laser ablation, and manual gold removal with desoldering tape. Chemical etching uses highly toxic cyanide reagents. Although it can achieve localized gold removal, it has problems such as environmental risks, personnel safety risks, difficult waste liquid treatment, easy lateral etching damage to surrounding coatings or substrates, and extreme difficulty in accurately locating the gold removal area. Machine polishing uses fiber pens or erasers for manual removal. Although the operation is simple, it is easy to cause solder pad contamination or scratches, poor manual consistency, and extremely low efficiency. Although laser etching meets the high-precision gold removal requirements of products, it has problems such as expensive equipment, complex process, and easy damage to the underlying nickel layer or heat-sensitive substrates such as ceramics if the heat input is not properly controlled. Manual gold removal with desoldering tape is currently the most commonly used manual processing method in the industry. Technicians often use solder resist to isolate the non-gold removal area and use a soldering iron tip to assist in heating the desoldering tape so that the desoldering tape can fully absorb the solder on the surface of the solder pad. While this method ensures complete gold removal from solder pads, the size limitations of the desoldering wick and the potential overheating from the soldering iron tip's auxiliary heat source can damage the pad substrate. Therefore, the desoldering wick gold removal solution cannot meet the efficiency requirements for simultaneous and rapid gold removal from large-area solder pads in production. Thus, a selective gold removal method with good selectivity, high efficiency, low cost, environmental friendliness, non-toxicity, and controllable heat-affected zone is needed for localized gold removal of high-density solder pads to improve production efficiency and quality. Summary of the Invention

[0004] To address the shortcomings of the existing technology, the technical problem to be solved by this invention is to provide a method for selective gold removal from high-density solder pads. This method utilizes the large inner surface area and dense pores of a high-mesh copper mesh to generate a strong capillary force. On the highly activated surface of the copper mesh, tin and copper instantly form an alloy compound. Its high thermal conductivity further locks the alloy tin within the micropores of the copper mesh, simultaneously drawing molten solder from multiple solder pads. This solves the problem of incomplete gold removal or damage to the solder pads during selective gold removal, achieving high-density selective and efficient gold removal from solder pads, and improving product reliability and stability.

[0005] One technical solution adopted by this invention is: providing a method for selective gold removal from high-density solder pads, comprising the following steps: S1: Solder paste is printed and soldered onto the pads on the substrate surface; S2: Fix a composite shielding fixture on the substrate surface and make the composite shielding fixture shield the non-gold removal area on the pad; S3: Heat the substrate and activate the pads with flux; S4: Place a copper mesh on the surface of the pads and use the copper mesh to absorb the solder formed by the melting of the solder paste; S5: Repeat step S4 until there are no protruding solder pads on the surface.

[0006] Furthermore, it also includes the following steps: S6: Inspect the substrate using a microscope. If the surface of the pads is smooth and flat, without any spikes, excess material, or gold residue, then the gold removal operation of the pads is complete.

[0007] Furthermore, the composite shielding fixture is made of a shielding steel sheet and a shielding polyimide sheet, with the shielding steel sheet having a thickness of 0.08-0.15mm and the shielding polyimide sheet having a thickness of 0.1-0.3mm.

[0008] Furthermore, step S1 includes the following sub-steps: S11: Apply solder paste to the areas on the pad surface that require gold removal using a conventional solder paste printing process; S12: The substrate coated with solder paste is soldered using a conventional soldering process, so that the gold plating layer of the solder pads reacts fully with the solder formed by the melting of the solder paste to form a gold-tin compound.

[0009] Furthermore, the thickness of the solder paste coating is 0.1-0.4 mm.

[0010] Furthermore, step S2 includes the following sub-steps: S21: Precisely align the composite masking fixture with the surface of the substrate pads so that the areas on the pads that need gold removal are fully exposed, while the areas that do not need gold removal are fully masked. S22: Use adhesive bonding to fix the composite shielding fixture to the substrate.

[0011] Furthermore, the heating of the substrate includes the following sub-steps: S31: Place the substrate on the preheating table and preheat it to a temperature of 140-160℃. S32: Place the preheated substrate on the heating table and heat it to a temperature of 220-250℃.

[0012] Furthermore, step S4 includes the following sub-steps: S41: Cut copper mesh to the corresponding size according to the pad size; S42: Place the copper mesh on the surface of the pad; S43: Add flux to the copper mesh; S44: Apply pressure to the top of the copper mesh to ensure full contact between the copper mesh and the solder on the pads.

[0013] Furthermore, the mesh count of the copper mesh is 200-350.

[0014] Furthermore, the flux is a liquid or solid flux.

[0015] The method for selective gold removal from high-density pads according to the present invention has at least the following beneficial effects: 1. The high-mesh copper mesh of this invention, with its high thermal conductivity, solderability, and special three-dimensional mesh structure, allows for the removal of gold-tin compounds from the substrate pads during the contact process between the copper mesh and the molten solder on the pads. The adhesion force of the liquid surface is greater than the cohesive force of the copper mesh itself, causing the solder to automatically climb and spread along the micropores, thus achieving the removal of gold from the substrate pads. This invention innovatively uses a high-mesh copper mesh, leveraging its large internal surface area and dense porosity to generate a strong capillary force. On the highly activated copper mesh surface, tin and copper instantly form an alloy compound. Its high thermal conductivity further locks the alloy tin within the micropores of the copper mesh, simultaneously drawing in molten solder from multiple pads, achieving selective and efficient gold removal from high-density pads, and improving product reliability and stability.

[0016] 2. This invention addresses the selective gold removal of high-density gold-plated pads. Operators use a high-mesh copper mesh for selective gold removal at high temperatures. A composite shielding fixture effectively blocks non-gold removal areas, preventing solder splatter from contaminating the interconnect gold areas. The overall operation time is approximately 2-3 minutes, suitable for high-volume gold removal production. The removed pads are smooth, flat, and free of spikes and burrs, meeting the quality requirements of QJ3267 and ensuring the long-term reliability of products in high-reliability fields. Attached Figure Description

[0017] The accompanying drawings, which are included to provide a further understanding of this application and form part of this application, illustrate exemplary embodiments and are used to explain this application, but do not constitute an undue limitation of this application. In the drawings: Figure 1 This is a flowchart of the method for selective gold removal from high-density pads according to the present invention.

[0018] Figure 2 This is a schematic diagram of the gold removal area of ​​the solder pads in this invention.

[0019] Figure 3 This is a schematic diagram of selective gold removal from high-density pads according to the present invention.

[0020] Explanation of reference numerals in the attached figures: Gold Removal Area -1; Non-Gold Removal Area -2; Heating Table -3; Solder Pads -4; Copper Mesh -5; Composite Shielding Fixture -6 Detailed Implementation The invention will now be further described with reference to the accompanying drawings.

[0021] Please see Figure 1 This is a flowchart of the method for selective gold removal from high-density solder pads according to the present invention. A method for selective gold removal from high-density solder pads includes the following steps: S1: Solder paste is printed and soldered onto the pads on the substrate surface. This S1 step involves applying solder paste to the pad surface and heating it (soldering) to melt the paste, causing a metallurgical reaction with the gold layer on the pad surface to form a gold-tin compound (such as AuSn4). This S1 step converts the gold layer into a removable compound, preparing it for subsequent desoldering and gold removal. It ensures the gold layer is effectively converted into a brittle compound, avoiding the difficulties of direct gold removal. By controlling the reaction conditions, it lays the foundation for subsequent gold removal steps, improving the thoroughness and efficiency of gold removal.

[0022] This S1 step may include the following sub-steps: S11: Apply solder paste to the areas on the pad surface that require gold removal using a conventional solder paste printing process. The conventional solder paste printing process in this S11 step is the conventional SMT process, such as printing the pads on the substrate surface using a conventional stencil printing process or a high-precision dot coating process. This ensures that the solder paste is accurately applied to the target pads, ensuring that each pad requiring gold removal is evenly covered. At the same time, the amount and distribution of solder paste are controlled, providing consistent conditions for subsequent reactions and avoiding incomplete gold removal or waste caused by insufficient or excessive coating.

[0023] S12: The substrate is soldered using a conventional soldering process, allowing the gold plating layer on the pads to fully react with the solder formed by the melting of the solder paste, forming a gold-tin compound. The conventional soldering process in step S12 is a typical SMT process, such as soldering the pads on the substrate surface using conventional reflow soldering or manual soldering. Heating melts the solder paste, causing it to chemically react with the gold layer, forming a brittle gold-tin intermetallic compound. This transforms the gold layer into an easily removable compound, creating conditions for the desoldering step. The thoroughness of the reaction improves gold removal efficiency and reduces the negative impact of the gold layer on the reliability of the solder joints.

[0024] In addition, the thickness of the solder paste can be controlled between 0.1-0.4mm, which ensures that there is enough solder to react with the gold layer without too much causing residue. It can also optimize the reaction efficiency and avoid the problems of too much solder (difficult to remove) or too little solder (insufficient reaction). It improves the repeatability of the process and the flatness of the pad surface.

[0025] S2: A composite masking fixture is fixedly placed on the substrate surface, and the fixture blocks the non-gold removal areas on the pads. This S2 step uses the composite masking fixture to precisely cover the non-gold removal areas on the substrate, exposing only the pads requiring gold removal. The fixture's perforated design enables selective masking, preventing solder splatter or contamination of non-target areas during the gold removal process. This improves the selectivity and precision of the gold removal process, protects the integrity and cleanliness of the non-gold removal areas, reduces the risk of contamination, and ensures consistent gold removal for high-density pads. It is suitable for substrates with complex layouts.

[0026] In some embodiments, the composite shielding fixture can be made of a shielding steel sheet and a shielding polyimide sheet. The shielding steel sheet is about 0.08-0.15mm thick, and the shielding polyimide sheet is about 0.1-0.3mm thick. The shielding tool combines softness and hardness, fills the gaps of slight deformation, effectively shields non-gold removal areas, prevents contamination in non-gold removal areas, and ensures effective selective gold removal.

[0027] This S2 step also includes the following sub-steps: S21: Precisely align the composite masking fixture with the surface of the substrate pads to fully expose the areas on the pads that need gold removal. This step S21 ensures that the fixture openings are aligned with the gold removal areas of the pads through precise alignment, achieving selective exposure, improving the positioning accuracy of the gold removal operation, avoiding accidental masking or insufficient exposure, and ensuring the selectivity and consistency of gold removal.

[0028] S22: Secure the composite shielding fixture to the substrate using adhesive bonding. Step S22 uses tape or fluid adhesive to fix the composite shielding fixture to the substrate, preventing displacement during heating or operation. This ensures the stability of the fixture and avoids contamination or operational errors caused by movement. The adhesive bonding method is simple, reliable, and suitable for mass production.

[0029] S3: The substrate is heated, and a small amount of flux is used to activate the exposed pads. This S3 step remelts the solder paste through heating, while the flux removes the oxide layer from the pad surface, activating the pads and solder surfaces and improving their wettability and flowability. Heating is typically performed in stages (e.g., preheating and main heating) to control heat input. The flux effectively removes oxides, preventing oxidation from hindering the solder absorption process; heating ensures the solder is in a molten state, facilitating copper mesh adsorption; overall, it improves the efficiency and reliability of the gold removal reaction and reduces the risk of heat damage.

[0030] The "heating the substrate" step in step S3 may also include the following sub-steps: S31: Place the substrate on the preheating table for preheating at a temperature of 140-160℃. In this step S31, the substrate is gradually heated by the preheating table, so that the substrate and solder paste are slowly heated, reducing thermal shock and preventing the substrate (especially heat-sensitive materials such as ceramics) from cracking or being damaged due to sudden temperature changes. Preheating improves the heating uniformity and prepares the substrate for subsequent main heating.

[0031] S32: Place the preheated substrate on the heating stage and heat it to a temperature of 220-250℃. This S32 uses the heating stage to completely melt the solder paste, reaching the reaction temperature required for gold removal. This temperature range ensures that the solder is fully melted, facilitating the absorption of solder by the copper mesh; it avoids excessively high temperatures (damaging the substrate) or excessively low temperatures (incomplete reaction), thus optimizing the gold removal effect.

[0032] S4: Place a copper mesh on the solder pad surface to absorb the molten solder paste. In this S4 step, the micron-sized pores and high thermal conductivity of the copper mesh allow for capillary action to absorb molten solder at high temperatures, simultaneously carrying away gold-tin compounds. The wettability of the copper mesh with the solder enables rapid solder retention, allowing for simultaneous gold removal from multiple pads and improving operational efficiency. The mesh structure of the copper mesh ensures thorough solder absorption, preventing solder residue. Compared to traditional solder wicks, heat conduction is more uniform, reducing the risk of localized overheating.

[0033] This S4 step may also include the following sub-steps: S41: Cuts copper mesh to the corresponding size according to the pad size; This S41 cuts the copper mesh according to the pad size to match the pad area and ensures complete coverage. It can improve the utilization rate of copper mesh and its adaptability to the pads, reduce material waste, while the cutting accuracy (0.1mm) ensures targeted desoldering and improves efficiency.

[0034] S42: Place the copper mesh on the surface of the solder pad; This step S42 initiates the solder absorption process by bringing the copper mesh into direct contact with the molten solder, utilizing the high thermal conductivity of the copper mesh to quickly transfer heat and promote capillary action; thus achieving rapid solder absorption.

[0035] S43: Add flux to the copper mesh; This S43 step removes the oxide layer on the surface of the copper mesh by using flux, activates its surface, and enhances its wettability with solder, thereby ensuring the solder absorption capacity of the copper mesh and avoiding adsorption failure caused by oxidation; The small amount of flux (0.1g weight precision) optimizes chemical activity and environmental friendliness.

[0036] S44: Apply pressure to the top of the copper mesh to ensure full contact between the copper mesh and the solder on the pads. This S44 step ensures close contact between the copper mesh and the solder through slight pressure, promoting capillary action, improving the thoroughness and speed of solder absorption, and avoiding solder residue caused by poor contact; pressure control prevents excessive squeezing from damaging the pads.

[0037] It is worth mentioning that the copper mesh in this solution is a high-mesh copper mesh, with a mesh count of 200-350. This mesh count range balances the adsorption force and the risk of pore blockage. The 200-350 mesh provides sufficient surface area and capillary force to ensure rapid solder lock-in while maintaining the structural integrity of the mesh.

[0038] In addition, clean, highly activated liquid or solid flux should be used, and the type can be selected according to process requirements, which improves flexibility; liquid flux provides uniform coverage, while solid flux releases its activity slowly during heating, both of which ensure activation effect and improve process adaptability.

[0039] S5: Repeat step S4 above until there is no protruding solder on the surface of the pads. This step S5 can be achieved by repeatedly replacing the copper mesh and repeating the desoldering operation to ensure that the solder on the surface of the pads is completely removed and reaches a smooth and flat state. This solution can ensure complete removal of solder by replacing 2-3 copper meshes, thereby ensuring the stability and consistency of gold removal quality, avoiding spikes, burrs or gold residue on the surface of the pads, and meeting the process requirements of high-reliability products (such as military and aerospace).

[0040] S6: Inspect the substrate using a microscope. If the pad surface is smooth and flat, without spikes, excess material, or gold residue, the gold removal operation is complete. This S6 step serves as a quality inspection step, verifying the surface condition of the pads after gold removal under a microscope to ensure compliance with standards (such as GJB548 or QJ3267). Through microscopic inspection, defects (such as gold residue or unevenness) are detected early, improving the long-term reliability and environmental adaptability of the product; and ensuring the traceability and quality control of the gold removal process.

[0041] The above description merely illustrates preferred embodiments of the present invention and is quite specific and detailed; however, it should not be construed as limiting the scope of the invention. It should be noted that those skilled in the art can make various modifications and improvements without departing from the inventive concept, and these all fall within the scope of protection of the present invention. Therefore, the scope of protection of this invention should be determined by the appended claims.

Claims

1. A method for selective gold removal from high-density solder pads, characterized in that, Includes the following steps: S1: Solder paste is printed and soldered onto the pads on the substrate surface; S2: Fix a composite shielding fixture on the substrate surface and make the composite shielding fixture shield the non-gold removal area on the pad; S3: Heat the substrate and activate the pads with flux; S4: Place a copper mesh on the surface of the pads and use the copper mesh to absorb the solder formed by the melting of the solder paste; S5: Repeat step S4 until there are no protruding solder pads on the surface.

2. The method for selective gold removal from high-density pads as described in claim 1, characterized in that, It also includes the following steps: S6: Inspect the substrate using a microscope. If the surface of the pads is smooth and flat, without any spikes, excess material, or gold residue, then the gold removal operation of the pads is complete.

3. The method for selective gold removal from high-density pads as described in claim 1, characterized in that, The composite shielding fixture is made of a shielding steel sheet and a shielding polyimide sheet. The thickness of the shielding steel sheet is 0.08-0.15mm, and the thickness of the shielding polyimide sheet is 0.1-0.3mm.

4. The method for selective gold removal from high-density pads as described in claim 3, characterized in that, Step S1 includes the following sub-steps: S11: Apply solder paste to the areas on the pad surface that require gold removal using a conventional solder paste printing process; S12: The substrate coated with solder paste is soldered using a conventional soldering process, so that the gold plating layer of the solder pads reacts fully with the solder formed by the melting of the solder paste to form a gold-tin compound.

5. The method for selective gold removal from high-density pads as described in claim 4, characterized in that, The thickness of the solder paste coating is 0.1-0.4 mm.

6. The method for selective gold removal from high-density pads as described in claim 1, characterized in that, Step S2 includes the following sub-steps: S21: Precisely align the composite masking fixture with the surface of the substrate pads so that the areas on the pads that need gold removal are fully exposed, while the areas that do not need gold removal are fully masked. S22: Use adhesive bonding to fix and limit the composite shielding fixture to the substrate.

7. The method for selective gold removal from high-density pads as described in claim 1, characterized in that, The heating of the substrate includes the following sub-steps: S31: Place the substrate on the preheating table and preheat it to a temperature of 140-160℃. S32: Place the preheated substrate on the heating table and heat it to a temperature of 220-250℃.

8. The method for selective gold removal from high-density pads as described in claim 1, characterized in that, Step S4 includes the following sub-steps: S41: Cut copper mesh to the corresponding size according to the pad size; S42: Place the copper mesh on the surface of the pad; S43: Add flux to the copper mesh; S44: Apply pressure to the top of the copper mesh to ensure full contact between the copper mesh and the solder on the pads.

9. The method for selective gold removal from high-density pads as described in claim 8, characterized in that, The copper mesh has a mesh count of 200-350.

10. A method for selective gold removal from high-density pads as described in claim 1 or 8, characterized in that, The flux is a liquid or solid flux.

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