A micro-corrosion solution and its application in revealing the Cu-Ni interface boundary of elemental metal coatings.

Abstract

This invention provides a micro-etching solution and its application in revealing the Cu-Ni interface boundary line of elemental metal coatings. The micro-etching solution comprises a hydrogen peroxide solution with a mass-volume percentage concentration of 2.5%-15%. The use of dilute hydrogen peroxide solution can clearly reveal the Cu-Ni interface boundary line of elemental metal coatings, enabling accurate measurement of the metal coating thickness and reducing SEM image measurement errors. The micro-etching solution of this invention has a simple composition, can be replaced by medical-grade hydrogen peroxide, is inexpensive, safe to use, has minimal hazard, and a long shelf life, thus having broad application prospects.

Description

Technical Field

[0001] This invention belongs to the field of metal surface treatment technology, and relates to a micro-corrosion liquid and its application in revealing the Cu-Ni interface boundary line of elemental metal coatings. Background Technology

[0002] On printed circuit boards (PCBs), Cu substrates are often treated with immersion gold to prevent oxidation. However, during the immersion gold process, gold wires are often generated, causing short circuits. Therefore, before immersion gold, the Cu substrate surface is treated with nickel plating to isolate the diffusion between Au and Cu. At the same time, the solder layer also needs to have a sufficient thickness of nickel layer.

[0003] For PCB Cu substrates treated with nickel-gold plating, cross-sectional slices (X-sections) are fabricated, and SEM is used to measure the nickel-gold plating thickness to confirm the plating quality. Since SEM backscattered electron (BSE) imaging relies on the intensity of the physical signal carried by BSE electrons received by the signal receiver, which alters the grayscale level of the image, areas with higher atomic numbers exhibit higher BSE yields and appear as bright areas in the image. Because Ni (atomic number 28) and Cu (atomic number 29) have similar atomic numbers, their image grayscale levels are similar during SEM imaging. Therefore, when nickel is plated onto the copper surface, the boundary between the Ni and Cu layers cannot be effectively distinguished during SEM imaging, making it impossible to measure the Ni layer thickness.

[0004] Currently, common metal etching solutions include H2O2-ammonia-water etching solutions or sulfuric acid-containing layering agents. However, H2O2-ammonia-water etching solutions used for etching metals have low hydrogen peroxide content, are complex to prepare, and the volatility of ammonia limits the effective etching time to only 2 hours, otherwise the etching effect will be affected. Furthermore, these etching solutions are mostly used for etching the boundary between electroplated copper and electroless copper. Sulfuric acid-containing layering agents, used for etching the surface of intermetallic compounds formed by welding, have poor short-term etching effects due to the slow oxidation of Ni and Cu by dilute sulfuric acid. Therefore, layering agents have poor time-limited effectiveness for Cu-Ni boundary etching. Even after 5 minutes of etching with a layering agent, the boundary between the Ni and Cu layers on the Cu-Ni cross-section remains unclear, with only slight corrosion marks on the copper layer and uneven corrosion.

[0005] The existing technologies described above cannot achieve clear layering at the Cu-Ni interface of a single-element metal plating layer. Therefore, when the thickness of the metal plating layer needs to be confirmed on a printed circuit board, accurate measurement is not possible. Thus, in this field, it is desirable to develop a micro-etching solution that can achieve clear layering at the Cu-Ni interface of a single-element metal plating layer. Summary of the Invention

[0006] To address the shortcomings of existing technologies, the present invention aims to provide a micro-etching solution and its application in revealing the Cu-Ni interface boundary line of elemental metal coatings, particularly providing a micro-etching solution for clearly revealing the Cu-Ni interface boundary line of elemental metal coatings and its application.

[0007] To achieve this objective, the present invention adopts the following technical solution:

[0008] On one hand, the present invention provides a micro-etching solution for revealing the boundary line of the Cu-Ni interface of elemental metal coatings, the micro-etching solution comprising a hydrogen peroxide aqueous solution with a mass-volume percentage concentration of 2.5%-15%.

[0009] In this invention, a dilute hydrogen peroxide solution is used to clearly reveal the boundary line at the Cu-Ni interface of the elemental metal plating layer, so that the thickness of the metal plating layer can be accurately measured. Only by accurately confirming the thickness of each plating layer, especially the Ni layer thickness, can we ensure that there is enough Ni thickness in the printed circuit board to prevent diffusion between Au and Cu, while also meeting the soldering requirements of the printed circuit board soldering layer.

[0010] The micro-corrosion solution of this invention has a simple composition and can be replaced by medical-grade hydrogen peroxide. It is inexpensive, safe to use, and poses minimal harm. Although hydrogen peroxide decomposes slowly at room temperature, this corrosion solution has a shelf life of up to six months, based on the expiration date of medical-grade hydrogen peroxide. The Cu-Ni metal interlayer boundary is clearer after corrosion, reducing SEM image measurement errors.

[0011] In this invention, the hydrogen peroxide solution is an aqueous solution of hydrogen peroxide. The concentration of the hydrogen peroxide solution used in this invention can be 2.5%-15%, such as 3%, 5%, 10%, 12%, or 15%, preferably 3-15%. If the concentration of the hydrogen peroxide solution is below 2.5%, the corrosion effect is not obvious due to the low concentration, and the Ni-Cu layer does not show a boundary line. If the concentration of the hydrogen peroxide solution is above 15%, honeycomb pits appear on the copper surface due to the excessive concentration, the surface morphology deteriorates, and loose copper oxide remains on the slice surface. Furthermore, special attention must be paid to the safety of using high-concentration hydrogen peroxide solutions.

[0012] In this invention, the micro-etching time using hydrogen peroxide solution is 5s-15min, for example 30s, 1min, 3min, 5min, 10min, or 15min. The Cu-Ni layer can be clearly distinguished after 5s of etching. To obtain the optimal surface morphology, the etching time can be extended to 5min, but it is recommended not to exceed 30min, because excessive etching time will result in obvious honeycomb pits on the copper surface. Therefore, the preferred etching time is 30s-15min, and more preferably 30s-5min.

[0013] In summary, in this invention, the Ni-Cu surface morphology is optimal after etching with a 2.5%-15% hydrogen peroxide etchant for 30s-15min. When the hydrogen peroxide concentration is below 1% and the etching time is less than 5s, there is no significant corrosion, and the Cu-Ni layer does not show a clear boundary. When the hydrogen peroxide concentration is below 2% and the etching time is 5s-30s, slight corrosion occurs in the Cu-Ni layer, but the boundary is not obvious. However, when the hydrogen peroxide concentration is above 15% and the etching time is above 15min, honeycomb pits appear on the copper surface, leaving loose copper oxide and deteriorating the surface morphology. Furthermore, higher concentrations of hydrogen peroxide have strong oxidizing properties, requiring safety precautions during use, and prolonged etching time also reduces work efficiency.

[0014] On the other hand, the present invention provides a method for revealing the boundary line of the Cu-Ni interface of a single-element metal coating, the method using the micro-etching solution described above.

[0015] Preferably, the method includes the following steps:

[0016] (1) Slice the Cu-Ni coated sample and grind and polish the cross section of the sliced ​​sample;

[0017] (2) Add hydrogen peroxide solution with a mass percentage concentration of 2.5%-15% to the cross section after step (1) for micro-corrosion, and observe the boundary line of Cu-Ni interface of elemental metal coating under scanning electron microscope.

[0018] In this invention, the polishing in step (1) can be done with sandpaper.

[0019] Preferably, the polishing in step (1) is done by polishing with 180 grit, 800 grit, 2000 grit and 2400 grit sandpaper in sequence.

[0020] In this invention, the polishing in step (1) is a commonly used polishing method in the art. For example, it can be polished with alumina polishing liquid to ensure that there are no obvious scratches on the surface of the slice cross section.

[0021] In this invention, hydrogen peroxide solution is uniformly spread on the cross-section of the slice.

[0022] Preferably, for the slice cross-section after step (1) treatment, the amount of hydrogen peroxide solution used in step (2) is 1.0-1.5 mL / cm², calculated based on the area of ​​the cross-section. 2 For example, 1.0 mL / cm 2 1.1 mL / cm 2 1.2 mL / cm 2 1.3 mL / cm 2 1.4 mL / cm 2 Or 1.5 mL / cm2 .

[0023] Preferably, the micro-corrosion time in step (2) is 1s-120min, for example 1s, 3s, 5s, 20s, 30s, 50s, 1min, 3min, 5min, 10min, 30min, 50min, 60min, 80min, 100min or 120min, preferably 30s-5min, and more preferably 30s-5min.

[0024] In this invention, after the hydrogen peroxide solution is used to perform micro-etching on the cross-section of the slice after step (1), the residual etching solution on the surface of the slice is wiped off with a clean, lint-free cloth.

[0025] In this invention, by using the low-concentration hydrogen peroxide solution to perform micro-etching on the cross-section of the slice for a short time (5-30s), Ni and Cu can form different appearance morphologies and show obvious boundary lines, which can be clearly distinguished under SEM electron microscope.

[0026] In this invention, because Ni is more reactive than Cu, after the metal coating cross-section is fabricated, Ni is rapidly oxidized by O2 in the air, forming a thin, dense oxide layer on its surface. In contrast, the oxidation of Cu by O2 in the air is relatively slow. When using a lower concentration of hydrogen peroxide to oxidize Ni and Cu, the thin, dense oxide film already formed on the Ni surface of the cross-section prevents further oxidation of Ni by hydrogen peroxide. However, for the Cu layer, hydrogen peroxide oxidizes it, forming a thicker, relatively loose, porous oxide layer (CuO), and a passivation layer cannot be formed to further prevent Cu oxidation on the surface. Therefore, due to the different surface morphologies, a clear boundary appears at the interface between the two elemental metals, Ni and Cu.

[0027] The method described in this invention utilizes low-concentration hydrogen peroxide for micro-corrosion, which has low environmental requirements, is safe and reliable, can be carried out at normal temperature and pressure, and has a short corrosion time.

[0028] On the other hand, the present invention provides the application of the micro-etching solution or the method for revealing the Cu-Ni interface boundary line of elemental metal plating as described above in the quality control of printed circuit boards.

[0029] Compared with the prior art, the present invention has the following beneficial effects:

[0030] In this invention, a dilute hydrogen peroxide solution is used to clearly reveal the boundary line at the Cu-Ni interface of the elemental metal coating, enabling accurate measurement of the metal coating thickness and reducing SEM image measurement errors. The micro-corrosion solution of this invention has a simple composition, can be replaced by medical hydrogen peroxide, is inexpensive, safe to use, poses minimal harm, has a long shelf life, and has broad application prospects. Attached Figure Description

[0031] Figure 1 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Example 1.

[0032] Figure 2 The image is a scanning electron microscope image of the cross section of the slice after step (1) in Example 1 without micro-etching.

[0033] Figure 3 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 1.

[0034] Figure 4 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 2.

[0035] Figure 5 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 3.

[0036] Figure 6 This is a scanning electron microscope image of the cross-section of the sample after micro-etching in Example 2.

[0037] Figure 7 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Example 3.

[0038] Figure 8 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Example 4.

[0039] Figure 9 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 4.

[0040] Figure 10 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 5.

[0041] Figure 11 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 6.

[0042] Figure 12 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 7.

[0043] Figure 13 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 8.

[0044] Figure 14This is a scanning electron microscope image of the cross-section of the sample after micro-etching in Example 5.

[0045] Figure 15 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Example 6.

[0046] Figure 16 This is a scanning electron microscope image of the cross-section of the sample after micro-etching in Example 7.

[0047] Figure 17 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 9.

[0048] Figure 18 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 10.

[0049] Figure 19 This is a scanning electron microscope image of the cross-section of the sample after micro-corrosion in Comparative Example 11.

[0050] Figure 20 This is a scanning electron microscope (SEM) image of the cross-section of the sample after etching with a delamination agent in Comparative Example 12.

[0051] Figure 21 This is a scanning electron microscope (SEM) image of the cross-section of the sample in Comparative Example 13 after etching with H2O2-ammonia-water etching solution. Detailed Implementation

[0052] The technical solution of the present invention will be further illustrated below through specific embodiments. Those skilled in the art should understand that the embodiments described are merely illustrative of the present invention and should not be construed as limiting the invention in any way.

[0053] Example 1

[0054] This embodiment provides a micro-etching solution for clearly revealing the boundary line of the Cu-Ni interface of elemental metal coatings. The micro-etching solution is a 3% hydrogen peroxide aqueous solution.

[0055] The method for revealing the Cu-Ni interface boundary line of elemental metal coatings using this micro-etching solution specifically includes the following steps:

[0056] (1) The Cu-Ni coating sample was sliced, and the cross section of the sliced ​​sample was polished in sequence with 180 mesh, 800 mesh, 2000 mesh and 2400 mesh sandpaper, and then polished with alumina polishing liquid (TROJAN AO-W alumina polishing liquid with alumina particle size of 0.05um).

[0057] (2) Add 3% hydrogen peroxide solution to the cross-section of the slice treated in step (1), spread it evenly, and calculate the amount of hydrogen peroxide solution added based on the area of ​​the cross-section as 1.0 mL / cm². 2After micro-etching for 30 seconds, the boundary line of the Cu-Ni interface of the elemental metal coating was observed under a scanning electron microscope, such as... Figure 1 As shown, Figure 2 The image shows a scanning electron microscope (SEM) image of the cross-section of the sample without micro-corrosion (i.e., the cross-section of the slice after step (1)). Figure 1 As can be seen, the boundary line of the Cu-Ni interface is clearly visible, while from... Figure 2 As can be seen, the boundary line of the Cu-Ni interface in the cross-section of the uncorroded sample is not obvious.

[0058] Comparative Example 1

[0059] The only difference from Example 1 is that the micro-corrosion solution used is a 0.5% hydrogen peroxide solution.

[0060] Comparative Example 2

[0061] The only difference from Example 1 is that the micro-corrosion solution used is a 1.0% hydrogen peroxide solution.

[0062] Comparative Example 3

[0063] The only difference from Example 1 is that the micro-corrosion solution used is a 2.0% hydrogen peroxide solution.

[0064] Example 2

[0065] The only difference from Example 1 is that the micro-corrosion solution used is a 5.0% hydrogen peroxide solution.

[0066] Example 3

[0067] The only difference from Example 1 is that the micro-corrosion solution used is a 10% hydrogen peroxide solution.

[0068] Example 4

[0069] The only difference from Example 1 is that the micro-corrosion solution used is a 15% hydrogen peroxide solution.

[0070] Comparative Example 4

[0071] The only difference from Example 1 is that the micro-corrosion solution used is a 20% hydrogen peroxide solution.

[0072] Comparative Example 5

[0073] The only difference from Example 1 is that the micro-corrosion solution used is a 25% hydrogen peroxide solution.

[0074] Comparative Example 6

[0075] The only difference from Example 1 is that the micro-corrosion solution used is a 30% hydrogen peroxide solution.

[0076] To investigate the effect of different hydrogen peroxide concentrations on Cu-Ni corrosion, the morphologies of Au-Ni-Cu samples with different hydrogen peroxide concentrations were compared after the same corrosion time (30 s). When the hydrogen peroxide concentration was below 1%, no clear boundary appeared at the Cu-Ni interface (Comparative Example 1, the cross-sectional image of the sample after micro-corrosion is shown in Figure 1). Figure 3 As shown), 1%-2% only showed slight corrosion in the copper-exchange region at the Cu-Ni interface (Comparative Examples 2 and 3, whose cross-sectional images after micro-corrosion are shown in Figure 1). Figure 4 and Figure 5 (as shown); hydrogen peroxide concentration 3%-15% (Examples 1-4, electron micrographs of the cross-section of the samples after micro-etching are shown). Figure 1 , Figure 6-8 As shown in the figure, the Cu-Ni interface gradually becomes more distinct, and the Cu-Ni interface becomes more prominent with increasing corrosion concentration. The copper surface is uniformly corroded, and the copper layer surface tends to be smooth. After the hydrogen peroxide concentration is greater than 15% (Comparative Examples 4-6, the electron micrographs of the cross-section of the samples after micro-corrosion are shown in the figure), the copper surface is uniformly corroded, and the copper layer surface tends to be smooth. Figures 9-11 As shown in the figure, although there is a clear boundary line at the Cu-Ni interface, the copper layer surface has honeycomb-like pits due to the excessive concentration of hydrogen peroxide. The higher the concentration, the more obvious the honeycomb phenomenon becomes. At the same time, a large amount of debris remains on the surface of the copper layer, and the morphology quality of the copper layer becomes worse and worse.

[0077] Comparative Example 7

[0078] The only difference from Example 1 is that the corrosion time is 1 second.

[0079] Comparative Example 8

[0080] The only difference from Example 1 is that the corrosion time is 5 seconds.

[0081] Example 5

[0082] The only difference from Example 1 is that the corrosion time is 1 minute.

[0083] Example 6

[0084] The only difference from Example 1 is that the corrosion time is 5 minutes.

[0085] Example 7

[0086] The only difference from Example 1 is that the corrosion time is 15 minutes.

[0087] Comparative Example 9

[0088] The only difference from Example 1 is that the corrosion time is 30 minutes.

[0089] Comparative Example 10

[0090] The only difference from Example 1 is that the corrosion time is 60 minutes.

[0091] Comparative Example 11

[0092] The only difference from Example 1 is that the corrosion time is 120 min.

[0093] To determine the optimal etching time with hydrogen peroxide, the morphology of Cu-Ni alloys after etching with the same concentration (3%) of hydrogen peroxide for different etching times was compared. After 1 second of hydrogen peroxide etching, no clear boundary appeared at the Cu-Ni interface, and only slight corrosion was observed in the copper-exit region at the Cu-Ni interface (Comparative Example 7, whose cross-sectional electron microscope image after micro-etching is shown below). Figure 12 (As shown); hydrogen peroxide etching for 5s-30s, the Cu-Ni interface gradually becomes clear, but the copper surface corrosion is uneven, and a small amount of uncorroded areas remain on the copper surface (electron micrographs of the cross-sections of the samples after micro-etching in Comparative Example 8 and Example 1 are shown in the figure). Figure 13 and Figure 1 As shown in the figure, the uncorroded area gradually decreases with increasing etching time; hydrogen peroxide etching for 1-15 min (Examples 5-7, electron microscope images of the cross-section of the samples after micro-etching are shown in the figure). Figure 14-16 As shown in the figure, a clear boundary line appears at the Cu-Ni interface, and the copper surface is uniformly etched, with the copper layer surface tending to be smooth; hydrogen peroxide etching for 30-120 min (Comparative Examples 9-11, the electron microscope images of the sample cross-section after micro-etching are shown in the figure). Figure 17-19 As shown, although a clear boundary line appears at the Cu-Ni interface, honeycomb-like pits appear on the surface of the copper layer due to the long-term corrosion of hydrogen peroxide. The longer the corrosion time, the more obvious the honeycomb appearance becomes. At the same time, a large amount of velvety debris appears on the copper surface, and the morphology quality of the copper surface becomes worse and worse.

[0094] Comparative Example 12

[0095] The only difference from Example 1 is that a layering agent is used instead of the hydrogen peroxide solution used in Example 1. The layering agent is an alcohol-sulfuric acid mixture (25 mL of undiluted alcohol + 1 mL of 10% (weight%) sulfuric acid). The cross-section of the slice treated in step (1) is etched for 2 min using this layering agent, and the scanning electron microscope image of its Cu-Ni interface is as follows. Figure 20 As shown, by Figure 20 It can be seen that there is no clear boundary at the Cu-Ni interface.

[0096] Comparative Example 13

[0097] The only difference from Example 1 is that an H2O2-ammonia-water etching solution was used instead of the hydrogen peroxide solution used in Example 1. The H2O2-ammonia-water etching solution consisted of a 50 mL ammonia solution formed by mixing 32% ammonia with pure water in a 1:1 (V:V) ratio, followed by the addition of 1.0 mL of 36% hydrogen peroxide solution. This etching solution was used to etch the cross-section of the slice treated in step (1) for 2 minutes. The scanning electron microscope image of the Cu-Ni interface is shown below. Figure 21 As shown, by Figure 21 It can be seen that due to the low content of H2O2, no obvious boundary appeared at the Cu-Ni interface, and the etching solution was complicated to prepare and had a short shelf life.

[0098] The applicant declares that this invention illustrates the micro-corrosion solution of the present invention and its application in revealing the Cu-Ni interface boundary line of elemental metal coatings through the above embodiments. However, the present invention is not limited to the above process steps, that is, it does not mean that the present invention must rely on the above process steps to be implemented. Those skilled in the art should understand that any improvements to the present invention, equivalent substitutions of the raw materials used in the present invention, addition of auxiliary components, selection of specific methods, etc., all fall within the protection scope and disclosure scope of the present invention.

Claims

1. A micro-etching solution for revealing the boundary line of the Cu-Ni interface in elemental metal coatings, characterized in that, The micro-corrosion solution is a hydrogen peroxide solution with a mass-volume percentage concentration of 2.5%-15%.

2. The micro-etching solution for revealing the boundary line of the Cu-Ni interface in elemental metal coatings according to claim 1, characterized in that, The hydrogen peroxide solution has a mass-volume percentage concentration of 3%-15%.

3. A method for revealing the boundary line of the Cu-Ni interface in a single-element metal coating, characterized in that, The method uses the micro-etching solution as described in claim 1 or 2 for revealing the boundary line of the Cu-Ni interface of elemental metal coatings.

4. The method according to claim 3, characterized in that, The method includes the following steps: (1) Slice the Cu-Ni coated sample and grind and polish the cross section of the sliced ​​sample; (2) Add the micro-corrosion solution to the slice section after step (1) for micro-corrosion. The micro-corrosion solution is a hydrogen peroxide solution with a mass-volume percentage concentration of 2.5%-15%. Then observe the boundary line of the Cu-Ni interface of the elemental metal coating under a scanning electron microscope.

5. The method according to claim 4, characterized in that, The polishing described in step (1) is polishing with sandpaper.

6. The method according to claim 4, characterized in that, The polishing in step (1) involves polishing with 180 grit, 800 grit, 2000 grit and 2400 grit sandpaper in sequence.

7. The method according to claim 4, characterized in that, The polishing in step (1) is performed using an alumina polishing slurry.

8. The method according to claim 4, characterized in that, Based on the area of ​​the cross-section after step (1), the amount of hydrogen peroxide solution used in step (2) is 1.0-1.5 mL / cm². 2 .

9. The method according to claim 4, characterized in that, The micro-corrosion time in step (2) is 1s-120min.

10. The method according to claim 9, characterized in that, The micro-corrosion time in step (2) is 30s-15min.

11. The method according to claim 9, characterized in that, The micro-corrosion time in step (2) is 30s-5min.

12. The application of the micro-etching solution for revealing the Cu-Ni interface boundary line of a single-element metal plating layer according to claim 1 or 2, or the method for revealing the Cu-Ni interface boundary line of a single-element metal plating layer according to any one of claims 3-11, in the quality control of printed circuit boards.

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