Application of measurement of electroplated copper layer thickness on copper wire in drop test
A variety of materials are often used on the external electrodes of electronic components to form a multi-layer composite coating structure through electroplating to meet the overall performance of the product1. The coating thickness of the external electrode of electronic components directly affects the working performance, stability, reliability and lifespan of electronic components2. Therefore, accurate sample preparation and testing of electronic component coatings directly affects the accuracy of reliability research and failure analysis results.
Many electronic components on the market require three coatings of Cu, Ni and Sn to be plated on copper electrodes in sequence (Figure 1).
Figure 1 Schematic diagram of the external electrode structure of a certain product
The picture comes from Sunlord internal
Take reliability research as an example: When studying the drop reliability level of a certain component, it was found that some samples fell off from the welded substrate, and the peeling and breaking points generally occurred in the electroplated Cu layer of the sample (Figure 2).
Figure 2 Related pictures of a product’s drop test. (a) Before falling, it was welded to the substrate; (b) After falling, it fell off the substrate; (c) Slice view of the failure location.The picture comes from Sunlord internal
In order to verify the impact of the electroplated Cu layer thickness on drop reliability, it is necessary to measure the electroplated Cu layer thickness of samples with different drop levels. However, since the copper electrode and Cu plating are made of the same material and are densely connected, it is impossible to use optical microscopy (difference in metal optical color), Scanning Electronic Microscope (SEM) (difference in backscattered electron contrast) or energy-dispersive X-ray spectroscopy ( Energy Dispersive X-Ray Spectroscopy (EDS) (element difference) to distinguish (Figure 3).
Figure 3 Image of the original morphology of copper wire and its coating. (a) 500X metallographic image; (b) 3000X secondary electron image; (c) 3000X backscattered electron image; (d) Mapping of Cu, Ni and Sn.The picture comes from Sunlord internal
Figure 3(a): Optical microscope picture. Only light yellow-brown Cu and a thin gray-brown Ni coating can be seen. The electroplated Cu layer cannot be distinguished.
Figure 3(b): SEM-SE image, the electroplated Cu layer cannot be distinguished.
Figure 3(c): SEM-BSE image, the electroplated Cu layer cannot be distinguished.
Figure 3(d): EDS-Mapping picture, which can only distinguish Cu from the electroplated Ni layer, but cannot distinguish the electroplated Cu layer.
Therefore, it is necessary to find or develop a convenient and accurate method to observe and measure the Cu coating, so as to obtain the relationship between the Cu coating thickness and the drop test results.
Based on the principles of metallographic corrosion and optical imaging, this article introduces a method to distinguish the interface between the Cu electrode and the electroplated Cu layer by exposing it through corrosion.
Experimental methods and principle description
Metallographic sample preparation: Use 400# sandpaper to grind the sample to the electrode cross-section area, and then gradually use 800#, 1200#, 2000#, 2500# and 4000# sandpaper to grind. Each grinding will remove the scratches formed by the previous grinding. . Then use 3μm diamond suspension for rough polishing for 5 minutes, and then use 1μm diamond suspension for fine polishing for 3 minutes. Finally, the samples were rinsed and dried.
Corrosion method: The corrosive solution is made by uniformly mixing 10g FeCl3.6H2O, 10ml HCL and 100ml deionized water. All chemical materials used are of analytical grade. The corrosion method is to immerse the sample in the corrosive liquid. After the time is up, use bamboo tweezers to take out the sample and rinse it with plenty of water to remove the residual corrosive liquid on the surface. Finally, the samples were dried using an air gun.
Principle of metallographic corrosion: Metallic materials will react chemically with specific corrosive liquids and dissolve. However, the free energy between different crystal grains, different phases, and grain boundaries in metals is different, especially the interface between the Cu electrode and the electroplating layer. Due to the presence of micropores or microcracks, the free energy is more active, so it is easy to A particularly narrow groove is formed by corrosion, thus distinguishing the two.
Results and discussion
After corrosion: The metallographic morphology of the copper wire and its coating is shown in Figure 4.
Figure 4 500X metallographic images of copper wire and its coating at different corrosion times. (a) Corrosion for 60 seconds; (b) Corrosion for 90 seconds; (c) Corrosion for 150 seconds.The picture comes from Sunlord internal
Figure 4(a): After corrosion for 60 seconds, the boundary between the Cu electrode and the electroplated Cu layer appears vaguely at the arrow.
Figure 4(b): After 90s of corrosion, the boundary becomes more obvious.
Figure 4(c): After 150s of corrosion, the boundary is clearly visible.
Summary: Cu electrodes and electroplated Cu layers can be distinguished from sliced samples that have been specially etched.
Since copper will undergo a redox reaction with Fe3+ ions in the corrosive solution but the Ni plating does not react, the reaction equation is Cu+2Fe3+=Cu2++2Fe2+, so a height difference will be formed between the Ni plating and the Cu plating under a metallographic microscope. form an obvious interface. Due to different processes between the Cu electrode and the electroplated Cu layer, the differences in grain size and grain orientation will lead to significant differences between the two. By using ferric chloride hydrochloric acid aqueous solution to corrode the polished copper wire containing Cu/Ni/Sn coating, an obvious interface is formed due to the different corrosion rates between the electroplated Cu layer, Ni layer and Cu electrode. This interface can be observed under a metallographic microscope, and finally the thickness of the electroplated Cu layer can be obtained by measuring the clear upper and lower interfaces of the electroplated Cu layer.
Table 1. Test results of electroplated Cu layer of a certain product
The picture comes from Sunlord internal
Applications in drop testing
By adopting the "Measurement Method of Electroplating Cu Layer on Copper Wire Surface" technology, it can effectively assist product design and improve the product's drop resistance. The number of drops of a certain type of product increased from 47 to 108 times, meeting and exceeding customer standards.
With the advancement of science and technology and the improvement of people's living standards, small electromechanical products, especially portable electronic products, have been greatly used in daily life. Due to the portability of the product, accidents often occur when consumers drop the product while carrying or using it, resulting in damage to the product. Moreover, as sophisticated high-tech products, portable electronic products are often expensive, and the hidden losses caused by their damage may even exceed the value of the product itself. Therefore, when consumers purchase products, they will prefer products with good quality if they meet the basic functions. Crash-resistant products. The drop impact resistance of products has become an important feature of product quality and the core competitiveness of products3.
Since the foundational paper "Dynamics of Package Crushing" published by Mindlin in 1945, the study of drop impact crashworthiness has been of concern to researchers, and tests have been widely conducted in the industry at both the product and integrated circuit board levels. Sunlord is committed to becoming an expert in the field of electronic components. We have conducted extensive and in-depth research on drop tests of electronic components and applied these results to product design. Only by mastering the relationship between product design parameters, drop impact parameters and crashworthiness, as well as the uncertainty transfer rules, can we obtain a robust product design solution (design parameters).
This method of measuring the thickness of the electroplated Cu layer provides more data support for product designers on reliability design parameters, ensures a good anti-drop design, helps to understand the product more thoroughly, and provides consumers with safer and more reliable products. The product.
Review Editor: Tang Zihong
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