2022 Volume 63 Issue 6 Pages 800-804
In the present study, plating films with three-dimensional structures were formed by using a plating bath mixed with Cu sulfate and Ni amidosulfate. Furthermore, the effect of the shape of plating films, which was changed by plating at different potentials, on the adhesive force with epoxy resin was investigated. The results show that when the ratio of the concentration of Ni amidosulfate to that of Cu sulfate was 150:15 (g/L) and the potential was set to −1.0 V to −2.0 V, dendritic plating films were formed. The average peak shear force of the specimen joined with epoxy resin between two Cu plates with the dendritic plating films formed at the potential of −1.5 V, was 180.1 N. The dendritic plating films had higher adhesive force than the smooth plating film through the anchoring effect.
Fig. 6 Peak shear force of adhesive joint specimens with Cu–Ni alloy plating films obtained by constant potential electroplating.
With the demand for higher speed, smaller size, and higher density of devices, the electronic component mounting technology, especially the wiring formation technology, is rapidly advancing.1–7) Cu is used for the wiring of semiconductor devices and circuit boards because of its low electrical resistance, no signal delay due to finer wiring patterns, and excellent migration resistance.8–10) This wiring is formed by Cu electroplating, and fine wiring patterns are realized by damascene technology. Ni has excellent corrosion resistance and diffusion barrier properties.11) Therefore, Ni plating is used for the metallization of lead frames and electrode pads on chips and circuit boards. It has also been developed as an interconnecting material for power semiconductors.12,13) Thus, Cu and Ni plating technology have very important roles in the electronics field.
In electroplating, it is possible to form a specially shaped plating film depending on additives or the concentration of metal ions in the plating solution.14–18) Cu–Ni alloy plating solution with specific Cu ion and Ni ion concentrations can form a projection-shaped three-dimensional electroplating film.19–23) By applying this plating film for wiring and lead frames, it is expected to improve adhesion between metal and mold resin for semiconductor packaging through the anchoring effect.
In this study, the effect of the shapes of Cu–Ni alloy plating films, which were changed by potential, was investigated using an electrochemical measurement. Moreover, Cu–Ni alloy plating film was formed on the Cu plate, and the adhesive force to epoxy resin was measured.
We prepared Cu sulfate, Ni amidosulfate, and additives, which are used in the wiring process of semiconductor and circuit board manufacturing. Table 1 shows the reagents and their concentrations in the Cu (No. 1) and Ni (No. 2), Cu–Ni alloy (No. 3) plating solutions.
Linear-sweep-voltammetry (LSV) was conducted to clarify the electrochemical reduction mechanism of Cu, Ni, and Cu–Ni. LSV was performed using an electrochemical analysis device (ECstat-101, EC FRONTIER CO., LTD.) and a three-electrode glass cell, where Pt spiral wire was used as a counter electrode and silver/silver chloride (Ag/AgCl) electrode as a reference electrode. A zincated Al plate with Ni plating was used as a working electrode with an exposed area of 5 × 5 mm2. Table 2 shows the reagents and their concentrations in the Ni plating solutions. Table 3 shows the conditions of Ni electroplating. LSV analysis was performed at a scan rate of 10 mV/s at room temperature for each plating solution shown in Table 1. The volume of the plating solution was approximately 0.05 L.
To evaluate the shape of the Cu–Ni alloy plating film, it was electroplated by chronoamperometry method using the plating solution No. 3 shown in Table 1. The electroplating was conducted at the constant potentials of −0.5, −1.0, −1.5, and −2.0 V, respectively.
2.3 Microstructural observationAfter electroplating, the top-view images of the Cu–Ni alloy plating films were obtained by field-emission scanning electron microscopy (FE-SEM). After that, cross-sections of the Al plates with Cu–Ni alloy plating films embedded in epoxy resin were polished with abrasive papers and Al2O3 suspension. Then, the cross-sectional shapes of the Cu–Ni alloy plating films were observed using an electron probe X-ray microanalyzer (EPMA).
2.4 Adhesive force testsA shear test was conducted to investigate the effect of the shape of the Cu–Ni plating film on the adhesive force to the epoxy resin. Pure Cu (C1100) plates of 10 × 10 × 1 mm3 and 20 × 20 × 1 mm3 were prepared. The Cu plates with an aperture area of 5 × 5 mm2 were electroplated using the Ni plating solution shown in Table 2 under the plating conditions shown in Table 3. Then, the Cu plates with Ni plating were electroplated with Cu–Ni alloy plating solution No. 3 shown in Table 1 at a potential of −0.5, −1.0, −1.5, and −2.0 V, and an aperture area of 5 × 5 mm2. After that, 3.0 ± 0.5 mg of two-part curing epoxy resin (Epomount 27-771 and 27-772, REFINETEC CO., LTD.) was applied to plating films of the 20 × 20 × 1 mm3 Cu plate. Then, the 10 × 10 × 1 mm3 Cu plate with plating films was placed on top of it and allowed to cure naturally for 24 h.
Figure 1 shows a schematic of the shear test of an adhesive joint specimen. The adhesive force of the adhesive joint specimen was measured using a shear testing machine (STR-1000, RHESCA CO., LTD.). The width of the shear tool tip is 6 mm. The shear tool tip was pushed from a height of 1.0 µm above the lower Cu plate at 3.0 mm/min until the specimen was broken. The test was conducted using three specimens per condition. After the test, the surfaces of the fractured specimens were examined by laser microscopy (VK-X150, KEYENCE Corp.).
Schematic of shear test of adhesive joint specimen.
Figure 2 shows the LSV curves of each plating solution shown in Table 1. In the Cu sulfate solution of No. 1, the current density increased as the potential became negative from 0 V. The value of current density increased slowly in the potential range of ECu,1 = −0.30 V to ECu,2 = −1.38 V. This is the diffusion-limited of Cu ions, and the Cu deposition rate becomes almost constant. The current density increased at potentials above ECu,2 = −1.38 V, and hydrogen was generated violently. In the Ni amidosulfate solution of No. 2, the starting point for the reduction of Ni ions was about −0.8 V, and the current density increased rapidly at later potentials. Therefore, Ni plating is controlled by charge transfer. However, as shown in the inset of Fig. 2, the current density decreased sharply after ENi = −1.82 V. This is because pH in the vicinity of the working electrode increased due to hydrogen generation, and the Ni hydroxide formed on the working electrode surface acted as a resistive film.24) In the Cu–Ni alloy solution of No. 3, a single peak at about ECu–Ni = −0.13 V is seen, which corresponds to the reduction of Cu2+ to metal Cu.25) Cu–Ni alloy plating is formed at potentials above −0.8 V, and the Ni deposition rate increases as the potential increases.
LSV curves in electroplating solutions containing Cu sulfate (No. 1), Ni amidosulfate (No. 2), and Cu sulfate and Ni amidosulfate (No. 3), inset shows overall view.
Figure 3 shows secondary electron (SE) images of the top and backscattered electron (BSE) images of cross-sectional views of Cu–Ni alloy plating films electroplated at a constant potential using the plating solution No. 3 shown in Table 1. At a potential of −0.5 V, the plating film had no special structure and was smooth morphology. At potentials of −1.0 V to −2.0 V, plating films had dendritic structures. The maximum height of the dendritic structure was 8 µm at a potential of −1.0 V, 23 µm at −1.5 V, and 40 µm at −2.0 V. From the top views, the larger negative potentials, the larger the dendritic plating clumps and the smaller the number of clumps. It can be seen that the structure of the Cu–Ni alloy plating film depends on the deposition potential. It has been reported that when plating by Cu–Ni alloy plating solution with a low Cu ion concentration and a high Ni ion concentration, the plating film grows with a three-dimensional structure.26–29) Since Cu is nobler than Ni, nuclei of Cu crystals are generated on the substrate in the early stage of current flow. Then, Ni is deposited on the substrate and surrounds the nucleus of the Cu crystal. During electroplating, Cu ions in the plating solution are depleted near the substrate, and Cu ions diffuse from the bulk solution. The Cu ions are reduced at the upper tip of the Cu crystal nucleus, which is the shortest diffusion distance. On the other hand, Ni plating film grows with an isotropic shape due to the abundance of Ni ions near the substrate.
SEM images of top and cross-sectional views of Cu–Ni alloy plating films.
Figure 4 shows the formation mechanism of Cu–Ni alloy plating film at different potentials. When the negative potential is between −1.0 V and −1.5 V, Cu ions diffused from the bulk solution reduce at the upper tip of the Cu crystal nucleus, and each three-dimensional structure grows. When the negative potential is −2.0 V, Ni deposition rate increases and Ni hydroxide is also deposited, but the structures grow vertically as well as when the potential is −1.0 V to −1.5 V. Cu ions within the diffusion radius in the Fig. 4 tend to deposit in specific structures where the ion diffusion distance is short.30) Furthermore, as the structure becomes higher, the diffusion radius R becomes larger and large clumps are formed due to the deposition of excessive Cu ions at the top of the structure. As a result, high aspect ratio structures are generated while the number of such structures decreases.
Formation mechanisms of Cu–Ni alloy plating film.
Figure 5 shows BSE images of Cu–Ni alloy plating films and the corresponding EPMA composition maps. In the EMPA composition maps in (a), Ni was detected in a linear pattern on the Al plate. This is due to the underlying Ni plating. On the other hand, only a small amount of Cu was detected. In the EMPA composition maps in (b), (c), and (d), the Cu element was mainly detected on the dendrites and trunk, while the Ni element was strongly detected on a plate surface and the root part of the dendritic-type plating structure. As mentioned above, when the Cu–Ni alloy plating solution containing a low Cu concentration is plating, Cu ions diffused from the bulk solution are preferentially reduced on the dendritic plating tip. Therefore, Cu content is high at the tip of the dendritic plating. However, there is less possibility of Cu ion reduction on the plate surface and the root part of the dendritic-type plating, resulting in a lower Cu content in these areas.
BSE images of cross-sectional views of Cu–Ni alloy plating films obtained by constant potential electroplating at (a) −0.5 V, (b) −1.0 V, (c) −1.5 V and (d) −2.0 V, and corresponding EPMA composition maps.
Figure 6 shows the measurement results of the peak shear force at delamination of the adhesive joint specimen with Cu–Ni alloy plating film obtained by the shear test. At the potential of −0.5 V, the average peak shear force was 17.5 N. At the potential of −1.0 V, the average peak shear force was 58.8 N. It was found that the dendritic plating films had higher peak shear force than the smooth plating film. Furthermore, at the potential of −1.5 V, the average peak shear force was 180.1 N, which was the highest among those obtained under all the conditions. At the potential of −2.0 V, the average peak shear force was 172.4 N, which was almost the same value as that at potential −1.5 V. As shown in Fig. 3, although the dendritic structures formed at the potential of −2.0 V were larger than those at the potential of −1.5 V, the peak shear force did not increase because the trunks of the dendritic structures were formed thinner and easier to break. From the above, the larger the height of the three-dimensional structures in the plating film, the higher the adhesive force.
Peak shear force of adhesive joint specimens with Cu–Ni alloy plating films obtained by constant potential electroplating.
Figure 7 shows the fractured surfaces of specimens after the shear test and corresponding surface topographies. In the figures, 20 × 20 × 1 mm3 Cu plate sides are shown. From the surface morphology, the area shown in yellow and red are epoxy resin residues, and the area shown in blue is the plating film. In all conditions, both the plating film and epoxy resin can be observed. At the potential of −1.0 V to −2.0 V, parts of the dendritic plating film were observed in the epoxy resin. Therefore, the fracture modes under these conditions were a mixture of fractures of the dendritic plating film and epoxy resin. This indicates that the adhesion with epoxy resin is improved through the anchoring effect of the Cu–Ni alloy plating film with dendritic structures.
OM images of top-views of fractured specimens after shear test and corresponding surface topographies.
In this study, plating films with three-dimensional structures were formed by controlling potential using a plating solution with a mixture of copper sulfate and nickel amidosulfate. The effect of the shape of plating films, which was changed by plating at different potentials, on the adhesive force was investigated. The results of this study were as follows:
This study was supported by JSPS KAKENHI under Grant Number JP20K15045.
