Formation and growth mechanism of thin Cu₆Sn₅ films in Sn/Cu and Sn-0.1AlN/Cu structures using laser heating

Cu metal has been used in electronic packaging field for lead-frame and pad on substrate due to the high strength, electrical conductivity, thermal conductivity and good processability. To establish a connection between the lead or pad and the device, Sn metals with outstanding mechanical performance have become the primary connection material for electronic interconnection. This is owing to their ability to react with Cu, forming IMCs that facilitate metallurgical bonding (Hu et al., 2021; Liu et al., 2021). Due to the brittle performance of IMCs, cracks initiated and propagated in excessively thick IMC layer in service leading to early solder joint failure (Zhang et al., 2021; Wang et al., 2022). Therefore, controlling the thickness of IMCs at Cu interface has become an important research topic in the field of electronic packaging.

Laser heating with high energy density laser beam as heat source is an efficient and precise soldering method. Laser-soldered lead-free solder can reduce the heating time and rapidly form solder joints, which can induce a thinner IMCs layer. There is a literature (Kunwar et al., 2020) investigated the interfacial Cu₆Sn₅ IMC in Sn-xAg-yCu/Cu structure, the morphology of IMC can be observed with types-prismatic and scalloped, which can be influenced by the laser power, scan speed and composition. Nishikawa and Iwata (2015) have investigated the IMC layer growth at Sn-3.0Ag-0.5Cu/Cu interface after laser soldering and isothermal aging. It is found that a relatively thin Cu₆Sn₅ layer is formed at the interface after laser soldering. The mechanical performance of the solder joints under laser soldering at 20 W for 40 s is superior to that of solder joints under reflowing. Huan et al. (2022) have studied the laser soldering of 8-SOIC to analyze the interfacial reaction between Sn-3.0Ag-0.5Cu and Cu or Ni/Al, the results indicate that the IMC thickness is less than 5 μm near Cu, the IMC thickness is less than 1 μm near Ni/Al. Abdullah and Idris (2021) have studied the IMC growth of Sn-xCu/ENIG (electroless nickel immersion gold) solder by laser soldering. The result shows that the IMC layer thickens while the equilibrium contact angle reduces with the amount of Cu increasing, and Sn-0.7Cu/ENIG exhibits more excellent performance as compared to other content of Cu. Liu et al. (2010) investigated the effect of the temperature of protection atmosphere on morphology of IMCs in laser reflowed Sn-2.0Ag-0.75Cu-3.0Bi solder joints. The results indicate that the needle-like AuSn₄ IMC and Au-rich phases were formed in Sn-2.0Ag-0.75Cu-3.0 Bi solder joint protected by N₂ atmosphere at room temperature. When the temperature was upper 100°C, the Au-rich phases almost disappeared, and orientation of the needle-like AuSn₄ IMCs changed obviously. Thus, the morphology of Au-Sn IMCs in the laser reflowed solder joints can be controlled by regulating N₂ atmosphere temperature. But until now, there is no detailed report on the study of laser heating inducing the formation of nanothin IMC layer.

In this paper, we fabricate the Sn/Cu and Sn-0.1AlN/Cu solder joints using laser soldering. The effect of laser power and heating time on the thickness of Cu₆Sn₅ layer and sizes of Cu₆Sn₅ grains with Sn/Cu and Sn-0.1AlN structures was investigated systematically. The objective is to establish a correction between nano-Cu₆Sn₅ films and laser parameters/AlN additives. A new method to enhance the reliability of solder joints on Cu substrate in electronic packaging is also aimed to be explored.

In this study, the Sn-AlN composite solder comprising Sn paste and reinforcing AlN particles with an average diameter ranging from 30 to 50 nm was used. The composite solder was fabricated by ultrasonic agitation to ensure the uniform distribution of nanoparticles in matrix. Solder paste (approximately 0.2 g) was put on the Cu substrate (25 mm × 25 mm × 0.5 mm) cleaned with C₂H₅OH solution. Then the laser heating under different laser power (200, 225 and 250 W) and heating time (2, 3 and 4 s) was used to form molten solder to react with Cu substrate as shown in Figure 1, after laser heating, 25%HNO₃ and 75% C₂H₅OH mixed solution were selected for the deep etching to prepare samples, the scanning electron microscope (SEM) and transmission electron microscope (TEM) were used to observe the morphologies of nanothin IMC layer and IMC grains. To minimize experimental errors, every composite with ten samples was selected for observation and the average value was taken.

The interfacial structure of Sn/Cu solder joints prepared at different laser powers (200, 225 and 250 W) and heating times (2, 3 and 4 s) is shown in Figure 2. The scallop-type Cu₆Sn₅ intermetallic compound layer can be observed parallel to the Cu substrate, it is evident that thickness of Cu₆Sn₅ layer from 635.6 to 890.9 nm is increasing with the increase of laser power from 200 to 250 W for 2 s heating time. Moreover, the scallop-type Cu₆Sn₅ grains with several microns in height during solid–liquid interdiffusion reaction can also be obtained in Cu/Sn-Ag/Cu structure with about 10 μm thickness solder (Gusak et al., 2020). Comparing with the influence of laser power, the influence of heating time is more intense, for 200 W laser soldering, it can be seen that the thickness of Cu₆Sn₅ layer grows from 635.6 to 1045.3 nm, as shown in Figure 2(a)–(c), which induce 1.64 times as heating times from 2 to 4 s. Moreover, when the laser power reaches 225 W, the Cu₆Sn₅ film is turned into needle-like structures and the Cu₆Sn₅ thickness increases from 720 to 1066.5 nm while the laser heating time rises from 2 to 4 s in Figure 2(d)–(f). As the laser power rises to 250 W, the growth rate of Cu₆Sn₅ films has increased significantly and the scallop-shaped Cu₆Sn₅ films are connected to form flake IMCs, as shown in Figure 2(g)–(i). The thickness of Cu₆Sn₅ films is increased to 2025.8 nm when the laser heating rise to 4 s. Thus, the Cu₆Sn₅ films thicken with the laser power or heating time extending.

Figure 3 shows the SEM morphologies of Sn-0.1AlN/Cu solder suffered laser heating (2, 3 and 4 s) under different laser power (200, 225 and 250 W). Likewise, it is found that the typical scallop-shaped Cu₆Sn₅ films at the Sn-0.1AlN interfacial surface and the IMC films get thicker as the laser heating time or power increased. After the laser heating time added from 2 to 4 s, the Cu₆Sn₅ films thickness of Sn-0.1AlN solder at 200 W increases from 524.2 to 1030 nm, as shown in Figure 3(a)–(c), which has a decrease of 1.5%–17.5% compared with Sn solder heated from 2 to 4 s. Under 225 W of laser power, the thickness of Cu₆Sn₅ films rises from 668.5 to 1050.9 nm with laser heating from 2 to 4 s, as can be seen in Figure 3(d)–(f), which is 1.5%–7.2% less than that of Sn solder under corresponding condition. When the laser power reaches 250 W, the Cu₆Sn₅ films thickness of Sn-0.1AlN composite solder heated for 2 and 4 s adds up to 792.3 and 1486.1 nm as shown in Figure 3(g)–(i), with a decrease of 11.1%–26.7% by compared with Sn solder under lasering heating for 2 and 4 s. By comparing and analyzing the interfacial thickness of two kinds of structures. It is found that the interfacial thickness of Sn-AlN/Cu is significantly smaller than that of Sn/Cu. The decrease ranged from 1.5% to 26.7%.

The molten Sn expands under laser heating on Cu surface, the increase of laser power will induce the increase of soldering temperature, which can enhance the atoms diffusion between Sn and Cu to improve the Cu-Sn reaction. The increase of laser heating time can also improve the atoms diffusion. An appropriate amount of Cu-Sn reaction can form a metallurgical connection, while an excess reaction induces crack initiation and reduces the structure reliability (Li et al., 2021). The IMC thickness of Sn/Cu and Sn-AlN/Cu structures has been obtained by Image-J software, as can be seen in Figure 4. On one hand, both IMC layer thickness of two samples is thin since the heating duration is very short which results in the diffusion. And reaction between molten solder and Cu substrate is insufficient. On the other hand, it is clear that the thickness of Cu₆Sn₅ IMC film of the Sn solder containing AlN nanoparticles is lower than that of pure Sn solder at the same laser soldering condition. Therefore, the nanothin Cu₆Sn₅ films can be found at the Sn/Cu and Sn-AlN/Cu structures at 200 W/225 W with 2 and 3 s, and at 250 W with 2 s.

In addition to the thickness of Cu₆Sn₅ layer being nano-sized, the diameter of Cu₆Sn₅ grains can also reach nanometer size. It is interesting to explore the top view of Cu₆Sn₅ grains, the morphologies of Cu₆Sn₅ grains at Sn/Cu and Sn-0.1AlN/Cu structures with different laser power and heating times are shown in Figures 5 and 6, respectively. The top-view morphologies show that the Cu₆Sn₅ grains grow continuously with the extension of heating time or the increase of laser power. When the laser heating time reaches 4 s, the Cu₆Sn₅ grains in Sn/Cu structure under 250 W of laser power are fused with each other and the Cu₆Sn₅ grain size increases significantly. Figure 7 exhibits the grain diameter of Sn/Cu and Sn-0.1 AlN structure suffered different laser heating time and laser power. It shows that the Cu₆Sn₅ grains sizes in Sn/Cu and Sn-0.1AlN/Cu systems are added along with the laser heating. Under 200 W, the Cu₆Sn₅ grain diameter of Sn-0.1AlN structure heated for 2 s is 752.8 nm, which is less than that (1039.3 nm) of Sn/Cu structure. After heating for 4 s, the Cu₆Sn₅ grain size of Sn-0.1AlN solder is 1164 nm, a decrease of 31.2% compared with that of Sn/Cu. When the laser power rises to 250 W, the diameter of Cu₆Sn₅ grain in Sn/Cu and Sn-0.1AlN/Cu structure heating for 4 s rose to 2877.5 and 1726.5 nm, with an increase of 156% and 87.6% as compared with that of corresponding structure heated for 2 s. The result demonstrates that the growth rate of Cu₆Sn₅ grains in Sn/Cu structure adding with AlN nanoparticles is slower than that without AlN nanoparticles. This exhibits that doping AlN nanoparticles can impede IMC grains growth. Furthermore, the growth of Cu₆Sn₅ grains is controlled by the Cu flux (Xiong et al., 2019). According to Gibbs–Thomson effect (Kim and Tu, 1996), the Cu concentration on the Cu₆Sn₅ grain surface in Sn solder is:

where C₀ is the equilibrium concentration of Cu atoms, $γ$ is the interface energy between molten solder and Cu₆Sn₅ IMC grains, V_m is the molar volume of Cu₆Sn₅ IMC grains, r is the radius of Cu₆Sn₅ IMC grains, R is the gas constant and T is the absolute temperature. Based on Fick's first law (Yao et al., 2017), the maturation flux of Cu is:

where D is the diffusion coefficient of Cu, and $λ$ is the dimensionless coefficient. During laser soldering process, the diameter of Cu₆Sn₅ grains is different, and a Cu concentration gradient will exist between the IMC grains, which results in Cu atoms spreading to the large grains from the small ones. Thus, the large grains are growing while the small grains are shrinking. In addition, the diffusion coefficient (D) can be described as follows:

where D₀ is diffusion constant, and Q is diffusion activation energy. Moreover, the diffusion activation energy represents an atom required to migrate from one place to another place (Sun et al., 2019). The AlN nanoparticles can be adsorbed on the Cu₆Sn₅ grains surface and in the solder matrix, which will impede the diffusion of Cu atoms. Therefore, AlN nanoparticles addition decreases the diffusion coefficient and the diffusion flux (J₁) of Cu atoms, causing the inhibition of IMC growth. Moreover, the element mapping of Sn-AlN/Cu with laser heating for 1 s under 200 W has been obtained, as shown in Figure 8, it is demonstrated that AlN nanoparticles are diffused on the Cu₆Sn₅ grains surface in the Sn-AlN/Cu structure, which represents the AlN nanoparticles absorbed on the grains surface can inhibit the diffusion of Cu and Sn to reduce the Cu₆Sn₅ growth.

To further identify the sizes of nanothin Cu₆Sn₅ film and Cu₆Sn₅ grains, FIB has been used to fabricate ultrathin Sn-AlN/Cu samples, TEM and EDS can be used to observe the elements distribution at the Sn-AlN/Cu interface, as shown in Figure 9. The Cu₆Sn₅ grains can be observed obviously, small Cu₆Sn₅ grains can reach 247.5 nm in height and large grains can reach 414.4 nm in height. The nanothin Cu₆Sn₅ film could enhance the reliability in the future solder joints with 1 μm height in miniature 3D chip stacking. From the HRTEM image of nanothin Cu₆Sn₅, the spacing interplanar measured is 0.211 nm, and identified as Cu₆Sn₅ (132) planar. In the nanothin Cu₆Sn₅ film, no obvious defect, such as Kirkendall voids can be found in this area, only three weeny Kirkendall voids with 30–40 nm diameter can be found. According to the published data from literatures (Vianco et al., 2022), solder joints with different materials, including Sn-Pb/Ni/Au, Sn/Cu and Sn-Ag-Cu/Cu-Ti, have an extensive network of Kirkendall voids formed along the interface, the Kirkendall voids can even 1 μm. Comparing with the published data, we can conclude that the formation of nanothin Cu₆Sn₅ film can enhance the reliability of structure. This further shows that by laser heating, although the heating time is very short, but can obtain a good nanothin Cu₆Sn₅ film on Cu substrate. Nanothin Cu₆Sn₅ films can be used as Cu protective films to improve the reliability of the new generation electronic products.

To further analyze the growth kinetics of nanothin Cu₆Sn₅ film, the relationship between the thickness of IMC (L) and heating time (t) can be expressed by the empirical equation (Liu et al., 2020), as shown in equation (4). In this experiment, the values of L and t can be obtained by measuring the thickness of nanothin Cu₆Sn₅ at different times, based on the logarithm of L and t, the n can be computed to be 0.78 ± 0.29, 0.56 ± 0.05 and 1.15 ± 0.11 for 200, 225 and 250 W in Sn/Cu structure. The n is 1.06 ± 0.29, 0.64 ± 0.05 and 0.93 ± 0.07 for 200, 225 and 250 W in Sn-AlN/Cu structure. From the calculation, it is found that the value of n is significantly larger, more than 0.5, and even up to about 1. In the growth investigation of IMC with prolonged heating (Zhang and Liu, 2020), the value of n is 0.5, it is found that parabolic growth tendency can present the growth of the IMCs growth, but in the present experiment, the thickness of the Cu₆Sn₅ film is nanothin, the thickness of the data is in the initial stage of the parabolic, and is in a very small distance, so the numerical fitting n will close to the linear relationship, as shown in Figure 10. Moreover, with the calculation of D, it is found that the diffusion coefficient is 0.34 ± 0.12, 0.49 ± 0.03 and 0.42 ± 0.06 nm/sⁿ for 200, 225 and 250 W in Sn/Cu structure, 0.23 ± 0.08, 0.43 ± 0.03 and 0.41 ± 0.04 for 200, 225 and 250 W in Sn-AlN/Cu structure, which also demonstrate the AlN nanoparticles can decrease the growth rate, to retard the growth of nanothin Cu₆Sn₅ films. Al₂O₃ nanoparticles as additives into Sn-0.3Ag-0.7Cu/Ni-Cu structure (Tikale and Prabhu, 2020), can retard the IMC growth from 4 to 11 μm after reflow soldering. Combining our research about the nanothin Cu₆Sn₅ film, the nanoparticles can be used as the barrier for elements diffusion to control the IMC growth:

where L is the IMC thickness, D is the diffusion coefficient, t is the heating time and n is the reaction constant.

The present study appears to be the first study to experimentally demonstrate the formation of nanothin Cu₆Sn₅ films on the Cu substrates with laser heating in Sn/Cu and Sn-AlN/Cu structures. The conclusions can be drawn as follows:

• The thickness of Cu₆Sn₅ film increased from 635.6 to 890.9 nm and 524.2 to 792.3 nm for Sn/Cu and Sn-AlN/Cu structures with the increase of laser power from 200 to 250 W for 2 s heating time. The increase of laser power and laser heating time can improve the atoms diffusion to enhance the Cu₆Sn₅ thickness.

• The diameters of Cu₆Sn₅ grains can also be enhanced with the increase of laser power and heating time. The diameters of Cu₆Sn₅ grains in Sn/Cu structure are all over 1000 nm, for Sn-AlN/Cu structure, the diameters are 752.8 and 909.9 nm with 200 W and 2 s/3 s.

• The growth rate of Cu₆Sn₅ film in Sn/Cu structure is obviously higher than that in Sn-AlN/Cu structure; moreover, the value of reaction constant n for both structures is significantly larger, more than 0.5, and even up to about 1.

This work was financially supported by Fujian Provincial “Minjiang Scholar” Distinguished Professor Talent Plan Project, Henan Provincial Distinguished Researcher Project and Natural Science Foundation of Jiangsu Province Project (BK20211351).