Rapid Publication
Investigation of the Mechanical Properties of Sn–58Bi/Sn–3Ag–0.5Cu Composite Solder
2023 Volume 64 Issue 11 Pages 2673-2676
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2023 Volume 64 Issue 11 Pages 2673-2676
A Sn–58Bi (SB)/Sn–3Ag–0.5Cu (SAC) composite solder was developed to overcome the challenges, e.g., the low ductility, deterioration of bonding properties and joint reliability, associated with SB eutectic solder and enhance its mechanical bonding properties. Specifically, four types of SB/SAC composite solder samples with different SB/SAC mixing ratios (100:0, 50:50, 20:80, and 0:100) were formulated. Two types of mechanical property investigations, i.e., ball shear and microhardness tests, were conducted to explore the influence of the SB/SAC mixing ratios on the mechanical properties of SB/SAC composite solder joints. The results indicated that the mechanical properties of the joint that containing both SB and SAC were superior to that with only SB or SAC. Furthermore, the mechanical properties of SB/SAC composite solder joint increased linearly with increasing SAC content. This improvement was attributable to the precipitation hardening and dispersion strengthening induced by the presence of fine intermetallic compounds and Bi-rich phase particles dispersed in the SB/SAC composite solder joint.
Solder plays a crucial role in the formation of reliable electrical and mechanical joints during the manufacturing and operation of electronic package devices. Sn–37Pb eutectic solder has been widely used as a bonding material for electrical devices on printed circuit boards (PCBs) owing to its ease of use, low melting temperature (Tm), excellent material properties, ductility, and wettability to Cu electrode. However, because lead can adversely influence human health and the environment, the use of Sn–37Pb has been restricted by international environmental regulations. Consequently, researchers have been actively seeking eco-friendly lead-free solder materials.1,2) Sn–3Ag–0.5Cu (SAC) solder has gained widespread acceptance as a substitute for Sn–37Pb eutectic solder in the electronic packaging industry, owing to its excellent electrical conductivity and solder joint strength. However, the Tm of SAC is 490–494 K, approximately 34–38 K higher than that of Sn–37Pb eutectic solder (Tm = 456 K). Consequently, the high reflow temperature during the bonding process may result in thermal damage to the PCBs or electronic components.
To prevent the risk of thermal damage to electronic devices, solder materials with low Tm, such as Sn–Bi, Sn–In, and Sn–Zn, have garnered attention. Among these low-Tm solder materials, Sn–58Bi (SB) eutectic solder has been widely used in the electronic package industry owing to its low Tm, excellent solderability, mechanical strength, and low cost.3) However, the widespread application of SB eutectic solder is limited by several challenges, such as the low ductility owing to the inherent brittleness of Bi, segregation and coarsening of the Bi phase at the bonding interface due to Sn consumption via the formation of an intermetallic compound (IMC), and deterioration of bonding properties and joint reliability because of the rapid growth of the IMC layer at the bonding interface attributed to its low Tm.4–6) Additionally, the low Tm of SB eutectic solder limits its operating regime to a narrow range of temperatures (lower than the Tm).7) These problems can likely be addressed by introducing additional reinforcing elements, such as Ag8) or Cu,9) or by increasing the Sn content to reduce the relative content of the brittle Bi element in the SB solder.10)
Considering these aspects, in this study, SB/SAC composite solder with various mixing ratios of SB and SAC was developed, aimed at addressing the problems associated with the SB eutectic solder and improving the mechanical bonding properties of the SB joint. Ball shear and microhardness tests were conducted to examine the influence of the SB/SAC mixing ratios on the mechanical bonding properties of the SB/SAC composite solder joint.
To fabricate the SB/SAC composite solder, Sn–58Bi (Tm = 412 K) and Sn–3Ag–0.5Cu (Tm = 492 K) solder pastes (Danyang Soltec Co.) with a diameter of 38 µm were used. To examine the influence of the SB/SAC mixing ratios on the mechanical bonding properties of the resulting solder, four types of SB/SAC composite solder samples were developed with varying mixing ratios (vol%): 100:0, 50:50, 20:80 and 0:100. To ensure uniform dispersion of the SB and SAC solder pastes in the SB/SAC composite solder, the solder pastes were subjected to mechanical mixing for 30 min using a stirring rod.
Ball shear and microhardness tests were performed to evaluate the mechanical bonding properties of the SB/SAC composite solder. For the ball shear test, a solder-mask-defined type PCB (sized 250 × 250 × 1 mm3) with a subsurface Cu sheet was prepared. The size and shape of the Cu pads were defined using a circular solder resist opening with a diameter of 300 µm and pattern pitch of 5 mm. To prepare the test specimens, a 100 µm-thick metal mask was aligned on the cleaned PCB. The prepared SB/SAC composite solder was selectively applied onto the Cu electrodes through stencil printing, and reflowed using a heating plate (SMHS-3, DAIHAN Scientific Co.). The reflow process for the specimen with only SB was implemented from room temperature to 453 K at a heating rate of 38 K/min, with a 2 min hold at the peak temperature. The specimens with both SB and SAC or only SAC were reflowed from room temperature to 533 K to melt the SAC, with a 2 min hold at the peak temperature.
To confirm the uniformity of solder ball formed by the SB/SAC composite solder with different SB/SAC mixing ratios, the prepared specimens were cross-sectioned, and their morphologies were inspected using an optical microscope (VHX-1000, Keyence Co.). Ball shear tests for 25 solder ball samples were conducted using a shear tester (PTR-1000, Rhesca Co.). Specifically, the test specimen was fixed on the jig of the shear tester. The shear tip was positioned close to the solder ball with a standoff height of 20 µm and then horizontally moved with a shear speed of 0.1 mm/s, in accordance with JESD22-B117A. To clarify the effects of the SB/SAC mixing ratios on the microhardness of the SB/SAC composite solder joints, the cross-sectioned SB/SAC composite solder ball joints were subjected to microhardness tests using a micro-Vickers hardness tester (MH-3, Metkon Instruments Ltd.). To this end, the cross-sectioned test specimen was placed on the jig of the microhardness tester. A diamond indenter was vertically pressed onto the SB/SAC composite solder surface with a compression load of 10 gf, maintained for 10 s. Measurements were obtained at nine random points for each test specimen. After the formation of the indentation on the SB/SAC composite solder surface, the diagonal length of the indentation was measured using an optical microscope to calculate the microhardness. Furthermore, to analyze the reasons for the changes in the mechanical properties of the SB/SAC composite solder with different SB/SAC mixing ratios, microstructure and fracture mode analyses were performed through field-emission scanning electron microscopy (FE-SEM, JSM-7001F, JEOL Ltd.) and energy-dispersive spectrometry (EDS, X-MaxN, Oxford Instruments Co.).
Figure 1 shows the results of the cross-sectional inspection conducted on the ball shear test specimens to verify the uniformity of the SB/SAC composite solder joints with different SB/SAC mixing ratios. Uniform spherical solder balls were obtained for all considered SB/SAC mixing ratios owing to appropriate wetting behavior and surface tension of the molten solder. The findings demonstrated that the SB/SAC mixing ratio did not considerably affect the wetting and joint formation behaviors of the molten SB/SAC composite solder.
Morphologies of the cross-sectioned SB/SAC composite solder joints with different SB/SAC mixing ratios: (a) 100:0, (b) 50:50, (c) 20:80, and (d) 0:100.
Figure 2 shows the results of the ball shear tests performed to clarify the influence of the SB/SAC mixing ratio on the mechanical properties of the SB/SAC composite solder joints. The shear strength of the joints with only SB (88.017 ± 2.958 MPa) was higher than that of the joints with only SAC (63.855 ± 3.564 MPa) because of the high content of brittle Bi in SB. With increasing SAC content in the SB/SAC composite solder joints, the shear strength of the joint linearly increased. For example, when the SAC contents were 50 and 80 vol%, the shear strength was 102.372 ± 3.281 MPa and 106.162 ± 3.894 MPa, respectively.
Shear strength of the SB/SAC composite solder joints with different SB/SAC mixing ratios.
Figure 3 shows the results of the microhardness test performed on the cross-sectioned SB/SAC composite solder joints. The microhardness of the joint containing only SB (242.148 ± 13.111 MPa) was higher than that the joint containing only SAC (199.852 ± 17.928 MPa). Moreover, the microhardness steadily increased with increasing SAC content in the SB/SAC composite solder joints. For example, when the SAC contents were 50 and 80 vol%, the microhardness was 302.710 ± 14.784 MPa and 364.546 ± 24.780 MPa, respectively. Notably, the microhardness of the SB/SAC composite solder containing 80 vol% SAC was 50% higher than that of the solder containing only SB. These results highlighted that the introduction of SAC in the SB/SAC composite solder can help enhance the mechanical properties of the SB/SAC composite solder.
Microhardness values of the SB/SAC composite solder joints with different SB/SAC mixing ratios.
Figure 4 shows the results of the microstructure analysis of the SB/SAC composite solder joint, conducted to identify the cause for the variation in the mechanical properties of the SB/SAC composite solder joint with the SB/SAC mixing ratio. The microstructure of the joints containing only SB exhibited a typical eutectic structure with alternating lamellae of a dark Sn-rich phase and a light gray Bi-rich phase, corresponding to their eutectic compositions (Fig. 4(a)).11) In the joints containing only SAC, Ag3Sn and Cu6Sn5 IMCs were distributed in a band-like pattern within a large Sn-rich phase region (Fig. 4(d)).12) In contrast, in the solder containing both SB and SAC (Figs. 4(b) and (c)), Ag3Sn and Cu6Sn5 IMC particles and Bi-rich phase particles were uniformly distributed across a large Sn-rich phase region. With increasing SAC content in the SB/SAC composite solder joints, the eutectic structure pertaining to the initial microstructure of SB completely disappeared, and the sizes of the Ag3Sn and Cu6Sn5 IMCs and Bi-rich phase particles in the Sn-rich phase decreased. These microstructural changes in the SB/SAC composite solder joints were attributable to the modification of the solder composition resulting from the increase in the SAC content. The introduction of SAC increased the Sn content, thereby transforming the eutectic composition of SB to a hypo-eutectic composition. During the cooling process, owing to this change in the composition, the primary Sn phase precipitated from the molten SB/SAC composite solder, and simultaneously, Ag3Sn and Cu6Sn5 IMCs were generated by the chemical reaction between the Sn, Ag, and Cu atoms. As the cooling temperature approached the eutectic temperature of SB, the fine Bi phase particles precipitated from the large primary Sn phase.13) Owing to this solidification mechanism of the molten SB/SAC composite solder, the fine Ag3Sn and Cu6Sn5 IMCs and Bi-rich phase particles were uniformly distributed in the solidified SB/SAC composite solder joints. The distribution of the fine IMCs and Bi-rich particles in the SB/SAC composite solder joints helped enhance their mechanical properties by preventing dislocation and delaying the plastic deformation through precipitation hardening and dispersion strengthening.14,15) These observations indicated that the improvement in the mechanical properties of the SB/SAC composite solder joints resulting from the increase in the SAC content can be attributed to the changes in the microstructure.
Microstructures of the SB/SAC composite solder joints with different SB/SAC mixing ratios: (a) 100:0, (b) 50:50, (c) 20:80, and (d) 0:100.
Figure 5 shows the fracture surface morphologies of the SB/SAC composite solder joints with different SB/SAC mixing ratios subjected to the ball shear test. The fracture surfaces of the solder containing only SB or only SAC were mainly composed of the solder bulk without any interfacial IMCs. This finding indicated that the fractures propagated primarily within the bulk solder region (Figs. 5(a) and (d)). In contrast, as shown in Figs. 5(b) and (c), the fracture surfaces of the solder containing both SB and SAC were composed of bulk solder and the Cu6Sn5 IMC phase, indicating that the fractures mainly propagated near the interfacial Cu6Sn5 IMC layer. Furthermore, the area of the Cu6Sn5 IMC region on the entire fracture surface increased as the SAC content increased in the SB/SAC composite solder joints. These observations confirmed that the fracture path of the SB/SAC composite solder joints shifted from the bulk solder region to the bonding interface between the Cu electrode and Cu6Sn5 IMC layer as the SAC content increased in the SB/SAC composite solder joints. In general, the failure mode in solder joints is influenced by the mechanical strength of the solder materials: In low-strength solders, fractures propagate within the bulk solder region. In contrast, in high-strength solder joints, failure occurs at the solder bulk/Cu6Sn5 IMC layer interface or inside the IMCs.16) Overall, the introduction of SAC in the SB/SAC composite solder can enhance the mechanical properties and failure mode of the solder joint.
Fracture surface morphologies of the SB/SAC composite solder joints with different SB/SAC mixing ratios: (a) 100:0, (b) 50:50, (c) 20:80, and (d) 0:100.
SB/SAC composite solder was prepared to address the challenges associated with SB eutectic solder and improve its mechanical bonding properties. Ball shear and microhardness tests were conducted to investigate the influence of the SB/SAC mixing ratios on the mechanical properties of the SB/SAC composite solder joints. The results demonstrated that the mechanical properties of the SB/SAC composite solder joints were improved owing to the reinforcement effects, specifically, precipitation hardening and dispersion strengthening induced by the fine Bi-rich phase and IMC particles distributed in the SB/SAC composite solder joints. These effects were attributable to the changes in the composition and microstructure of the SB/SAC composite solder joints owing to the introduction of SAC in the SB solder. Future work will be focused on exploring the bonding properties and reliability of SB/SAC composite solder joints.
This research was supported by the National Research Foundation of Korea (NRF) grant funded by the Korea government (MSIT) (No. 2022R1A2C1010405) and Basic Science Research Program funded by the Ministry of Education (No. 2022R1A6A3A01086880).
