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(a) SEM cross-section of the as reflowed Cu/IMCs/SAC solder joint at two different magnifications. (b) Sn, Cu and Ag EDS maps of the Cu/IMCs/SAC solder joint. (c) Temperature profile of the heating experiment, showing different time intervals (points 1-9) through the heating cycle.
Source publication
The complex reaction between liquid solder alloys and solid substrates has been studied ex-situ in a few studies, utilizing creative setups to “freeze” the reactions at different stages during the reflow soldering process. However, full understanding of the dynamics of the process is difficult due to the lack of direct observation at micro- and nan...
Contexts in source publication
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... TEM lamellar of approximately 16 µm × 25 µm was prepared using a focused ion beam (FIB) technique on a FEI Scios FIB-dual beam scanning electron microscope (SEM), similar to a technique described elsewhere [20]. The sample was extracted from a region of interest (ROI) at the Cu/IMCs/SAC interface ( Figure 1a) and welded to a Cu TEM half grid by Pt deposition. Subsequently, the area of the lamellar containing the Cu/IMCs/SAC interfaces were thinned to 500 nm. ...
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... sample was tilted to a low-index zone axis of this grain and selected area electron diffraction (SAED) patterns were obtained. The sample was then heated according to the temperature profile shown in Figure 1c. A high-resolution video recorder is used to capture the evolution of the selected Cu 6 Sn 5 grain during the heating experiment at a frame rate of 1 frame per second. ...
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... sample was tilted to a low-index zone axis of this grain and selected area electron diffraction (SAED) patterns were obtained. The sample was then heated according to the temperature profile shown in Figure 1c. A high-resolution video recorder is used to capture the evolution of the selected Cu6Sn5 grain during the heating experiment at a frame rate of 1 frame per second. ...
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... SEM cross-section of the SAC305 solder ball reflow soldered on to the OSP Cu pad is shown in Figure 1a along with a higher magnification image displaying the Cu/IMCs/SAC interface. The Cu 6 Sn 5 IMC has a scalloped-like morphology. ...
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... has an atomic number of 47 which is relatively close to the atomic number of Sn, therefore the contrast between Ag 3 Sn and the Sn-rich matrix is low. The brighter spots in the Ag-LA EDS map in Figure 1b corresponds to the fine Ag 3 Sn plates, while the Sn-LA and Cu-KA maps help identify the Sn-rich SAC305 solder, the Cu 6 Sn 5 IMC and the Cu substrate. The Cu 3 Sn IMC layer in the as reflowed joint was thin and was not visible due to the resolution limit of the SEM. ...
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... SEM cross-section of the SAC305 solder ball reflow soldered on to the OSP Cu pad is shown in Figure 1a along with a higher magnification image displaying the Cu/IMCs/SAC interface. The Cu6Sn5 IMC has a scalloped-like morphology. ...
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... has an atomic number of 47 which is relatively close to the atomic number of Sn, therefore the contrast between Ag3Sn and the Sn-rich matrix is low. The brighter spots in the Ag-LA EDS map in Figure 1b corresponds to the fine Ag3Sn plates, while the Sn-LA and Cu-KA maps help identify the Sn-rich SAC305 solder, the Cu6Sn5 IMC and the Cu substrate. The Cu3Sn IMC layer in the as reflowed joint was thin and was not visible due to the resolution limit of the SEM. ...
Citations
... The IMC layer uniformly forms between the interface of Ga and the Cu6Ni substrate, and as the temperature increases, the IMC formation speed increases significantly, and the growth is in the direction of the Cu6Ni substrate which is similar to the observation for SAC/Cu joints observed under in situ TEM. 26 It can be seen from Supplementary Video 1 that the IMC growth rate at 104°C is much slower than that at 150°C. The large ...
... The change in the substrate surface morphology has also been observed for the solid−liquid reaction between SAC305 and Cu substrates. 26 However, the slow change of the substrate proves that at 100°C, the reaction rate between Ga and Cu6Ni is much lower than that at 150°C. This indicates that the IMC growth rate is heavily dependent on the reaction temperature during heating. ...
... We estimated growth-rate constants of (Cu, Pd) 6 Sn 5 IMCs of about 0.9 nm/ √ s and 0.8 nm/ √ s for the upper and lower interface, respectively, after 500 h at 150 • C. A prominent difference was seen on the Cu30/100-SAC interface (compared to the same sample after bonding and temperature cycling), where more copper was consumed to enhance the Cu 3 Sn layer. Meanwhile, the Cu diffusion to Cu-Sn-Pd creates vacancies leading to Kirkendall voids, as also observed by [46]. The SEM micrographs show an uneven Cu 3 Sn-layer thickness across the interface; the maximum thickness was about 1.5 µm. ...
Flip-chip bonding is a key packaging technology to achieve the smallest form factor possible. Using copper as a direct under-bump metal and performing bonding under little force and at a low temperature eliminates the processing step for the deposition of a suitable wetting metal and offers an economical solution for electronic chip packaging. In this paper, various samples with copper and nickel–gold surface finishes are used to apply an in-house solder bumping, flip-chip bonding and reflow process to exhibit the bump-bond feasibility. Native oxides are reduced using process gases, and copper surface protection and solder wetting are achieved using copper formate. Lead-free 40 µm solder balls were bumped on 80 µm copper pads and 120 µm copper pillars to demonstrate a full intermetallic Cu–Cu bond as a base study for stacking applications. Using a low-force bonding technique, various chips with different dimensions were bonded at 0.5–16 MPa, followed by a reflow step at a maximum temperature of 270 ∘C. Then, 30 µm solder balls are utilized to bump the samples with NiAu and Cu bond pads at 50 µm pitch. A mean shear strength of 44 MPa was obtained for the 30 µm Cu samples. To the best of our knowledge, 30 µm solder bumping directly on the copper pads by producing copper formate is a novel research contribution.
