Fig 1 - uploaded by C. R. Kao
Content may be subject to copyright.
Schematic drawing showing two types of solder joints, flip-chip joints and BGA joints, used in microelectronic packaging
Source publication
The interfacial reactions between Sn-based solders and two common substrate materials, Cu and Ni, are the focuses of this
paper. The reactions between Sn-based solders and Cu have been studied for several decades, and currently there are still
many un-resolved issues. The reactions between Sn-based solders and Ni are equally challenging. Recent stu...
Contexts in source publication
Context 1
... by definition involves the chemical reaction(s) between the solder and the two surfaces to be joined together [1], and consequently the importance of understanding the chemical reactions between solders and bonding surfaces cannot be overemphasized. Figure 1 is a schematic illustration showing the key elements of the solder joints in an up-to-date electronic package. Here, the chip is connected to a ball grid array (BGA) substrate through an array of solder joints (flip-chip joints) with a diameter of around 90 lm. ...
Context 2
... regions on the surface of a chip that are to be in direct contact with the solders are the so-called un- der bump metallurgy (UBM) regions. Copper is the most popular choice for the surface layer of the UBM, mainly due to its good wetting property with solders [2,3]. In Fig. 1, Cu is selected for illustration as the sur- face layer of the UBM, and the Ni layer beneath serves as the diffusion barrier layer. During assembly or normal service of the device, the Cu layer will be consumed completely, exposing the Ni layer to the solder. Those regions on the BGA and PCB substrates that are to be in contact with ...
Context 3
... the solder rapidly during soldering [4][5][6][7][8][9][10][11][12][13][14][15], exposing the Ni layer below. In the cases of the other three surface finishes, Sn, Ag, or OSP will be dissolved into the solder (immersion Sn and immersion Ag) or be displaced from the interface (OSP) during soldering, leaving the Cu layer exposed to the solder. In Fig. 1, Au/Ni is used as the surface finish for the BGA substrate, and OSP for the PCB substrate. Summarizing all the popular material choices for the UBM and the surface finishes, one can expect that the most common material sequences across a post-assembly solder joints are, Cu/ solder/Cu, Cu/solder/Ni, and Ni/solder/Ni. The Ni layer here ...
Context 4
... Ag is inert as far as the interfacial reaction is concerned [73,75,87]. Accordingly, the Sn- Cu-Ni ternary phase diagram is sufficient for the present purpose. The Sn-Cu-Ni isotherm had been measured by two independent groups [113,114], and the results are reasonably consistent. The 240°C Sn-Cu-Ni isotherm basing on these two studies is shown in Fig. 11. The Sn-rich corner of this isotherm is shown in Fig. 5. There is some evidence for the existence of a ternary compound (Ni26Cu29Sn45, atomic percent) [115]. If this compound is indeed stable, then the isotherm in Fig. 11 is only a metastable isotherm [29,41]. Never- theless, as far as soldering is concerned, the isotherm shown in Fig. ...
Context 5
... [113,114], and the results are reasonably consistent. The 240°C Sn-Cu-Ni isotherm basing on these two studies is shown in Fig. 11. The Sn-rich corner of this isotherm is shown in Fig. 5. There is some evidence for the existence of a ternary compound (Ni26Cu29Sn45, atomic percent) [115]. If this compound is indeed stable, then the isotherm in Fig. 11 is only a metastable isotherm [29,41]. Never- theless, as far as soldering is concerned, the isotherm shown in Fig. 11 is still adequate, as results from most soldering reaction experiments were observed to follow the phase relationships shown in Fig. ...
Context 6
... in Fig. 11. The Sn-rich corner of this isotherm is shown in Fig. 5. There is some evidence for the existence of a ternary compound (Ni26Cu29Sn45, atomic percent) [115]. If this compound is indeed stable, then the isotherm in Fig. 11 is only a metastable isotherm [29,41]. Never- theless, as far as soldering is concerned, the isotherm shown in Fig. 11 is still adequate, as results from most soldering reaction experiments were observed to follow the phase relationships shown in Fig. ...
Context 7
... (Ni26Cu29Sn45, atomic percent) [115]. If this compound is indeed stable, then the isotherm in Fig. 11 is only a metastable isotherm [29,41]. Never- theless, as far as soldering is concerned, the isotherm shown in Fig. 11 is still adequate, as results from most soldering reaction experiments were observed to follow the phase relationships shown in Fig. ...
Context 8
... the reaction between SnAgCu solder balls and Ni substrate (Fig. 12), the original amount of Cu in the solder ball before reflow equals the remaining Cu in the solder plus the Cu which is incorporated into the intermetallic(s). Neglecting the Cu atoms in those intermetallic particles located inside the solder, one can obtain the following equation [89][90][91]: Fig. 8 XRD patterns for the reaction ...
Context 9
... Cu atoms in those intermetallic particles located inside the solder, one can obtain the following equation [89][90][91]: Fig. 8 XRD patterns for the reaction products in Fig. 7(a)-(d). The Ni signals originated from the Ni layer beneath the intermetallic compounds [73,106] combinations ranging from the BGA to flip-chip dimensions is plotted in Fig. 13. For example, if 2 lm (Cu 1-x Ni x ) 6 Sn 5 , which is the thickness commonly seen in real solder joints, forms in the 100 lm/80 lm combination, the Cu concentration will drop by as large as 0.51 wt.%. Under such a condition, the resi- due Cu concentration in solder will be less than 0.3 wt.% for most SnAgCu solder compositions listed ...
Context 10
... joint size dependence can be clearly seen in Fig. 14, where the drop of the Cu concentration is plot- ted against the joint size. In short, the Cu concentration drop rapidly increases as the joint becomes ...
Context 11
... experimental evidence of the solder vol- ume effect is presented in Fig. 15 [89][90][91]. The experi- mental setup in Fig. 15 was used with d pad being kept constant at 375 lm, and d joint being varied from 760 to 500, and to 300 lm. In other words, three different d joint /d pad values were used. The solder used here was Sn3Ag0.6Cu, and the samples had been reflowed with a typical profile (235°C peak ...
Context 12
... experimental evidence of the solder vol- ume effect is presented in Fig. 15 [89][90][91]. The experi- mental setup in Fig. 15 was used with d pad being kept constant at 375 lm, and d joint being varied from 760 to 500, and to 300 lm. In other words, three different d joint /d pad values were used. The solder used here was Sn3Ag0.6Cu, and the samples had been reflowed with a typical profile (235°C peak temperature, 90 s molten solder duration). Using Eq. 1 and ...
Context 13
... varied from 760 to 500, and to 300 lm. In other words, three different d joint /d pad values were used. The solder used here was Sn3Ag0.6Cu, and the samples had been reflowed with a typical profile (235°C peak temperature, 90 s molten solder duration). Using Eq. 1 and the (Cu 1-x Ni x ) 6 Sn 5 thicknesses measured from the three cases shown in Fig. 15 (1.2, 1.5, and 2.2 lm, respectively), one can calculate the Cu concentration drops to be 0.02, 0.05, and 0.48 wt.% for 760, 500, and 300 lm d joint , respec- tively. As a result, the remaining Cu concentrations of the solders after reflow were 0.58, 0.55, and 0.12 wt.% for d joint = 760, 500, and 300 lm, respectively. As expected, the ...
Context 14
... calculate the Cu concentration drops to be 0.02, 0.05, and 0.48 wt.% for 760, 500, and 300 lm d joint , respec- tively. As a result, the remaining Cu concentrations of the solders after reflow were 0.58, 0.55, and 0.12 wt.% for d joint = 760, 500, and 300 lm, respectively. As expected, the intermetallic compound for the first two cases shown in Fig. 15(a) and (b) was still (Cu 1-x Ni x ) 6 Sn 5 . However, for the 300 lm case, the Cu concentration was so low that a new layer of (Ni 1- y Cu y ) 3 Sn 4 had nucleated beneath the (Cu 1-x Ni x ) 6 Sn 5 layer that formed in the early stage of the reaction when the Cu concentration was still high. In Fig. 15(c), a series of voids can be seen, ...
Context 15
... compound for the first two cases shown in Fig. 15(a) and (b) was still (Cu 1-x Ni x ) 6 Sn 5 . However, for the 300 lm case, the Cu concentration was so low that a new layer of (Ni 1- y Cu y ) 3 Sn 4 had nucleated beneath the (Cu 1-x Ni x ) 6 Sn 5 layer that formed in the early stage of the reaction when the Cu concentration was still high. In Fig. 15(c), a series of voids can be seen, separating these two inter- metallic compounds. As will be shown in the next sec- tion, an advanced development, when the reflow time is increased longer (see Fig. 18) or the solder joint is shrunk even smaller (see Fig. 17), of this effect is the total separation (spalling) of the upper (Cu 1-x Ni x ) 6 ...
Context 16
... had nucleated beneath the (Cu 1-x Ni x ) 6 Sn 5 layer that formed in the early stage of the reaction when the Cu concentration was still high. In Fig. 15(c), a series of voids can be seen, separating these two inter- metallic compounds. As will be shown in the next sec- tion, an advanced development, when the reflow time is increased longer (see Fig. 18) or the solder joint is shrunk even smaller (see Fig. 17), of this effect is the total separation (spalling) of the upper (Cu 1-x Ni x ) 6 Sn 5 layer from the interface. That is, the consequence of the shifting equilibrium phase leads to the massive spalling of (Cu 1-x Ni x ) 6 Sn 5 ...
Context 17
... formed in the early stage of the reaction when the Cu concentration was still high. In Fig. 15(c), a series of voids can be seen, separating these two inter- metallic compounds. As will be shown in the next sec- tion, an advanced development, when the reflow time is increased longer (see Fig. 18) or the solder joint is shrunk even smaller (see Fig. 17), of this effect is the total separation (spalling) of the upper (Cu 1-x Ni x ) 6 Sn 5 layer from the interface. That is, the consequence of the shifting equilibrium phase leads to the massive spalling of (Cu 1-x Ni x ) 6 Sn 5 ...
Context 18
... the interface Spalling refers to the detachment of a compound from the interface into one of the reacting phases. One classical example, shown in Fig. 16, is the spalling of Cu 6 Sn 5 during the reaction of eutectic PbSn solder with Au/Cu/Cu-Cr thin film [3,116]. Spalling here was due to the exhaustion of the Cu film, and the poor wetting between Cu 6 Sn 5 and the remaining Cr (or SiO 2 ) caused the compound to detach itself from the interface. The ''massive spalling' introduced in this ...
Context 19
... massive spalling has a higher tendency to occur in smaller joints because these joints will experience a lager Cu concentration drop. Fig. 17 shows an example of massive spalling in its early stage. The reflow con- ditions were the same as those in Fig. 15, but d pad and d joint were 175 and 200 lm, respectively. As can be seen in Fig. 17, a new (Ni 1-y Cu y ) 3 Sn 4 layer had formed over the Ni substrate, pushing the original (Cu 1-x Ni x ) 6 Sn 5 layer away from the ...
Context 20
... massive spalling has a higher tendency to occur in smaller joints because these joints will experience a lager Cu concentration drop. Fig. 17 shows an example of massive spalling in its early stage. The reflow con- ditions were the same as those in Fig. 15, but d pad and d joint were 175 and 200 lm, respectively. As can be seen in Fig. 17, a new (Ni 1-y Cu y ) 3 Sn 4 layer had formed over the Ni substrate, pushing the original (Cu 1-x Ni x ) 6 Sn 5 layer away from the interface, and a gap had appeared between these two layers. It should be stressed again that even though the reflow ...
Context 21
... massive spalling has a higher tendency to occur in smaller joints because these joints will experience a lager Cu concentration drop. Fig. 17 shows an example of massive spalling in its early stage. The reflow con- ditions were the same as those in Fig. 15, but d pad and d joint were 175 and 200 lm, respectively. As can be seen in Fig. 17, a new (Ni 1-y Cu y ) 3 Sn 4 layer had formed over the Ni substrate, pushing the original (Cu 1-x Ni x ) 6 Sn 5 layer away from the interface, and a gap had appeared between these two layers. It should be stressed again that even though the reflow conditions were the same, such spalling did not occur for the larger joints in Fig. 15(a) ...
Context 22
... be seen in Fig. 17, a new (Ni 1-y Cu y ) 3 Sn 4 layer had formed over the Ni substrate, pushing the original (Cu 1-x Ni x ) 6 Sn 5 layer away from the interface, and a gap had appeared between these two layers. It should be stressed again that even though the reflow conditions were the same, such spalling did not occur for the larger joints in Fig. 15(a) and (b). Longer reflow time also favors the spalling, as shown in Fig. 18. Here, the sample had the same configuration as that of Fig. 15(c), but the reflow time was increased from 90 s to 20 min. Longer reflow time had increased the separation between (Ni 1-y Cu y ) 3 Sn 4 and (Cu 1-x Ni x ) 6 Sn 5 from a series of voids in Fig. 15(c) ...
Context 23
... Ni substrate, pushing the original (Cu 1-x Ni x ) 6 Sn 5 layer away from the interface, and a gap had appeared between these two layers. It should be stressed again that even though the reflow conditions were the same, such spalling did not occur for the larger joints in Fig. 15(a) and (b). Longer reflow time also favors the spalling, as shown in Fig. 18. Here, the sample had the same configuration as that of Fig. 15(c), but the reflow time was increased from 90 s to 20 min. Longer reflow time had increased the separation between (Ni 1-y Cu y ) 3 Sn 4 and (Cu 1-x Ni x ) 6 Sn 5 from a series of voids in Fig. 15(c) to a gap in Fig. ...
Context 24
... from the interface, and a gap had appeared between these two layers. It should be stressed again that even though the reflow conditions were the same, such spalling did not occur for the larger joints in Fig. 15(a) and (b). Longer reflow time also favors the spalling, as shown in Fig. 18. Here, the sample had the same configuration as that of Fig. 15(c), but the reflow time was increased from 90 s to 20 min. Longer reflow time had increased the separation between (Ni 1-y Cu y ) 3 Sn 4 and (Cu 1-x Ni x ) 6 Sn 5 from a series of voids in Fig. 15(c) to a gap in Fig. ...
Context 25
... joints in Fig. 15(a) and (b). Longer reflow time also favors the spalling, as shown in Fig. 18. Here, the sample had the same configuration as that of Fig. 15(c), but the reflow time was increased from 90 s to 20 min. Longer reflow time had increased the separation between (Ni 1-y Cu y ) 3 Sn 4 and (Cu 1-x Ni x ) 6 Sn 5 from a series of voids in Fig. 15(c) to a gap in Fig. ...
Context 26
... (b). Longer reflow time also favors the spalling, as shown in Fig. 18. Here, the sample had the same configuration as that of Fig. 15(c), but the reflow time was increased from 90 s to 20 min. Longer reflow time had increased the separation between (Ni 1-y Cu y ) 3 Sn 4 and (Cu 1-x Ni x ) 6 Sn 5 from a series of voids in Fig. 15(c) to a gap in Fig. ...
Context 27
... very revealing example of the massive spalling is shown in Fig. 19, where a d joint /d pad = 760 lm/600 lm joint had been reflowed at 235°C for 5 min. In this particular sample, a 1.2( ± 0.1) lm Au layer had been coated over the Ni layer of the substrate before reflow. The images in Fig. 19(a) and (b) were obtained from the same specimen, but the solder in Fig. 19(b) had been etched away. As can be ...
Context 28
... very revealing example of the massive spalling is shown in Fig. 19, where a d joint /d pad = 760 lm/600 lm joint had been reflowed at 235°C for 5 min. In this particular sample, a 1.2( ± 0.1) lm Au layer had been coated over the Ni layer of the substrate before reflow. The images in Fig. 19(a) and (b) were obtained from the same specimen, but the solder in Fig. 19(b) had been etched away. As can be seen here, the detached Au-bearing (Cu 1-x Ni x ) 6 Sn 5 layer was almost continu- ously. The extensive spalling shown in this pair of micrographs demonstrates the justification of naming the phenomenon massive spalling. It should ...
Context 29
... example of the massive spalling is shown in Fig. 19, where a d joint /d pad = 760 lm/600 lm joint had been reflowed at 235°C for 5 min. In this particular sample, a 1.2( ± 0.1) lm Au layer had been coated over the Ni layer of the substrate before reflow. The images in Fig. 19(a) and (b) were obtained from the same specimen, but the solder in Fig. 19(b) had been etched away. As can be seen here, the detached Au-bearing (Cu 1-x Ni x ) 6 Sn 5 layer was almost continu- ously. The extensive spalling shown in this pair of micrographs demonstrates the justification of naming the phenomenon massive spalling. It should be pointed out that our recent data suggests that Au does have the effect ...
Context 30
... the solder joints became even smaller (300 lm), the supply of Cu became very limited. For Fig.19 (b) 100 µm (a) 760 ...
Context 31
... investigate the cross-interaction during the reflow stage more carefully, the solder joints that were assembled by two different paths are compared. As illustrated in Fig. 21, in Path I, the solder (Sn3.5Ag) was attached to the Cu substrate in the first reflow, and then the Ni substrate was attached to the other side of the solder joint in the second reflow. In path II, the solder was attached to the Ni substrate first (the first reflow), and in the second reflow Cu substrate was attached to the other side ...
Context 32
... (aging) Figure 26 shows the microstructures of the interfaces in Fig. 22 after aging at 160°C for 1,000 h. The Ni/ solder interface and the solder/Cu interface that had Fig. 21 Illustration showing the two sequences that the solder joints were assembled been soldered only once and consequently had no cross-interaction during aging were shown in the left column for comparison. In the middle column and the right column, the corresponding interfaces for the Path I and Path II after aging are shown, respectively. ...
Context 33
... compound thickness. As the Cu concen- tration decreases, the equilibrium phase at the inter- face may change, and the local thermodynamic equilibrium at the interface is no longer static. Path I Path II Fig. 22 Micrographs showing the solder/Cu and Ni/solder interfaces for Path I (left column) and path II (right column) illustrated in Fig. 21. The micrographs in (a) and (d) had been tilted by ...
Context 34
... of the solder joints. No cross-interaction Path I Path II Fig. 26 Micrographs showing the microstructures of the interfaces in Fig. 22 after aging at 160°C for 1,000 h. Each micrograph here corresponds to the after-aging microstructure of the interface that has the same label in Fig. 22 Ni Fig. 25 Glancing angle XRD pattern of the interface in Fig. 21(d) and (e). The crystal structures of the Ni 3 Sn 4 compound [ Fig. 21(d)] and the (Cu 1-x Ni x ) 6 Sn 5 compound [Fig. 21(e)] were positively ...
Context 35
... showing the microstructures of the interfaces in Fig. 22 after aging at 160°C for 1,000 h. Each micrograph here corresponds to the after-aging microstructure of the interface that has the same label in Fig. 22 Ni Fig. 25 Glancing angle XRD pattern of the interface in Fig. 21(d) and (e). The crystal structures of the Ni 3 Sn 4 compound [ Fig. 21(d)] and the (Cu 1-x Ni x ) 6 Sn 5 compound [Fig. 21(e)] were positively ...
Citations
... However, the faster growth of IMCs accompanying the formation of Kirkendall voids gives rise to critical reliability issues [13]. The void formation caused by the Kirkendall effect is a well-known phenomenon in Sn-based solder/Cu joints [14][15][16]. Due to the large difference of atomic diffusivities, the Cu atoms diffuse rapidly in the Cu 3 Sn phase rather than the reverse diffusion of Sn into Cu 3 Sn [13]. The out-diffusion of Cu atoms leaves vacancies on the departure sites, while the refilling of vacancies fails due to the slower diffusion rate of Sn. ...
Sn-3Ag-0.5Cu (SAC305)- and Sn-9Zn-based alloys (Sn-Zn-X, X = Al, In) are lead-free solders used in the fabrication of solder joints with Cu metallization. Electroplating is a facile technology used to fabricate Cu metallization. However, the addition of functional additive molecules in the plating solution may result in impurity residues in the Cu electroplated layer, causing damage to the solder joints. This study investigates the impurity effect on solder joints constructed by joining various solder alloys to the Cu electroplated layers. Functional additives are formulated to fabricate high-impurity and low-impurity Cu electroplated samples. The as-joined solder joint samples are thermally aged at 120 °C and 170 °C to explore the interfacial reactions between solder alloys and Cu. The results show that the impurity effect on the interfacial reactions between SAC305 and Cu is significant. Voids are massively formed at the SAC305/Cu interface incorporated with a high impurity content, and the Cu6Sn5 intermetallic compound (IMC) grows at a faster rate. In contrast, the growth of the Cu5Zn8 IMC formed in the SnZn-based solder joints is not significantly influenced by the impurity content in the Cu electroplated layers. Voids are not observed in the SnZn-based solder joints regardless of the impurity content, indicative of an insignificant impurity effect. The discrepancy of the impurity effect is rationalized by the differences in the IMC formation and associated atomic interdiffusion in the SAC305- and SnZn-based solder joints.
... When compared with silver or gold, Cu possesses a superior cost advantage. Cu thus is widely adopted in the metallization processes of ULSICs [1,2], micro-electronic circuits, printed circuits, and the like, mainly in the semiconductor and photoelectronic industries. However, once a Cu layer is deposited onto a substrate, Cu scattering is prone to creep into the substrate; the scattered Cu often interacts with the silicon dioxide or silicon in the substrate, producing oxides that often detrimentally increase the resistivity [3] of the related electronic components. ...
A new type of copper (Cu)-rhodium (Rh)-alloy, Cu(Rh), films is developed by co-sputtering copper and rhodium onto silicon (Si) substrates under an argon (Ar) atmosphere. The new films are next annealed at 600 and 670 °C, or alternatively at 100 and 450 °C, for 1 h. Longer annealing to the films, for up to 8 days, is also conducted to explore resistivity variation. The resistivity of the new 300-nm-thick film is 2.19 μΩ cm after annealing at 670 °C for 1 h and drifts to 2.26 and 2.14 μΩ after annealing at 400 and 450 °C, respectively, for 200 h. A 2.7-μm-thick Sn layer is then thermally evaporated atop the new film for stable flip-chip solder joints; their metal and Cu-Sn intermetallic compound (IMC) growth processes vs. various annealing periods are tested. After annealing at 670 °C, the new 300-nm-thick film’s adhesive strength reaches 44.2 ± 0.01 MPa, which is 11 ~ 12-fold that of their pure Cu counterpart. Some key test results of the new film are disclosed herein, including its X-ray diffraction (XRD) patterns, transmission electron microscopy (TEM) images, secondary-ion mass spectrometry (SIMS), time-dependent dielectric-breakdown (TDDB) lifetime curves, and adhesive strength. The new film’s antibacterial efficacy arrives at an antibacterial ratio of approximately 100% against Staphylococcus aureus ( S. aureus ) BCRC 10451 for the 300-nm-thick film and approximately 99.82% for the 8 nm film, far superior to that of a pure Cu film, which is 0 with the same annealing temperature range. The new film, hence, seems to be a remarkable candidate material for various industrial applications, such as ultra-large-scale integrated circuits (ULSIC), micro-electronic circuits, printed circuits, flip-chip technology, medical care concerning antibacteria, and the like.
Graphical Abstract
A new type of copper (Cu)-rhodium (Rh)-alloy, Cu(Rh), films is developed by co-sputtering copper and rhodium onto silicon (Si) substrates under an argon (Ar) atmosphere and then annealing the new films at 600 and 670 °C, or alternatively at 100 and 450 °C, for 1 h. Longer annealing to the films, for up to 8 days, is also conducted to explore resistivity variation. The resistivity of the new 300-nm-thick film is 2.19 mW cm after annealing at 670 °C for 1 h and drifts to 2.26 and 2.14 mW after annealing at 400 and 450 °C, respectively, for 200 h. A 2.7-μm-thick Sn layer is next thermally evaporated atop the new film for stable flip-chip solder joints; their metal and Cu-Sn intermetallic compound (IMC) growth processes vs. various annealing periods are tested. After annealing at 670 °C, the new 300-nm-thick film’s adhesive strength reaches 44.2 ± 0.01 MPa, which is 11~12-fold that of their pure Cu counterpart. Some key test results of the new film are disclosed herein, including its X-ray diffraction (XRD) patterns, transmission electron microscopy (TEM) images, secondary-ion mass spectrometry (SIMS), time-dependent dielectric-breakdown (TDDB) lifetime curves, and adhesive strength. The new film’s antibacterial efficacy arrives at an antibacterial ratio of approximately 100% against Staphylococcus aureus ( S. aureus ) BCRC 10451 for the 300-nm-thick film and approximately 99.82% for the 8-nm film, far superior to that of a pure Cu film, which is 0 with the same annealing temperature range. The new film, hence, seems to be a remarkable candidate material for various industrial applications, such as ultra-large-scale integrated circuits (ULSIC), micro-electronic circuits, printed circuits, flip-chip technology, medical care concerning antibacteria, and the like.
... Solder joints are essential components in various electronic devices and systems, serving as critical connectors that establish both electrical and mechanical connections between components and circuitry. These joints are created by melting a solder material, typically an alloy, which then solidifies to form a durable and conductive bond [17]. The quality and reliability of solder joints are crucial to ensure the optimal performance and reliability of electronic assemblies. ...
... At the reflow temperature, it dissolves in the molten solder. Owing to the decreased reaction rate of Ni-Sn [52], a thinner (Cu, Ni) 6 Sn 5 IMC layer of 1.58 μm was developed in the SAC305 solder joint. Simultaneously, a tertiary phase of Ni 3 Sn 4 occurs in the SnPb solder joints. ...
SAC305(Sn-3.0Ag-0.5Cu) has gained the highest acceptance as a solder alloy. The fatigue performance of solder joints has become an essential reliability assessment criterion due to the transition to more reliable lead-free alloys from highly predictable lead-based alloys. This study examines the shear fatigue characteristics of sandwich test vehicles for SnPb and SAC305 at different temperatures using both Organic Sol-derability Preservative (OSP) and Electroless Nickel-Immersion Silver (ENIG) surface finishes. The fatigue experiments were performed using an Instron micromechanical tester at a constant strain rate, and micro-structural analysis was carried out using Scanning Electron Microscopy (SEM) to identify the Intermetallic Compound (IMC) morphology and failure mode. The stress-strain (hysteresis) loops of SnPb and SAC305 were measured, and the fatigue life of the specimens was estimated using the strain-life equation at various temperatures. SAC305 was observed to have a better fatigue life than SnPb, particularly at higher strain levels or testing temperatures. The OSP surface finish demonstrated superior fatigue properties compared to the ENIG surface finish. Additionally, elevated testing temperatures were found to accelerate fatigue failure in solder joints. The Arrhenius model was utilized to develop a general empirical model that predicts the fatigue life of SAC305 and SnPb solder alloys with OSP and ENIG surface finishes as a function of the strain level and testing temperature.
... These above results indicate that the B-doping in Co(B) deposits has a notable effect on IMC formation, including preventing IMC spalling, suppressing IMC growth, enhancing interfacial stability, and improving solder joint reliability. Extensive investigations have been conducted on the suppression effect of minor alloying additions on the growth of Cu 3 Sn at the interface between leadfree solders and Cu pads [35][36][37]. For example, the addition of 0.1 wt.% Ni to Sn-3.5 wt.% Ag significantly inhibits the Cu 3 Sn growth and Kirkendall voids, leading to improve the mechanical properties and reliability of micro solder joints [34]. ...
... However, it should be noted that the addition of Ni enhances the growth of the porous (Cu,Ni) 6 Sn 5 phase consisting of small grains due to its lower Gibbs free energy. Similarly, Ho et al. reported that minor Pd addition in solder inhibits the Cu 3 Sn growth [35]. This effect is attributed to the dissolution of Pd atoms in the Cu 6-Sn 5 phase, forming a ternary (Cu,Pd) 6 Sn 5 phase with lower Gibbs free energy compared to the binary Cu 6 Sn 5 . ...
This study investigated the interfacial reactions of Sn and Sn–3.0 wt.% Ag–0.5 wt.% Cu (SAC305) with electroplating Co(B) deposits of varying B contents (0.8 at.%–3.2 at.%) at 250 °C. The B content of the Co(B) deposits was adjusted by adding different DMAB concentrations. The results showed that increasing the B content of the Co(B) deposits above 1.3 at.% B caused the microstructures to change from larger column grains to finer nanograins, and the crystallinity decreased and shifted towards an amorphous structure. In the soldering reactions of Co and Co–0.8 at.% B with Sn (or SAC305), only CoSn3 was formed, and it exhibited a high and linear growth rate. However, when the B content exceeded 1.3 at.%, in addition to the dominant CoSn3 phase, a thin reaction layer was formed, namely Co-Sn-B layer with Sn–33 at.% Co–2.5 at.% B. This layer was identified as the nanocrystalline mixed phase of CoSn3, Co3B, and Sn. The growth rate of the total reaction layer was greatly suppressed by about 90%, and it was nearly proportional to the cubic root of aging time. This strong inhibition was attributed to the Co-Sn-B phase acting as a diffusion barrier that altered the mechanism of the CoSn3 formation. Importantly, unlike the massive spalling of CoSn3 observed in Co(P) deposits, no such spalling occurred in the Co(B) deposits. Additionally, in the reactions of the Co(B) deposits with SAC305, the interfacial microstructures were similar to those of the reactions with Sn, but the intermetallic compound (IMC) growth was more sluggish. The strong inhibiting effect on the IMC growth was not only due to B-doping of Co(B) but also the presence of Cu in the SAC305 solder.
... Hence, Cu has gradually replaced Al in widespread industrial applications, particularly in ultra-large-scale integration (ULSI) manufacture using Cu metallization methods. 1,2) Conversely, Cu is prone to diffusing into Si or SiO 2 substrates resulting in Cu silicides, e.g., Cu 3 Si particles, which are detrimental to silicon and can catalyze silicon's oxidation at room temperature. 3) This is a major setback of Cu metallization. ...
Copper (Cu) alloy thin films deposited on barrierless substrates via sputtering and annealing processes have been essential for numerous microelectronic products and continue to be so in the new nanometer-range manufacture era. The search for better new films thus is crucial for further technical and manufacture advancement. The requirements on the new films lie in their stability in existence under high-temperature manufacture environments, low electric resistivity, less leakage current under various electric fields, and sufficient adhesion strength. For the search and advancement, I have developed a new type of films by co-sputtering impurities of niobium, Nb, and zirconium, Zr, with Cu within a vacuum chamber without any gas or with nitrogen (N) under low pressure, resulting in new Cu(NbZr) or Cu(NbZrNx) films whose fabrication processes and test results are detailed herein. The new type of films displays good physical features and seems desirable for microelectronic manufacture and to material science, too.
Fullsize Image
... 21 Kirkendall effect is another well-known cause for the void formation in the Sn-based solder/Cu joints. [26][27][28][29] The reflow reaction between Sn-based solders and Cu produced two IMCs, Cu 6 Sn 5 and Cu 3 Sn, at the joint interface as a result of atomic interdiffusion and interfacial reactions. 30,31 The growth of Cu 3 Sn was governed by the reaction between Cu 6 Sn 5 and Cu (Cu 6 Sn 5 + 9Cu→5Cu 3 Sn), while faster diffusion of Cu left excessive vacancies in the vicinity of the Cu 3 Sn/Cu interface. ...
SnAgCu and Ni-containing SnAgCu alloys are Pb-free solders widely used to join with Cu to construct solder joints. Electrodeposition is a technology commonly used to fabricate Cu but co-deposition of organic impurities originating from additives is an inevitable reliability issue. This study investigates the impurity effect on the voiding propensity in the two solder joints (SnAgCu/Cu and SnAgCu-Ni/Cu) subjected to thermal aging at 200°C. Results show that a high level of impurity incorporation causes massive void propagation along the SnAgCu/Cu and SnAgCu-Ni/Cu interface. Reduction of the impurity concentration by precise control of the additive formulas can weaken the impurity effect and effectively suppress the void propagation. The weakening phenomenon of the impurity effect is more pronounced in the SnAgCu-Ni/Cu joint, indicating that suppression of the Cu3Sn growth as well as Kirkendall voids by Ni addition is also helpful in reducing the influences of impurities.
... This is caused by the continuing reaction between metallic Sn (from the solder) and Cu (from the baseplate) at elevated temperatures. However, any hints of Kirkendall porosity [12] were not (yet) detected. No other effects of prolonged aging such as coarsening of the microstructure were observed. ...
In this work, for several SiC MOSFETs manufacturers and technologies (planar and trench), we monitor the threshold voltage hysteresis as well as the concentration of voids in the solder die attach area as a function of power cycling number. The results show that, while the semiconductor aging is recognizable in increasing Tj,max, the aging does not have a strong impact on the threshold voltage and the VTH hysteresis does not change significantly upon aging. For Pb-rich solders, a reduction of the solder-voids area fraction with increasing power cycles was observed.
... They were mostly composed of Sn-based alloys with melting points as low as nearly 100°C to as high as 400°C. The eutectic or nearly-eutectic Sn-Ag-(Cu) solders with melting points around 220°C have been popularly used as the mid-temperature solders [3][4][5]. For the low-temperature reflowing process, the eutectic In-48at.%Sn ...
This study investigated the interfacial reactions of Co with molten In and eutectic In–48at.%Sn solders at different temperatures. For the In/Co reactions at 350 °C, the intermetallic compound (IMC) of CoIn3 was formed with a uniform and dense structure. It exhibited a unique linear growth with aging time, suggesting that the CoIn3 formation was reaction-controlled. In the eutectic In–Sn/Co reactions at 350 °C, the formed Co(Sn,In)2 phase also displayed a linear growth and its growth rate (~ 5 μm/h) was higher than that of the In/Co reactions (~ 2 μm/h). Moreover, in these two interfacial systems, both the CoIn3 and Co(Sn,In)2 IMCs revealed the so-called cruciform pattern, which is the important feature of the reaction-controlled linear growth. It was the first time to find the special linear growths in the Co/In and In–48at.%Sn/Co reactions. The reactions were carried out at lower temperatures and the microstructures of the reaction phases had a significant change. In the In/Co reactions at 250 °C, the CoIn3 showed a microstructure of isolated particulates and a significant grain coarsening occurred due to the Ostwald ripening. At 180 °C, the CoIn3 grains was unstable at the interface and drastically spalled into the molten In. A similar phenomenon and the microstructural evolutions were also observed in the eutectic In–Sn/Co reactions. It was attributed to the fact that the diffusion and dissolution of the formed phase became dominant at lower reaction temperatures. Based on the detailed results, the correlated reaction mechanism was discussed in detail.
... In Fig. 10b, J chem-Cu and J TM-Cu are reversed, while J chem-Ni and J TM-Ni are in the same direction, which should result in the enhanced diffusion of Ni atoms from the Ni hot end to the Cu cold end, but Ni atoms cannot penetrate the eutectic laminated Sn-Bi solder region. In contrast, if Ni concentration was too high, the solubility of Cu in Sn would decrease significantly [21]. The decrease in the solubility of Cu in Sn near the Ni end leads to a smaller concentration of Cu in this region than in the solder region, which induced much larger J chem-Cu than J TM-Cu and a large total diffusion flux J Cu of Cu atoms. ...
In this work, the atomic migration behavior in Sn-58Bi solder joints was investigated under the condition of a temperature gradient. The effect of different substrates including Cu, Ni and Co on microstructural evolution in the solder was also compared. It was found that the main migrating element in Sn-58Bi solder was Bi in the direction of temperature gradient to produce certain Bi-rich phases at the cold end. With the change of hot end from Cu to Ni or Co, the thermal migration rate of Bi phase and the growth rate of interfacial Intermetallic compounds (IMC) were significantly depressed, which can be attributed to the decrease of the temperature gradient in the solder joint. In particular, there is no obvious asymmetric growth during the growth of IMC at both ends of the interface, which is different from other Sn-based Pb-free solders. Finally, finite element simulation analysis also verified the excellent thermal migration resistance of Co and Ni substrates.
