Cu soldering with a micro interconnected height

Materials Characterization 131 (2017) 49–63

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A study on interfacial phase evolution during Cu/Sn/Cu soldering with a micro interconnected height

MARK

Peng Yao⁎, Xiaoyan Li, Xiaobo Liang, Bo Yu, Fengyang Jin, Yang Li College of Materials Science and Engineering, Beijing University of Technology, No. 100 Ping Le Yuan, Chaoyang District, Beijing 100124, PR China

A R T I C L E I N F O

A B S T R A C T

Keywords: Interfacial phase evolution A micro interconnected height Full Cu3Sn solder joints Interfacial reaction flux Ripening flux Growth process of Cu6Sn5 grains

In this study, the interfacial phase evolution during Cu/Sn/Cu soldering (260 °C, 1 N) with a micro interconnected height of 6 μm was analyzed. During soldering, the Cu6Sn5 precipitated first along Cu/Sn interfaces in a planar shape. After the appearance of Cu6Sn5, the Cu3Sn emerged between Cu and Cu6Sn5 in a planar shape as well. Then, until residual Sn was completely consumed, the Cu6Sn5 layers at opposite sides continued to grow with a change from the planar shape to a scallop-like shape. In the meantime, the Cu3Sn layers continued to grow with a round-trip change from the planar shape to a wave-like shape. After the total consumption of residual Sn, the Cu3Sn grew at the expense of Cu6Sn5 until the formation of full Cu3Sn solder joints at 300 min. Further, concrete reasons for the interesting shape change in both Cu6Sn5 layers and Cu3Sn layers were given. With the soldering time increasing from 10 min to 60 min, the morphology of Cu6Sn5 grains agreed with the shape of Cu6Sn5 layers well. As the growth of Cu6Sn5, a ripening process without the dependence on the morphology of Cu6Sn5 grains occurred. The dependence of mean radius of Cu6Sn5 grains on the soldering time was based on the relation of R = C1tk at different time segments. The morphology of Cu6Sn5 grains affected Cu flux, leading to different growth mechanism at these time segments. From 10 min to 30 min, the constant k was calculated to be 0.53, which was due to the growth of Cu6Sn5 being only supplied by the interfacial reaction flux. From 40 min to 60 min, the constant k was calculated to be 0.35, which was due to the growth of Cu6Sn5 being supplied by both the interfacial reaction flux and the ripening flux. Compared with the growth of Cu6Sn5 grains at 30 min, the growth of Cu6Sn5 grains at 40 min was supplied by an additional Cu flux (the ripening flux), which led to the constant k being 3.82 from 30 min to 40 min. Moreover, the growth process of Cu6Sn5 grains was thought to be the accumulation of a growth period of “formation of small grains on the surface of big grains → growth of small grains along the preferential growth direction of corresponding big grains → mergence of small grains on corresponding big grains → formation of new big grains on previous big grains → mergence between new big grains and previous big grains”.

1. Introduction In recent years, electronic products, such as smart phones, panel computers, etc., have played a more and more important role in people's daily life. With the rapid development of electronic products, the industry of electronic manufacture, especially the manufacture of sophisticated electronic products, has become a very important development field in many countries of the world [1–3]. Therefore, as an indispensable part of electronic products, electronic packaging, which not only provides electronic products with pathways for electric and heat conduction but also blocks impacts of external environment to electronic products, has gained an increasing number of attentions [4–9]. The interconnection technology, which is one of key technologies in electronic packaging, plays a bridge for chips, components and systems

⁎

Corresponding author. E-mail address: [email protected] (P. Yao).

http://dx.doi.org/10.1016/j.matchar.2017.06.033 Received 8 April 2017; Received in revised form 21 June 2017; Accepted 28 June 2017 Available online 29 June 2017 1044-5803/ © 2017 Elsevier Inc. All rights reserved.

[10–11]. Concretely, the interconnection technologies mainly include wire bonding, flip chip, BGA, TSV, etc. [12–15]. Essentially, in most cases, the interconnection is achieved through soldering process because of advantages such as low demand for heat source and causing little stress. After soldering, the solder joints formed mainly provide electrical, thermal and mechanical support to connected parts [16–18]. Because of the trend realizing multi-functionality and portability for electronic products, there are more function modules within electronic products, and the volume of electronic products decreases. As a result, the interconnected height of solder joints decreases. Even, the interconnected height has been reduced from several hundreds of micrometers to less than dozens of micrometers in many cases [19]. Under the scale less than dozens of micrometers, the solders can be completely reacted with substrates to form full intermetallic compounds (IMCs)

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micro interconnected height less than 10 μm. Unfortunately, existing studies about this only focused on the initial exploration of soldering process [32–35]. Thus, in this study, some efforts were made to enrich studies regarding soldering with such a micro interconnected height. Concretely, the widely used Cu-Sn system, with a micro interconnected height of 6 μm, was adopted to analyze the interfacial phase evolution during soldering. Actually, the analysis of interfacial phase evolution during soldering was the analysis of interfacial phase evolution during the formation of full Cu3Sn solder joints, which was due to the interfacial structure of joints experiencing a change from Cu/Cu-Sn IMCs/ Sn/Cu-Sn IMCs/Cu to Cu/Cu3Sn/Cu during soldering. Undoubtedly, the analysis of interfacial phase evolution is quite significant, because the interfacial phase evolution can lead to the change of reliabilities of joints. Moreover, it should be also noted that previous researchers always studied the phase evolution during soldering by merely analyzing either the interfacial phase transition or the morphology transition of interfacial IMCs, which resulted in the overgeneralization. In fact, analyzing any one of these two aspects can obtain insights not given by analyzing the other. In our study, the interfacial phase transition as well as the morphology transition of Cu6Sn5 was analyzed together in order to comprehensively disclose the interfacial phase evolution during Cu/ Sn/Cu soldering.

joints during soldering. Due to the relatively high melting points of IMCs, full IMCs solder joints are able to service under high temperature, which is of great significance for electronic packaging. Despite the fact that the IMCs are brittle in nature, full IMCs solder joints are not necessarily less reliable than joints with the conventional interfacial structure of substrates/IMCs/solders/IMCs/substrates. For one thing, compared with solders (e.g. Sn-based solders), the IMCs have much higher mechanical strength and creep resistance. For another, the surrounding environment of IMCs in full IMCs joints is different from that of IMCs in joints with the conventional interfacial structure. According to literature [20], it is known that the shear strength of Sn-3.5Ag/Cu joints was primarily controlled by the mechanical properties of solder, not the thickness of IMCs formed between the solder and the substrate. Under the background of electronic packaging, previous researchers have conducted many studies regarding solder joints [21–31]. In these existing studies, the interconnected height of solder joints mainly ranged from several hundreds of micrometers to dozens of micrometers, while the solders included Sn-based solders, pure Sn and pure In, etc. Furthermore, the substrates included Cu, Ag and Ni, etc. Overall, these existing studies mainly focused on interfacial reaction and microstructure of solder joints [21–25], growth kinetics of IMCs within solder joints [26–28] and reliabilities of solder joints [29–31]. J. W. Yoon et al. studied the interfacial reaction of eutectic Sn-0.7Cu/Ni BGA solder joints during isothermal long-term aging [21]. It was found that the IMC formed at the interface was (Cu, Ni)6Sn5 after reflowing, and only this (Cu, Ni)6Sn5 layer was observed after aging at temperature between 70 °C and 150 °C. In addition, the solder/Ni interface exhibited a duplex structure of (Cu, Ni)6Sn5 and (Ni, Cu)3Sn4 after isothermal aging at 170 °C for 50 days. R. Zhang et al. found that the growth direction of Cu3Sn grains was parallel to the Cu6Sn5 grain boundaries in the middle layer of IMCs joints [22]. Also, they found a preferred orientation of Cu3Sn (100) crystal plane being parallel to the Cu substrate, which was unrelated to the orientation of Cu substrates. F. Y. Ouyang et al. showed a two-stage growth behavior of Ag3Sn in micro-scale Pbfree solder alloys under a temperature gradient [26]. At the first stage, the thickness change of Ag3Sn was controlled by chemical potential gradient, while force driving thermomigration became dominant at the second stage. Y. H. Tian et al. found that Cu2In was a high-quality phase which could improve the mechanical properties of Cu/In/Cu joints [29]. After shear test, they also found that fractures in Cu/In/Cu joints soldered at 260 °C happened at Cu11In9 layers, while fractures in Cu/In/ Cu joints soldered at 360 °C occurred at the interface between Cu2In layers and Cu7In3 layers. However, in practical electronic packaging, especially 3D packaging, the interconnected height of solder joints has been decreased to less than 10 μm in some cases. Under such a micro interconnected height, the formation of full IMCs joints becomes easier than that with the interconnected height of dozens of micrometers, which is due to the further reduced interconnected height. Apparently, the further decrease in the interconnected height, from dozens of micrometers to less than 10 μm, leads to many changes of interfacial reaction conditions. As a result, it is very necessary to conduct studies regarding soldering with a

2. Experimental Details High pure polycrystalline Cu foils, with the size of 5 mm × 5 mm × 1 mm, were used as substrates, while pure Sn was used as the solder. The Sn solder layers with a thickness of 3 μm were deposited on the surface (area = 5 mm × 5 mm) of Cu substrates using electroplating. Before the electroplating of Sn solder layers was conducted, for surface flatness of Cu substrates, the surface of Cu substrates was firstly ground using #800, #1000, #1500, #2000 and #3000 abrasive papers, respectively. Further, mechanical polishing was conducted on the surface of Cu substrates using diamond polishing paste with the grain diameter of 0.5 μm. After electroplating, the electroplated Sn layers were respectively cleaned by acetone and deionized water, which was followed by the drying of Sn layers using an air blower. Afterwards, two Cu substrates with the electroplated Sn solder were aligned using a special clamp to form the Cu-6 μmSn-Cu sandwich structure. Fig. 1 gives the schematic illustration for the formation of Cu6 μmSn-Cu sandwich structure. The Cu-6 μmSn-Cu sandwich structure was then placed in a tube furnace to conduct soldering. During soldering, Ar was used as protection gas. The soldering process was conducted under a certain temperature and time. For soldering with a micro interconnected height of 6 μm, the application of pressure was quite necessary to ensure the quality of solder joints. On the one hand, the pressure makes the Cu6 μmSn-Cu sandwich structure contact closely. On the other hand, oxidization film on Sn surface can be damaged by the pressure, which contributes to improving the wettability of liquid Sn during soldering. Thus, in this study, a pressure of 1 N was applied along the thickness direction of Sn layers during soldering. Fig. 2 presents the schematic

Fig. 1. Schematic illustration for the formation of Cu-6 μmSn-Cu sandwich structure.

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spectroscopy (EDS) was used to conduct the identification of different phases. In addition, in order to determine the average radius of Cu6Sn5 grains (μm) and the average number of Cu6Sn5 grains per unit area (cm2 ), several top-view SEM images were taken at different positions within joints soldered at a certain time. The equivalent radius of Cu6Sn5 grains within each top-view SEM image was calculated as

R=

s , πN

(1)

where R represents the equivalent radius, s represents the actual area of each SEM image (μm2) and N denotes the number of Cu6Sn5 grains within the image area. The number of Cu6Sn5 grains within the image area was counted manually. After the equivalent radius of Cu6Sn5 grains within each top-view SEM image was obtained, the number of Cu6Sn5 grains per unit area (cm- 2) was calculated as

Fig. 2. Schematic illustration of applying pressure on the Cu-6 μmSn-Cu sandwich structure.

illustration of applying pressure on the Cu-6 μmSn-Cu sandwich structure during soldering. For the soldering temperature, it was said that a high temperature could cause considerable stress in the connected materials during practical production of electronic products [36]. What's more, in electronic packaging, the performance of sensitive chips will be degraded, and warping resulted from the mismatch of thermal expansion coefficient will occur in wafers with a high soldering temperature. Based on this and consideration of the melting point of Sn (232 °C), a relatively low temperature, 260 °C, was chosen as the soldering temperature. After soldering was finished, solder joints were cooled in air. In order to analyze the interfacial phase transition during Cu/6 μmSn/Cu soldering (260 °C, 1 N), joints with the soldering time of 10 min, 20 min, 30 min, 40 min, 50 min, 60 min, 80 min, 100 min, 180 min, 240 min and 300 min were respectively needed. Then, metallographic cross-sections of these solder joints were needed. First of all, these solder joints were mounted in epoxy resin. After mounting, the mounted samples were respectively ground using #800, #1000, #1500, #2000 and #3000 abrasive papers. Finally, the samples were polished mechanically using 0.5 μm diamond polishing paste. For studying the morphology transition of Cu6Sn5 during soldering, joints with the soldering time of 10 min, 20 min, 30 min, 40 min, 50 min and 60 min were respectively needed. Then, the method of melting separation was used to split these joints. Fig. 3 gives the schematic illustration of melting separation of these joints. Concretely, these joints were placed on a heating plate. Given that the melting points of Sn and Cu6Sn5 were 232 °C and 415 °C respectively, the temperature of heating plate was set as 250 °C. Under the temperature of 250 °C, the residual Sn solder which located at the middle part of joints melted. Meanwhile, thrust force was applied on the top side and bottom side of joints. As the middle part of joints merely included the residual Sn solder, the joints were split by the applied thrust force. After the solder joints were split, the separated joints were deeply etched in 10 vol% HNO3 solution with ultrasonic waves for the purpose of removing the Sn solder covered on the surface of Cu6Sn5. Then, the morphology of Cu6Sn5 during soldering could be observed. It should be noted that the entire separating process for a joint merely lasted for 10s. Further, it was thought that the lasting time of only 10s could not influence the Cu6Sn5 within joints. The interfacial phase transition, as well as the morphology transition of Cu6Sn5, was observed using a scanning electron microscope (SEM) with backscattered electron signal. Also, an energy disperse

N =

N × 108, s

(2)

where N represents the number of Cu6Sn5 grains per unit area. 3. Results and Discussion 3.1. Interfacial Phase Transition During Soldering With the purpose of analyzing the interfacial phase transition during Cu/Sn/Cu soldering with a micro interconnected height, cross-sectional SEM images of Cu/6 μmSn/Cu joints (260 °C, 1 N) for different soldering time are presented in Fig. 4. In the meantime, the EDS results of interfacial phases are also given in Fig. 4. When the soldering time was 10 min, 20 min, 30 min, and 40 min respectively, the thickness of some interfacial phases was quite small. In order to observe these phases clearly, SEM images of high magnification are used in Fig. 4 for joints with the soldering time ranging from 10 min to 40 min (Fig. 4a–d). According to Fig. 4a and L, it is known that three phases existed in the interfacial region with the soldering time of 10 min. Concretely, the three different phases were residual Sn, Cu6Sn5 and Cu3Sn, respectively. In other words, two kinds of IMCs, Cu3Sn and Cu6Sn5, were formed in the interfacial region after soldering at 260 °C and 1 N. The residual Sn, which located in the middle of interfacial region, accounted for a large portion of this region, while the Cu6Sn5 layer and the Cu3Sn layer existed between Cu and Sn. Despite of quite small undulations at Cu6Sn5/ Sn interfaces, the Cu6Sn5 layer kept a planar shape as a whole. The Cu3Sn layer, which was actually adjacent to both Cu and Cu6Sn5, also presented a planar shape. According to Cu-Sn binary phase diagram (Fig. 5), it is inferred that interfacial reactions during Cu/Sn/Cu soldering at 260 °C could occur as Eqs. (3), (4) and (5). Apparently, the Cu6Sn5 could be formed as Eq. (3), while the Cu3Sn could be obtained as both Eqs. (4) and (5).

6Cu + 5Sn → Cu6 Sn5

(3)

3Cu + Sn → Cu3 Sn

(4)

9Cu + Cu6 Sn5 → 5Cu3 Sn

(5)

However, it should be noted that there was a sequence regarding the formation of Cu3Sn and Cu6Sn5, which was due to different driving forces of formation for these two IMCs. At the beginning of phase

Fig. 3. Schematic illustration of melting separation of solder joints.

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Fig. 4. Cross-sectional SEM images of joints soldered at 260 °C, 1 N for different time and EDS results of interfacial phases. a 10 min b 20 min c 30 min d 40 min e 50 min f 60 min g 80 min h 100 min i 180 min j 240 min k 300 min L EDS results for interfacial phases at 10 min.

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Fig. 5. Cu-Sn binary phase diagram.

transition, the phase with the highest driving force of formation usually precipitates first. According to literature [37], it is known that the driving force of formation for Cu6Sn5 was higher than that for Cu3Sn. Thus, at the metastable Cu/Sn interface during soldering of 260 °C, the preferential formation of Cu6Sn5 happened. Further, the Cu6Sn5 was experimentally confirmed by some previous studies as the first phase that precipitated at the interface of liquid Sn and solid Cu during soldering [38–39]. After the formation of Cu6Sn5, the Cu3Sn appeared because of the thermodynamical feasibility for its precipitation at Cu/ Cu6Sn5 interfaces. During the period of soldering time increasing from 10 min to 40 min, the Cu6Sn5 at opposite sides kept growing into the residual Sn (Fig. 4a–d). With the continuous growth of Cu6Sn5, a very interesting change in the shape of Cu6Sn5 also occurred during this period. In general, the Cu6Sn5 still presented the planar shape with the soldering time of 20 min, which was similar to the planar Cu6Sn5 at 10 min (Fig. 4a and b). However, compared with the undulations of Cu6Sn5/Sn interfaces at 10 min, there existed more distinct undulations along Cu6Sn5/Sn interfaces at 20 min. When the soldering time was increased to 30 min, some shallow valleys appeared along Cu6Sn5/Sn interfaces (Fig. 4c). As a result, the Cu6Sn5 had presented a scallop-like shape at 30 min. In other words, the Cu6Sn5 was transformed from the planar shape to the scallop-like shape. For the scallop-like Cu6Sn5, there were some differences in the size of its scallops, which was due to the difference in the grain size of Cu used in this study. However, it should be noted that the scallop-like shape at 30 min was different from that of Cu6Sn5 formed through reflowing process between Cu and Sn-based solders [40–44]. Actually, the scallop-like Cu6Sn5 at 30 min had a relatively flat top, while the scallop-like Cu6Sn5 in these literatures has a round top. Then, with the soldering time increasing to 40 min, it is found from Fig. 4d that the scallop-like Cu6Sn5 had also possessed the round top just as the Cu6Sn5 in literatures [40–44]. Apparently, there was a change for the Cu6Sn5 from the planar shape at 10 min and 20 min to the scallop-like shape at 30 min and 40 min. Several studies have proved that the Cu6Sn5 layer was actually composed of Cu6Sn5 grains, and only one Cu6Sn5 grain was included along the thickness direction of Cu6Sn5 layers [45–48]. So it was thought that the requirement of force equilibrium, at Cu6Sn5 grain boundaries during soldering, caused this interesting shape transition. Fig. 6 shows the schematic illustration of force equilibrium at Cu6Sn5 grain boundaries with the soldering time of 10 min, 20 min, 30 min and 40 min. The force equilibrium relationship could be expressed mathematically as

Fig. 6. Schematic illustration of force equilibrium at Cu6Sn5 grain boundaries under different soldering time. a 10 min b 20 min c 30 min d 40 min.

γgb = 2γCu6 Sn5

Sn cos θCu6 Sn5 Sn ,

(6)

where γgb denoted the grain boundary energy of Cu6Sn5, γCu6Sn5/Sn represented the Cu6Sn5/Sn interfacial energy and θCu6Sn5/Sn denoted the semidihedral angle at the Cu6Sn5/Sn interface. According to literature [49], it is known that the grain boundary energy was dependent on temperature. Thus, during soldering of 260 °C, the γgb kept as a constant, which meant that the γgb at 10 min, 20 min, 30 min and 40 min was the same (γgb , 10min = γgb , 20min = γgb , 30min = γgb , 40min). Nevertheless, according to the Gibbs-Thomson effect, the microstructure including interfaces unremittingly adjust to reduce interfacial energy for purpose of making the interfacial energy lowest with the phase transition process proceeding, which led to a continuous decrease of Cu6Sn5/Sn interfacial energy during soldering of 260 °C. As a result, the γCu6Sn5/Sn at 10 min, 20 min, 30 min and 40 min showed a descending trend (γCu6Sn5/Sn,10min > γCu6Sn5/ Sn,20min > γCu6Sn5/Sn,30min > γCu6Sn5/Sn,40min). Then, based on the force equilibrium relationship at Cu6Sn5 grain boundaries, it was known that the cosθCu6Sn5/Sn at 10 min, 20 min, 30 min and 40 min presented an ascending relationship (cosθCu6Sn5/Sn,10min < cosθCu6Sn5/Sn,20min < cosθCu6Sn5/Sn,30min < cosθCu6Sn5/Sn,40min), which indicated that the θCu6Sn5/Sn at 10 min, 20 min, 30 min and 40 min had a descending trend (θCu6Sn5/Sn,10min > θCu6Sn5/Sn,20min > θCu6Sn5/Sn,30min > θCu6Sn5/Sn,40min). Finally, it was found that the relatively large θCu6Sn5/ Sn at 10 min was declined progressively to the relatively small θCu6Sn5/Sn at 40 min with the shape change in Cu6Sn5 from 10 min to 40 min. Obviously, this was in agreement with what had been observed experimentally. Fig. 7 53

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Fig. 8. Schematic illustration of scallop-like Cu6Sn5 wetting Sn GBs.

with the soldering time of 80 min, as shown in Fig. 4g. According to J. W. Cahn's study [54], it is known that minor phases wetted GBs completely when major phases were distributed as droplets in minor phases. Thus, at 80 min, the emergence of residual island-like Sn (the major phase) in the Cu6Sn5 (the minor phase) had also proved the complete wetting of Sn GBs by the Cu6Sn5 occurred. After the appearance of island-like Sn, the Cu6Sn5 continued to grow into the Sn islands with the increase of soldering time until the total consumption of Sn solder at 100 min (Fig. 4h). Apart from the Cu6Sn5 and Sn, with the soldering time increasing from 10 min to 100 min, the Cu3Sn layer unremittingly grew, presenting a continuous thickening trend. However, during this period, what should be noted was that there existed several changes regarding the shape of Cu3Sn layers. It has been mentioned in the previous part that the Cu3Sn layer presented a planar shape with the soldering time of 10 min, while the Cu6Sn5 also kept the planar shape at 10 min (Fig. 4a). With regard to reasons of obtaining the planar Cu3Sn, it was thought that the planar Cu6Sn5 had resulted in the formation of planar Cu3Sn. Fig. 9a gives the schematic illustration which depicts why the planar Cu3Sn was formed. During soldering of 260 °C, it was known that the preferential precipitation of Cu6Sn5 occurred in the interfacial region. For Cu atoms along Cu/Cu6Sn5 interfaces, they could be divided into two parts. Concretely, one part of Cu atoms diffused into liquid Sn, while another part remained at the Cu/Cu6Sn5 interfaces to join interfacial reactions forming Cu3Sn. The number of Cu atoms, which located at different positions along Cu/Cu6Sn5 interfaces, could be regarded as equal because of the identical interfacial status at these positions during soldering. To Cu atoms diffusing into liquid Sn, they had to traverse the Cu6Sn5 layer before entering into liquid Sn. In other words, the Cu6Sn5 layer acted as a barrier for Cu atoms diffusing into liquid Sn. However, the Cu6Sn5 presenting the planar shape indicated that the thickness of Cu6Sn5 was uniform, which resulted in a fact that Cu atoms at different positions along Cu/Cu6Sn5 interfaces needed to traverse a roughly equal distance of Cu6Sn5 before entering into liquid Sn. Further, this meant that Cu atoms at these positions had a roughly equal diffusion resistance to overcome prior to getting into liquid Sn. As a result, the number of Cu atoms diffusing into liquid Sn at these positions was roughly identical. Obviously, this manifested that the number of Cu atoms remained at these positions to participate in reactions forming Cu3Sn was approximately equal, which caused the formation of planar Cu3Sn. Then, as the soldering time was increased to 20 min, the Cu3Sn kept the planar shape (Fig. 4b). Nevertheless, with the Cu6Sn5 becoming scallop-like at 30 min, it was found that the Cu3Sn had presented a wave-like shape, as shown in Fig. 4c. So it was known that the Cu3Sn layer was changed from the planar shape to the wave-like shape, which was accompanied by the change in Cu6Sn5 from the planar shape to the scallop-like shape. The appearance of wave-like Cu3Sn meant the thickness of Cu3Sn had become uneven. Concretely, the Cu3Sn at the

Fig. 7. Values of θCu6Sn5/Sn from the soldering time of 10 min to the soldering time of 40 min.

gives the values of θCu6Sn5/Sn with the soldering time increasing from 10 min to 40 min. As shown in Figs. 4a and 7, the planar Cu6Sn5, which had quite small undulations along Cu6Sn5/Sn interfaces, favored the largest θCu6Sn5/Sn of 85° at 10 min. At 20 min, the planar Cu6Sn5, with more distinct undulations along Cu6Sn5/Sn interfaces than that at 10 min, favored a reduced θCu6Sn5/Sn of 79°, as presented in Figs. 4b and 7. As shown in Figs. 4c and 7, with the soldering time of 30 min, the scallop-like Cu6Sn5, which had the flat top, owned a further reduced θCu6Sn5/Sn of 66°. When the soldering time was increased to 40 min, the scallop-like Cu6Sn5 with the round top had the smallest θCu6Sn5/Sn of 31.5°, as indicated in Figs. 4d and 7. Consequently, we inferred that it was the requirement of force equilibrium at Cu6Sn5 grain boundaries during soldering of 260 °C, through changing the relatively large θCu6Sn5/Sn (85°) at 10 min to the relatively small θCu6Sn5/Sn (31.5°) at 40 min in a continuously decreasing way, which primarily resulted in the transformation in Cu6Sn5 of “planar shape with quite small undulations at 10 min → planar shape with more distinct undulations at 20 min → scallop-like shape with a flat top at 30 min → scallop-like shape with a round top at 40 min”. As the soldering time was increased to 50 min, the scallop-like Cu6Sn5 at opposite sides was further grown into a larger size (Fig. 4e). Although the Cu6Sn5 obtained a further growth at 50 min, it was found that the residual Sn still occupied the middle part of interfacial region. For the residual Sn in the middle part, with the increase of soldering time, the continuous growth of scallop-like Cu6Sn5 into it could also be seen as a wetting transition process of its grain boundaries (GBs), which was similar to the GBs wetting transition in literatures [50–52]. In these literatures, Straumal et al. found that the GBs wetting transition, which referred to the change from incomplete GBs wetting to complete GBs wetting, could be processed not only by liquid phases wetting GBs [50] but also by solid phases wetting GBs [51–52]. Obviously, the GBs wetting transition in our study belonged to the solid phase (Cu6Sn5) wetting grain boundaries. Fig. 8 gives the schematic illustration of scallop-like Cu6Sn5 wetting Sn GBs. With the soldering time increasing, the scallop-like Cu6Sn5 at opposite sides continued to grow. When the soldering time was 60 min, it was found that the Cu6Sn5 at opposite sides merged with each other at a very few positions, as shown in Fig. 4f. What's more, it should be noted that there were the formation and growth of necks in the merged areas, as marked by blue arrows in Fig. 4f. The appearance of such necks suggested that a coarsening process, whose main mechanism was physical coalescence springing from anisotropic mass flow [53], occurred when the Cu6Sn5 at opposite sides merged with each other. Then, as the soldering time was further increased, such coarsening processes happened in an increasing number of Cu6Sn5 scallops at opposite sides. As a result, the residual Sn was isolated as island-like shape 54

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Fig. 9. Schematic illustration of mechanism for forming Cu3Sn with different shapes during soldering. a Planar shape b planar shape to wave-like shape c wavelike shape to planar shape.

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compared with the relatively thin part of Cu3Sn, the relatively thick part had greater diffusion resistance, which indicated that more Cu atoms diffused to interfaces between Cu6Sn5 and relatively thin part of Cu3Sn. Finally, when the Sn was totally consumed, along interfaces between Cu6Sn5 and relatively thin part of Cu3Sn, there were more Cu atoms involving reactions to produce Cu3Sn compared with that along interfaces between Cu6Sn5 and relatively thick part of Cu3Sn. As a consequence, the recovery from the wave-like shape to the planar shape happened at 100 min. After soldering of 100 min, the solder joints had become full IMC joints composed of Cu3Sn and Cu6Sn5 (Fig. 4h). As shown in Fig. 4h, i and j, with the soldering time further increasing, it was found that the Cu3Sn became thicker while the Cu6Sn5 became thinner, which resulted from the growth of Cu3Sn at the expense of Cu6Sn5. By 300 min, the Cu6Sn5 had disappeared completely, and the full Cu3Sn solder joints had been obtained (Fig. 4k).

bottom of Cu6Sn5 grains grew thicker than the Cu3Sn at the bottom of junction of two Cu6Sn5 grains. Fig. 9b gives the schematic illustration of mechanism for the change from the planar Cu3Sn to the wave-like Cu3Sn. After the precipitation of Cu3Sn layers, the process of Cu atoms diffusing into liquid Sn could be divided into two stages. At the first stage, Cu atoms, which located along Cu/Cu3Sn interfaces, diffused to Cu3Sn/Cu6Sn5 interfaces. During this stage, Cu atoms had to traverse the Cu3Sn layer, which meant the Cu3Sn acted as a diffusion barrier. Later, at the second stage, Cu atoms along Cu3Sn/Cu6Sn5 interfaces diffused into liquid Sn through traversing the Cu6Sn5, which indicated that the Cu6Sn5 acted as a diffusion barrier. But it should be noted that not all of the Cu atoms along Cu3Sn/Cu6Sn5 interfaces diffused into liquid Sn during the second stage. There was a portion of Cu atoms remained at the Cu3Sn/Cu6Sn5 interfaces for forming Cu3Sn. In the first stage, when the Cu3Sn layer was planar, for Cu atoms at different positions along Cu/Cu3Sn interfaces, the planar Cu3Sn meant it was the diffusion barrier with a roughly equal thickness. Thus, at these positions, the number of Cu atoms diffusing to Cu3Sn/Cu6Sn5 interfaces was roughly equal, which led to the roughly identical number of Cu atoms at different positions along Cu3Sn/Cu6Sn5 interfaces. In the second stage, when the Cu6Sn5 was changed from the planar shape to the scallop-like shape, there was a relatively thick Cu6Sn5 layer for Cu atoms, which located at the bottom of Cu6Sn5 grains, to traverse. Rather, Cu atoms, which were at the bottom of Cu6Sn5 grain junction, merely needed to traverse a relatively thin Cu6Sn5 layer before entering into liquid Sn. Thus, it was known that the junction of two Cu6Sn5 grains served as an easier diffusion path for Cu atoms entering into liquid Sn, which led to Cu atoms at the bottom of Cu6Sn5 grain junction diffusing into liquid more easily. In other words, at the bottom of Cu6Sn5 grain junction, there were more Cu atoms diffusing into liquid Sn compared with that at the bottom of Cu6Sn5 grains. As a consequence, at the bottom of Cu6Sn5 grains, there were more Cu atoms involving reactions for forming Cu3Sn compared with that at the bottom of Cu6Sn5 grain junction, which had led to the formation of wave-like Cu3Sn. Afterwards, it was found that the Cu3Sn layers grew with the wavelike shape until the soldering time of 80 min (Fig. 4d–g). As the soldering time increasing to 100 min, with the complete consumption of liquid Sn, the Cu3Sn layer was transformed to the planar shape with a uniform thickness once again, as shown in Fig. 4h. Apparently, this demonstrated a recovery in the shape of Cu3Sn happened. Fig. 9c presents the schematic illustration regarding mechanism of this recovery in shape. For Cu atoms diffusing to Cu3Sn/Cu6Sn5 interfaces, the Cu3Sn layers acted as the diffusion barrier. When the Cu3Sn presented the wave-like shape, the Cu3Sn acted as the diffusion barrier with different thickness at different positions. Concretely, the relatively thick part of Cu3Sn acted as the thick diffusion barrier, while the relatively thin part of Cu3Sn acted as the thin diffusion barrier. As a result, there were more Cu atoms diffusing to interfaces between Cu6Sn5 and relatively thin part of Cu3Sn, compared with that diffusing to interfaces between Cu6Sn5 and relatively thick part of Cu3Sn. For Cu atoms along Cu3Sn/Cu6Sn5 interfaces, one part of them diffused into liquid Sn with another part remaining at the interfaces for producing Cu3Sn. Together with the Cu3Sn being wave-like, the Cu6Sn5 presented the scallop-like shape. Moreover, the relatively thin part of Cu3Sn located at the bottom of Cu6Sn5 grain junction, while the relatively thick part of Cu3Sn positioned at the bottom of Cu6Sn5 grains. Hence, along the interfaces between Cu6Sn5 and relatively thin part of Cu3Sn, there were more Cu atoms diffusing into liquid Sn compared with that along the interfaces between Cu6Sn5 and relatively thick part of Cu3Sn. This resulted in an approximately equal number of Cu atoms remained at different positions along Cu3Sn/Cu6Sn5 interfaces for producing Cu3Sn. When the residual Sn was consumed completely, the Cu6Sn5 at opposite sides merged totally. At this moment, for Cu atoms diffusing into Cu6Sn5, they mainly diffused to Cu3Sn/Cu6Sn5 interfaces in order to form Cu3Sn, while the wave-like Cu3Sn acted as the diffusion barrier. Also,

3.2. Morphology Transition for Cu6Sn5 During Soldering In the above section, the interfacial phase transition, during Cu/Sn/ Cu soldering with a micro interconnected height of 6 μm, was analyzed through observing cross-sectional SEM images of joints with different soldering time. It was known that both Cu6Sn5 and Cu3Sn were formed in the interfacial region during soldering. Further, in this section, the morphology transition for Cu6Sn5 was studied by analyzing top-view SEM images of Cu6Sn5 with different soldering time (Fig. 10). Noteworthily, Fig. 10 also includes the image of Cu6Sn5 with the soldering time of 60 min. However, we have known from Fig. 4f that the Cu6Sn5 (melting point: 415 °C) at opposite sides merged with each other at a very few positions when the soldering time was 60 min, which indicated splitting joints of 60 min through the method of melting separation (temperature: 250 °C) seemed infeasible. Actually, the merged area of opposite Cu6Sn5 at 60 min was so small that the application of somewhat large thrust force, when the Sn melted, could result in the fracture of merged area. In other words, the method of melting separation was still able to split joints of 60 min. Overall, the morphology of Cu6Sn5 was not influenced by the fracture happened in the very few merged parts. Table 1 gives statistical analysis results of data series regarding the radius of Cu6Sn5 grains and the number of Cu6Sn5 grains per unit area under different soldering time. As shown in Fig. 10 and Table 1, with the soldering time increasing, the Cu6Sn5 grains grew larger, presenting a larger mean radius, while the mean number of Cu6Sn5 grains per unit area decreased. This indicated a ripening process occurred among the Cu6Sn5 with its growth during soldering, which was similar to the growth of Cu6Sn5 during soldering between Cu and Sn-based solders [55–59]. But in these literatures, the ripening process was accompanied by the growth of scallop-like Cu6Sn5 grains, while Cu6Sn5 grains, with different soldering time in our study, do not all show the scallop-like shape. Thus, it was inferred that the ripening process, happened in the growth of Cu6Sn5 during Cu/6 μmSn/Cu soldering at 260 °C, 1 N, had no dependence on the shape of Cu6Sn5 grains. With the soldering time of 10 min and 20 min, the Cu6Sn5 grains presented a plate-like shape, as shown in Fig. 10a and b. The top of different plate-like Cu6Sn5 grains at 10 min was almost flat, and they almost located at the same horizontal plane. This was in accordance with the Cu6Sn5 layer at 10 min presenting the planar shape with quite small undulations in the cross-sectional SEM image (Fig. 4a). Comparatively, the top of different plate-like Cu6Sn5 grains at 20 min had humps, which led to the Cu6Sn5 layer at 20 min presenting the planar shape with more evident undulations in the crosssectional SEM image (Fig. 4b). When the soldering time was increased to 30 min, evident grooves appeared between Cu6Sn5 grains, so the Cu6Sn5 grains became scallop-like, as shown in Fig. 10c. But the scallop-like Cu6Sn5 grains at 30 min were slightly extraordinary, because they do not have a round top. Rather, they had relatively a flat top, which agreed with what we had observed in the cross-sectional 56

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Fig. 10. Top-view SEM images showing the morphology of Cu6Sn5 in the joints soldered at 260 °C, 1 N for different time. a 10 min b 20 min c 30 min d 40 min e 50 min f 60 min.

Table 1 Statistical analysis results of data series regarding the radius of Cu6Sn5 grains and number of Cu6Sn5 grains per unit area under different soldering time. Soldering time/min

10 20 30 40 50 60

Number of Cu6Sn5 grains per unit area/cm− 2

Radius of Cu6Sn5 grains/μm Mean

Min

Max

SD

Mean

Min

Max

SD

1.1 1.5 2.0 6.0 6.5 6.9

0.9 1.5 1.7 5.9 6.3 6.0

1.3 1.6 2.3 6.3 7.2 7.4

0.2 0.1 0.2 0.2 0.3 0.5

26,171,875 14,013,672 7,910,156 878,906 754,123 672,743

19,042,968 12,597,656 5,859,375 813,802 618,490 585,938

38,671,875 14,941,406 10,839,844 911,458 813,802 878,906

9,376,221 915,576 2,096,314 41,175 72,545 108,289

coordinate, the linear relations under the log-log coordinate follow: R = C1tk, where R is the mean radius of Cu6Sn5 grains, C1 and k are the constants, and t is the soldering time. According to the data in Table 1, the constant k was calculated to be 0.53 with the soldering time increasing from 10 min to 30 min. When the soldering time was increased from 40 min to 60 min, the constant k was calculated to be 0.35, which was in consistent with the findings of Kim and Tu [56]. What's more, at the time segment from 30 min to 40 min, the constant k was calculated to be 3.82 with the mean radius of Cu6Sn5 grains experiencing a remarkable increase from 2.0 μm to 6.0 μm. However, it should be noted that the Cu6Sn5 grains presented the plate-like shape and the scalloplike shape with the relatively flat top from 10 min to 30 min, while the Cu6Sn5 grains presented the scallop-like shape with the round top from

SEM image (Fig. 4c). Then, with the soldering time increasing to 40 min, 50 min and 60 min respectively, the Cu6Sn5 grains remained scallop-like, as shown in Fig. 10d, e and f. Unlike Cu6Sn5 grains at 30 min, the scallop-like Cu6Sn5 grains at 40 min, 50 min and 60 min owned the round top. Also, this was in consistent with corresponding cross-sectional SEM images (Fig. 4d–f). From the analysis above, it can be known that the mean radius of Cu6Sn5 grains increased with the soldering time increasing no matter whether the morphology of Cu6Sn5 grains was plate-like or scallop-like. Fig. 11 presents the dependence of mean radius of Cu6Sn5 grains on the soldering time. Obviously, under the log-log coordinate, the mean radius of Cu6Sn5 grains and the soldering time presented different linear relations during different time segments. Under the conventional 57

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Cu6Sn5 grains is (the detailed derivation can be seen in literature [56]) 2

r3 =

DC0 ∫ ⎛ 3γNΩA LRT ⎜

+

⎝

ρAΩV (t ) ⎞ dt . 4πmNP (t ) ⎠ ⎟

(10)

However, when the Cu6Sn5 grains present the plate-like shape, the surface of Cu6Sn5 grains is planar, so r = ∞, as shown in Fig. 12b. According to Eq. (7), one can get (11)

Cr = C0, which results in

(12)

J1 = 0.

Moreover, for the plate-like Cu6Sn5 grains, there is B = πr 2 (Fig. 12b), where r represents the radius of plate-like Cu6Sn5 grains. As a result, one can also get

J2 = Fig. 11. Dependence of mean radius of Cu6Sn5 grains on the soldering time.

J1 =

2ΩγDC0 1 ⋅ , 3LRT r 2

ρNA AV (t ) 1 J2 = ⋅ , mNP (t ) B

(13)

Apparently, this indicates the growth of plate-like Cu6Sn5 grains is only supplied by the interfacial reaction flux (J2). For a plate-like Cu6Sn5 grain with the radius of r , the total number of Cu atoms/s diffusing into this plate-like Cu6Sn5 grain, E, is

40 min to 60 min, as shown in Fig. 10. Thus, it was inferred that the morphology of Cu6Sn5 grains acted as a factor influencing the growth mechanism of Cu6Sn5 grains. Concretely, it was thought that the morphology of Cu6Sn5 grains affected Cu flux for the growth of Cu6Sn5 grains, which led to different k being obtained at different time segments. Fig. 12 shows the schematic diagram of Cu flux among Cu6Sn5 grains with different morphologies. Based on the Gibbs-Thomson effect and the analysis by Kim and Tu [56], the Cu flux shown in Fig. 12 can be obtained as follows (the detailed derivation was presented in literature [56]):

2Ωγ ⎞, Cr = C0 exp ⎛ ⎝ rRT ⎠

ρNA AV (t ) 1 ⋅ . πmNP (t ) r 2

E = πr 2⋅

ρNA AV (t ) 1 ρNA AV (t ) ⋅ = . πmNP (t ) r 2 mNP (t )

(14)

If the number of moles of Cu6Sn5 in a plate-like grain with the radius of r is Q, then the number of Cu atoms in Cu6Sn5, QCu, is

QCu = 6NA Q = 6NA

V , Ω

(15)

πr 2h

is the volume of this plate-like Cu6Sn5 grain. It should where V = be noted that h represents the thickness of this plate-like Cu6Sn5 grain. According to literature [60], it is known that the radius of Cu6Sn5 was proportional to the thickness of Cu6Sn5. Hence, one can get h = Fr , where F is a constant. Then, V = πr 3F leads to

(7)

(8)

QCu = 6NA (9)

V πr 3F = 6NA Ω Ω

(16)

and

where Cr represents the concentration of Cu in the Sn solder at the surface of Cu6Sn5 grains, J1 represents the ripening flux, J2 represents the interfacial reaction flux, γ represents interfacial energy per unit area between Cu6Sn5 and Sn solder, r represents the curvature radius of Cu6Sn5 grains, Ω represents the molar volume of Cu6Sn5, R is the gas constant, T is the temperature, NA is the Avogadro's constant, D is the diffusivity of Cu in Sn solder, V(t) is the consumption rate of Cu during soldering, NP(t) is the total number of Cu6Sn5 grains at the interface, ρ is the density of Cu, A is the total area of Sn solder/Cu interface, m is the atomic mass of Cu, L is a constant, C0 is the equilibrium concentration of Cu in Sn, and B denotes the area of Cu6Sn5 surface at which the flux J2 arrives. When the Cu6Sn5 grains present the scallop-like shape, B = 2πr2. Then, based on Eqs. (7)–(9), the growth equation for scallop-like

dQCu 18NA πF 2 dr = r . dt Ω dt

(17) dQCu , dt

the growth rate of plateBased on the continuity equation, E = like Cu6Sn5 grains whose radius is r can be expressed as follows:

r2

ΩρA V (t ) dr = , dt 18πFm NP (t )

(18)

then, the growth equation for plate-like Cu6Sn5 grains is

r3 =

ΩρA 6πFm

∫ NVP((tt)) dt.

(19)

As shown in Fig. 10a, b and c, the Cu6Sn5 grains presented the platelike shape at 10 min and 20 min, while they presented the scallop-like Fig. 12. Schematic diagram of Cu flux among Cu6Sn5 grains with different morphologies. a Scallop-like Cu6Sn5 grains b plate-like Cu6Sn5 grains.

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shape with the relatively flat top at 30 min. Actually, the scallop-like shape at 30 min was quite close to the plate-like shape, which was due to the relatively flat top as well as shallow grooves between Cu6Sn5 grains. Then, the curvature radius of Cu6Sn5 grains at 30 min could be also regarded as infinite, which resulted in the ripening flux J1 being zero. In other words, at the time segment from 10 min to 30 min, the growth of Cu6Sn5 grains was only supplied by the interfacial reaction flux, and it followed the Eq. (19). It was found by Wang and Liu [61] that V(t) could be expressed as V(t) = 1.5 × 10− 5t− 0.76 for Sn/Cu soldering at 250 °C. Since there is no data available for V(t) during soldering of 260 °C, and the difference in temperature of only 10 °C is not considered to influence V(t) remarkably, we also took V(t) = 1.5 × 10− 5t− 0.76 in our study. Fig. 13 shows the dependence of mean number of Cu6Sn5 grains per unit area on the soldering time, which was respectively based on the relation of N (t ) = C2 t n (C2 and n are the constants) at different time segments. At the time segment from 10 min to 30 min, the constant n was calculated to be − 1.10. Substituting the two variable functions V(t) and NP (t ) = 25N (t ) into the Eq. (19), one can obtain r ~t 0.45, which is consistent with our experimental

Fig. 13. Dependence of mean number of Cu6Sn5 grains per unit area on the soldering time.

Fig. 14. Special morphologies founded in the plate-like Cu6Sn5 grains of local positions and EDS results.

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results of R ~ t0.53 because of both constant k being around 0.5. When the Cu6Sn5 grains presented the scallop-like shape with the soldering time increasing from 40 min to 60 min (Fig. 10d–f), the growth of Cu6Sn5 grains followed the Eq. (10), and it was supplied by both the ripening flux and the interfacial reaction flux. During this time segment, the constant n was calculated to be −0.70. Further, substituting the two variable functions V(t) and NP (t ) = 25N (t ) into the Eq. (10), r ~ t1/3 can be obtained. Certainly, this is also in accordance with our experimental results of R ~ t0.35. At 30 min, the growth of Cu6Sn5 grains was merely supplied by the ripening flux. At 40 min, the growth of Cu6Sn5 grains was supplied by both the ripening flux and the interfacial reaction flux. Obviously, compared with the growth of Cu6Sn5 grains at 30 min, an additional Cu flux, the ripening flux, joined the growth of Cu6Sn5 grains at 40 min, which resulted in the mean radius of Cu6Sn5 grains increasing greatly from 2.0 μm at 30 min to 6.0 μm at 40 min. Furthermore, as shown in Fig. 10c–f, some special morphologies were also observed in the scallop-like Cu6Sn5 grains of local positions. These special morphologies could be divided into three types which are respectively marked in rectangle boxes with different colours. In addition to these special morphologies, there still existed several other special morphologies in the Cu6Sn5 grains of local positions within joints soldered at 260 °C, 1 N. Interestingly, when all the special morphologies were analyzed together, it was found that they could reflect the growth process of Cu6Sn5 grains. Fig. 14 shows the special morphologies founded in the plate-like Cu6Sn5 grains of local positions, while Fig. 15 presents the special morphologies founded in the scalloplike Cu6Sn5 grains of local positions. As shown in Figs. 14a and 15a, for either plate-like Cu6Sn5 grains at local positions or scallop-like Cu6Sn5 grains at local positions, it was captured that there were many grains with quite small size on their surface. What should be noted is that the morphology shown in Fig. 14a belongs to the type of morphology marked by the green rectangle box in Fig. 10c. Further, it was determined by EDS that the composition of these small grains was also Cu6Sn5, as presented in Figs. 14e and 15f. The appearance of these small Cu6Sn5 grains indicated that the nucleation of Cu6Sn5 grains occurred on the surface of big Cu6Sn5 grains. It was said by several studies that the nucleation of Cu6Sn5 grains belonged to heterogeneous nucleation [62–64]. With the supply of Cu flux during soldering, the big Cu6Sn5 grains had to keep growing. However, the big Cu6Sn5 grains might have different grain orientations, which resulted in their different preferential growth directions. Then, their preferential growth might be blocked because of their adjacent grains acting as the barrier. Under this occasion, the growth of big Cu6Sn5 grains sought other growth directions. At other growth directions, the energy needed for growth was much higher. Certainly, there existed such a possibility that the nucleation energy of Cu6Sn5 grains was less than the energy for growing along directions except for the preferential growth direction. Thus, the nucleation of Cu6Sn5 grains could happen on the surface of big Cu6Sn5 grains. As is widely known, the nucleation of grains can be regarded as the beginning of crystal growth. This meant the growth process of Cu6Sn5 grains, during Cu/6 μmSn/Cu soldering, did not attach to just one course of nucleation and growth. It was thought that the small Cu6Sn5 grains, formed on one big Cu6Sn5 grain, had the same grain orientation just as the big Cu6Sn5 grain, which resulted in the same preferential growth direction of these small Cu6Sn5 grains. Consequently, the growth of these small Cu6Sn5 grains along their preferential growth direction could not be influenced by their neighboring grains. After the formation of small Cu6Sn5 grains, with the supply of Cu flux, the formed small Cu6Sn5 grains began to grow along their preferential growth direction, as shown in Fig. 14b. It should be noted that the red arrows in Figs. 14 and 15 denotes projection directions, shown in the two-dimension plane, of growth directions for small Cu6Sn5 grains appeared on big Cu6Sn5 grains. Then, with more Cu atoms, small Cu6Sn5 grains continued to grow along their preferential growth directions because of the existence of constitutional supercooling in the growth front of small Cu6Sn5 grains. So the small

grains became thicker and longer, and the elongation direction was their growth direction, as shown in Figs. 14c and 15b. As the growth of small Cu6Sn5 grains proceeding, some small grains on a big Cu6Sn5 grain had contacted with each other, and the contacted Cu6Sn5 grains merged with each other at some locations, as shown in Figs. 14d and 15c. Afterwards, as the supply of Cu flux, more small grains on a big Cu6Sn5 grain merged, and the merged parts on a big Cu6Sn5 grain continued to merge with each other, as shown in Fig. 15d and e. The morphologies shown in Fig. 15d and e respectively belong to the types of morphologies marked by red rectangle boxes and blue rectangle boxes in Fig. 10d–f. Eventually, with more Cu atoms, all the small grains on a big Cu6Sn5 grain had merged, and then all the merged parts on a big Cu6Sn5 grain merged into a new big Cu6Sn5 grain. In other words, the new formed Cu6Sn5 grains had covered the previous big Cu6Sn5 grains. Under consideration of only one Cu6Sn5 grain being included along the thickness direction of Cu6Sn5 layers [45–48], it was thought that a mergence also occurred between the new formed big Cu6Sn5 grains and the previous big Cu6Sn5 grains. As a result, the Cu6Sn5 grains were thought to experience a growth period. J.C. Gong et al. studied the nucleation and growth of Sn dendrites which were controlled by nucleation energy and supercooling [65]. However, although the formation and growth behaviors of small Cu6Sn5 grains in our study were similar to the nucleation and growth of Sn dendrites in their study, the formation and growth behaviors of small Cu6Sn5 grains in our study were thought to be controlled by Cu flux and constitutional supercooling. Whether the Cu6Sn5 grains were plate-like or scallop-like, they seemed to have similar growth period, as shown in Figs. 14 and 15. Undoubtedly, this indicated that the growth process of Cu6Sn5 grains had no dependence on the morphology of Cu6Sn5 grains. In summary, the growth process of Cu6Sn5 grains, during Cu/6 μmSn/Cu soldering at 260 °C, 1 N, could be regarded as the accumulation of a growth period of “formation of small grains on the surface of big grains → growth of small grains along the preferential growth direction of corresponding big grains → mergence of small grains on corresponding big grains → formation of new big grains on previous big grains → mergence between new big grains and previous big grains”. Fig. 16 gives the model which depicts a growth period of Cu6Sn5 grains. Noteworthily, such a growth period was deemed to happen in a very short time, which meant the special morphologies in Figs. 14 and 15 were not easily captured. Moreover, in the model shown in Fig. 16, the growth of different Cu6Sn5 grains during soldering was thought to keep synchronous for simplicity. Actually, the growth of different Cu6Sn5 grains didn't keep at the same stage. This explained why these special morphologies could be observed at local positions. However, there is no denying the fact that the growth process of Cu6Sn5 grains was indeed the accumulation of such growth period. 4. Conclusions In this study, the interfacial phase evolution during Cu/Sn/Cu soldering (260 °C, 1 N) with a micro interconnected height of 6 μm was analyzed. Firstly, the interfacial phase transition during soldering was studied through observing cross-sectional SEM images of joints with different soldering time. Then, the morphology transition of Cu6Sn5 during soldering was studied through observing top-view SEM images of Cu6Sn5 within joints soldered at different time. The concrete conclusions are as follows: During soldering of 260 °C and 1 N, the precipitation of Cu6Sn5 first happened along the Cu/Sn interfaces, and the Cu6Sn5 layers presented a planar shape with quite small undulations along Cu6Sn5/Cu3Sn interfaces. After the formation of Cu6Sn5, the Cu3Sn appeared because of the thermodynamical feasibility for its precipitation at Cu/Cu6Sn5 interfaces, and the Cu3Sn layers presented a planar shape as well. It was thought that the planar Cu6Sn5 had resulted in the appearance of planar 60

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Fig. 15. Special morphologies founded in the scallop-like Cu6Sn5 grains of local positions and EDS results.

on the soldering time was based on the relation of R = C1tk at different time segments. The morphology of Cu6Sn5 grains affected Cu flux for the growth of Cu6Sn5 grains, which led to different k being obtained at different time segments. When the soldering time was increased from 10 min to 30 min, the constant k was calculated to be 0.53, which was due to the growth of Cu6Sn5 being only supplied by the interfacial reaction flux. At the time segment from 40 min to 60 min, the constant k was calculated to be 0.35, which was due to the growth of Cu6Sn5 being supplied by both the interfacial reaction flux and the ripening flux. Compared with the growth of Cu6Sn5 grains at 30 min, an additional Cu flux, the ripening flux, participated in the growth of Cu6Sn5 grains at 40 min. Thus, the mean radius of Cu6Sn5 grains was increased remarkably from 2.0 μm at 30 min to 6.0 μm at 40 min, which led to the constant k being calculated to be 3.82. The growth process of Cu6Sn5 grains could be regarded as the accumulation of a growth period of “formation of small grains on the surface of big grains → growth of small grains along the preferential growth direction of corresponding big grains → mergence of small grains on corresponding big grains → formation of new big grains on previous big grains → mergence between new big grains and previous big grains”.

Cu3Sn. Next, both the Cu3Sn and the Cu6Sn5 continued to grow with the soldering time increasing. The Cu6Sn5 layers experienced a change of “planar shape with quite small undulations → planar shape with more distinct undulations → scallop-like shape with a flat top → scallop-like shape with a round top”, which was due to the requirement of force equilibrium at Cu6Sn5 grain boundaries during soldering. The Cu3Sn layers were first transformed from the planar shape to a wave-like shape, which resulted from the Cu6Sn5 layers becoming scallop-like. Because of the growth of Cu6Sn5 scallops at opposite sides, they merged with each other, and the residual Sn was therefore isolated as an islandlike shape. Further, with the complete consumption of residual Sn, a shape recovery from the wave-like shape to the planar shape occurred in the Cu3Sn layers. After the residual Sn was consumed totally, the Cu3Sn grew at the expense of Cu6Sn5. Finally, full Cu3Sn solder joints were obtained with the soldering time of 300 min. The morphology of Cu6Sn5 grains with different soldering time was consistent with the shape of Cu6Sn5 layers observed in corresponding cross-sectional SEM images. Moreover, a ripening process, which had no dependence on the morphology of Cu6Sn5 grains, occurred with the growth of Cu6Sn5 during soldering. With the soldering time increasing from 10 min to 60 min, the dependence of mean radius of Cu6Sn5 grains 61

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