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Cu lap joints
Measurement 66 (2015) 195–203
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Corrosion measurement on shear strength of Cu/Sn–9Zn/Cu lap joints Muhammad Ghaddafy Affendy a, Muhamad Zamri Yahaya a, Fakhrozi Che Ani b, Ahmad Azmin Mohamad a,⇑ a b
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Jabil Circuit Sdn. Bhd., Bayan Lepas Industrial Park, 11900 Bayan Lepas, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 21 November 2013 Received in revised form 20 October 2014 Accepted 6 February 2015 Available online 14 February 2015 Keywords: Corrosion Shear strength Lap joints NaCl Morphology
a b s t r a c t Corrosion of the Cu/Sn–9Zn/Cu lap joints in 3.5 wt.% NaCl solution showed reduction in the ultimate shear strength with increasing immersion time. The highest ultimate shear strength value obtained was at 122.67 MPa before immersion while the lowest value was 85.32 MPa after immersion for 28 days. Corrosion products consist of tin (II) chloride and zinc hydroxide chloride had been obtained due to the active dissolution of tin. The formation of pits observed on the lap joints marked as stress concentration points which are detrimental toward the shear strength. Upon longer immersion period, the pit coalescence and propagation led to the existence of long, deep cracks. The fracture paths nucleates from the cracks clearly established the mechanism of the fracture formation for the Cu/Sn–9Zn/ Cu lap joints. The fracture analysis together with the presence of the corrosion by-products fully reiterated that corrosion activities was the main factor for the decrease in shear strength of Cu/Sn–9Zn/Cu lap joints. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The dependence on Sn–Pb solders as electronic interconnection materials is drastically brought down due to its harmful and toxic consequences on the environment and human health [1–3]. Numerous researches have been done on various Pb-free solders families such as the Sn– Cu [4,5], Sn–Ag [6,7], Sn–Ag–Cu [8,9] and Sn–Zn [10–12] to search for a suitable candidate to replace the conventional Sn–Pb solder. From all the Pb-free solder systems, the autistic Sn–9Zn solder is considered to be a promising alternative as it has a melting temperature of 198 °C [13], which is almost similar to the conventional Sn–Pb solders. Aside from having economic advantages [14], Sn–9Zn solder has excellent mechanical properties that can be ⇑ Corresponding author. Tel.: +60 4 599 6118; fax: +60 4 594 1011. E-mail address: [email protected] (A.A. Mohamad). http://dx.doi.org/10.1016/j.measurement.2015.02.016 0263-2241/Ó 2015 Elsevier Ltd. All rights reserved.
significantly important to form solder joints in electronic devices [15–17]. Solder joints, according to Li et al. [18], provide electrical conductivity and suitable mechanical strength for devices to perform smoothly. However, as of today, the reliability of solder joints remains an unceasing concern in the electronic packaging industry [19]. Miniaturization of electronic devices increased the demand for higher mechanical integrity of the solder joints as finer pitch interconnection were incorporated [20,21]. In conjunction with these requirements, the Sn–9Zn solder alloy is able to offer excellent mechanical properties, particularly the shear strength. These merits of Sn–9Zn solder alloy had resulted in the incorporation of various Sn–Zn solder family in a wide range of commercial electronics products [22,23]. A study conducted by Shoji et al. [24], reported that the shear strength of Sn–9Zn solder is higher than the
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conventional Sn–Pb solder. Thus, performing the shear strength measurements on Sn–9Zn solder joints would provide a proper estimation on the interconnection life for future applications [25]. In addition, the lap joints configuration is essential in simulating real solder joints in order to investigate the mechanical properties of solders toward various parameters such as alloying elements [26], composite solders [27] and corrosion [28]. On a more practical perspective, the solder joints are vulnerably exposed to corrosive environments such as moisture, air pollutants and oceanic environments, throughout its real-time applications [10,12,29]. The exposure, in a period of time, permits corrosion to take place between them. The tendency for oxidation of the Sn–9Zn solder alloy is a setback which confines its other merits from potential electronics applications [30,31]. These corrosion tendencies of the Sn–9Zn solder alloy will significantly affect the end properties of the solder joint. In addition, reduction in interconnection density between components in current electronic devices also reduces the size of moisture droplet for corrosion to occur, making it more susceptible to corrosion attack [32]. Consequently, this may lead to loss of mechanical integrity of solder joints and lastly, operational failure. In the Sn–Zn system, Liu et al. [33] have found out that pits are formed at solder/substrate interfaces when immersed in 3.5 wt.% NaCl solution. Furthermore, a study reported that the increase of pit size after immersion caused the drop of interfacial adhesion strength of Sn–9Zn–1.5Ag/Cu joints [34]. Therefore, the study on corrosion behavior of the Sn–9Zn solder alloy in 3.5 wt.% NaCl solution is significant in understanding the failure mechanism by corrosion activities of the solder alloy. In this study, the effects of corrosion on shear strength of Cu/Sn–9Zn/Cu lap joints in 3.5 wt.% NaCl solution have been studied. Shear strength of specimens were obtained with specimens possessing shear-lap joint geometry since such geometry is better suited for obtaining the shear strength. Apart from the shear strength evaluations, phase determination, failure analysis, microstructural and elemental composition characterizations were performed to support the results obtained.
2. Experimental The Sn–9Zn solder alloy used was made via the melting process of tin (Sn) pieces and zinc (Zn) flakes according to its required composition. The melting process was done in a box furnace under an oxygen-free nitrogen environment at 500 °C with a heating rate of 10 °C min 1. The melted alloy was left for soaking process for 30 min, before the furnace temperature decreases to end the process. Later, the melted solder alloy was left to be cooled off at room temperature (25 °C) on a flat ceramic slab. Sn–9Zn solder pellets with a diameter of 5 mm each and a thickness of 0.5 mm were formed with the assistance of a custom-made solder puncher. The Cu/Sn–9Zn/Cu lap joint was made from two Cu pads (50.0 5.0 0.5 mm in dimensions) and a Sn–9Zn solder pellet. The Cu/Sn–9Zn interface was fixed at
0.25 mm2 (0.5 0.5 in dimensions). Prior to the re-melting and solidification process, the components were ground and polished with common metallographic practices. Remelting process was made on a hot plate in the presence of a ZnCl-based flux. Temperature above 200 °C was maintained for about 30 s with the highest temperature reaching 220 °C during re-melting. The soldered joint was left to solidify at room temperature. Fig. 1 shows the schematic diagram of a Cu/Sn–9Zn/Cu lap joint. In order to study the effects of corrosion on the lap joints, the immersion method was preferred. A Cu/Sn– 9Zn/Cu lap joint was immersed in a 3.5 wt.% NaCl solution under different immersion periods. The lap joint was immersed in a polyethylene container covered with a fabric mesh to allow oxygen ventilation and also to avoid impurities from entering the immersion assembly. The shear strength of Cu/Sn–9Zn/Cu lap joints was determined by performing the shear strength evaluation using a universal testing machine (INSTRON 5900 Testing System). The evaluation was performed on lap joints before the immersion as well as after immersion according to the different immersion periods. A crosshead speed of 2 mm/min was set throughout the evaluation at room temperature [35]. The shear stress–strain graph was plotted, while the ultimate shear strength (USS) and strain at USS values of lap joints were obtained from the Bluehill 3 Testing Software and compared with respect to its immersion time. Upon failure, fracture micrographs were taken on the fractured surface and cross-section of Cu/Sn–9Zn/Cu lap joints before and after immersion in 3.5 wt.% NaCl solution. The observation views for the lap joints can be seen in Fig. 1. Phase determination analysis of Cu/Sn–9Zn/Cu lap joints was conducted via X-ray diffraction (XRD) by using Bruker AXS D9 diffractometer. It was performed with the scanning angle from 10° to 180°, Cu K-alpha (0.1542 nm) wavelength at a scanning rate of 2 deg/min. The samples of pure Sn, Zn, and Cu/Sn–9Zn/Cu lap joint before and after immersion in 3.5 wt.% NaCl solution were evaluated. A Hitachi TM3000 tabletop scanning microscope (SEM) was used to study the microstructural changes of Cu/Sn–9Zn/ Cu lap joints. The surface and cross-section micrographs of lap joints before and after immersion were taken. Elemental composition analysis on post-immersion microstructure was done by an energy dispersive X-ray (EDX) instrument coupled to a Zeiss Supra 35VP Field Emission Scanning Electron Microscope (FESEM) system.
View 1 Sn-9Zn Cu View 3
View 2
Fig. 1. Schematic diagram of a Cu/Sn–9Zn/Cu lap joint and microstructural observation View 1, 2, and 3. Dashed line represents cross-section direction for View 3 observation.
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3. Results and discussions 3.1. Effects of corrosion on shear strength The failures behavior of the lap joints can be examines by the representative stress–strain curves of Cu/Sn–9Zn/ Cu lap joints from the shear strength evaluations shown in Fig. 2. The curves of non-corroded Cu/Sn–9Zn/Cu lap joint was represented by Fig. 2a, while Fig. 2b–f represents the lap joints after immersion in different periods in 3.5 wt.% NaCl solution. For all of the curves, with exception to Fig. 2f, three distinctive regions were observed; the elastic, flow hardening and failure regions. On the contrary, Fig. 2f only exhibited the elastic and failure regions. The strain values of Cu/Sn–9Zn/Cu lap joints from the evaluations were greatly affected by the immersion periods. The strain value of lap joint before immersion occurred was 7.43 mm/mm, while the lap joint after immersion for 28 days obtained the lowest strain value at 0.29 mm/mm as in Fig. 2. By comparing these two values, the shear strain of lap joints decreased as well with increasing immersion time. This decrease can be pointedly seen by the decreasing length of the flow hardening region in the stress–strain curves. The shear strength of Cu/Sn– 9Zn/Cu lap joints is defined by the values of USS obtained from the stress–strain curves. Lap joint prior to immersion recorded the highest value of USS at 128.50 MPa while the lowest value was recorded at 85.32 MPa after 28 days of immersion. From the trend shown in Fig. 3, the shear strength of Cu/Sn–9Zn/Cu lap joints decreased with increasing immersion time. Aside from that, the yield strengths attained in the elastic region from all the curves were comparable to one another. Further investigation on the failures of the Cu/Sn–9Zn/ Cu lap joints before and after 28 days of immersion in 3.5 wt.% NaCl solution was made possible by the fracture surfaces micrographs in Fig. 4 which were obtained from View 1. In Fig. 4a, a typical ductile failure was clearly observed for the lap joint which can be seen by the smooth deformation across the surface in line with the shear
Fig. 3. Ultimate shear strength of lap joints (a) before and after (b) 1, (c) 7, (d) 14, (e) 21, and (f) 28 days of immersion in 3.5 wt.% NaCl solution.
Fig. 4. View 1 of fractured on Sn–9Zn surface micrographs of Cu/Sn–9Zn/ Cu lap joints (a) before and (b) after 28 days after immersion in 3.5 wt.% NaCl solution.
Fig. 2. Stress–strain graph of lap joints (a) before and after (b) 1, (c) 7, (d) 14, (e) 21, and (f) 28 days of immersion in 3.5 wt.% NaCl solution.
direction. Furthermore, rough and deep dimples, represented by the dark regions were noticeable on the Sn– 9Zn surface as well, similar to feature were observed for Sn–9Zn solder/Cu joint at different solidification rate [36]. However, from Fig. 4b, the deformation that occurred for the lap joints immersed for 28 days did not adhere to the shear direction at all. The lap joint surface appeared
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to be rough and surrounded with pits (dark regions) and corrosion products (gray regions). The distribution of pits and corrosion products was in a randomly manner, without any proper orientation. The dimples produced in this microstructure were abundant, shallower and smaller in size. This is the exact characteristic of a brittle failure experienced by the lap joint. Changes in physical structures of the Cu/Sn–9Zn/Cu lap joints before and after 28 days of immersion can be observed by the fracture cross-sectional micrographs represented by Fig. 5 which were been obtained from View 2. In both Fig. 5a and b, it is observed that the Sn–9Zn solder layer remained attached to the Cu substrate after the shear strength evaluations. The Sn–9Zn solder layer in Fig. 5a was more compacted and denser than the layer in Fig. 5b. In addition to that, throughout the solder layer in Fig. 5b, the Sn–9Zn solder was transformed into a rough and flaky microstructure. Dark pits were evident in the layer, hence producing a loose, uncompact, and porous solder layer. The corrosion products from the solder alloy formed an uncompact layer which covers the surface of the Sn– 9Zn solder joint [37]. The stress–strain curves clearly indicates that the immersion time possessed significant influences on the failures behavior of the joints. The Cu/Sn–9Zn/Cu lap joints experienced a typical ductile failure with exception for the lap joint immersed for 28 days with portrays brittle failure. Ductile fractures can be well represented by the three regions apparent in Fig. 2a–e. The stress–strain curves obtained in this study is comparable to the Sn–9Zn solder results done by El-Daly et al. [38]. During the shear strength evaluation, loads from opposite directions were
applied on the Sn–9Zn solder. The Sn–9Zn solder region withstood a large value of stress which can be seen within the elastic region of the stress–strain curves. Later, as soon as the yield strength of the solders has been achieved, the flow hardening region began. The flow hardening region as
Fig. 6. X-ray diffraction spectra of (a) Sn, (b) Zn, and Cu/Sn–9Zn/Cu lap joint (c) before immersion and (d) after 28 days after immersion in 3.5 wt.% NaCl solution.
(a)
(b) IMC
IMC
Fig. 5. View 2 of fractured cross-sectional micrographs of Cu/Sn–9Zn/Cu lap joints (a) before and (b) after 28 days after immersion in 3.5 wt.% NaCl solution.
Fig. 7. View 2 of surface micrographs of (a) Sn–9Zn solder alloy and (b) Cu/Sn9Zn/Cu lap joint before immersion in 3.5 wt.% NaCl solution.
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Fig. 8. View 2 of Sn–9Zn surface micrographs of post-immersed Cu/Sn– 9Zn/Cu lap joint in 3.5 wt.% NaCl solution after 28 days magnified at (a) 50 and (b) 150 (yellow-dashed box). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Sn–9Zn region. The peaks of pure Sn and Zn were identified and used as references for the analysis on lap joints (Fig. 6a and b). The XRD pattern of the Cu/Sn–9Zn/Cu lap joint prior to immersion in Fig. 6c contains several Sn and Zn peaks as main constituents which corresponded to its pure forms [15,40–42]. There were thirteen peaks of Sn observed, whereas seven peaks were detected for Zn. In Fig. 6d, there are additional phases present in the XRD patterns of Cu/Sn–9Zn/Cu lap joint after immersion for 28 days. The peaks were identified as tin (II) chloride (SnCl2) and zinc hydroxide chloride (Zn5(OH)8Cl2H2O). The peaks of SnCl2 were detected at 34.92°, 42.92°, and 55.49° (as shown in insert in Fig. 6d), while Zn5(OH)8Cl2 H2O peaks were observed at 42.46°, 49.48° and 52.06°. Besides that, there were two Sn peaks present in the lap joint after the immersion as well. The SnCl2 and Zn5(OH)8 Cl2H2O peaks are the only additional phases detected upon immersing the Sn–9Zn solder joints in 3.5 wt.% NaCl solution [37,43]. It is important to note that the lap joint after 28 days of immersion was chosen so that clear and profound phase identifications can be done. The reason being that with increasing immersion time, the amount of new additional products deposited on the surface will be increased as well. As mentioned earlier, the phases identified were SnCl2 and Zn5(OH)8Cl2H2O. The presence of these new peaks in the XRD patterns is mainly attributed to the corrosion process between the Cu/Sn–9Zn/Cu lap joints and the 3.5 wt.% NaCl solution during the immersion.
illustrated in curves resembled a steady state stress–strain behavior. A steady state stress–strain behavior is indicative of plastic deformation [39]. Reduction on the plastic deformation regions with increased immersion period further reflects the changes in failure behavior from ductile to brittle fractures. These phenomena can basically be explained by the alteration in the microstructural condition of the solder alloys due to the deteroriation caused by the corrosion activities. Failure of the lap joints which are non-directional as in Fig. 4b was resulted by the reduction of the surface area on the fracture plane. These reduction were represented by pores in the solder alloys, which will obstruct stress distribution in the solder alloy resulting in instantaneous fracture. The tranformation of the Sn–9Zn solder alloys from dense to flaky physical state in Fig. 5 indicates severe corrosion attack on the lap joint immersed for 28 days. Such changes in the physical structures of the solder alloys significantly effects the mechanical integrity of the solder joints which in this case was projected by the poor performance in mechanical properties of the corroded lap joints. 3.2. Phase determination analysis Fig. 6 presents the XRD spectras of Sn, Zn, Cu/Sn–9Zn/ Cu lap joint before and after immersion in the 3.5 wt.% NaCl solution. For the Cu/Sn–9Zn/Cu lap joints, the XRD analysis was focused specifically at the sandwiched
Fig. 9. View 2 of surface micrographs of (a) SnCl2 and (b) Zn5(OH)8Cl2 H2O, corrosion products of lap joints after immersion in 3.5 wt.% NaCl solution for 28 days with their corresponding elemental composition analysis.
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3.3. Microstructure of corrosion products on lap joint Fig. 7 shows the surface micrographs of a Sn–9Zn solder alloy and a Cu/Sn–9Zn/Cu lap joint from View 2 prior to immersion. Two noticeable regions can be observed on the micrograph in Fig. 7a, namely; the b-Sn matrix and the Zn-rich phase. The bright gray regions are the b-Sn matrix phase which is primarily solidified phases while the dark phases are fine needle-like Zn-rich phase dispersed in the b-Sn matrix [11,44]. In order for the solder to achieve its excellent mechanical properties, the eutectic Sn–9Zn composition has to be fulfilled. This can be verified by the typical eutectic lamella microstructure observed in the micrographs [45]. Whereas in Fig. 7b, the SEM micrograph shows the Cu/Sn–9Zn/Cu lap joint from View 2 before immersion in 3.5 wt.% NaCl solution. It displays the Sn–9Zn solder sandwiched between two Cu pads. The eutectic lamellae structure can still be seen while a thin layer of intermetallic compound (IMC) was observed at
(a)
Cu B
the Sn–9Zn/Cu interfaces. Presence of IMC is essential in joints and possesses the strengthening effect on solder alloys [46,47]. In addition, the surface is smooth and there is no discrete microstructural change present on the lap joint compared to the solder alloy. On the contrary, prominent microstructural changes were observed from the Cu/Sn–9Zn/Cu lap joint after immersion for 28 days in Fig. 8. Large deposits of corrosion products, lightly-colored in appearance, covered the Sn– 9Zn region as shown in Fig. 8a. Besides that, pits were formed as well on the Sn–9Zn layer, represented by the dark regions. As compared to the Sn–9Zn region prior immersion, the surface region altered from a smooth to a rough structure. Elongated and thick cracks were noticeable throughout the Sn–9Zn layer. A higher magnified micrograph of the corroded surface of lap joint can be seen in Fig. 8b. Higher magnification revealed two distinctive microstructures formed on the surface of the Cu/Sn–9Zn/
(b)
(c)
Pits
Sn-9Zn
Cu A
(d)
(e)
(f)
Corrosion products
300 X 1000 μm Fig. 10. One part of cross-sectional micrographs of lap joints from View 3 (a) before and after (b) 1, (c) 7, (d) 14, (e) 21, and (f) 28 days of immersion in 3.5 wt.% NaCl solution.
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Cu lap joints, as seen in Fig. 9. In Fig. 9a, the microstructure looked to be lightly-colored, with a coral-like, translucent exterior. The distribution of this microstructure was throughout the entire Sn–9Zn region. The elemental composition analysis of this particular microstructure indicated high percentages of Sn and Cl traced. Hence, it can be deduced that the corrosion product formed was SnCl2, as also been identified from the XRD analysis. In addition, another microstructure shown in Fig. 9b has a sharp, long and needle-like structure. This microstructure can be found surrounded by the SnCl2. It is believed that the microstructure formed was Zn5(OH)8Cl2H2O due to the needle-like structure, which is one of the signature forms of oxidized zinc [48,49]. Apart to that, the microstructure identified agrees well with results obtained from both XRD and EDX analyses. Microstructural changes on the lap joints were prominent due to the corrosion attack occurred during the immersion in the NaCl solution. Lower corrosion potential of the Sn–9Zn solder makes the alloy prone to corrosion attack rather than the Cu substrate. For that matter itself, the Sn–9Zn solder acted as an anode, releasing Sn2+ and Zn2+ ions from the solder surface. The presence of pits in
Surface view (from View 2) (ai)
the micrograph can be attributed to the active dissolution of Sn in the presence of halide ions [50]. In accordance to that, the leaching of ions too may well be the factor of pit formation [37]. The pits were well represented by the dark regions observed in the micrographs. Longer immersion period allows the corrosion activities to attack the Sn–9Zn solder region even deeper than the surface. Cross-sectional micrographs before and after immersion of the Cu/Sn–9Zn/Cu lap joints as in Fig. 10 were obtained from View 3 to further investigate the penetration of these corrosion attacks. In Fig. 10a, the micrograph shows a control sample with no sign of corrosion. The solder surface was smooth and no microstructural change was observed. However, once the corrosion process took place during immersion, corrosion products and pits were formed on the surface, as seen in Fig. 10b. Sn and Zn ions were released and reacted with Cl ions to form corrosion products. In return, pits began to form on the solder surface. As the immersion time increased, corrosion attack by the Cl increased as well. Thus, the resultant corrosion products increased in quantity, while the pits were penetrating deeper into the Sn–9Zn solder (Fig. 10c–e). In
(ci)
(bi)
Cu B
Cu B
Cu B
Cu A
Cu A
Pits
Cu A
Cross-section view (from View 3) (aii)
(cii)
(bii) Cu B
Cu B
Cu B
Cu A
Cu A
Pits
Cu A
Fig. 11. Sequence of fracture path formation (a) crack nucleation, (b) crack enlargement, and (c) crack coalescence and propagation at (i) View 2 and (ii) View 3.
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Fig. 10f, the corrosion products detached themselves from the Sn–9Zn surface, leaving behind large pits exposing the Sn–9Zn/Cu interfaces. These exposed fresh surface of the Sn–9Zn/Cu interfaces allows further corrosion attack along these path, reducing overall area of the Sn–9Zn/Cu interfaces. The reduction of the bonding area between the Sn– 9Zn solder and the Cu substrate weakened the lap joint configuration. This is due to that shear strength is indirectly proportional to the surface area parallel to the applied load. Thus, explaining the reduction of shear strength on the immersed lap joints which had been measured from the shear test. 3.4. Fracture path formation The reduction in USS values can be attributed to the fracture path which had been favored by the immersion period of the lap joints in the 3.5 wt.% NaCl solution. Through the morphology observations, it can be viewed that longer immersion period promotes easier fracture path formation on the Sn–9Zn/Cu interfaces. The mechanism of these fracture path formation of Cu/Sn–9Zn/Cu lap joints can be summarized in Fig. 11. Firstly, in Fig. 11ai from View 2, pits were produced on the surface of the Sn–9Zn solder as a result from the dissolution of Sn-rich phase from the solder alloy. Similar pit observation can be seen from View 3 in Fig. 11aii. Pits acted as stress concentration points, disrupting the distribution of load applied on the lap joint. Due to this, pits were the nucleatic sites of crack initiation. These pits then became enlarged (Fig. 11bi and bii), and coalesced together to form miniature cracks. The fracture ensued by the rapid propagation of crack across the solder region (Fig. 11ci and cii), until the lap joint split into two parts. The increase of immersion time allowed further Sn-rich phase dissolution, in return producing larger and deeper pits on the surface, which can be seen in the microstructure analysis. As the depth of pits increased, this accelerated the crack initiation and propagation process. Cracks formed weakened the solder region by reducing the contact area along the fracture path, thus lower force is required for deformation explaining the reduction of the lap joints USS values. 4. Conclusion The effects of corrosion on the shear strength of Cu/Sn– 9Zn/Cu lap joints in 3.5 wt.% NaCl solution have been investigated. It is evident that corrosion of lap joints decreased the values of USS with increasing immersion time. The decrease of USS is mainly attributed by the effects of corrosion, which are the formation of pits. Throughout the corrosion process, Sn and Zn ions leached out to form corrosion products of SnCl2 and Zn5(OH)8Cl2 H2O. The leaching out of these ions produced pits, which act as stress concentration points. Cracks nucleated from these pits, and propagated across the solder region. As the immersion time increased, the pits grew in size and penetrated even deeper into the solder, severely weakening the Sn–9Zn/Cu interfacial joints. Fracture analysis of
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Measurement journal homepage: www.elsevier.com/locate/measurement
Corrosion measurement on shear strength of Cu/Sn–9Zn/Cu lap joints Muhammad Ghaddafy Affendy a, Muhamad Zamri Yahaya a, Fakhrozi Che Ani b, Ahmad Azmin Mohamad a,⇑ a b
School of Materials and Mineral Resources Engineering, Universiti Sains Malaysia, 14300 Nibong Tebal, Penang, Malaysia Jabil Circuit Sdn. Bhd., Bayan Lepas Industrial Park, 11900 Bayan Lepas, Penang, Malaysia
a r t i c l e
i n f o
Article history: Received 21 November 2013 Received in revised form 20 October 2014 Accepted 6 February 2015 Available online 14 February 2015 Keywords: Corrosion Shear strength Lap joints NaCl Morphology
a b s t r a c t Corrosion of the Cu/Sn–9Zn/Cu lap joints in 3.5 wt.% NaCl solution showed reduction in the ultimate shear strength with increasing immersion time. The highest ultimate shear strength value obtained was at 122.67 MPa before immersion while the lowest value was 85.32 MPa after immersion for 28 days. Corrosion products consist of tin (II) chloride and zinc hydroxide chloride had been obtained due to the active dissolution of tin. The formation of pits observed on the lap joints marked as stress concentration points which are detrimental toward the shear strength. Upon longer immersion period, the pit coalescence and propagation led to the existence of long, deep cracks. The fracture paths nucleates from the cracks clearly established the mechanism of the fracture formation for the Cu/Sn–9Zn/ Cu lap joints. The fracture analysis together with the presence of the corrosion by-products fully reiterated that corrosion activities was the main factor for the decrease in shear strength of Cu/Sn–9Zn/Cu lap joints. Ó 2015 Elsevier Ltd. All rights reserved.
1. Introduction The dependence on Sn–Pb solders as electronic interconnection materials is drastically brought down due to its harmful and toxic consequences on the environment and human health [1–3]. Numerous researches have been done on various Pb-free solders families such as the Sn– Cu [4,5], Sn–Ag [6,7], Sn–Ag–Cu [8,9] and Sn–Zn [10–12] to search for a suitable candidate to replace the conventional Sn–Pb solder. From all the Pb-free solder systems, the autistic Sn–9Zn solder is considered to be a promising alternative as it has a melting temperature of 198 °C [13], which is almost similar to the conventional Sn–Pb solders. Aside from having economic advantages [14], Sn–9Zn solder has excellent mechanical properties that can be ⇑ Corresponding author. Tel.: +60 4 599 6118; fax: +60 4 594 1011. E-mail address: [email protected] (A.A. Mohamad). http://dx.doi.org/10.1016/j.measurement.2015.02.016 0263-2241/Ó 2015 Elsevier Ltd. All rights reserved.
significantly important to form solder joints in electronic devices [15–17]. Solder joints, according to Li et al. [18], provide electrical conductivity and suitable mechanical strength for devices to perform smoothly. However, as of today, the reliability of solder joints remains an unceasing concern in the electronic packaging industry [19]. Miniaturization of electronic devices increased the demand for higher mechanical integrity of the solder joints as finer pitch interconnection were incorporated [20,21]. In conjunction with these requirements, the Sn–9Zn solder alloy is able to offer excellent mechanical properties, particularly the shear strength. These merits of Sn–9Zn solder alloy had resulted in the incorporation of various Sn–Zn solder family in a wide range of commercial electronics products [22,23]. A study conducted by Shoji et al. [24], reported that the shear strength of Sn–9Zn solder is higher than the
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conventional Sn–Pb solder. Thus, performing the shear strength measurements on Sn–9Zn solder joints would provide a proper estimation on the interconnection life for future applications [25]. In addition, the lap joints configuration is essential in simulating real solder joints in order to investigate the mechanical properties of solders toward various parameters such as alloying elements [26], composite solders [27] and corrosion [28]. On a more practical perspective, the solder joints are vulnerably exposed to corrosive environments such as moisture, air pollutants and oceanic environments, throughout its real-time applications [10,12,29]. The exposure, in a period of time, permits corrosion to take place between them. The tendency for oxidation of the Sn–9Zn solder alloy is a setback which confines its other merits from potential electronics applications [30,31]. These corrosion tendencies of the Sn–9Zn solder alloy will significantly affect the end properties of the solder joint. In addition, reduction in interconnection density between components in current electronic devices also reduces the size of moisture droplet for corrosion to occur, making it more susceptible to corrosion attack [32]. Consequently, this may lead to loss of mechanical integrity of solder joints and lastly, operational failure. In the Sn–Zn system, Liu et al. [33] have found out that pits are formed at solder/substrate interfaces when immersed in 3.5 wt.% NaCl solution. Furthermore, a study reported that the increase of pit size after immersion caused the drop of interfacial adhesion strength of Sn–9Zn–1.5Ag/Cu joints [34]. Therefore, the study on corrosion behavior of the Sn–9Zn solder alloy in 3.5 wt.% NaCl solution is significant in understanding the failure mechanism by corrosion activities of the solder alloy. In this study, the effects of corrosion on shear strength of Cu/Sn–9Zn/Cu lap joints in 3.5 wt.% NaCl solution have been studied. Shear strength of specimens were obtained with specimens possessing shear-lap joint geometry since such geometry is better suited for obtaining the shear strength. Apart from the shear strength evaluations, phase determination, failure analysis, microstructural and elemental composition characterizations were performed to support the results obtained.
2. Experimental The Sn–9Zn solder alloy used was made via the melting process of tin (Sn) pieces and zinc (Zn) flakes according to its required composition. The melting process was done in a box furnace under an oxygen-free nitrogen environment at 500 °C with a heating rate of 10 °C min 1. The melted alloy was left for soaking process for 30 min, before the furnace temperature decreases to end the process. Later, the melted solder alloy was left to be cooled off at room temperature (25 °C) on a flat ceramic slab. Sn–9Zn solder pellets with a diameter of 5 mm each and a thickness of 0.5 mm were formed with the assistance of a custom-made solder puncher. The Cu/Sn–9Zn/Cu lap joint was made from two Cu pads (50.0 5.0 0.5 mm in dimensions) and a Sn–9Zn solder pellet. The Cu/Sn–9Zn interface was fixed at
0.25 mm2 (0.5 0.5 in dimensions). Prior to the re-melting and solidification process, the components were ground and polished with common metallographic practices. Remelting process was made on a hot plate in the presence of a ZnCl-based flux. Temperature above 200 °C was maintained for about 30 s with the highest temperature reaching 220 °C during re-melting. The soldered joint was left to solidify at room temperature. Fig. 1 shows the schematic diagram of a Cu/Sn–9Zn/Cu lap joint. In order to study the effects of corrosion on the lap joints, the immersion method was preferred. A Cu/Sn– 9Zn/Cu lap joint was immersed in a 3.5 wt.% NaCl solution under different immersion periods. The lap joint was immersed in a polyethylene container covered with a fabric mesh to allow oxygen ventilation and also to avoid impurities from entering the immersion assembly. The shear strength of Cu/Sn–9Zn/Cu lap joints was determined by performing the shear strength evaluation using a universal testing machine (INSTRON 5900 Testing System). The evaluation was performed on lap joints before the immersion as well as after immersion according to the different immersion periods. A crosshead speed of 2 mm/min was set throughout the evaluation at room temperature [35]. The shear stress–strain graph was plotted, while the ultimate shear strength (USS) and strain at USS values of lap joints were obtained from the Bluehill 3 Testing Software and compared with respect to its immersion time. Upon failure, fracture micrographs were taken on the fractured surface and cross-section of Cu/Sn–9Zn/Cu lap joints before and after immersion in 3.5 wt.% NaCl solution. The observation views for the lap joints can be seen in Fig. 1. Phase determination analysis of Cu/Sn–9Zn/Cu lap joints was conducted via X-ray diffraction (XRD) by using Bruker AXS D9 diffractometer. It was performed with the scanning angle from 10° to 180°, Cu K-alpha (0.1542 nm) wavelength at a scanning rate of 2 deg/min. The samples of pure Sn, Zn, and Cu/Sn–9Zn/Cu lap joint before and after immersion in 3.5 wt.% NaCl solution were evaluated. A Hitachi TM3000 tabletop scanning microscope (SEM) was used to study the microstructural changes of Cu/Sn–9Zn/ Cu lap joints. The surface and cross-section micrographs of lap joints before and after immersion were taken. Elemental composition analysis on post-immersion microstructure was done by an energy dispersive X-ray (EDX) instrument coupled to a Zeiss Supra 35VP Field Emission Scanning Electron Microscope (FESEM) system.
View 1 Sn-9Zn Cu View 3
View 2
Fig. 1. Schematic diagram of a Cu/Sn–9Zn/Cu lap joint and microstructural observation View 1, 2, and 3. Dashed line represents cross-section direction for View 3 observation.
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3. Results and discussions 3.1. Effects of corrosion on shear strength The failures behavior of the lap joints can be examines by the representative stress–strain curves of Cu/Sn–9Zn/ Cu lap joints from the shear strength evaluations shown in Fig. 2. The curves of non-corroded Cu/Sn–9Zn/Cu lap joint was represented by Fig. 2a, while Fig. 2b–f represents the lap joints after immersion in different periods in 3.5 wt.% NaCl solution. For all of the curves, with exception to Fig. 2f, three distinctive regions were observed; the elastic, flow hardening and failure regions. On the contrary, Fig. 2f only exhibited the elastic and failure regions. The strain values of Cu/Sn–9Zn/Cu lap joints from the evaluations were greatly affected by the immersion periods. The strain value of lap joint before immersion occurred was 7.43 mm/mm, while the lap joint after immersion for 28 days obtained the lowest strain value at 0.29 mm/mm as in Fig. 2. By comparing these two values, the shear strain of lap joints decreased as well with increasing immersion time. This decrease can be pointedly seen by the decreasing length of the flow hardening region in the stress–strain curves. The shear strength of Cu/Sn– 9Zn/Cu lap joints is defined by the values of USS obtained from the stress–strain curves. Lap joint prior to immersion recorded the highest value of USS at 128.50 MPa while the lowest value was recorded at 85.32 MPa after 28 days of immersion. From the trend shown in Fig. 3, the shear strength of Cu/Sn–9Zn/Cu lap joints decreased with increasing immersion time. Aside from that, the yield strengths attained in the elastic region from all the curves were comparable to one another. Further investigation on the failures of the Cu/Sn–9Zn/ Cu lap joints before and after 28 days of immersion in 3.5 wt.% NaCl solution was made possible by the fracture surfaces micrographs in Fig. 4 which were obtained from View 1. In Fig. 4a, a typical ductile failure was clearly observed for the lap joint which can be seen by the smooth deformation across the surface in line with the shear
Fig. 3. Ultimate shear strength of lap joints (a) before and after (b) 1, (c) 7, (d) 14, (e) 21, and (f) 28 days of immersion in 3.5 wt.% NaCl solution.
Fig. 4. View 1 of fractured on Sn–9Zn surface micrographs of Cu/Sn–9Zn/ Cu lap joints (a) before and (b) after 28 days after immersion in 3.5 wt.% NaCl solution.
Fig. 2. Stress–strain graph of lap joints (a) before and after (b) 1, (c) 7, (d) 14, (e) 21, and (f) 28 days of immersion in 3.5 wt.% NaCl solution.
direction. Furthermore, rough and deep dimples, represented by the dark regions were noticeable on the Sn– 9Zn surface as well, similar to feature were observed for Sn–9Zn solder/Cu joint at different solidification rate [36]. However, from Fig. 4b, the deformation that occurred for the lap joints immersed for 28 days did not adhere to the shear direction at all. The lap joint surface appeared
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to be rough and surrounded with pits (dark regions) and corrosion products (gray regions). The distribution of pits and corrosion products was in a randomly manner, without any proper orientation. The dimples produced in this microstructure were abundant, shallower and smaller in size. This is the exact characteristic of a brittle failure experienced by the lap joint. Changes in physical structures of the Cu/Sn–9Zn/Cu lap joints before and after 28 days of immersion can be observed by the fracture cross-sectional micrographs represented by Fig. 5 which were been obtained from View 2. In both Fig. 5a and b, it is observed that the Sn–9Zn solder layer remained attached to the Cu substrate after the shear strength evaluations. The Sn–9Zn solder layer in Fig. 5a was more compacted and denser than the layer in Fig. 5b. In addition to that, throughout the solder layer in Fig. 5b, the Sn–9Zn solder was transformed into a rough and flaky microstructure. Dark pits were evident in the layer, hence producing a loose, uncompact, and porous solder layer. The corrosion products from the solder alloy formed an uncompact layer which covers the surface of the Sn– 9Zn solder joint [37]. The stress–strain curves clearly indicates that the immersion time possessed significant influences on the failures behavior of the joints. The Cu/Sn–9Zn/Cu lap joints experienced a typical ductile failure with exception for the lap joint immersed for 28 days with portrays brittle failure. Ductile fractures can be well represented by the three regions apparent in Fig. 2a–e. The stress–strain curves obtained in this study is comparable to the Sn–9Zn solder results done by El-Daly et al. [38]. During the shear strength evaluation, loads from opposite directions were
applied on the Sn–9Zn solder. The Sn–9Zn solder region withstood a large value of stress which can be seen within the elastic region of the stress–strain curves. Later, as soon as the yield strength of the solders has been achieved, the flow hardening region began. The flow hardening region as
Fig. 6. X-ray diffraction spectra of (a) Sn, (b) Zn, and Cu/Sn–9Zn/Cu lap joint (c) before immersion and (d) after 28 days after immersion in 3.5 wt.% NaCl solution.
(a)
(b) IMC
IMC
Fig. 5. View 2 of fractured cross-sectional micrographs of Cu/Sn–9Zn/Cu lap joints (a) before and (b) after 28 days after immersion in 3.5 wt.% NaCl solution.
Fig. 7. View 2 of surface micrographs of (a) Sn–9Zn solder alloy and (b) Cu/Sn9Zn/Cu lap joint before immersion in 3.5 wt.% NaCl solution.
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Fig. 8. View 2 of Sn–9Zn surface micrographs of post-immersed Cu/Sn– 9Zn/Cu lap joint in 3.5 wt.% NaCl solution after 28 days magnified at (a) 50 and (b) 150 (yellow-dashed box). (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
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Sn–9Zn region. The peaks of pure Sn and Zn were identified and used as references for the analysis on lap joints (Fig. 6a and b). The XRD pattern of the Cu/Sn–9Zn/Cu lap joint prior to immersion in Fig. 6c contains several Sn and Zn peaks as main constituents which corresponded to its pure forms [15,40–42]. There were thirteen peaks of Sn observed, whereas seven peaks were detected for Zn. In Fig. 6d, there are additional phases present in the XRD patterns of Cu/Sn–9Zn/Cu lap joint after immersion for 28 days. The peaks were identified as tin (II) chloride (SnCl2) and zinc hydroxide chloride (Zn5(OH)8Cl2H2O). The peaks of SnCl2 were detected at 34.92°, 42.92°, and 55.49° (as shown in insert in Fig. 6d), while Zn5(OH)8Cl2 H2O peaks were observed at 42.46°, 49.48° and 52.06°. Besides that, there were two Sn peaks present in the lap joint after the immersion as well. The SnCl2 and Zn5(OH)8 Cl2H2O peaks are the only additional phases detected upon immersing the Sn–9Zn solder joints in 3.5 wt.% NaCl solution [37,43]. It is important to note that the lap joint after 28 days of immersion was chosen so that clear and profound phase identifications can be done. The reason being that with increasing immersion time, the amount of new additional products deposited on the surface will be increased as well. As mentioned earlier, the phases identified were SnCl2 and Zn5(OH)8Cl2H2O. The presence of these new peaks in the XRD patterns is mainly attributed to the corrosion process between the Cu/Sn–9Zn/Cu lap joints and the 3.5 wt.% NaCl solution during the immersion.
illustrated in curves resembled a steady state stress–strain behavior. A steady state stress–strain behavior is indicative of plastic deformation [39]. Reduction on the plastic deformation regions with increased immersion period further reflects the changes in failure behavior from ductile to brittle fractures. These phenomena can basically be explained by the alteration in the microstructural condition of the solder alloys due to the deteroriation caused by the corrosion activities. Failure of the lap joints which are non-directional as in Fig. 4b was resulted by the reduction of the surface area on the fracture plane. These reduction were represented by pores in the solder alloys, which will obstruct stress distribution in the solder alloy resulting in instantaneous fracture. The tranformation of the Sn–9Zn solder alloys from dense to flaky physical state in Fig. 5 indicates severe corrosion attack on the lap joint immersed for 28 days. Such changes in the physical structures of the solder alloys significantly effects the mechanical integrity of the solder joints which in this case was projected by the poor performance in mechanical properties of the corroded lap joints. 3.2. Phase determination analysis Fig. 6 presents the XRD spectras of Sn, Zn, Cu/Sn–9Zn/ Cu lap joint before and after immersion in the 3.5 wt.% NaCl solution. For the Cu/Sn–9Zn/Cu lap joints, the XRD analysis was focused specifically at the sandwiched
Fig. 9. View 2 of surface micrographs of (a) SnCl2 and (b) Zn5(OH)8Cl2 H2O, corrosion products of lap joints after immersion in 3.5 wt.% NaCl solution for 28 days with their corresponding elemental composition analysis.
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3.3. Microstructure of corrosion products on lap joint Fig. 7 shows the surface micrographs of a Sn–9Zn solder alloy and a Cu/Sn–9Zn/Cu lap joint from View 2 prior to immersion. Two noticeable regions can be observed on the micrograph in Fig. 7a, namely; the b-Sn matrix and the Zn-rich phase. The bright gray regions are the b-Sn matrix phase which is primarily solidified phases while the dark phases are fine needle-like Zn-rich phase dispersed in the b-Sn matrix [11,44]. In order for the solder to achieve its excellent mechanical properties, the eutectic Sn–9Zn composition has to be fulfilled. This can be verified by the typical eutectic lamella microstructure observed in the micrographs [45]. Whereas in Fig. 7b, the SEM micrograph shows the Cu/Sn–9Zn/Cu lap joint from View 2 before immersion in 3.5 wt.% NaCl solution. It displays the Sn–9Zn solder sandwiched between two Cu pads. The eutectic lamellae structure can still be seen while a thin layer of intermetallic compound (IMC) was observed at
(a)
Cu B
the Sn–9Zn/Cu interfaces. Presence of IMC is essential in joints and possesses the strengthening effect on solder alloys [46,47]. In addition, the surface is smooth and there is no discrete microstructural change present on the lap joint compared to the solder alloy. On the contrary, prominent microstructural changes were observed from the Cu/Sn–9Zn/Cu lap joint after immersion for 28 days in Fig. 8. Large deposits of corrosion products, lightly-colored in appearance, covered the Sn– 9Zn region as shown in Fig. 8a. Besides that, pits were formed as well on the Sn–9Zn layer, represented by the dark regions. As compared to the Sn–9Zn region prior immersion, the surface region altered from a smooth to a rough structure. Elongated and thick cracks were noticeable throughout the Sn–9Zn layer. A higher magnified micrograph of the corroded surface of lap joint can be seen in Fig. 8b. Higher magnification revealed two distinctive microstructures formed on the surface of the Cu/Sn–9Zn/
(b)
(c)
Pits
Sn-9Zn
Cu A
(d)
(e)
(f)
Corrosion products
300 X 1000 μm Fig. 10. One part of cross-sectional micrographs of lap joints from View 3 (a) before and after (b) 1, (c) 7, (d) 14, (e) 21, and (f) 28 days of immersion in 3.5 wt.% NaCl solution.
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Cu lap joints, as seen in Fig. 9. In Fig. 9a, the microstructure looked to be lightly-colored, with a coral-like, translucent exterior. The distribution of this microstructure was throughout the entire Sn–9Zn region. The elemental composition analysis of this particular microstructure indicated high percentages of Sn and Cl traced. Hence, it can be deduced that the corrosion product formed was SnCl2, as also been identified from the XRD analysis. In addition, another microstructure shown in Fig. 9b has a sharp, long and needle-like structure. This microstructure can be found surrounded by the SnCl2. It is believed that the microstructure formed was Zn5(OH)8Cl2H2O due to the needle-like structure, which is one of the signature forms of oxidized zinc [48,49]. Apart to that, the microstructure identified agrees well with results obtained from both XRD and EDX analyses. Microstructural changes on the lap joints were prominent due to the corrosion attack occurred during the immersion in the NaCl solution. Lower corrosion potential of the Sn–9Zn solder makes the alloy prone to corrosion attack rather than the Cu substrate. For that matter itself, the Sn–9Zn solder acted as an anode, releasing Sn2+ and Zn2+ ions from the solder surface. The presence of pits in
Surface view (from View 2) (ai)
the micrograph can be attributed to the active dissolution of Sn in the presence of halide ions [50]. In accordance to that, the leaching of ions too may well be the factor of pit formation [37]. The pits were well represented by the dark regions observed in the micrographs. Longer immersion period allows the corrosion activities to attack the Sn–9Zn solder region even deeper than the surface. Cross-sectional micrographs before and after immersion of the Cu/Sn–9Zn/Cu lap joints as in Fig. 10 were obtained from View 3 to further investigate the penetration of these corrosion attacks. In Fig. 10a, the micrograph shows a control sample with no sign of corrosion. The solder surface was smooth and no microstructural change was observed. However, once the corrosion process took place during immersion, corrosion products and pits were formed on the surface, as seen in Fig. 10b. Sn and Zn ions were released and reacted with Cl ions to form corrosion products. In return, pits began to form on the solder surface. As the immersion time increased, corrosion attack by the Cl increased as well. Thus, the resultant corrosion products increased in quantity, while the pits were penetrating deeper into the Sn–9Zn solder (Fig. 10c–e). In
(ci)
(bi)
Cu B
Cu B
Cu B
Cu A
Cu A
Pits
Cu A
Cross-section view (from View 3) (aii)
(cii)
(bii) Cu B
Cu B
Cu B
Cu A
Cu A
Pits
Cu A
Fig. 11. Sequence of fracture path formation (a) crack nucleation, (b) crack enlargement, and (c) crack coalescence and propagation at (i) View 2 and (ii) View 3.
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Fig. 10f, the corrosion products detached themselves from the Sn–9Zn surface, leaving behind large pits exposing the Sn–9Zn/Cu interfaces. These exposed fresh surface of the Sn–9Zn/Cu interfaces allows further corrosion attack along these path, reducing overall area of the Sn–9Zn/Cu interfaces. The reduction of the bonding area between the Sn– 9Zn solder and the Cu substrate weakened the lap joint configuration. This is due to that shear strength is indirectly proportional to the surface area parallel to the applied load. Thus, explaining the reduction of shear strength on the immersed lap joints which had been measured from the shear test. 3.4. Fracture path formation The reduction in USS values can be attributed to the fracture path which had been favored by the immersion period of the lap joints in the 3.5 wt.% NaCl solution. Through the morphology observations, it can be viewed that longer immersion period promotes easier fracture path formation on the Sn–9Zn/Cu interfaces. The mechanism of these fracture path formation of Cu/Sn–9Zn/Cu lap joints can be summarized in Fig. 11. Firstly, in Fig. 11ai from View 2, pits were produced on the surface of the Sn–9Zn solder as a result from the dissolution of Sn-rich phase from the solder alloy. Similar pit observation can be seen from View 3 in Fig. 11aii. Pits acted as stress concentration points, disrupting the distribution of load applied on the lap joint. Due to this, pits were the nucleatic sites of crack initiation. These pits then became enlarged (Fig. 11bi and bii), and coalesced together to form miniature cracks. The fracture ensued by the rapid propagation of crack across the solder region (Fig. 11ci and cii), until the lap joint split into two parts. The increase of immersion time allowed further Sn-rich phase dissolution, in return producing larger and deeper pits on the surface, which can be seen in the microstructure analysis. As the depth of pits increased, this accelerated the crack initiation and propagation process. Cracks formed weakened the solder region by reducing the contact area along the fracture path, thus lower force is required for deformation explaining the reduction of the lap joints USS values. 4. Conclusion The effects of corrosion on the shear strength of Cu/Sn– 9Zn/Cu lap joints in 3.5 wt.% NaCl solution have been investigated. It is evident that corrosion of lap joints decreased the values of USS with increasing immersion time. The decrease of USS is mainly attributed by the effects of corrosion, which are the formation of pits. Throughout the corrosion process, Sn and Zn ions leached out to form corrosion products of SnCl2 and Zn5(OH)8Cl2 H2O. The leaching out of these ions produced pits, which act as stress concentration points. Cracks nucleated from these pits, and propagated across the solder region. As the immersion time increased, the pits grew in size and penetrated even deeper into the solder, severely weakening the Sn–9Zn/Cu interfacial joints. Fracture analysis of
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