Creep corrosion cracking

Creep corrosion cracking of Sn–3.0Ag and Sn–0.5Cu solder alloys in NaClsolutionKen’ichi Yokoyama a ,⇑,Akihide Nogami a ,Jun’ichi Sakai b ,caDepartment of Materials Science and Engineering,Kyushu Institute of Technology,1-1Sensui-cho,Tobata-ku,Kitakyushu 804-8550,Japan bFaculty of Science and Engineering,Waseda University,3-4-1Okubo,Shinjuku-ku,Tokyo 169-8555,Japan cKagami Memorial Laboratory for Materials Science and Technology,Waseda University,2-8-26,Nishiwaseda Shinjuku-ku,Tokyo 169-0051,Japana r t i c l e i n f o Article history:Received 24December 2013Accepted 5May 2014Available online 14May 2014Keywords:A.Electronic materials A.TinB.SEMC.Intergranular corrosion C.Stress corrosiona b s t r a c tThe surface crack nucleation of Sn–3.0Ag and Sn–0.5Cu solder alloys has been examined by performing sustained tensile-loading tests in 0.9mass%NaCl solution at room temperature.For Sn–3.0Ag alloy,many cracks nucleate and propagate on the side surface of the specimen,similarly to Sn–3.0Ag–0.5Cu alloy reported previously.For Sn–0.5Cu alloy,such cracks are not observed,and ordinary creep deformation occurs in the solution.The effect of sustained applied stress,i.e.,creep,on the dissolution of ions is smal-ler for Sn–0.5Cu alloy than for Sn–3.0Ag alloy.The present results suggest that there are differences in the susceptibility to cracking under applied stress in a solution,i.e.,creep corrosion cracking,among lead-free solder alloys.Ó2014Elsevier Ltd.All rights reserved.1.IntroductionLead-free solders including Sn–Ag–Cu,Sn–Ag and Sn–Cu alloys exhibit better corrosion resistance than conventional tin–lead solders in NaCl solutions [1–6].In addition,the creep resistance of the above lead-free solder alloys is much greater than that of eutectic tin–lead solder because of their dispersion-strength mechanism [7–10].Intermetallic compound particles such as Ag 3-Sn play an important role in the creep behavior of Sn–Ag–Cu-based or Sn–Ag-based alloys.In particular,the particles are considered to cause an increase in the time to creep rupture but a decrease in the creep strain [11].However,from the results of a sustained tensile-loading test in 0.9mass%NaCl solution,the fracture mode of Sn–3.0Ag–0.5Cu alloy is not ordinary creep rupture [12].This frac-ture is probably caused by overload due to surface crack nucleation at and propagation along grain boundaries accompanied by the preferential dissolution of Ag and Cu ions [12],in spite of the ade-quate corrosion resistance without an applied load in the solution.Therefore,the surface cracking behavior is especially important in the case of this fracture mode.We have suggested that surface crack nucleation is related to both creep and corrosion,i.e.,creep corrosion cracking [12];hence,it must not be confused with conventional stress corrosion cracking.Moreover,surface crack nucleation cannot necessarily be predicted from only corrosion behavior without applied stress or creep behavior under atmospheric conditions.Among the lead-free solder alloys,such surface cracking has currently only been observed for Sn–Ag–Cu alloy.It should be investigated exper-imentally whether or not the interaction of cracking with creep and corrosion occurs for other solder alloys such as Sn–Ag and Sn–Cu alloys.If such cracking occurs,the methods of evaluating the reliability of lead-free solder alloys should be reviewed.Con-versely,the case of no cracking will help clarify the mechanisms of creep corrosion cracking in detail.The purpose of the present study is to investigate the surface cracking behavior of Sn–3.0Ag and Sn–0.5Cu alloys subjected to a sustained tensile-loading test in NaCl aqueous solution at room temperature.The findings obtained by the present study will be useful for clarifying the cracking mechanisms and for evaluating the reliability of lead-free solder alloys.2.Experimental procedureFor comparison with the results for Sn–3.0Ag–0.5Cu alloy,the present study was performed under the same experimental condi-tions as those reported previously [12].Commercially available Sn–3.0Ag and Sn–0.5Cu (mass%)solder alloy wires of 0.50mm diameter were cut into specimens.Percent-ages in this paper refer to mass percentages unless otherwise stated.The surfaces of the specimens were carefully finished with 600-grit SiC paper and ultrasonically cleaned in acetone for 5min./10.1016/j.corsci.2014.05.0040010-938X/Ó2014Elsevier Ltd.All rights reserved.⇑Corresponding author.Tel.:+81938843356;fax:+81938843351.E-mail address:yokken@post.matsc.kyutech.ac.jp (K.Yokoyama).The microstructure of the specimens in a cross-sectional area was examined by optical microscopy and scanning electron microscopy(SEM)with an electron probe microanalyzer(EPMA) after etching with ethanol solution containing hydrochloric acid. The secondary phase of the specimens was examined using an X-ray diffractometer operated at40kV and20mA with Cu K a radiation of wavelength k=0.154056nm.To measure the mechanical properties,tensile tests were performed at room temperature(25±2°C)under atmospheric conditions using an Instron-type machine at a strain rate of 8.33Â10À4/s.Tensile creep tests were also conducted under various sustained loads at room temperature under atmospheric conditions.The applied stress was calculated as the ratio of the applied load to the initial cross-sectional area.The measurement of creep strain was terminated after2Â104s for experimental reasons.The potentiodynamic polarization curves of the specimen without applied stress were measured at least three times from the corrosion potential towards the anodic and cathodic directions with a scan rate of20mV/min at room temperature in neutral0.9 mass%NaCl aqueous solution under aerated conditions.The counter electrode and reference electrode used were a platinum electrode and a saturated calomel electrode(SCE),respectively. The measurement was started10min after immersion in the solution.A sustained tensile-loading test was carried out under an applied stress of9MPa in the test solution at room temperature. The applied stress was calculated as the ratio of the applied load to the initial cross-sectional area.The test solution used was 50ml of0.9mass%NaCl solution under aerated conditions.The immersion length of each specimen in the solution was50mm. Immediately after a given time(10–120h),the specimen was removed from the solution,cleaned with acetone and dried in ambient air.The side surfaces of the tested specimens were exam-ined by SEM.The changes in corrosion potentials were also mea-sured.The amounts of Sn,Ag and Cu ions dissolved in the solution during sustained tensile-loading tests were determined by inductively coupled plasma(ICP)spectroscopy measurements. The standard deviation was calculated from the results for at least three specimens.3.Results and discussion3.1.Microstructure characterizationThe microstructures of Sn–3.0Ag and Sn–0.5Cu alloys are shown in Fig.1(a)and(b),respectively.The grains of both specimens were approximately2–4l m in size and equiaxial in shape.Fine precip-itates were distributed along grain boundaries and frequently existed in grains.Fig.2shows SEM images and element mappings of the specimens.The Ag-rich white phases for Sn–3.0Ag alloy and Cu-rich white phases for Sn–0.5Cu alloy were distributed mainly along grain boundaries.The X-ray diffraction(XRD)results obtained from a side surface of the specimens are shown in Fig.3.The XRD peaks of Ag3Sn were identified for Sn–3.0Ag alloy, whereas those of Cu6Sn5were identified for Sn–0.5Cu alloy.3.2.Creep behavior under atmospheric conditionsFig.4shows representative tensile stress–strain curves of Sn–3.0Ag and Sn–0.5Cu alloys under atmospheric conditions at room temperature.The tensile strengths of Sn–3.0Ag and Sn–0.5Cu alloys were approximately28and25MPa and their tensile strains were approximately0.6and0.5,respectively.Mechanical proper-ties(e.g.tensile strength and yield stress)may be less important in the practical application,because the creep deformation of lead-free solder alloys is observed under applied stresses for below the yield stress,even at room temperature.Fig.5(a)and(b)show typical creep curves of Sn–3.0Ag and Sn–0.5Cu alloys under atmospheric conditions at room tempera-ture,respectively.The arrow in thefigure denotes the result for a nonfractured specimen within2Â104s.The creep strain to frac-ture for Sn–3.0Ag alloy(approximately0.8)was larger than that for Sn–0.5Cu alloy(approximately0.3–0.5).For Sn–3.0Ag alloy, the creep strain to fracture was larger than the tensile strain to fracture,whereas,for Sn–0.5Cu alloy,the creep strain to fracture was smaller than the tensile strain to fracture.The time to fracture was slightly longer for Sn–3.0Ag alloy than for Sn–0.5Cu alloy under an applied stress of15MPa and above.Under an applied stress of up to9MPa,creep scarcely occurred for both alloys,at least in the early stage of the sustained tensile-loading test.The times to fracture under an applied stress of9MPa for Sn–3.0Ag and Sn–0.5Cu alloys were94.5±14.7and415.5±63.4h(n=5), respectively.Under a lower applied stress,Sn–3.0Ag alloy appears to be affected by creep deformation than Sn–0.5Cu alloy.3.3.Polarization behavior without applied stressThe polarization curves of Sn–3.0Ag and Sn–0.5Cu alloy specimens immersed in0.9mass%NaCl solution under aerated conditions without applied stress are shown in Fig.6.The polariza-tion behavior was only slightly different between Sn–3.0Ag and Sn–0.5Cu alloys.In addition,the polarization behavior of both alloys was not significantly different from that of Sn–3.0Ag–0.5Cu alloy reported previously[12].The corrosion potential and corrosion current density of both alloys were approximately À0.55V(vs.SCE)and2mA/m2,respectively.These values agree(a)10 µm(b)Optical metallography images of(a)Sn–3.0Ag and(b)Sn–0.5Cugeneral microstructure.K.Yokoyama et al./Corrosion Science86(2014)142–148143with those in previous studies [2,13–15].Hence,both specimens probably had satisfactory corrosion resistance without applied stress.In the noble direction,the active dissolution of both specimens occurred;corrosion products with a platelet like or needlelike shape were distributed on the surface with different orientations,as reported previously [2,13–15].It appears that the present results are consistent with those obtained in previous studies [2,13–15].From the above results,the microstructures,tensile,creep and corrosion behavior of both specimens in the present study appear to have typical characteristic of Sn–Ag and Sn–Cu alloys.3.4.Sustained tensile-loading test in solutionRepresentative side surfaces of Sn–3.0Ag and Sn–0.5Cu alloys in 0.9mass%NaCl solution under an applied stress of 9MPa are shown in Figs.7and 8,respectively.On the side surfaces of the Sn–3.0Ag (Fig.7(a))and Sn–0.5Cu (Fig.8(a))alloys before the immersion test,scratches due to SiC paper finishing were observed.For Sn–3.0Ag alloy under the applied stress in thesolution,when the immersion time exceeded approximately 10–20h,surface cracks perpendicular to the loading direction were observed (Fig.7(b)).The initial crack length almost coincided with the grain size as shown in Fig.1.The cracks propagated with increasing immersion time (Fig.7(c)).Only small amounts of mac-roscopic creep deformation and corrosion products were observed.Under the applied stress in the solution,the surface crack nucle-ation and propagation behavior of Sn–3.0Ag alloy were almost the same as those of Sn–3.0Ag–0.5Cu alloy reported previously [12].The time to crack nucleation of Sn–3.0Ag alloy was slightly shorter than that of Sn–3.0Ag–0.5Cu alloy (approximately 20–30h)[12].However,we should carefully discuss whether the crack nucleation of Sn–Ag-based alloys,compared with that of Sn–Ag–Cu-based alloys,tends to occur in nature,because crack nucleation seems to strongly depend on the microstructure of lead-free solder alloys (e.g.,grain size,type and distribution of intermetallic compound particles,or defects such as dislocations).Sn Ag(a) SEM5 µm5 µmSnCu(b) SEM Fig.2.Element mappings of cross sections of (a)Sn–3.0Ag and (b)Sn–0.5Cu alloys.50607080902 theta (degree)β-Sn Ag 3Snβ-SnCu 6Sn 5patterns of (a)Sn–3.0Ag and (b)Sn–0.5Cu alloys.510152025303500.10.20.30.40.5Tensile strainT e n s i l e s t r e s s (M P a )Sn-3.0AgSn-0.5CuFig.4.Typical stress–strain curves of Sn–3.0Ag and Sn–0.5Cu of 8.33Â10À4/s.The strain was calculated from the displacement and the initial gauge length.144K.Yokoyama et al./Corrosion Science 86(2014)142–148For Sn–0.5Cu alloy shown in Fig.8(b),surface cracks were not observed even for a longer immersion time (120h).The changes in the topography of the side surface such as distortions of scratches due to SiC paper finishing mainly resulted from creep deformation.The effects of corrosion were only slightly observed in the SEM images.It is likely that 0.9mass%NaCl solution has no effect on crack nucleation for Sn–0.5Cu alloy under applied stress.The changes in the corrosion potentials of Sn–3.0Ag and Sn–0.5Cu alloys in the 0.9%NaCl solution without and under an applied stress of 9MPa are shown in Fig.9(a)and (b),respectively.For Sn–3.0Ag alloy,without the applied stress,the corrosion poten-tial gradually shifted from À0.6to À0.4V (vs.SCE)over 40–50h and then was stable.Under the applied stress,the corrosion poten-tial often shifted rapidly to approximately À0.45V (vs.SCE)after 10–20h.The time to the rapid shift of the corrosionpotential(a)crack nucleation(b)20 µmcrack propagationl o a d i n g d i r e c t i o n(c)l o a d i n g d i r e c t i o nSEM images of typical side surfaces of Sn–3.0Ag alloy (a)before and sustained tensile-loading test for (b)20h and (c)50h under applied stress mass%NaCl solution.K.Yokoyama et al./Corrosion Science 86(2014)142–148145appears to correspond to the time to crack nucleation.For Sn–0.5Cu alloy,the corrosion potential always shifted gradually from À0.6to À0.4V (vs.SCE)over 40–50h irrespective of the existence of applied stress.The cause of the increase in corrosion potential may be the change of the surface oxide film of the specimen and should be investigated in the future.The amounts of Sn,Ag and Cu ions dissolved in 0.9%NaCl solu-tion without and under an applied stress of 9MPa after immersion for various periods are shown in Fig.10.For Sn–3.0Ag alloy,the dissolution was enhanced by the applied stress.When the immer-sion time exceeded 10–20h,the amount of dissolved ions mark-edly increased under the applied stress.This immersion time corresponds to the time to crack nucleation as shown in Fig.7.For Sn–0.5Cu alloy,the increase in dissolution due to the applied stress was smaller than that for Sn–3.0Ag alloy,particularly,in the early stage of immersion.Moreover,the amounts of dissolution for Sn–0.5Cu alloy were smaller than those for Sn–3.0Ag alloy for the same immersion time.The amount of dissolved Cu ion varied widely without applied stress.These results are consistent with the SEM images (Fig.8)and changes in corrosion potential (Fig.9).In addition,the amount of dissolved Ag or Cu ions was nearly one-quarter of that of dissolved Sn ion.Thus,on the basis of elemental ratio for Sn–3.0Ag or Sn–0.5Cu alloys,the preferential dissolution of Ag or Cu probably occurred,similarly to the result of Sn–3.0Ag–0.5Cu alloy reported previously [12].One important finding of the present study is that the creep cor-rosion cracking of Sn–3.0Ag alloy occurs,similarly to that of Sn–3.0Ag–0.5Cu alloy [12],whereas no cracking of Sn–0.5Cu alloy was observed.This is supported by the fact that it is difficult to pre-dict cracking from only corrosion behavior without applied stress or creep behavior under atmospheric conditions.The behavior of creep corrosion cracking for Sn–3.0Ag alloy in the present study was very similar to that for Sn–3.0Ag–0.5Cu alloyreported previously [12].Although there are minor differences in the mechanism,the general mechanism of creep corrosion cracking for Sn–3.0Ag alloy appears to be similar to that for Sn–3.0Ag–0.5Cu alloy,as described previously [12].Therefore,the following discussion will focus on additional mechanisms of creep corrosion cracking.From the present result of no cracking of Sn–0.5Cu alloy,one of the reasons for the nucleation of surface cracks may be the corrosion potential between the Sn matrix and the intermetallic compound particles (i.e.,Ag 3Sn or Cu 6Sn 5).Tsao and Chen [16]demonstrated that the corrosion potentials of pure Sn,ingots of Cu 6Sn 5and Cu 3Sn,and pure Cu were approximately À0.83,À0.46,À0.31and À0.19V (vs.SCE)in 3.5%NaCl solution,respec-tively.With decreasing amount of Cu in the Sn/Cu system,the cor-rosion potential shifts from that of pure Cu in the less noble direction,thereby suppressing dissolution.On the other hand,the corrosion potential of pure Ag,compared with that of pure Cu,is generally located in the more noble direction.We have confirmed in a separate experiment that the corrosion potentials of pure Sn,Cu and Ag are approximately À0.55,À0.17and À0.05V (vs.SCE)in 0.9%NaCl solution under aerated conditions at room tempera-ture,respectively [17].The corrosion potential of Ag 3Sn appears to be located near that of pure Ag rather than that of pure Sn because of the large amount of Ag in the Sn/Ag system.Hence,it is likely that dissolution is enhanced for Sn–3.0Ag alloy,particu-larly at grain boundaries where Ag 3Sn is precipitated.In fact,Mori(a)20 µml o a d i n g d i r e c t i o n(b)SEM images of typical side surfaces of Sn–0.5Cu alloy (a)before and sustained tensile-loading test for 120h under applied stress of 9MPa in 0.9solution.146K.Yokoyama et al./Corrosion Science 86(2014)142–148et al.[18]concluded that the dissolution of Sn from Sn–3.5Ag alloy is accelerated by the presence of Ag.Song and Lee[19]and Wang et al.[20]also reported that the presence of Ag3Sn in Sn–Ag–Cu alloys accelerates the dissolution of Sn because of microgalvanic corrosion between the Sn matrix and Ag3Sn.Bui et al.[21]sug-gested that better corrosion-resistant lead-free solder alloys should have a lower Ag content.In the present study,under the applied stress in the solution, interactions between creep and corrosion probably exist.Under atmospheric conditions,the creep deformation was larger for Sn–3.0Ag alloy than for Sn–0.5Cu alloy(Fig.5).For Sn–0.5Cu alloy, the creep strain was smaller than the tensile strain,suggesting lit-tle effect of creep.As shown in Figs.8and9,the effect of applied stress,i.e.,creep,on the corrosion behavior of Sn–3.0Ag alloy is probably greater than that of Sn–0.5Cu alloy.Ding et al.[22] reported that for Sn–3.5Ag eutectic solder alloy subjected to in situ tensile tests at a low strain rate under SEM,the defects accu-mulated around secondary particles by a diffusion process,thereby causing cracking at grain boundaries.For Sn–3.0Ag and Sn–3.0Ag–0.5Cu alloys[12],it is likely that the multiplicative effect of creep and microgalvanic corrosion results in the nucleation of many cracks and their propagation at grain boundaries.For lead-free sol-der alloys,large creep deformation and a large electrochemical potential between the matrix and precipitates at grain boundaries appear to make the alloys prone to creep corrosion cracking.4.ConclusionsWe have demonstrated that for Sn–3.0Ag solder alloy,many surface cracks are rapidly nucleated by interactions between creep and corrosion in0.9mass%NaCl solution under applied stress in the same manner as for Sn–3.0Ag–0.5Cu solder alloy reported pre-viously.In contrast,for Sn–0.5Cu solder alloy,no surface cracks are observed and ordinary creep deformation occurs even in the solu-tion.The susceptibility to creep 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