SEM and EDX study of intermetallics in a copper-tin system and the oxidation behaviour of tin

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1 SEM nd EDX of intermetllis SEM nd EDX study of intermetllis in opper-tin system nd the oxidtion ehviour of tin Syed Nsimul Alm, Mns Kumr Mishr Deprtment of Metllurgil nd Mterils Engineering, Ntionl Institute of Tehnology- Rourkel, Rourkel, Oriss, Indi Introdution In the eletronis industry led-tin (P-Sn) solder is very importnt mteril ut P is toxi nd hs dverse effets on the environment nd humn eings. Due to the hrmful effets of P the use of the P-Sn solder lloys re eing voided nd new P-free solder lloys re eing used for eletroni pplitions. Severl lternte solder lloys re eing investigted nd their vrious properties re eing studied [1, 2]. Here we hve tried to understnd the vrious intermetllis tht re formed when P-Sn solder solidifies on opper-rih eletril ontt. The molten solder, whih is P-Sn lloy, forms intermetllis with Cu in the eletril omponent. At high tempertures the diffusion of Cu nd Sn inreses nd s result intermetllis of Cu-Sn re formed. The rittle nture of the intermetllis often leds to frture of the omponent. The ommon intermetllis formed re Cu 3 Sn nd Cu 6 Sn 5. The vrious Cu-Sn intermetllis formed during the solidifition of Cu-Sn lloys hving different ompositions were lso studied. Additionlly the mirostruture of n eletril wire mde of Cu-Sn lloy fused due to the pssge of high voltge ws studied [3-5]. Mterils nd Methods Smple preprtion Cu power nd Sn grnules were proured from RFCL Limited (RANKEM), Indi. To nlyze the intermetllis formed during solidifition of P- Sn solder on Cu-Sn eletril wire ommeril solder lloy ws wrpped round Cu-Sn wire nd heted to 500 o C for 1 h. The mirostruture nd vrious phses formed in the resulting Cu- 20wt.%Sn nd Sn-5wt.%Cu lloys were nlyzed. Metlli Sn ws oxidized in muffle furne t 1000 o C for 1 h nd 2 h in presene of ir in order to study the oxidtion ehvior of Sn. Snning eletron mirosopy nd energydispersive X-ry spetrosopy The mirostruture nd the formtion of intermetllis in vrious Cu-Sn lloys nd of P-Sn solder solidified on Cu-Sn eletril wire were studied using JEOL JSM-6480LV snning eletron mirosope (SEM) equipped with n INCAPentFET-x3 energy-dispersive X-ry (EDX) mironlysis system with high-ngle ultr-thin window detetor nd 30 mm 2 Si(Li) rystl. Results nd Disussion Metls used s eletril ontts nd solder The dverse effets of P on environment hs led to the need for new solder lloys in order to reple P-Sn solder lloys. Some of the lloys eing investigted to reple the P-Sn solder lloys re from the Sn-Ag-Cu system. It is importnt to understnd the Cu-Sn system for etter understnding of the Sn-Ag-Cu lloys. The solidifition of the molten solder is n importnt spet. Intermetlli ompounds suh s Cu 6 Sn 5, Cu 3 Sn nd Cu 5 Sn hve een found to form during solidifition of the molten Cu-Sn lloy. These intermetllis re rittle in nture nd filure is likely to tke ple in this region [6-8]. Eletril wires used in homes re often mde of Cu-Sn lloys. We hve lso tried to study the Cu-Sn lloy system. We hve exmined Cu-Sn eletril ondutor whih hd fused due to high voltge. Mirostruturl nlysis of the fused setion of the wire shows the formtion of rod shped intermetlli ompounds (Figure 1,). The rod-like strutures were only seen in the fused setion of the wire nd were not seen in other res of the wire. Figure 1 is the SEM imge Figure 1 () Snning eletron mirosope imge of n eletril wire ontining Cu nd Sn, fused y the pplition of high voltge. (, ) SEM imges of needle-like Cu-Sn intermetlli ompound in fused wire ontining Cu nd Sn. (d) EDX spetrum nd tle of elementl vlues of the needle-like strutures mrked ( *) in (). (e) EDX spetrum nd tle of elementl vlues of the needle-like strutures mrked ( *) in (). d e MirosopyndAnlysis Jnury

2 SEM nd EDX of intermetllis of the wire where rod-like strutures were not seen. The omposition of the rods in oth the EDX nlysis in Figure 1d nd Figure 1e suggests tht the possile intermetlli ompound is Cu 5 Sn. The intermetlli ompound ws formed when the ondutor ttined high temperture due to exessive flow of urrent through the wire nd then ooling in ir. Due to the exessive high temperture the wire lso melted. The rod-like struture of Cu-Sn intermetlli ompounds suh s Cu 6 Sn 5 hve lso een reported erlier. The size of the rods ws out 10 µm. The lrge perentge of oxygen is possily due to the oxidtion of the wire in ir t the high temperture. It ws shown from the EDX nlysis tht the tomi perentge of oxygen ws very high, whih indites tht possily some mount of Cu nd Sn ws oxidized. Cu-20wt.% Sn lloy To understnd the intermetllis tht were formed in the Cu-Sn system, the Cu-20wt.%Sn omposition ws hosen. It ws heted t 800 o C for 2 h nd ooled in the furne. At this temperture Sn ws ompletely melted s its melting point is o C wheres Cu remined solid s its melting temperture is 1083 o C. The SEM imges in Figure 2 revel tht there were regions whih were highly dense ontining oth Cu nd Sn. This is the intermetlli Cu 6 Sn 5, wheres in other res loose Cu prtiles ould e found. EDX of these res reveled 100%Cu nd the sene of Sn. From the EDX nlysis in Figure 2, the Cu:Sn stoihiometri rtio of the densely solidified region ws found to e Cu 6 Sn 5.79 nd Cu 6 Sn 6.83 in two different regions. This is very lose to the omposition of the intermetlli Cu 6 Sn 5. The SEM imge in Figure 2 shows the elongted shpe of the intermetlli Cu 6 Sn 5 seen in the mirostruture of the Cu-20wt.%Sn lloy. Cu 6 Sn 5 is very well known intermetlli of the Cu-Sn system nd hs n orthorhomi rystl struture. At 800 o C Sn is ompletely molten. Cu toms diffuse into Sn resulting in the formtion of the intermetlli. The molten lloy ws ooled inside the furne nd lower ooling rte hs een found to e fvourle in the formtion of the intermetlli ompound Cu 6 Sn 5. Cu 6 Sn 5 hs two rystl strutures. At high temperture Cu 6 Sn 5 hs hexgonl rystl struture wheres t lower temperture Cu 6 Sn 5 hs monolini struture. At room temperture the rystl struture of Cu 6 Sn 5 is monolini. The llotropi trnsformtion from monolini Cu 6 Sn 5 (h ), whih is stle t lower temperture, to hexgonl Cu 6 Sn 5 (h), stle t higher temperture, tkes ple t 186 o C. During ooling the hexgonl to monolini trnsformtion leds to volume hnge nd hene rking of the intermetlli Cu 6 Sn 5 tkes ple. Intermetllis like Cu 5 Sn nd Cu 6 Sn 5 ould e found t the Sn-Cu interfe. Cu diffuses into Sn to form the intermetlli ompound Cu 6 Sn 5. Grdully the soluility of Cu in Sn redues nd stops when Sn eomes supersturted with Cu. The exess Cu remins in elementl form round the intermetlli ompound Cu 6 Sn 5. After long time the diffusion of Cu will slow down nd the growth rte of Cu 6 Sn 5 phse will derese [6-11]. Sn-5 wt. % Cu lloy Sn-5wt.%Cu omposition ws lso hosen for study. It ws heted to 500 o C for 2 hours. The molten lloy ws ooled in the furne. Cu 6 Sn 5 () Cu rih re (, * ) (d) Intermetlli ompound Cu 6 Sn 5 (, ** ) (e) Intermetlli ompound (, * ) Figure 2 Cu-20wt.%Sn omposition heted to 800 o C for 2 h nd ooled in the furne showing the formtion of intermetlli Cu 6 Sn 5. (, ) SEM imges nd (, d, e) EDX nlyses of speifi res in these imges (indited y sterisks). Figure 3 () SEM imge nd (-) EDX spetr nd elementl vlues of Sn-5wt.%Cu omposition heted to 500 o C for 2 h. Figure 4 (,) SEM imges nd () EDX spetrum nd elementl vlues from mrked ( * ) region in () of Sn-5wt.%Cu omposition heted to 500 o C for 2 h showing the formtion of SnO 2 strutures in the solidified lloy. 8 Jnury 2013 MirosopyndAnlysis

3 SEM nd EDX of intermetllis ould e tred in the solidified lloy. There ws lso indition of the formtion of SnO 2 strutures in the solidified lloy. The SEM imge in Figure 3 shows some res where Sn ws found to e oxidized. The SEM imge in Figure 4 - lso suggests the formtion rod-like strutures of SnO 2. The EDX nlysis of Figure 3 suggests the formtion of intermetlli Cu 6 Sn 5. On the other hnd the EDX nlysis in Figure 3 of the few right regions showed tht Cu ws lmost sent. These rod-like strutures in this region were found to ontin signifint mount of O nd Sn. The EDX nlysis of the rod-like strutures suggests tht the tin oxide strutures were highly non-stoihiometri. SEM imges in Figure 4, lerly show the rod-like strutures of SnO 2 tht were formed y oxidtion of the Sn-5wt.%Cu smple due to the presene of ir in the furne. These strutures were not seen in the Cu-20wt%Sn omposition. EDX nlysis in Figure 2 shows some mount of Cu in these rod-like strutures [12-14]. Solidifition of P-Sn Solder on Cu The interfil retion etween opper nd the solder ws lso studied. A opper-rih wire ws rought into ontt with molten P-Sn solder. The molten solder ws llowed to solidify on the Cu wire in ir. Copper from the opper wire diffused into the Sn in the P-Sn lloy. A similr retion lso took ple when P-Sn solder ws used to solder opper-rih eletril omponents. P-Sn formed euteti system with omposition of Sn-37wt.%P nd the euteti temperture s 183 o C. This omposition hs the lowest melting temperture in the P-Sn system nd so the P-Sn euteti solder is very ommonly used. The EDX nlysis of the solder fter solidifition in ir nd the opper wire is shown long with the SEM imges in Figure 5 -d. It n e onluded from the EDX nlysis in Figure 5 tht Cu diffused into Sn in the solder only to the very low extent of 2.21 t.%. There ws no tre of the formtion of intermetlli ompounds etween Sn nd Cu. This ws possily due to the fst ooling of the molten P-Sn solder in ir so Cu did not get enough time to diffuse into Sn nd form n intermetlli with Sn. From the EDX nlysis given in Figure 5d it n e onluded tht in the interior portion of the solidified solder there ws no tre of Cu in the solder. The perentge of oxygen in the interior portion of the solidified solder ws round t.%. This lrge perentge of oxygen in the solidified solder suggests tht there is possile oxidtion of Sn nd P in the solder. There ws lso no tre of diffusion of Sn or P into Cu. The SEM imge in Figure 6 shows lerly the interfe etween the solidified solder nd the opper wire. From the EDX nlysis in Figure 6 we found lrge perentge of oxygen in the Cu wire, round t.%. The solidifition of molten solder on opper wire ws lso studied y melting ommeril solder lloy in furne nd solidifying the molten solder on the opper wire. A ommeril solder lloy ws wrpped round Cu wire nd heted to 500 o C for 1 h in ir in muffle furne. At 500 o C the solder is expeted to e ompletely molten. The melting temperture of P is o C nd tht of Sn is o C. The molten solder ws ooled in the furne. The SEM imge in Figure Figure 5 (, left) SEM imge of the solidified P-Sn solder on the Cu wire. (, elow) Shemti showing the solidifition of P-Sn solder on Cu wire. (, d) EDX spetr nd elementl vlues of the solidified P-Sn solder () in the interior of P-Sn solder (*) nd (d) ner the Cu wire (**). Figure 6 () SEM imge of the solidified P-Sn solder on the Cu wire () EDX spetrum nd elementl vlues of the Cu wire ner the solidified P-Sn solder (*). d MirosopyndAnlysis Jnury

4 SEM nd EDX of intermetllis 7 shows the surfe of the solidified solder. Crks ould e seen in the surfe of the solder. Figure 7 shows the region of ontt etween the solidified solder nd the opper wire. EDX nlysis in Figure 7 nd e suggests tht there ws diffusion of Cu into the solder. The perentge of Cu in the solidified solder ws found to e 4.71 t.% in the interior regions of the solder nd t.% ner the opper wire. There ws indition of oxidtion of oth the solder nd the opper wire. The EDX nlysis in Figure 7d suggests tht there ws possile formtion of tin oxide s there were regions on the solder where only tin nd oxygen ould e tred. The tomi perentge of oxygen in oth the solder nd the opper wire ws found to e very high [15]. d e f Oxidtion ehviour of tin Tin oxide (SnO 2 ) is mteril of high tehnologil importne. SnO 2 is n-type semiondutor hving wide nd gp. It hs optoeletroni nd gs sensing pplitions. Nnodevies developed from SnO 2 re eoming more nd more populr. Tin oxide is hemilly very stle. Here SnO 2 ws synthesized y oxidtion of tin in ir in muffle furne. The ojetive ws to understnd the oxidtion ehvior of Sn. Nnostrutured SnO 2 ould lso e seen in the oxidized smples [16-17]. Oxidtion of metlli Sn t 1000 o C for 1 h showed the formtion of tin oxide rods nd nnosturtures. The melting point of Sn is 232 o C wheres its oiling point is 2603 o C. At 1000 o C Sn is expeted to melt ompletely ut it will e in the liquid stte s it vporizes t muh higher temperture. The SEM imges in Figure 8, show the SnO 2 rods tht hve een formed y oxidtion of Sn in muffle furne t 1000 o C for 1 h in ir. Some of these SnO 2 rods hd lengths of µm. The EDX nlysis in Figure 8 shows tht the tomi perentge of oxygen in these tin oxide strutures ws very high. The tin oxide formed here ws not stoihiometri SnO 2. The tin oxide strutures were in the form of tringulr rods, whih hs lso een reported y others [18]. Even t temperture of 1000 o C nd holding time of 1 h the numer of tin oxide strutures ws found to e very low. Nnostrutures of tin oxide ould lso e seen in the smple. The SEM imge in Figure 9 shows tin oxide nnostrutures on the tin rih prtiles. These nnostrutures hd mximum length of round 3 µm. The EDX nlysis in Figure 9 shows tht the tomi perentge of oxygen ws very high in the tin oxide nnostrutures. Sn ws lso oxidized in ir in muffle furne t 1000 o C for 2 h. The SEM imge in Figure 10 of this oxidized Sn shows the sene of ny tin oxide mirostrutures. Oxidtion of Sn t 1000 o C for 2 h led to more stoihiometri tin oxide. EDX nlysis in Figure 10 shows tht the tomi perentges of Sn nd O were nd 69.43, respetively. This suggests tht with inresing time of oxidtion t 1000 o C the metlli Sn ould e onverted to stoihiometri SnO 2 [19-22]. Figure 7 (, ) SEM imges of ommeril solder wrpped round Cu wire nd heted to 500 o C for 1 h. (-f) EDX elementl vlues of vrious regions of the smple (mrked with the orresponding letter).. Figure 8 (, ) SEM imges of Sn oxidized in ir t 1000 o C for 1 h. () EDX spetrum nd elementl vlues of the rodlike strutures. Conlusions 1. Intermetllis suh s Cu 6 Sn 5 nd Cu 5 Sn ould e formed in the vrious Cu-Sn lloys. 2. Needle-like strutures of intermetlli Cu 5 Sn ould e seen in fused Cu-Sn lloy eletri wire. The elongted morphology of the intermetlli 10 Jnury 2013 MirosopyndAnlysis

5 SEM nd EDX of intermetllis Cu 6 Sn 5 ould e seen in the mirostruture of solidified Cu-20wt.%Sn lloy. 3. In the solidified Sn-5 wt.%cu lloy the formtion of Cu 6 Sn 5 intermetlli ould e tred. 4. Solidifition of P-Sn Solder on Cu eletril wires did not show ny formtion of intermetllis lthough there ws diffusion of Cu into Sn whih ould e seen in the solder in ontt with the opper wire. There ws no indition of diffusion of Sn into Cu. 5. Oxidtion of Sn in ir in muffle furne led to the formtion of SnO 2 strutures. Rod-like SnO 2 strutures ould e seen in the solidified Sn-5 wt. %Cu lloy. Tin oxide nnostrutures ould e formed when metlli Sn ws oxidized in ir t 1000 o C for 1 h. The SnO 2 tht ws formed ws highly non-stoihiometri. Tringulr rods of SnO 2 ould e formed. Oxidtion of metlli Sn for period of 2 h t 1000 o C led to the formtion of more stoihiometri SnO 2. Referenes 1. Wng, F., O Keefe, M., Brinkmeyer, B., Journl of Alloys nd Compounds 477(1 2): , Wood, E.P., Nimmo, K.L., Journl of Eletroni Mterils 23(8): , Lin, D., Wng, G. X., Srivtsn, T. S., Al-Hjri, M., Petrroli, M., Mterils Letters 53(4 5): , Nogit, K., Intermetllis 18(1): , Bkshi, V. U., Bkshi, U. A., Bsi Eletril Engineering, Tehnil Pulitions, Srisukho, P., Ruethitnnon, P., Wongpreedee, K., 37th Congress on Siene nd Tehnology of Thilnd. 7. Shen, J., Liu, Y. C., Go, H. X., Journl of Mterils Siene 42(14): , Feng,G., Hiroshi,N., Tdshi,T.,Trnstions of JWRI 34(2), Tsi, M.Y., Yng, S.C., Wng, Y.W., Ko, C.R., Figure 9 () SEM imge of Sn oxidized in ir t 1000 o C for 1 h () EDX spetrum nd elementl vlues of the rodlike strutures. Figure 10 () SEM imge of Sn oxidized in ir t 1000 o C for 2 h () EDX spetrum nd elementl vlues of the surfe. Journl of Alloys nd Compound 494(1 2): , Wng, K. K., Gn, D., Hsieh, K. C., Chiou, S.Y., Thin Solid Films 518(6): , Knhnomi, C., Mutoh, Y., Sript Mterili 50(1):83 88, Wei, C. C., Liu, P. C., Chen, C., Lee, J. C.B., Wng, I P., J. Appl. Phys. 102, Cho, S., Yu, J., Kng, S. K., Shih, D., Journl of Mterils of The Minerls, Metls nd Mterils Soiety 57(6): 50-52, Mour, I.T.L., Silv, C. L.M., Cheung, N., Goulrt, P. R., Gri, A., Spinelli, J. E., Mterils Chemistry nd Physis 132(1): , Anderson, I. E., Cook, B. A., Hrring, J., Terpstr, R. L., Foley,J. C., Unl, O., Mterils Trnstions 43(8): , Di, Z. R., Pn, Z. W., Wng Z. L., J. AM. CHEM. SOC. 124: , Guo, Z., Chen, X., Xu, W. H., Li, J., Yng, G. M., Li, M. Q., Liu, J. H., Hung, X. J., Mterils Tody 14(1 2):42 49, Pdmvthy, R., Rjendrn, K. V. AZojomo, (ISSN X) DOI: /zojomo Chen, Z.W., Du, J., Zhng, H.J., Jio, Z., Wu, M.H., Shek, C.H., Wu, C.M.L., Li, J.K.L., At Mterili 57: , Btzill, M., Dieold, U., Progress in Surfe Siene 79: , Lim, H.N., Nurzulikh, R., Hrrison, I., Lim, S.S., Tn, W.T., Yeo, M.C., Int. J. Eletrohem. Si. 6(9): , Johri, A., Rn, V., Bhtngr, M. C., Nnomter. nnotehnol. 1(2): 49-54, John Wiley & Sons, Ltd iogrphy Syed Nsimul Alm reeived BTeh in 1997 from the Indin Institute of Tehnology t Khrgpur, Indi, in metllurgil nd mterils engineering nd n MS in photonis nd eletroni mterils from the Chemil nd Nuler Engineering Dept t the University of Msshusetts, Lowell, USA. He did his PhD t the Dept of Metllurgil nd Mterils Engineering, Indin Institute of Tehnology t Khrgpur, Indi in the field of mehnil lloying. He is urrently n ssistnt professor t the Ntionl Institute of Tehnology-Rourkel, Indi. His interests re in nnomterils, X-ry diffrtion nd mirosopy. strt This study is minly sed on the intermetllis tht re formed in the Cu- Sn system. The im ws to understnd the solidifition of Cu-Sn lloys, the vrious intermetllis formed nd their morphology. The P-Sn solders re eing repled y the Sn-Cu, Sn-Ag or Sn-Ag- Cu lloys due to the hrmful effets of P in nture. Here the intermetllis formed etween Cu-Sn during solidifition of the molten solder on the Cu eletril ontts is lso studied. The intermetllis re rittle in nture nd this leds to frture of the solder. In order to understnd the Sn-Cu or Sn-Ag-Cu led free solder lloys it is essentil to is understnd the solidifition of the molten lloys. This is why few ompositions of Cu-Sn lloy re seleted nd the vrious intermetllis formed during the solidifition of the molten Cu-Sn lloys re nlyzed. Snning eletron mirosope (SEM) nd energy dispersive X-ry spetrosopy (EDX) hve een used to nlyze the vrious smples. Investigtions hve een done to find out the intermetllis tht re formed when the molten lloy solidifies. The oxidtion ehvior of Sn ws lso studied. knowledgements We would like to thnk the stff in the SEM nd XRD lortories t the Ntionl Institute of Tehnology-Rourkel, Indi. We would lso like to thnk everyone involved t the Ntionl Institute of Tehnology- Rourkel for their useful feedk nd help. orresponding uthor detils Dr. Syed Nsimul Alm, Metllurgil nd Mterils Engineering, Ntionl Institute of Tehnology Rourkel, Rourkel, Oriss Pin , Indi Tel: Emil: nsimullm@yhoo.om Mirosopy nd Anlysis 27(1):15-20 (AM), 2013 MirosopyndAnlysis Jnury