Thermodynamic Database for Phase Diagrams in Micro-Soldering Alloys

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1 1164 Journl of ELECTRONIC MATERIALS, Vol. 28, No. 11, 1999 Ohnum, Liu, Ohtni, Specil nd Issue Ishid Pper Thermodynmic Dtse for Phse Digrms in Micro-Soldering Alloys I. OHNUMA, X.J. LIU, H. OHTANI, nd K. ISHIDA Deprtment of Mterils Science, Grdute School of Engineering, Tohoku University, Aoym 02, Sendi , Jpn A thermodynmic dtse for the clcultion of phse digrms in microsoldering lloy systems, which include the elements P, Bi, Sn, S, Cu, Ag, nd Zn hs een developed using the CALPHAD method. The vrious thermodynmic prmeters for descriing the Gis energies of the different constituent phses hve een evluted y optimizing experimentl dt on phse oundry compositions nd thermochemicl properties such s ctivity, het of mixing nd enthlpy of formtion. The resulting dtse provides the mens wherey the liquidus nd solidus surfces, isotherml nd verticl section digrms, phse percentges nd the mole frction of the phse constitutions etc., in multicomponent soldering lloys cn e redily clculted. Relted informtion such s the surfce tension nd viscosity of the liquid phse cn lso e predicted, thus rendering the dtse s vlule tool for developing ledering nd led-free solders. Key words: Phse digrms, dtse, thermodynmics, P-ering solders, P-free solders INTRODUCTION Recent progress in the re of surfce mounting technology directed towrds improvements in the field of interconnection nd pckging of modern electronic components nd devices hs creted need for the development of vrious microsoldering mterils. Current mnufcturing technologies re centered round the P63mss%Sn eutectic lloy with 183 C s eutectic melting temperture. Aprt from the demnds for development of lloys with melting tempertures different from the ove, dditionl requirements rising from the environmentl nd helth issues concerning the toxicity of P, hve necessitted the development of new P-free solders. Design nd development of such P-free solders cn e significntly speeded up y the vilility of relile thermodynmic dtse for the prediction of phse equiliri, clcultion of liquidus, solidus nd equilirium compositions, volume frctions of constituent phses etc. in the multi-component lloys concerned. The present pper descries the recent progress chieved in the cretion nd vlidtion of dtse (Received Mrch 3, 1999; ccepted My 5, 1999) for clcultion of phse digrms in the micro-soldering lloy systems, whose min component elements re P, Bi, Sn, S, Cu, Ag nd Zn. The dtse hs een constructed y the CALPHAD (CALcultion of PHAse Digrms) method. 1 5 CALPHAD METHOD The sics of the well-known CALPHAD method hve een descried in previous ppers 1 4 nd ooks. 5 The outline scheme of the CALPHAD method is shown in Fig. 1. The Gis energies of the liquid nd solid solution phses re descried y the regulr solution model, where the Gis energy of phse in the A-B-C ternry system, for instnce, is expressed s G = G A x A + G B x B + G C x C + RT(x A ln x A + x B ln x B + x C ln x C ) + L AB x A x B + L BC x B x C + L AC x A x C + L ABC x A x B x C (1) where G i is the Gis energy of the pure component i in the respective reference stte, x i the mole frction of component i nd L ij temperture nd composition dependent interction energy. The Gis energies of pure component i in its different phse sttes; i.e., 1164

2 Thermodynmic Dtse for Phse Digrms in Micro-Soldering Alloys 1165 Tle I. Thermodynmic Assessments of Ternry Systems Experimentl Experimentl System Informtion System Informtion Fig. 1. Scheme of CALPHAD method. lttice stility prmeters for i, re tken from the SUITE (Scientific Group Thermodt Europe) 6 dtse. The Gis energies of intermetllic phses with some soluility rnge such s the β (SnS) nd the γ (Ag 5 Zn 8 ) phses re descried y the sulttice model. 7 According to this model, the β(sns) phse, for exmple, is divided into two sulttices, I nd II, where the sulttice I is occupied predominntly y S toms nd the sulttice II predominntly y Sn toms; i.e., (S,Sn) I (Sn,S) II. The Gis energy of the β phse is expressed y G = β G yi yii + β G yi yii + β G yi y S: S S S Sn: Sn Sn Sn S: Sn S + β G yi yii RT yi yi yi yi Sn: + ( ln + ln S Sn S S S Sn Sn II II II II ex + y ln y + y ln y ) + G S S Sn β where G ij : is the Gis energy of the hypotheticl β phse in which ll the sites in sulttice I re occupied y the element i nd ll the sites of sulttice II re occupied y the element j. The site frction of elements in ech sulttice is denoted y y i. G ex is the excess energy term, descried y n eqution similr to Eq. 1, consisting of pproprite interction energy prmeter terms. The quntity G ex in some phses, however, where the homogeneity rnge is rther limited cn e tken to e equl to zero. The phse equiliri t high pressures cn lso e clculted in some lloy systems. The Gis energy chnge of element i t pressure P is given y ( )= ( )+( ) φ1 φ2( x T) Sn II n (2) φ1 φ2 φ1 φ2 φ1 φ2 G x, T, P G x, T P P V (3) i i 0 i where G, is the Gis energy difference i φ1 φ etween φ 1 nd φ 2 phses, V 2 i the difference in molr volume of the element i etween φ 1 nd φ 2 phses, nd P 0 the tmospheric pressure. The required interction energy prmeters for Ag-Bi-Cu none Bi-Cu-Zn mny Ag-Bi-P few Bi-P-S mny Ag-Bi-S none Bi-P-Sn mny Ag-Bi-Sn mny Bi-P-Zn mny Ag-Bi-Zn few Bi-S-Sn mny Ag-Cu-P mny Bi-S-Zn few Ag-Cu-S few Bi-Sn-Zn mny Ag-Cu-Sn mny Cu-P-S few Ag-Cu-Zn mny Cu-P-Sn none Ag-P-S mny Cu-P-Zn mny Ag-{-Sn mny Cu-S-Sn mny Ag-P-Zn mny Cu-S-Zn few Ag-S-Sn few Cu-Sn-Zn mny Ag-S-Zn none P-S-Sn mny Ag-Sn-Zn mny P-S-Zn mny Bi-Cu-P few P-Sn-Zn few Bi-Cu-S few S-Sn-Zn few Bi-Cu-Sn none Bi-In-Sn mny In-S-Sn mny inputting into the Gis energy expressions re evluted y optimizing experimentl dt on phse oundry compositions nd thermochemicl properties such s ctivity, het of mixing nd enthlpy of formtion s shown in Fig. 1. THERMODYNAMIC DATABASE The present thermodynmic dtse developed for micro-soldering lloy systems contins the relevnt thermodynmic prmeters for clculting the phse equiliri in unry, inry nd ternry lloy systems consisting of seven elements P, Bi, Sn, S, Fig. 2. Contents of thermodynmic dtse for micro-soldering lloys.

3 1166 Ohnum, Liu, Ohtni, nd Ishid Fig. 3. Clculted () stle phse digrm; nd () metstle liquid/(p) phse equiliri in the P-Sn inry system. Cu, Ag nd Zn. Thermodynmic ssessments of some lloy systems contining In re lso included in the dtse. For some importnt lloy systems for which there ws little or no experimentl phse oundry dt, experimentl work for the determintion of phse equiliri such s liquidus, solidus, isotherml nd verticl sections, etc. ws crried out y DSC (differentil scnning clorimetry), x-ry diffrction nd EDS (energy dispersion x-ry spectroscopy) mesurements. Since mny of the previous references on the experimentl determintions of phse digrms in micro-soldering lloy systems were for work efore the 1950s, dditionl experimentl work ws undertken in severl lloy systems to check the reliility of the experimentl phse equiliri reported in these erlier references. This reconfirmtion of the greement etween the clculted nd the oserved phse equiliri ws required to otin etter estimtion of the thermodynmic prmeters. Tle I shows the ternry lloy systems tht were thermodynmiclly ssessed in the present work. The evluted thermodynmic prmeters hve een rrnged within the frmework of the Thermo-Clc softwre, which ws originlly developed y Sundmn et l. 8 This thermodynmic dtse cn e used to otin informtion relting to the following: stle nd metstle phse equiliri, the position of the T 0 line, volume frctions of the phse constituents under specified conditions, vlues of vrious thermodynmic quntities such s ctivity, mixing enthlpy, Gis Fig. 4. () P-Sn inry phse digrm t 2.5 GP; nd () effect of pressure on phse oundries t 85.5 t.% Sn.

4 Thermodynmic Dtse for Phse Digrms in Micro-Soldering Alloys 1167 Fig. 5. Verticl section digrms of the P-Sn-S system t () 10 t.% Sn; nd () 30 t.% P. energy of formtion, driving forces for phse trnsformtion etc., s illustrted in Fig. 2. In ddition, physicl properties such s surfce tension nd viscosity in the liquid stte, which re very relevnt to soldering, cn lso e clculted from the dtse. Tnk nd Hr 9 hve exmined the prediction of surfce tension nd viscosity in liquid lloys y numer of uthors. Tnk nd Iid 10 hve developed successful formlism sed on the Butler s theory 11 for clculting the surfce tension, while Seethrmn nd Sichen 12 hve shown tht viscosity cn e estimted using their recent model. Both models re sed on the correct estimtion of the Gis energy of the liquid phse. EXAMPLES OF CALCULATION P-Bering Solders The P-Sn se lloys re extensively used in soldering, ecuse of their low cost, good mechnicl properties, wettility, deformility, etc. The stle nd metstle phse equiliri in P-Sn inry lloy re shown in Fig The formtion of the metstle two-phse seprtion in the fcc phse cn e forecst y oserving the shpe of this phse oundry. Actully, misciility gp is predicted to exist s shown y the chin line in Fig. 3. Figure 4 shows the phse equiliri t high pressures, where the ppernce of stle re for the high-pressure ε phse is Fig. 6. Clculted liquidus surfce in the () P-Bi-Sn; nd () P-Bi-S systems.

5 1168 Ohnum, Liu, Ohtni, nd Ishid Fig. 7. () Clculted verticl section digrm of the P-Sn-20mss%Bi-10mss%S lloy; nd () effect of S ddition on the P-Bi-Sn liquidus projection. Fig. 8. () Liquidus surfce; nd () the phse frction vs. temperture plot in the Sn-Ag-Zn system. shown in the composition rnge of 70 ~ 80 t.% Sn. Antimony is n importnt dditive for P-Sn solders which prevents emrittlement cused y the low temperture trnsformtion of Sn. Figure 5 shows the clculted verticl section digrms for the P- Sn-S system, superimposed with experimentl dt to confirm the vlidity of the clcultion. 13 The greement etween the clculted nd oserved trnsitions is quite stisfctory. The other importnt ternry P-se soldering lloys elong to the P-Bi-Sn nd P-Bi-S systems, whose liquidus surfces re shown in Fig. 6. The eutectic point of the P-Bi-Sn system t 92 C is denoted y E 1 in Fig. 6. This lloy system is clssified s soft solder in prcticl pplictions. The thermodynmic ssessments of the P-Sn-S, P-Bi-Sn, P-Bi-S, nd Sn-Bi-S ternry systems enle the clcultion of phse equiliri in the P- Sn-Bi-S quternry system. Since there is very little phse digrm informtion for this system, the phse oundries were experimentlly determined y DSC mesurements. Figure 7 shows the comprison etween the clculted nd the oserved phse equiliri in verticl section of P-Sn-20mss%Bi- 10mss%S lloy. The clcultions re sed only on the thermodynmic prmeters of the four component ternry systems without inclusion of n dditionl quternry interction prmeter. The fct tht very good greement etween experiments nd

6 Thermodynmic Dtse for Phse Digrms in Micro-Soldering Alloys 1169 Fig. 9. () Liquidus surfce in the Sn-Bi-Ag system; nd () liquidus projection in the Sn-Bi-In system. Fig. 10. () Verticl section digrm of the Sn-Bi-Zn system t 70 mss% Sn; nd () the ctivitiy of Zn in the liquid phse. clcultion exists, demonstrtes tht the CALPHAD method is n extremely useful tool in the construction of complex multicomponent phse digrm, when no experimentl informtion is ville in higher order systems. Figure 7 shows the chnge in the liquidus projection when smll mount of S is dded to the P-Bi-Sn system. The primry β phse field ppers in the neighorhood of the P-Bi-Sn ternry eutectic point, nd the eutectic melting temperture of the lloy rises steeply. The ddition of S promotes the formtion of the intermetllic β phse compound long with Sn. Precipittion of such hightemperture phse in the solder results in loss of fluidity nd the emrittlement of the solder, which explins the empiricl oservtion. P-Free Solders Interest in P-free solders hs gthered momentum in recent times nd mny P-free solder lloys hve een proposed. 14 But none of the proposed lloys, mostly inry nd ternry ones, hve een le to stisfy ll the required mteril properties such s melting temperture, mechnicl properties, cost, mnufcturility, etc. Significnt efforts hve een mde to design Sn-se solders ecuse they hve rnges of melting tempertures similr to those of P- Sn solders. In view of this, the Sn-Bi-X, Sn-Ag-X, Sn- Zn-X, Sn-S-X nd Sn-In-X systems might e considered s ses for the new system of P-free solder lloys. Severl exmples of the phse digrms nd

7 1170 Ohnum, Liu, Ohtni, nd Ishid Fig. 11 () Verticl section digrm of the Sn-In-S system t 60 t.% Sn; nd () the enthlpy of mixing in the liquid phse t 663 C. Fig. 12. () Liquidus surfce; nd () verticl section digrm of the Sn-Bi-S system t 80 t.% Sn. thermodynmic properties of these Sn-se systems clculted from the dtse re shown. Figure 8 shows the liquidus surfce of the Sn-Ag- Zn system, where the ternry eutectic point is locted t 216 C, t composition Sn-4mss%Ag-1mss%Zn. 15 Figure 8 shows the vrition of the phse frction with temperture in this eutectic lloy. Figure 9 shows the liquidus surfce in the Sn-Bi-Ag system. Figure 9 shows the liquidus projection in the Sn-Bi- In system. The Sn-Bi-In lloy, which hs two ternry eutectic points, E 1 nd E 2 t 58 C nd 81 C, respectively, s shown in Fig. 9, 16 is suitle for use in system tht requires low melting processing. Figure 10 shows the clculted verticl section digrm 17 for the 70mss%Sn-Bi-Zn lloy system long with the experimentl dt 18 for comprison. The thermodynmic ctivities of components in the liquid phse in this system re shown in Fig. 10, where the numericl vlues re the recommended ctivity dt of Zn sed on EMF meusrements. 19 Figure 11 shows the clculted verticl section digrm for the 60t.%Sn- In-S system. DSC mesurements were conducted on lloys of this system to confirm the reliility of the dtse. 20 Figure 11 shows the comprison of experimentl nd clculted enthlpy of mixing in the liquid phse of the Sn-In-S system. 21 Figure 12 shows the clculted liquidus surfce nd the verticl section phse digrm t 80t.%Sn 22 for the Sn-Bi-S

8 Thermodynmic Dtse for Phse Digrms in Micro-Soldering Alloys 1171 Fig. 13. () Viscosity; nd () surfce tension in the Sn-Bi-S system t 900 K. system. The melting temperture of compositions in the region with smll mount of S re lmost equl to tht of the eutectic lloy of the P-Sn system currently in use, which mens tht these lloys could e strong cndidtes for use s P-free solders. The viscosity nd surfce tension of Sn-Bi-S lloys in the liquid stte estimted, using the recently proposed clcultion models 9,10,12 nd the Gis energies of the liquid phse from the thermodynmic dtse, re shown in Fig. 13. This kind of informtion would e useful for ssessing the melting ehvior nd the mnufcturility of solders from this system. CONCLUSION Recent progress in the updting nd vlidtion of the dtse for clcultion of phse digrms in micro-soldering lloy systems, which include the elements P, Bi, Sn, S, Cu, Ag nd Zn is presented. In ddition to using erlier experimentl informtion, dt generted from experimentl determintions of phse equiliri in severl inry, ternry nd quternry systems y DSC, EDS nd x-ry techniques hve lso een included in creting the dtse. This dtse hs een constructed y the CALPHAD method nd contins informtion, not only for clcultion of phse digrms ut lso for extrcting thermodynmic informtion such s ctivity, het of formtion enthlpy of mixing, surfce energy etc. Severl phse digrms of the P-ering nd P-free solders hve een presented. This dtse cn e used s powerful tool for developing new P-free solders. ACKNOWLEDGEMENT The uthors wish to thnk K. Okud, H. Somet, H. Ymzki, Y. Ishiwt, K. Urushiym, M. Miyshit, S. Ishihr, Y. Inohn, nd D.V. Mlkhov for their help with the experimentl works nd clcultions. They lso wish to thnk Dr. L. Chndrsekrn of DERA, U.K. for helping in the presenttion of the mnuscript. The support from Csio Science Promotion Foundtion is lso cknowledged. REFERENCES 1. T. Nishizw, Mter. Trns. JIM 33, 713 (1992). 2. U.R. Kttner, JOM 49, 14 (1997). 3. H. Ohtni nd K. Ishid, Thermochimic Act 214, 69 (1998). 4. K. Ishid nd H. Ohtni, Computtionl Mterils Design, ed. T. Sito (Heidelurg, Germny: Springer-Verlg, 1999). 5. N. Sunders nd A.P. Miodownik, CALPHAD (Lusnne, Switzerlnd: Pergmon, 1998). 6. A.T. Dinsdle, CALPHAD 15, 317 (1991). 7. M. Hillert nd L.I. Stffnsson, Act Chem. Scnd. 24, 3618 (1970). 8. B. Sundmn, B. Jnsson, nd T.O. Anderson, CALPHAD 9, 153 (1985). 9. T. Tnk nd S. Hr, Mteri Jpn 36, 47 (1997) (in Jpnese). 10. T. Tnk nd I. Iid, Steel Reserch 65, 21 (1994). 11. J.A.V. Butler, Proc. R. Soc. Lond. A135, 348 (1932). 12. S. Seethrmn nd D. Sichen, Metll. Mter. Trns. B 25B, 589 (1994). 13. H. Ohtni, K. Okud, nd K. Ishid, J. Phse Equiliri 16, 416 (1995). 14. J. Glzer, Intern. Mter. Rev. 40, 65 (1995). 15. H. Ohtni, M. Miyshit, nd K. Ishid, J. Jpn. Inst. Met. 63, 685 (1999). 16. S. Ishihr, H. Ohtni, nd K. Ishid, Astrcts of the Jpn. Inst. Metls 121, 386 (1997). 17. D.V. Mlkhov, X.J. Liu, I Ohnum, nd K. Ishid, to e sumitted to J. Phse Equiliri. 18. S.D. Muzffr, J. Chem. Soc. 125, 2341 (1923). 19. W. Ptk nd Z, Moser, Bull. Acd. Polon. Sci. 19, 1 (1971). 20. S. Ishihr, H. Ohtni, T. Sito, nd K. Ishid, J. Jpn. Inst. Met. 63, 695 (1999). 21. B. Gther, P. Schroter, nd B. Blchnik, Z. Metllkde. 76, 523 (1985). 22. H. Ohtni nd K. Ishid, J. Electron. Mter. 23, 747 (1994).