THERMAL PROCESS OF VACUUM FLUXLESS LASERSOLDERING AND ANALYSIS ON SOLDER SPREADING AND WETTING

CHINESE JOURNAL OF MECHANICAL ENCINEERINGVolume 13 Number 22000127THERMAL PROCESS OF VACUUM FLUXLESS LASERSOLDERING AND ANALYSIS ON SOLDERSPREADING AND WETTINGOWang Chunqing Li Mingyu Sun FujiangFeng WufengNational key laboratory of welding, Harbin Institute of TechnologyAbstract Cn order to study the mechanism of yacuum fluxless soldering on the conditors of laser heating, themethod of measuring tempenature by the themnocouple is used to analyze the spreading and wetting proces ofboth fluxless Sn-Pb solder in the vacuum suroundings and fux Sn- Pb solder on Cu pad. Solder spreading andwetting aete by the soldering themal process is also discused acoding to the thermodymamics principle.Resuls show that vacuum Qluxless soldering demands higher temperature, and the fall of the solder surfaceension is the important factor achieving fluxless laser solderning.Key words: Fluxless soldering Soldering hermal process Spreading and wetting0 INTRODUCTIONIn order to protect environment, freon is forbidden to be applied in welding field. However it isimpossible for Aux remains on joints soldered to be completely cleaned in soldening process for superfinematerial. So it will erode the joint metal and badly influence joint reliability of products. Recently, as thefluxless soldering process without cleaning ater soldering can assure the joint reliability, it has beenregarded'.At present, joint perfomnance between solder and soldered metal surface is mainly studied in fluxlesssoldering methods by means of both protecting the joint metal with inert atmnosphere from being re-oxidizedand wiping of the oxide on the matrix surface oxide in the soldering, which are key problems forachieving fluxless soldering. The fluxless soldeing has been achieved in the surface mount technologyfield, but the process and mechanism need further studying.In this thesis, the laser soldering process for fhuxless Sn- Pb solder in vacuum suroundings is studiedby comparing the spreading and wetting behavior on Cu pad with that of flux Sn-Pb solder. Meanwhile ，spreading and wetting mechanism of the fluxless Sn- Pb solder is studied by analyzing Ihe themal pocessof the solder spreading and welting on Cu pad.1 EQUIPMENT AND TEST METHOD中国煤化工1.1 Vacuum equipmentHCNMHGThe re-oxidation of the melt solder surface in Sorucng pwis Betuy pitents solder from spreadingand wetting on the soldered metal surface. In order to avoid making oxide on the metall surface in thesoldering process ，vacuum laser soldering equipment is designed, which is shown in Fig. 1a.JThis project is supperted by National Natural Science Foundation of China (59671063). Manuseript received on March25，999; rerised manwserip July 15, 1999Wang Chunging et al : Themal process of vacur fuxless laser soldering and analysis1280n solder spreadirng and weting1.2 Test methodOxides of solder and Cu pad are wiped off by mechanical ways, wastage on the surface of metal iscleaned before soldering. Then pads and solder are put into vacuum room. YAG laser is used as heatingsource for solder to in heating solder to spread and wet on Cu pad. Sizes of component and the pre-establishment way of solder are shown as Fig. lb.YAG LeserSubtratea=b= 4 mmPadSolder massm,= 16 ngLensQuartz glnss ISolder ballDeviceVacunin room(a)Fig.1 Equipment and device of fluxless Laser solder in vacum protection2 MATERIAL OF EXPERIMENT AND COMPOSTON OF SOI DERCopper is selected as matenal of the pad during the test of solder wetting and spreading undervacuum protection. 32Sn 68Pb solder is selected as the material to be soldered. According to the alloyphase diagram of the eutectic solder, it is known that the liquid phase temperature of 32Sn-68Pb solder isabout 275 C. Therefore 32Sn 68Pb solder can be completely melted at 275 C .3WEITING AND SPREADING EXPERIMENT OF SOLDER AND RESULTSWetting and spreading test for flux Sn-Pb solder on the Cu pad in vacum Toom in Imade to study theheating process in which fuxless laser soldering is compared with flux soldering. Perfect weldingparameters have been found from the results of the test. The spreading and wetting results with differenthealing parameters are shown in Fig.2. The vacuum degree is atmosphere and the laser power is 10 w .The solder can not completely spread and wet on the pad when the laser heating time is 0.7 s, andthe spreading and wetting area is yery smnall. It is shown in Fig. 2a. When the laser heating tirne is 0.8s, the spreading and wetting area increase. It is shown in Fig.2b. But when the laser heating time isadded up to 0.9s and 1.1 s, the change of the spreadins and wetting Area is ery lttle, which is shown中国煤化工in Fig.2c and Fig. 2d.I input energy of the laser is overfull duringMHCNMHGesultinbeingapttoforminter-mnetallic compound in the interface, which is more britle, and will influence the reliability of thejoint. So on the conditions of melting solder wet perfectly, Lhe less is energy of the laser input, the betteris the test esult of the spreading and welting. It is proved that the suitable heating parameters of fluxsolder laser soldering are P = 10 W, t =0.90 s.Result of fluxless Sn-Pb solder spreading and wetting test on Cu pad, which is cmied out under vacumCHINESE JOURNAL OF MECHANICAL ENGINEERING .，129，protection (0.06 Pa) with heating parumeterP= 10 W, I =0.90s，has been shown inFig.3a. Sn-Pb solder just begin to melt butcan hardly wet on Cu pad. The result showsthat the ability of the spreading and wettingof fluxless solder is quite different with thatof flux. solder. Flusless sokder can moth wetunder the same heating condition whileunder which flux solder can perfectly wet.Then heating time is increased and the result(a) P=10W 1=0.70s(b) P=10W t=0,80 sof the spreading and wetting is obeenved.When heating time reaches 1 s，fluxlesssolrer spread and wet slightly, but far fromideally, as shown in Fig. 3b. When heatingtime reaches 1.05 s，the wetting conditionchanges obviously and almost reach the bestspreading that flux solder can reach, shownas Fig.3c. While continuing to increaseheating time to 1.1 s, the pad will be bumt(c) P=10W r=0.90。《d) P=10W t=1.10sdue to overfull energy input of laser. Sosuitable parameters of fluxless laser solderingFig.2 Result of flux solder wetting testare P=10W, 1=1.05 8.(a) P=10W 1=0.90 s(b) P=10W t=1.00 s(e)P=10W 1=1.05sFig.3 Resalt of fluules solder wetting testBy analyzing the resuls of the previous test, it can be found that Sn-Pb solder can perfectly wet andfluxless soldering can be accomplished under vacuum protection. But input energy must be inereased inorder to promote pefecly wetting on the pad, becaus中国煤化工Tain the same result asflux solder do under the same condition.:CTHCNMHGSolder melts under the ffect of laser firstly and Then accomplishes the process of solder wetting onthe pad by the surface tension between melting solder and Cu pad. Under the condition of same material，the temperature is more important than others to influence the surface tension. Hence, in order to studythe mechanisrm of solder spreading and welting protected by vacuum, ollowing studies on the heatingproess of万布数据ting in the prerious test are needed.Wang Chunqing et al:Thermal pnes of vacum fuxles laser soldering and analyslis130●on solder spreading and wetting4 INSPECTION AND ANALYSIS OF THE HEATING PROCESS OF FORMINGSOLDERING JOINT4.1 Method to inspect joint temperature4500rThe temperature variation of laser soldering旨3600joint is inspected based on the mechanism ofthermoouple to inspect temperature. Because the2 700-size of the joint inspected is small, the hread ofNi and Cu whose diameter is 0. 1 mm are applied1 800-to make up of thermocouple. The signal outputfrom thermocouple is amplified by amplifier and900-then input into memorial oscillograph ( TektronixTDS340A). Fig.4 shows the signed result of0120240 360480 600amplified signal output, which corresponds to theTemperature 0/Cvariation of temperature from thermocouple.Fig.4 Siged resul d output sigul by the temooupdle4.2 Results of inspecting temperature of jointFirst, the heating process of fux solder wting is inspected. In order 10 compare with fluslessheating process, the heating paraneters are P=10W, 1=1.008. .Fig.5 is the inspecting result of the heating procss of solder spreading and weting. From Fig.S, ican be seen that solder wet perfecly. The process of the variation of the temperature during solderspreading and weting can be divided ino three parts, AB， BC and CD. Part AB shows that thetemperature increases rapidly under the efee of laser heating source. When it reaches the melting pointof slder, the tempernture changes slowly to the efe of phase tansfom, which makes part BC lok likea platform. After the solder mels completely, the temperature increases mapidly again, as shown by partCD. The highest temperature of the flux laser sldering can reach is 280 C.3r2D1B(a) Result of solder spreading and wetting中国煤化工:iation curveMHCNM HGfig.5 Heating process of fir solder spreating and wettingFlux slder heaing process shows hat the tempenature variation speed of flux laser soldering jint isslow. When the heating lempenture in over solder liquid phase temperature, ideal sprading and wtting resultcan be ach方片数据at is, solder can well spread and wet on the metal suface under lower tempernture.The heatng process of fluxless solder spreading and wetting is also inspected, and the result isCHINESE JOURNAL OF MECHANICAL ENGINEERING●131●shown in Fig.6. Laser heating parameters are P=10w, l= 1.05 s, vacuum degree is 0.06 Pa.2Sampling time t/s(a) Result of solder spreading and wetting(b) Solder tenperature variation curveFig.6 Heating process of fluxless solder spreading and wettingIn can be seen from Fig.6a that the solder shows good spreading and wetting ability on the padsurface. Fig.6b shows the temperature variation process of solder spreading and wtting. The temperatureof fuxless solder heated rises shaply, and shows no phase trunsformn temperature variation process. Thesolder temperature is abouu 450 C while solder perdetly spreads and wets on the ped. The temperatureinspecting result shows that the temperature of fluxless laser sldering joint varies sharply, which is muchhigher than the solder melting point at the end of sodering. That is, it nceds higher temperature underthe fluxless condition to make solder perfecly spread and wet on the pad surface.Following conclusions can be drawn from all tests. Linder the same laser heating condition (heatingtime is longer when flux solder is used)，the temperature of flux laser soldering is lower, during soldermeling process the temperature varies sharply. The reason is that when laser heats the solder, the lowmeling point fhux melts mpidly and vltilies, it takes away part of the calorie and prevents thetempenature from rising. It is also found from tests that the melting solder surface is very bright with theprotecion of flux，then it intensively elcts the laser beam. Thus the laser enengy rally aborbed bysolder is small. On the contrary, when fluxless solder is used, the relectivity of solder is lower. So thetemperature rises sharply and the soldering temperature is higher.5 EFFECT OF FLUXLESS SOLDERING HEATING PROCESS ON SOLDER SPREADINGAND WETTINGAccording to the test rsuls of spreding and wetting and analyis result of solder joint heating process,the tempernture of fules solder joints must be higher than that of fux ones s0 as to achieve the perfe:spreading and wtting cpbilty of Sn-Pb solder on the Cu ped. According to the Young equation, thespreadability and wetability mainly lie in the quantityslder and Cu.中国煤化工’During the soldering procss, the temperatureMHCNMHGit can be treated as nophase transfom. Thus, the surface lension variationlitle ffct on spreadingand weting process of solder. The inluence of vacum degree and temperature on the surface lension ofSn-Pb solder is analyred as follws.5.1The infuence of vacum degree on solder surface tensionFrom内方数据dynamics point of view, the liquid surface tension is the surface Gibbs free enengy,namelyw ang Chunqing et al: Thermal process of vacuurm fluxless Laser soldering and analysis132，m solder spreading and wettings = G.= H,-0S。(1)where σ 一Surface tension G3Surface Gbbs free energy0- TemperatureS.- Surface entropy H,-- Surface enthalpyFor liguid(aG)a0=-S(2)U,aHk(3)U, is surface themmal-dynamic energy, soU:= σ-(韵)(4)Namely, when temperature is steady, the relation of U, and σ is linear .The increment of system energy per area ergendered from interface of gas and liquid isAU,=一π(n一n)FA<5)24 2where n] 一Volumic nunber of molecules in liquidng Areic number of molecules in the vapor phaseBalarced distant of liquid molecularConstantObviously, ng decreases in vacuum due to the reduction of gas molecular. Under this condition(n; - ng) is greater than that in atmosphere , so system energy increases, and the surface Lension of theliquid solder nises conespondingly. So Fig.3a shows soldering in vacum with flux and jdeal heatingparameters, the solder can hardly spread and wet on the Cu pad. In order to make the solder oD the padwell spread and wet the surface tension of solder must be lessened.5.2Influence of temperabure on the surface tensionFrom the view of molecular thermal dynamics, with the increasing of the solder temperature, thethenal movenent of molecular in the system aggrayates, the distance between moleculars inereases, andthe density decreases. So the pull to the moleculars at surface is lessened .The half- experienced formula of the surface tension of the liquid metal yersus temperature isσo(1 - 0/0.)"(6)where σ0 一Surface tension when 0 = 0Exponent (about 1 in liquid Imnetal)Critical temperature ( at this tine the interface disappearsAs what can be seen, the surface Lension of the liquid solder descends when the temperatureascends. Therefore in vacuum, using a stronger laser can make the Sn-Pb solder spread on the Cu padwell and then wet, So fluxless soldering may be achieved中国煤化工Through the themal dynamic analysis above it car-acum fluxless solderingthat the vacum condition can forbid re-oxidation of theMHCNMHprcss.Itcaninecrasethe effect of the surface tension of the liquid solder so much that the spreading and wetting on the Cu pad isharmed. So in order to obtain good spreading and wetting behavior of Sn-Pb solder on the Cu pad, the inputenengy of the laser heating source must be added. At this time the temperature of the melting metal is farbeyond the melting point of the solder and reaches 450 C. Al higher temperature he sunface tension of theliquid solder decreases, which is enough to improve the spreading and wetting of the solder on. the pad.CHINESE JOURNAL OF MECHANICAL ENGINEERING133●6 APPLICATION TEST IN SMTOn the basis of the analysis to fluxless solderng opention and the joining mechanism, a slderingtest of SMD on pad is made. Resistance 823 and 32Sn- 68P are chosen as SMD component and soldermaterial repectively. Process paramelters are listed as fllows, vacum degree is 0.06 Pa, sldering timeis2.5s and laser power is 10 w. Solder joint obtained is shown in Fig.7.23(m) Morpbology of the soldering joint(b) Cross-section of the soldering jointFig.7 Solder joint of fluxles sldeingThe mophology of the sodering joint is shown in Fig. 7a. Lnder fuxles condition al the edge of thepad and the apparatus metal there is a good spreading and weting, the soldering joint has a finemophology. Fig.7b is the cnos-ectional diagram of Fig.7a. It can be seen under the fluxless conditionthe solder has a good packing behavior, and the basic requirement o1 SMT sldeing joint is stisied.7 CONCLUSIONS(1)In the dilute vacum ( about 0.06 Pa), using laser heaing soure, the fluxless slderng onelectrical apparatus can be achieved, but need a higher energy than flux soldering.(2)Under vacum enionment, the higet tempernture ol the fules laer sodering procss is about450 C, far higher than its meling point. Only at this time the slder can spead and wet on the ped well.(3)At higher temperature the fall of the soder su -中国煤化工factor that inluencesthe spreading and wetting behavior of fluxless liquid ::fYHCNM HG" poletion.References1 Li Mingyu, Wang Chunping. The newest status of fules sldering. Welding, 1998(2):2-52 N Koopman, S Bobbio. Fluxless soldeing in air and nitnogen, IEEE, 19933 Gu Xir万分数据’li Wwailang. Surface chemistry . Beijing: Publishing Company, 19944 Ding Yingu. Qin Gunlin. Solid sufuce cheniaty. Shanghai Shanghai Science Tchnology Pubisehing Compeny, 1988