Reliabiliy Design Technology for Power Semiconducor Modules Akira Morozumi Kasumi Yamada Tadashi Miyasaka 1. Inroducion The marke for power semiconducor modules is spreading no only o general-purpose inverers, servo moor drives, NC machine ools and elevaors bu also o new applicaions hrough he realizaion of elecric vehicles and renewable energy sysems. Fuji Elecric has developed various power modules in response o marke needs, and as he marke expands in he fuure, he required performance for power modules will surely become diversified and advanced. This paper inroduces our effors o lenghen he power cycling lifeime of IGBTs (insulaed gae bipolar ransisors), which is of grea imporance o he marke. as well as he solder join on he underside of a silicon chip. 2.2 Failure mechanism of he T j power cycling es Figure 2 shows he srucure of an IGBT module made by Fuji Elecric. As in he pas, is used a he join beween he silicon chip and he DCB subsrae. The failure mechanisms resuling from power cycling ess of modules using Pb-based solder are as below. When T j is approximaely 100 C or above, cracks occur a he inerface beween he silicon chip and he aluminum wire as shown in Fig. 3, due o shear sress generaed by mismached hermal expansion. Propagaion of hese cracks leads o wire bond lif-off and 2. Lifeime Evaluaion Technology and Failure Mechanism Fig.1 Operaing condiions of power cycling ess 2.1 Power cycling es The power cycling es is used o esimae he real operaion lifeime of IGBT modules. This es is repeaed unil a IGBT module fails due o hermal sress generaed from he rise and fall of he IGBT chip s juncion emperaure T j caused by urning on and off an elecrical load, where he IGBT module has been mouned on an air-cooled hea sink. Types of power cycling ess include he T j power cycling es and he T c power cycling es (hermal faigue lifeime es). In he T j power cycling es, he juncion emperaure is raised and lowered wihin relaive shor cycles as shown in Fig. 1. This es is used mainly o evaluae he lifeime of he aluminum wire bond and he solder join on he underside of a silicon chip. On he oher hand, he T c power cycling es is a hea cycle es whereby in one cycle, curren is urned on unil he case emperaure (T c ) reaches an arbirary value and hen is urned off from ha ime poin unil he case emperaure reurns o he original value prior o when curren was on, as shown in Fig. 1. This es is applied mainly o evaluae he lifeime of he solder join beween he DCB subsrae and copper base plae T j T C I C T I C T ON OFF ON T j T j power cycling OFF T C power cycling T C T f T f 54 Vol. 47 No. 2 FUJI ELECTRIC REVIEW
failure. This failure form is shown in Fig. 4. Wire meling failure is no observed on he emier bonding pad, and he lif-off surface can be plainly seen. This fac proves ha he wire bond has been broken by meal faigue. In addiion, via non-desrucive inspecion using an acousic microscope and cross-secional view of he solder join, cracks of approximaely 1mm aligned in parallel wih he inerface and originaing from he silicon chip ouer surface are observed. On he oher hand, when T j is less han approximaely 80 C, cracks occur in he solder join, due o shear srain generaed by he mismach of hermal expansion beween he DCB subsrae and he silicon chip. The juncion emperaure increase due o propagaion of hese cracks leads o failure of he IGBT. This failure form is shown in Fig. 4. The emier bonding pad shows races of burning due o wire meling failure. Meanwhile, cracks in he solder join are observed hroughou he silicon chip. The wire meling failure is caused by mel-off of he wire due o gradual emperaure increase of he silicon chip resuled from increasing hermal resisance accompanied by propagaion of he cracks. Consequenly, he power cycling lifeime wih Fig.2 Copper bonding Ceramic Copper bonding Verical srucure of IGBT module Silicone sof gel Silicon chip Aluminum bond wire DCB subsrae convenional srucures depends on he wire bond srengh because here is almos no damage o he solder iself in he range of relaively high operaing emperaures, namely when T j is abou 100 C or greaer, and depends on he solder join in he range of relaively low operaing emperaures, when T j is abou 80 C or less. 3. Improvemen of T j Power Cycling Lifeime In chaper 2, i was explained ha o prolong he power cycling lifeime, improvemens in he wire bond lifeime and he solder join lifeime are required for he high T j range and low T j range, respecively. Since mos applicaions are in a relaively low emperaure range such as below 100 C, he improvemen of power cycling lifeime in his emperaure range, in oher words, improvemen of he lifeime of he solder join is a pracical necessiy. 3.1 Improvemen of solder join lifeime In order o prolong he hermal faigue lifeime of a solder join, minimizing shear srain generaed in he solder join is mos effecive. In general, i is said ha lead-rich solder has a longer lifeime in he higher shear srain range ( γ ), and in-rich solder has a longer lifeime in he lower γ range. I can be esimaed ha applicaion of a in-rich solder would be effecive a he solder join beween he silicon chip and he DCB subsrae, since he generaed shear srain herein is relaively low, namely less han 1% according o he resuls of finie elemen mehod (FEM) analysis. Furhermore, he srain generaed a he solder join is relaed o he elasic modulus of he solder, and solder wih higher yield srengh (resising Copper base plae Fig.4 Failure form afer power cycling es of modules using Fig.3 Failure mechanism of power cycling es wih Pb-based T j 100 C Aluminum wire (Al) Shear sress (Si) DCB subsrae CTE (coefficien of hermal expansion) Al : 23.6 10 6 / C Sl : 3.5 10 6 / C T j 80 C Aluminum wire (Si) Shear srain DCB subsrae (Al 2 O 3 ) CTE (coefficien of hermal expansion) Si : 3.5 10 6 / C Al 2 O 3 : 7 10 6 / C Surface of emier bonding pad Surface of emier bonding pad join (acousic image of solder inerface) T j 100 C join (acousic image of solder inerface) T j 80 C Cross secion of cracks in solder join Cross secion of cracks in solder join Reliabiliy Design Technology for Power Semiconducor Modules 55
srengh agains plasic deformaion) generaes lower srain. Accordingly, in-rich solder is more effecive o realize a longer lifeime under he same T j because of is higher yield srengh. 3.1.1 Examinaion of new lead-free Wih he precondiion ha lead-free solder should be used in due consideraion of he environmen, he Sn/Ag based has been seleced as he basic composiion o be examined. The reason is ha Sn/Ag based has relaively beer balance of properies among he lead-free solders. However, leadfree solder no limied only o he Sn/Ag based solder alloy has he disadvanage of inferior weabiliy compared wih. Accordingly, Fuji Elecric has developed a new lead-free Sn/Ag based which possesses an excellen level of mechanical properies and he same weabiliy as Pbbased solder by opimizing various addiional elemens and heir quaniies. The properies of his new solder alloy are shown in Table 1. 3.1.2 Esimaion of solder join lifeime wih FEM analysis Two-dimensional elasic-plasic hermal sress analysis uilizing FEM was performed for boh he newly developed Sn/Ag based solder and he Pb-based solder o deermine he srain generaed a he solder join and esimae he lifeime. The resuls of analysis are shown in Table 2. When he resuls are compared under he same emperaure condiion, i can be seen ha he new Sn/ Ag based has a smaller srain and longer lifeime compared wih he. This resul is aribued o he mechanical properies of he Table 1 Mechanical properies Weabiliy Table 2 s New Sn/Ag solder Properies of newly developed lead-free Sn/Ag solder alloy Iems Convenional Yield srengh (MPa) 53 38 22 Elongaion (%) 19 18 25 Creep rae (% h) 0.09 0.20 1.30 Spreading (%) 100 50 100 Conac angle ( ) Weing force (mn) Resul of hermal sress FEM analysis for solder join T j ( C) 50 New Sn/Ag (%) Sn3.5Ag solder alloy Convenional Pb-based 17 27 9 0.39 0.36 0.41 Number of cycles o failure of solder join 0.14 2.0 10 7 80 0.22 2.7 10 5 50 0.23 2.7 10 5 80 0.36 5.0 10 4 new Sn/Ag based, having 2.4 imes higher yield srengh and higher creep srengh han he Pbbased solder. 3.2 Examinaion of T j power cycling lifeime (wih new Sn/Ag based solder) In order o clarify he power cycling lifeime of IGBT modules using Sn/Ag based solder, ess are performed under various operaing emperaures T j. The lifeime is defined o be 1% of he unreliabiliy rae (F()), obained by ploing he number of cycles o failure on Weibull probabiliy paper. The operaing emperaures, which are arbirarily se, are measured a he join, case and hea sink, using IR hermography and hermocoupling. Figure 5 shows he resuls of he 1,200V-75A series IGBT module a 60 C, 80 C and 110 C of T j. I can be undersood ha he lifeime is 1.3 10 6 cycles, 1.7 10 5 cycles and 3 10 4 cycles a 60 C, 80 C and 110 C of T j, respecively. 3.3 Failure mechanism of modules using Sn/Ag solder According o failure form inspecion of power cycling ess of 60 C and 110 C T j, inense wire meling failure on he emier wire bonding pad and severe solder damage were observed afer he power cycling es a 110 C T j. On he oher hand, no wire meling failure and very sligh solder damage were observed afer he power cycling es a 60 C T j, however, he generaion of cracks was observed in he solder bu i was very sligh. From hese failure forms, i is esimaed ha he power cycling lifeime of new Sn/Ag based solder depends on ha of he solder join when T j is higher han abou 110 C, and depends on ha of he wire bonds when T j is lower han abou 60 C. This fac differs from he failure mechanism of menioned in secion 2.2. In addiion, he crack propagaion form in he solder join also differs beween new Sn/Ag based solder and Pb-based solder. In he case of ha is easy o deform plasically, cracks propagae along he acing Fig.5 Unreliabiliy (%) Weibull plo of power cycling es resuls of modules using Sn/Ag solder (1,200V-75A) 99.5 95 70 40 10 T j = 110 C T j = 80 C T j = 60 C 1 0.1 10 4 10 5 10 6 Number of cycles o failure 56 Vol. 47 No. 2 FUJI ELECTRIC REVIEW
direcion of shear sress saring from he fille, where sress is concenraed as shown in Fig. 6. However, in he case of new Sn/Ag based solder wih high yield srengh, he propagaion of cracks is nearly concenric, originaing almos direcly under he silicon chip cener. Furhermore, a characerisic of he cracks generaed in new Sn/Ag based solder is ha hey are verical or reiculaed cracks, are parallel o he hick direcion of he solder, and propagae selecively along he in grain boundary. From hese facs, i is supposed ha deerioraion of is caused by plasic deformaion due o srain, while deerioraion of Sn/Ag based solder is caused by hermal damage due o he grain growh of in. 4. Power Cycling Lifeime Curve of Modules Using Sn/Ag lifeimes are judged as almos he same. 4.2 Reliabiliy for power cycling of modules using Sn/Ag solder As menioned in secion 4.1, he power cycling lifeime curve of IGBT modules using new Sn/Ag based solder has an inflecion poin a 50 C T j. As shown in Table 3, hese modules achieve a longer lifeime han convenional modules ha use. In paricular, he difference in lifeimes is remarkable a he relaively low operaion emperaure range of less han 100 C. This achievemen of longer lifeime in he Fig.7 Relaion of power cycling lifeime and failure mode New Sn/Ag 4.1 Relaion beween lifeime of wire bond/solder join and a module s lifeime From consideraion of he above, i is undersood ha he power cycling lifeime of modules using new Sn/Ag based solder is comprised of he lifeimes of he wire bond, he solder join and he area in which hese coexis as shown in Fig. 7. Thereupon, faigue lifeime of he wire bond and he solder join are calculaed from deailed analysis of he failure lifeime in power cycling ess and from FEM sress analysis of he wire bond and solder join. Figure 8 is obained by ploing he calculaed lifeimes of each join and he lifeime of IGBT modules on he same graph. This graph indicaes ha he lifeime of a solder join is in close proximiy o he lifeime of he module. On he oher hand, he lifeime of wire bond inersecs he module lifeime a abou 50 C T j and falls below he module lifeime a lower emperaures. Consequenly, faigue damage of he solder join rarely occurs in he range of T j less han 50 C, and he module lifeime depends on he lifeime of wire bond faigue. While, in he range of T j greaer han 50 C, he lifeime of solder joins exceeds he module lifeime in his examinaion, bu in acualiy, boh Fig.8 Number of cycles o failure Power cycling lifeime curve of modules using new Sn/Ag 10 9 10 8 Pb-based join lifeime area Wire bond lifeime area T j ( C) Wire bond lifeime area Coexisen area of wire bond and solder join lifeime join lifeime area Fig.6 propagaion form of solder join afer power cycling es Number of cycles o failure 10 7 10 6 10 5 Lifeime of wire bond (300µm Al wire) Lifeime of solder join (new Sn/Ag ) s Sn/Ag solder Failure form of Sn/Ag solder (acousic image of Sn/Ag solder inerface) 10 4 Module lifeime (F ()= 1% line) 10 3 10 50 100 200 T j ( C) Reliabiliy Design Technology for Power Semiconducor Modules 57
Table 3 Operaing emperaures Power cycling lifeime of modules under esimaion of F()=1% Modules using new Sn/Ag solder Modules using convenional T j =100 C 2.5 10 4 1.7 10 4 T j =60 C 1.3 10 6 1.8 10 5 T j =30 C 8.0 10 7 5.0 10 6 power cycling es is aribued o improved mechanical properies of he new Sn/Ag based solder and o mainaining a good weabiliy equivalen o Pb-based solder. 5. Conclusion In he preceding chapers, our effors o increase he power cycling lifeime has been presened in he conex of reliabiliy design for power semiconducor modules. New Sn/Ag lead-free solder wih excellen mechanical properies and weabiliy has been newly developed, and can conribue o improved reliabiliy of he modules. Through he longer power cycling lifeime, miniaurizaion and price-reducion of he devices are expeced. Fuji Elecric will endeavor o coninue o respond o he needs of a marke ha is becoming increasingly more demanding. 58 Vol. 47 No. 2 FUJI ELECTRIC REVIEW
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