Predicting and Avoiding Die Attach,

Transcription

1 Predicting and Aviding Die Attach, Wire Bnd, and Slder Jint Failures Craig Hillman June 15, D Pwer Electrnics Raleigh, NC 1

2 Why Die Attach/Wire Bnds/Slder Jints? Intercnnects tend t be the primary driver fr reliability f pwer mdules Influenced by the rbustness f tday s pwer devices (IGBT and FET, Si/GaN/SiC) and the tendency t twards cnservative design (derating margin) Active endurance tests f direct-bnded-cpper based MOSFET pwer mdules shwed that the aluminum bnds and the die attach are the mst critical design elements regarding prduct reliability Rbert Bsch,

3 Hw t Avid Die Attach, Wire Bnd Failures? Step 1: Gd quality cntrl Step 2: Perfrm reliability predictin using physics f failure (PF) 3

4 What is Physics f Failure (PF)? Als knwn as reliability physics Cmmn Definitin: The prcess f using mdeling and simulatin based n the fundamentals f physical science (physics, chemistry, material science, mechanics, etc.) t predict reliability and prevent failures 4

5 Physics f Failure: Mdeling and Simulatin What are we mdeling / simulating? Reliability (t > 0) = Material Change r Material Mvement Fundamental Material Mechanisms Diffusin Oxidatin/Reductin Creep Fatigue 5

6 Material Mvement: Diffusin Mtin f electrns, atms, ins, r vacancies thrugh a material Typically driven by a cncentratin gradient (Fick s Law) Can be driven by ther frces (electrmtive frce, stress) 6

7 Material Mvement: Creep The tendency f a slid t permanently defrm when subjected t a fixed lad Crllary: Tendency f a slid t relieve stress when laded at a fixed displacement Metals: Driven by mvement f defects within the crystalline structure Dislcatins (edge r screw) Grain Bundaries 7

8 Material Mvement and M&S All physics f failure mdels can be cndensed int answers t three questins Hw large is the stress? At what rate is this stress driving material mvement? At what time will this material mvement induce failure? 8

9 PF-Based Reliability Predictin Mst physics-f-failure (PF) based mdels are semi-empirical The basic cncept is still valid Requires calibratin Calibratin testing shuld be perfrmed ver several rders f magnitudes Allws fr the derivatin f semi-empirical cnstants, if necessary The purpse f PF is t limit, but nt eliminate, the influence f material and gemetric parameters E.g., Slder: Testing must be re-perfrmed fr each package family (ball array devices, gullwing, leadless, etc.) 9 9

10 Hw t Perfrm PF? Stress/Strain/Energy One f the key differentiatins in PF techniques is hw t capture the stress/strain/energy within the intercnnect 0.5 r L base wire 6 1 2base D D 1 D L T Optin 2: Analytical Equatins Optin 1: Finite element mdel Mst manufacturers will use Optin 1 at sme pint during the develpment prcess (especially fr cmplex gemetries) Hwever, Optin 2 is preferred fr tradeff analysis and fr users f the pwer mdules 10

11 Hw t Perfrm PF? Damage Accumulatin Lts f discussin abut key damage parameter (stress, strain, elastic, plastic, plasticity, creep, energy) Basquin (high-cycle) Cffin-Mansn (lw-cycle) Paris (high-cycle) Darveaux (lw-cycle) 11

12 Hw t Perfrm PF? What s Imprtant? One f the mst critical aspects f PF is including primary parameters and excluding secndary effects Can smetimes drive the semi-empirical nature f PF 12

13 Predicting Reliability f Die Attach 13

14 Predicting the Reliability f Die Attach Die attach tends t have nly ne failure mde Thermal cycle fatigue due t pwer cycling Frequency f the pwer cycle can play a very critical rle If the pwer cycle frequency is high enugh, the failure site will shift frm the die attach t the wire bnd (thermal inertia) Key Challenge: Defining failure Mst die attach cnfiguratins d nt cnduct electricity Die attach is primarily a thermal path 14

15 Die Attach Fatigue (Englemaier, 1982) ( L d 2 W d 2 )( CTE 2h die CTE DBC ) T Strain range at the die/die attach interface h = die thickness W = die width L = die length = cefficient f thermal expansin (CTE) T = change in temperature 15

16 Die Attach Fatigue (Time t Failure) N ( ) f f c Cffin-Masn based lw-cycle fatigue damage mdel Tin-based slders (SnPb, SAC305, Sn3.5Ag, etc.) tend t have fatigue expnents arund 2 t 2.5 SnPb: ~2; Sn3.5Ag: 2.2; SAC305: ~2.4 Fatigue expnents fr new die attach slders have nt yet been widely validated Nansilver, BiAgX, Sn25Ag10Sb ( J ally) 16

17 Nansilver: Is it Better Than Slder? Sintering with nansilver is challenging (and expensive) Requires pressure Fr large die, pst sintering stress relief step is ften required Viding must be cntrlled 17

18 Nansilver: Is it Better Than Slder? (cnt.) Fr the mst part, the industry agrees nansilver is mre reliable, but there can be issues L. Melchr, Dctral Dissertatin M. Beierlein, Adv. Pack. Cnf

19 Nansilver: Is it Better Than Slder? (cnt.) Different publicatins prvide different values fr fatigue cnstants and expnents Authr Cnstant Value Expnent M. Knerr 1.6E-01 Plastic Strain -3.0 Y. Tan 5.8E+11 Shear Stress -9.4 X. Li 1.6E-09 Shear Strain -7.6 Y. Tan, et. al., Cnf. n Fracture,

20 Vibratin Fatigue Lifetime under mechanical cycling is divided int tw regimes Lw cycle fatigue (LCF) High cycle fatigue (HCF) LCF is driven by inelastic strain (Cffin-Mansn) 2N p f c -0.5 < c < -0.7; 1.4 < - 1 /c > 2 HCF is driven by elastic strain (Basquin) f b e N f E < b < -0.12; 8 > - 1 /b > 20 f 20 20

21 Predicting Reliability f Wire Bnds 21

22 Reliability: When d Wire Bnds Fail? Expsure t elevated temperature Intermetallic frmatin Expsure t elevated temperature/humidity Crrsin Expsure t temperature cycling Lw cycle fatigue 22

23 Reliability at Elevated Temperatures Nt an issue in aluminum-aluminum wire bnd system The lack f intermetallic frmatin and differential diffusin makes it relatively immune t purple plague Prir studies have fund little change in resistance after 1000 hurs at 300C Bigger issue in mixed metal systems, like gld-aluminum Frmatin f brittle AuAl2 (purple plague) at 350C Diffusin f gld int Au 5 Al 2 causes Kirkendall viding at lwer temps 23

24 Reliability Predictin (Elevated Temperature) Is an abslute reliability predictin f wire bnd reliability at elevated temperature pssible? Shrt answer: NO Diffusin behavir is very sensitive t bnding temperature, quality f bnd, aluminum ally, aluminum bnd pad thickness, and encapsulant chemistry Lw bnding temperature Si in Al-Cu bnd pad Thin bnd pad (~1 um) Brmide-free flame retardants Can change abslute and relative (acceleratin factr) time t failure 24

25 Reliability Predictin Temperature (cnt.) Fr gld-aluminum, predictin is primarily by extraplatin frm test results using Arrhenius and a cnservative activatin energy (0.9 ev) Hwever, there is sme questin as t the presence f a minimum temperature t f H Aexp kt Peridically reprted as 125C fr unencapsulated and 85C fr encapsulated Observed in ther systems (tin-cpper and whiskers) 25

26 Other Systems (Cu-Al) Cpper-aluminum frms intermetallics at a much slwer rate Mst cmmn activatin energy f ev Micrn reprted 0.63 ev Mlding cmpund has little effect L. England, ECTC, 2007 HJ Kim, IEEE CPT, 2003 L Levine, Update n High Vlume Cpper Ball Bnding C. Breach, The Great Debate: Cpper vs. Gld Ball Bnding 26

27 Other Systems (Cu-Al)(cnt.) Au-Al Cu-Al Cu-Al can shw imprved perfrmance ver Au-Al Nt t the extent expected based n intermetallic grwth Different failure mde (gradual vs. sudden) 27

28 Shear Strength at Elev Temp Gld Wire a. b. Cpper Wire Cu Cu Shear strength f Au and Cu ball bnds n Al pads At lwer temperatures (<150C) they are similar in strength lss J. Onuki, M. Kizumi, I. Araki. IEEE Trans. On Cmp. Hybrids & Manfg. Tech. 12 (1987)

29 Cu-Al and Elevated Temperature Cncerns Different intermetallics frm at different temperatures Can a 150C/200C test be extraplated t 85C? Fracture mde with pure Cu changed frm bulk Cu t interfacial failure Sme indicatins that xidatin f the wedge bnd may be a critical weak pint S. Na, T. Hwang, J. Kim, H. Y, and C. Lee, Characterizatin f IMC grwth in Cu wire ball bnding n Al pad metallizatin, ECTC, IEEE,

30 Wire Bnds and Temperature/Humidity Degradatin primarily ccurs in the gld wire bnd / aluminum bnd pad system Driven by galvanic crrsin Absence f galvanic cuple in aluminum/aluminum and cpper/aluminum systems tends t limit crrsive behavir Presence f halides, especially chlride, can accelerate crrsive behavir Getters in the mlding cmpund tend t reduce this risk 30

31 Wire Bnds and Temperature/Humidity (cnt.) The mechanism f Au-Al crrsin has been primarily described by Peck s Law Peck s Law fr Temperature and Humidity Empirical study f THB/HAST (85/85, 110/85 & 5-70 VDC) t life A RH 0 n f ( v)exp E a kt t life = time t failure, A 0 = material cnstant RH = relative humidity, n = empirical cnstant (2.66) E a = activatin energy (0.79eV), k = Bltzmann cnstant T = temperature, f(v) = vltage functin (pwer law, ~1.5) 31

32 Cpper Wire Bnd and Temperature/Humidity Cpper is nt as nble as gld Nble catings (palladium) can cme ff during bnding Palladium (Pd) cating can als create galvanic cuple with cpper Studies have shwn early failures during temp/humidity testing Sme dependency n mlding cmpund (need lwer ph, lwer halgen cntent) Uncertain if JEDEC test with acceleratin factr based n Peck s equatin (based n aluminum/gld) is still valid Halgen-Free Mlding Cmpunds H. Clauberg, Chip Scale Review, Dec

33 Cpper Wire Bnd and Temperature/Humidity (cnt.) T. Bettcher believe early failures are due t galvanic crrsin f Cu-rich intermetallics (Cu 9 Al 4 ) (EPTC 2010) Induces the frmatin f cpper xides between the intermetallic and the cpper bnd wire Initial failures during JEDEC HTRB and Autclave testing were reversed by increasing the amunt f intermetallic thrugh annealing Small ande (intermetallic) relative t cathde greatly increases crrsin rate 33

34 Status f Cu-WB Durability-Reliability Research inemi P2B inemi Phase 2B: HAST 130degC/85% Rel. Humidity fr 384 hurs n lse parts Tested every 4 days at 96, 192, 288 and 384 hurs. Failure pattern suggests mre f a durability issue (hwever, hw relevant is 384 hurs under HAST?) 34

35 Wire Bnds and Temperature Cycling (Wedge Flexure) Driven by differences in cefficient f thermal expansin (CTE) Flexing mtin results create micrcracks at the heel f the wirebnd Mdel based n thery f curved beams 0.5 r L base wire 6 1 2base D D 1 D L Strain at the heel f the wire (assumes bnd pads at same height) T r = wire radius D = half wire span L = wire length = cefficient f thermal expansin (CTE) T = change in temperature 35

36 Wedge Flexure (cnt.) Cffin-Masn Based Lw-Cycle Fatigue N f C m C and m empirically determined t be 1.0 and (-1.4) respectively fr aluminum wedge bnds 36

37 Wire Bnds and Temperature Cycling (Axial Tensin) Wire encapsulated in mlding cmpund can experience tensile stresses in the wire due t differential expansin and cntractin Mdulus f mlding cmpund is ignred because f its minimal cntributin N f C m 37

38 Wire Bnds and Temperature Cycling (Shear) Shear stresses between the substrate (s), the bnd pad (p) and the wire (w) r is wire radius A is crss-sectinal area W is width G is shear mdulus b is thickness l is length 38

39 Cmpeting Failure Mechanisms 39

40 Validatin (Aluminum-Aluminum System) 40

41 Cpper Wire and Temperature Cycling Pwer mdule industry believes cpper wire is mre rbust than aluminum Changes being implemented fr electric drivetrain Part f imprvement is believed t be due t reduced temperature variatin frm imprved thermal cnductivity Part f imprvement culd be due t recrystallizatin Can result in self-healing N. Tanabe, Jurnal de Physique IV, 1995 D. Siepe, CIPS 2010 Part f imprvement culd be mre rbust fatigue behavir 41

42 Cpper vs. Gld Temperature Cycling Cpper superir based n these publicatins G. Pasquale, J. Micrelectrmech Sys.,, 2011 N. Tanabe, Jurnal de Physique IV,

43 Bending Stress (MPa) Aluminum vs. Cpper Temperature Cycling Cpper seems superir 100 Aluminum Cpper Fatigue Failure Cycles (N) J. Bielen, EurSime, 2006 N. Tanabe, Jurnal de Physique IV,

44 Thermal Cycling Reliability and Assembled Parts Lse Cu wire-bnded parts have passed cmpnent level thermal cycle tests, but have failed during thermal cycling f autmtive E/E mdules Believed, but nt cnfirmed, t be due t additinal expansin-cntractin stresses frm the CTE mismatch part and PCB 44

45 Predicting Reliability f Slder Jints 45

46 Slder Jint Fatigue Knwing the critical drivers fr slder jint fatigue, we can develp predictive mdels and design rules Vlume f Slder Thickness f Slder Slder Fatigue Prperties CTE f Cmpnent Elastic Mdulus (Cmpliance) f Cmpnent Length f Cmpnent CTE f Bard Elastic Mdulus (Cmpliance) f Bard 46

47 Predictive Mdels Physics f Failure (PF) Mdified Engelmaier fr Pb-free Slder (SAC305) Semi-empirical analytical apprach Energy based fatigue Determine the strain range () C T C is a crrectin factr that is a functin f dwell time and temperature, L D is diagnal distance, is cefficient f thermal expansin (CTE), T is temperature cycle, h is slder jint height L h D s 47

48 48 Predictive Mdels Physics f Failure (PF)(cnt.) Determine the shear frce applied t the slder jint F is shear frce, L is length, E is elastic mdulus, A is the area, h is thickness, G is shear mdulus, and a is edge length f bnd pad Subscripts: 1 is cmpnent, 2 is bard, s is slder jint, c is bnd pad, and b is bard Takes int cnsideratin fundatin stiffness and bth shear and axial lads a G A G h A G h A E L A E L F L T b c c c s s s

49 Predictive Mdels Physics f Failure (PF)(cnt.) Determine the strain energy dissipated by the slder jint W 0.5 F A s Calculate cycles-t-failure (N 50 ), using energy based fatigue mdels N f W 1 49

50 Slder Jint Validatin Energy-based analytical equatin shws strng crrelatin t bth test and field failures BGA When crrelatin is nt bserved, typically driven by the presence f an axial lading cnditin (cnstraints, ptting) Requires use f cmpatibility f displacements F T 2 1 A E A E 2 50

51 Summary / Cnclusin The field f reliability predictin is nt stagnant Driven by need fr new materials, new technlgies Driven by demand fr faster time t market (can nt test everything) Driven by limited resurces (can nt FEA everything!) Be aware when knwledge is sufficient, validated by a physical understanding and testing, t prceed with mdeling and simulatin 51