Chapter 7 Mechanical Characterization of the Electronic Packages

Transcription

1 Chapter 7 Mechanical Characterization of the Electronic Packages 1

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4 Thermal Mismatch Si (CTE=~3 ppm/c) Underfill (CTE=7 ppm/c) EU solder (CTE=5 ppm/c) Substrate (CTE=15~0 ppm/c) Thermal mismatch in electronic package is due to the different coefficients of thermal expansion (CTE) in dissimilar materials (Si/solder, Si/underfill, Si/substrate) or the temperature gradient Thermal stress and the associated thermal strains will arise in the connection joint or all interconnections. Thermal fatigue failures result from the thermal mismatch during thermal cycle (uniform temperature environment) or power cycle (power on/off, program running) 4

5 Thermal Fatigue Low cycle fatigue (LCF): below 10 6 cycles, plasticity encountered with obvious plastic deformation. High cycle fatigue(hcf): above 10 6 cycles, strains are elastic and without obvious plastic deformation. Thermal fatigue is a classic case of low cycle fatigue (LCF) For solder, the strain encountered in thermal cycle can be a few times as large as the yield strain 5

6 Fatigue Life Prediction Procedure M F Shear Stress Shear Strain Estimation F M 1. Determine system forces and deformations. Determine local stresses and strains in solder 3. Estimate fatigue life Local stresses/strains are critical to the susceptible element in packages (solder joint) Local shear strains in solder joint dominate the failure mode and are used fro the prediction of the thermal fatigue life 6

7 Thermal Stress/Strain T = L = h C = Temperature rise Beam length Joint height Chip "1" Joint "C" α = CTE Substrate "" τ = Shear Stress L h G ( α α ) c c 1 T 3 h A c c + 1E I c c γ = Shear Strain ( ) L α 1 α T h c 1. E1, E, I1, I are very large values. Shape of solder is right circular cylinder 7

8 Coffin-Manson Relationship L h c N f ε f ' Formation 1 = = 0.35 C = 0.44 γ ' ε f 1 C γ Large temperature excursions cause different damage mechanism which depends on overstress and strong variation of the properties of solder Solder joint quality which may cause fracture in solder that fails this model High frequency/low temperature make solder behavior like an elastic material, thus plastic strain is not close to the local strain 8

9 Example 5 mm 1 Temperature range: 35C to 85C Solder height effect on fatigue life: C hc 100 um dia. Height (um) N f (cycle) α = (7.6) ppm / C = 4.4 ppm / C T = (85 35) = 50C; ε f ' = 0.35; c = 0.44 γ = = N f = = 500 9

10 Preferred Solution Thermal stress: By decreasing: By increasing: Thermal strain: By decreasing: By increasing: τ α, T, L, G c, E c Ac h c, I c γ α, T, L h c The height of solder joint is suggested the higher the better α can be lowered by selected a proper substrate E, Smaller suggested using softer solder material c G c DNP(L): the distance between a solder joint and the neutral point of the chip (chip center) 10

11 11 Quick Estimation of Solder Height Quick Estimation of Solder Height The solder volume of a spherical solder joint: Height of a furstrum of a crone: Vertical loading per joint: ( ) [ ] 3 6 c s s s r r H H V + + = π [ ] [ ] B A A B A A H s = π A 3V = r c r s B + = ( ) 3 s s c c c r r r r V H + + = π ( ) s c s n H H H H F f = ( ) ( ) ( ) { } = c s c c s s s c s s c s s s s c r r H r r H H r r H r r H r H H F γπ H s H c

12 Different Solder Volume Input Data r r c s f n = 0.005cm = 50um = 0.005cm = 50um = 4.9dyne = 0.005gf γ = 35dyne / cm Final Height & Deposition Height +/- Variation: CASE 1 CASE CASE 3 V ( um 3 ) H s (um) H c (um) H (um) π X 50 X π X 50 X π X 50 X

13 Example 600um 400um TRADE-OFF BETWEEN FATIGUE, THERMAL AND PROCESSING PROCESSING VARATION Standard joint height: 15um ; deposition thickness: 15. um Standard deviation 10% of specified thickness (within 3-sigma deviation) Max. joint height = 19.3um Min. joint height = 10.6um FATIGUE AND THERMAL RESISTANCE VARIATION Joint height Max. Mean Min. Fatigue life (cycles, delta T = 60C) Thermal resistance (C/W)

14 Microstructure of 63Sn/37Pb Eutectic Solder The coarsened region is inherently weaker and through which cracks propagate to final solder failure The heterogeneous coarsened band is approximately parallel to the imposed shear strain Quantitative modeling of the microstructure change is not possible 14

15 Au/Ni Metallization on a Cu Pad Solder Ball Die Cu Pad/Trace Solder Mask Solder Ball 15

16 Finite Element Analysis of Solder Fatigue Life 16

17 Analysis Algorithm for Solder Fatigue Fatigue Life Data Output Prediction Model ANSYS Modeling * Material Properties Analysis Procedure Solder ball profile Solder Material Specific Temp. cycle Geometry consideration Boundary conditions Bump+Underfill composite properties Component properties Solution Methodology Converge criteria 17

18 FEA Modeling Schemes Referred from: Finite Element Modeling of BGA Packages for Life Prediction 000 Electronic Components and Technology Conference, pp Modeling Method Modeling Desciption Advantage/Disadvantage Remark Nonlinear slice model 1.Octant symmetry, untilizes inly a diagonal slice of the package.the model imposes symmetric boundary conditions on the slice plane coinciding with the true symmetry plane Reduce computation time Most Conservative Nonlinear global model with linear super model 1.Package and board are modeled as two super element and all of the solder balls as three-dimensional finite elements.except for the critical joints, the solder balls are modeled with a coarse mesh Avoid the assumptions associated with the boundary conditions of the slice model Not acceptable Linear global model with nonlinear submodel 1.Linear model of substrate and board and all of the solder balls using threedimensional finite elements.the global model includes only linear material properties, whereas the submodel includes nonlinear material behavior 3.The linear global model is s Permit the simulation of any thermal cycle using only one set of global model results Most fidelity Nonlinear global model with nonlinear submodel 1.Nonlinear global model with a very coarse mesh for the substrate and board and for the solder balls.providing the critical solder joint for the subsequent nonlinear submodeling Displacements become the coundary conditions for the nonlinear submodel of the critical joint in accordance with the thermal cycling Nonlinear global model 1.Global model employs a relatively coarse mesh for all of the components of a package except for the critical joints.it is not feasibile to model all of solder joints if the package consists of a large number of solder joints Time consuming Selected Modeling Methodologies: Nonlinear slice model: single chip package Linear global model with nonlinear sub-model: SiP or MCM packages 18

19 Approaching Architectures Finite Element Solution Domain ANSYS Environment Viscoplasticity Anand s Model (VISCO107) Plasticity σ-ε curves (SOLID185) Experimental+Analytical Solution Domain Darveaux (plastic work) N 0 da dn = K 1 3 = K K W ave K4 W ave a α = N 0 + da dn α N ff = Coffin-Manson (Equivalent plastic strain) Elasto-plastic + Creep Creep Model (SOLID185) ε p = CN m f Approaching Method Property Implement Advantages Disadvantagess 1. popular for life prediction of eutectic solder 1. limitation of material informations (LF) Viscoplasticity Anand's model. has been proven and widely used. only plastic work can be read out 3. nonlinear plasticity & creep involved 3. life prediction variables are difficult obtained Plasticity 1. stress-strain curves can be experimented (LF) 1. only plasticity behavior without creep effect Temp. dependent. Coffin-Manson variables can be determined stress-strain curves 3. both two kinds of life prediction methods can be used Elasto-plastic + Creep 1. can be used for low frequency fatigue analysis 1. difficult converge and time consuming Temp. dependent elastic. both two kinds of life prediction methods can be used. not real plasticity behavior involved modulus and creep function 3. creep functions can be determined by experiments Plasticity (high-cycle fatigue) +Creep (low-cycle fatigue) = Viscoplasticity (in-elastic fatigue) 19

20 Fatigue Analysis using ANSYS Code FEM: Plasticity & Elasto-plastic +Creep ε = ε + ε in eq pl eq cr eq After nth cycles in in in ( ε eq ) = ( ε eq ) ( ε eq ) +1 i in in in ( ε eq ) = ( ε eq ) = L = ( ε eq ) n n+ 1 saturated N f = B i in ( ε ) C eq saturated Modified Coffin-Manson Law Plasticity: equivalent plastic strain grows in the ramp duration and stays in the hold-time (dwell) Elasto-plastic + Creep: creep strain accumulates more in the hold-time duration (dwell) 0

21 Two-stage Analysis Method C*** SOLDER!MATERIAL PROPERTIES OF SOLDER BALL MP,EX,1,30E3 MP,NUXY,1,0.4 MP,ALPX,1,4.7E-6 Equivalent Plastic Strain TB,BKIN,1,3 TBTEMP,73! Temperature = 73 TBDATA,1,46,(55-46)/( )! Stresses at temperature = 73 (0) TBTEMP,33! Temperature = 33 TBDATA,1,34,(39-34)/( )! Stresses at temperature = 33 (50) TBTEMP,373! Temperature = 393 TBDATA,1,18,(0-18)/( )! Stresses at temperature = 393 (100) Equivalent Creep Strain C*** SOLDER!MATERIAL PROPERTIES OF SOLDER BALL MP,EX,1,30E3 MP,NUXY,1,0.4 MP,ALPX,1,4.7E-6 Stage 1: Plasticity analysis -Temp. dependent stress-strain curves -Nonlinear kinematic strain hardening Stage 1: Plasticity + Creep analysis -Temp. dependent stress-strain curves -Isotropic strain hardening -Creep function TB,BISO,1,3 TBTEMP,73! Temperature = 73 TBDATA,1,46,(55-46)/( )! Stresses at temperature = 73 (0) TBTEMP,33! Temperature = 33 TBDATA,1,34,(39-34)/( )! Stresses at temperature = 33 (50) TBTEMP,373! Temperature = 393 TBDATA,1,18,(0-18)/( )! Stresses at temperature = 393 (100) TB,CREEP,1,,,8!CREEP MODEL TBDATA,1,143., ,1.8888,

22 Temperature Cycle Profile 183 Temp Dwell period of 5 mins Ramp rate = 10 /min 100 (C) (D) (G) (H) (K) (L) 5 0 (A) (B) (E) (F) (I) (J) Time 1 st cycle nd cycle 3 rd cycle Remark: Board Level TC: 0~100C; 5 mins dwell and ramp rate with 10C/min

23 Stress-strain strain Curves of Solder Joint Stress-Strain Curves of Eutectic Solder (63Sn/37Pb) 80.0 Kinematic Hardening B Temp. = 0 C 70.0 Temp. = 50 C Temp. = 100 C 60.0 Bauschinger Effect A 50.0 Stress (MPa) O C Strain Remark: To used bi-linear stress-strain model instead of multi-linear model can make a balance between computation accuracy and time consuming Kinematic hardening effect must be considered into the FEA analysis D Yield Surface O S F σ σ 0 3 3

24 Material Properties Material Type Temp. (K) Elastic Modulus (MPa) Poisson's Ratio Yield Strength (MPa) FR-4 Board x,y 0.8 xz,yz 19 x,y elastic z 0.11 xy 70 z Solder Mask elastic 30 Copper Pad BT Laminate x,y 0.39 xz,yz 15 x,y elastic z 0.11 xy 5 z Underfill < elastic > Silicon Chip elastic.3 < -10 TIM 0.38 elastic > Adhesive < 49 7/73,4/98,0.7/33,0.09/348,0. Temp.-dependent /373,0.075/ > 49 nonlinear 16 Heat Spreader elastic 18 Solder creep function: & ε creep T [ ( )] e = 143. sinh σ CTE (ppm/k) for 63Sn/37Pb solder j+1 j+1 Solder bump + Underfill j+1 j+1 U S U S U E eq, G eq, v eq, eq Composite Algorithm Total volume: V Bump volume: V Underfill volume: E + V s u Equivalent material properties ns = Vs / V nu = Vu / V ν = ν n + ν n = eq Eunu Esns eq u u s s j j j j 4

25 Finite Element Model & BC s Single-chip Module Modeling Symmetric BC s Symmetric BC s UX=0 Fix UY=0 Coupling UY Slice Model Coupling UX Substrate opening: 0.5 mm PCB opening: 0.4 mm Standoff height: 0.4 mm Remark: Diagonal slice modeling for single-chip module Solder ball profile is determined by program prediction Coupling constraints make structure behave plane strain condition 5

26 Accumulated Plastic and Creep Strain Accumulated equivalent plastic strain (1 st cycle) Accumulated equivalent plastic strain ( nd cycle) Accumulated equivalent plastic strain (3 rd cycle) Accumulated equivalent creep strain (1 st cycle) Accumulated equivalent creep strain ( nd cycle) Accumulated equivalent creep strain (3 rd cycle) 6

27 Example of Case Studying (c) cycles (a) 411 cycles (d) cycles (b) 1897 cycles (a) Substrate Edge / Substrate Side (c) Chip Edge / Substrate Side N i PEEQ CEEQ IEEQ IEEQ N i PEEQ CEEQ IEEQ IEEQ Cycle E E Cycle E E Cycle E-15.85E Cycle E E Cycle E E Cycle E E (b) Substrate Edge / PCB Side (d) Chip Edge / PCB Side N i PEEQ CEEQ IEEQ IEEQ N i PEEQ CEEQ IEEQ IEEQ Cycle E E Cycle E E Cycle E E Cycle E E Cycle E E Cycle E E Fatigue life at substrate edge / substrate side: 441 cycles Fatigue life at substrate edge / PCB side:1897 cycles (ASE:000 cycles ) Fatigue life at chip edge / substrate side: cycles Fatigue life at chip edge / PCB side:18036 cycles 7

28 Prediction of Solder Ball Profile PBC pad open: mm PBC pad open: mm PBC pad open: mm PBC pad open: mm Programming by CCU (000) Original data are followed by KC Substrate pad open PCB pad open Standoff height Case mm mm mm Case 0.55 mm mm mm Case mm mm mm Case mm mm mm Pitch: 1 mm

29 PCB Pad Opening Effect Time (min) Pad=0.38 mm Pad=0.4 mm Pad=0.475 mm Pad=0.5 mm E E E E E E E E E E E E E-0.467E-0.060E E E-0.753E-0.85E-0.01E E E-0.85E-0.754E E E-0.85E-0.754E E E E E E E E E E E E E E E E E E E E E E E E E-0 cycle1.790e-0.753e-0.85e-0.01e-0 cycle.738e-0.63e-0.10e E-0 cycle3.783e-0.636e-0.118e E-0 PEEQ.760E-0.630E-0.119E E-0 Time (min) Pad=0.38 mm Pad=0.4 mm Pad=0.475 mm Pad=0.5 mm E E E E E E E E E E E E E E E E E E E E E-15.0E-15.0E-15.0E E E E E E E E E E E E E E E E E E E E E E E E E-15 cycle1 1.33E E E E-15 cycle 1.33E E E E-15 cycle3 1.33E E E E-15 CEEQ 1.33E E E E Predictition ASE Pad Open 0.38 mm 0.40 mm mm 0.50 mm Prediction ASE Data Difference 16.3% 7.% 1.6% 1.1% Pad=0.38 mm Pad=0.4 mm Pad=0.475 mm Pad=0.5 mm 9

30 Moire Interferometry Method 30

31 Moire Interferometry Method 31

32 Moire Interferometry Method 3

33 Shadow Moire Method 33

34 Shadow Moire Method 34

35 Shadow Moire Method 35

36 Electrical Speckle Pattern Interferometer (ESPI) 36

37 Electrical Speckle Pattern Interferometer (ESPI) 37

38 Three Point Bending Stiffness is a measure of how easily an object will bend when put under. It is often necessary to have stiff components, and this is achieved through the combination of design, ie the geometry, and material selection. The main material property that affects the stiffness is the Young's modulus, which has a large range of values for different materials. The three-point bend can also allow us to find the Young's Modulus (E) of the material once the second moment of area (I) is known. This is done by relating the vertical displacement δ, to the load W (= Mg) using the formula: δ = WL 3 /48EI where L = distance between the supports. A strip of balsa undergoing a 3-point bending test 38

39 Derivation of equation under 3-point 3 Bending Starting with take moments about left hand end: and integrate: At x = L/, dy/dx = 0, hence C1 = WL/16 Integrate again: At x = 0, y = 0, hence C = 0. y is at a maximum at x = L/, so and 39

40 3-Point Bending for Fracture Intensity 40

41 Four Point Bending 41

42 Four Point Bending Instrument 4

43 4-Point Bending Application Four point bending test With four point bending fixture a constant bending moment is achieved between the two indenters. In three point bending the moment increases linearly from support to the indenter. The strain (and stress) in four point bending varies linearly across the test specimen. e = M y / EI, where y is distance from the center of test sample Because the bending moment is constant between the indenters also the strain is. When the reliability of solder joints is tested 4 point bending test is good because all joints between the indenters are under equal loading. 43