0-7803-XXXX-X/04/$17.00 © 2004 IEEE 20th IEEE SEMI-THERM Symposium

Simulation Model Development for Solder Joint Reliability for High Performance FBGA Assemblies

Haiyu Qi1, Mikyoung Lee1, Michael Osterman1

Kyujin Lee2, Seyong Oh2 Tim Schmidt3

1 CALCE Electronic Products and Systems Center at University of Maryland, College Park, MD, 20770

2 IPT Samsung Electronics, Giheungeup, Yongin, Gyeonggido, Korea 3 Trilion Quality Systems, West Conshohocken, PA, 19248

Abstract New construction of fine pitch plastic ball grid array

(FBGA) has been investigated through experimentation and physics of failure (PoF) analysis based on the reliability point of view. In this study, a three dimensional FEA model was developed to understand the thermomechanical behavior of FBGA under cyclic thermal loading environments. Experimental measurement was also carried out and the package warpage information was recorded by high-resolution digital CCD cameras with a 3-D image correlation method to validate this FEA model. The validated FEA model was used to calculate the inelastic strain of FBGA package that is related to the fatigue life of solder joint.

Keywords FBGA, FEA, Warpage, Fatigue Life

1. Introduction Miniaturization and specific needs of portable products

are drivers for the introduction of chip scale packages. The fine pitch ball grid array (FBGA) package is a small ball grid array package, which can be considered to be a chip scale package. It has been developed for requirements in the area of high-end logic devices when high pin counts and high speed are required.

Figure 1 shows the vertical structure of a FBGA package with 2-layer PCB. It is constructed of a substrate onto which a die is mounted and an array of balls is attached. The die is encapsulated for protection. Configurations for a FBGA package include microBGA, die-up substra te BGA, die-down on center bond substrate BGA, and die down bumped die substrate BGA [1]. The most recent definition for a FBGA package from JEDEC requires the ball pitches to be 0.8 mm or less.

With the existing thin small outline packages (TSOP), the resulting inductance due to the lead frame can cause signal integrity problems when using high-speed SDRAM devices in large memory arrays. With FBGA packages, manufacturers of SDRAM devices get the advantage of using a near chip size package with minimal impact on the performance of the device.

In this study, the attention is focused on the thermomechanical modeling of the solder joint of a fine pitch ball grid array package. The mismatch in thermal expansion coefficients between the chip, FBGA package, solder balls, and the circuit board to which the FBGA is mounted, can induce substantial stress in the solder joint in thermal cycling. The accumulated inelastic strain may cause fatigue failures in the solder joint after a limited number of thermal cycles. The observed failure modes for FBGA are fatigue cracks that initiate and propagate through the solder joint near the substrate metallization pads and the PWB copper pads [2-5] or delamination at the interface between joined dissimilar material [6]. To make reliable products using FBGA, potential failure modes and failure mechanism must be identified.

The objective of this study is to develop the simulation model specification that can estimate the fatigue failure of the solder joint under cyclic thermal loading environments and can be used for the optimized design specification of FBGA package in order to achieve the highest thermal fatigue reliability as possible. For that, this study is divided into three parts: FEA model development, model verification, and solder joint reliability prediction.

2. Finite element analysis (FEA) model development Figure 2 shows the cross-sectional view of a FBGA44

soldered on a printed circuit board and Figure 3 shows solder ball arrangement and dimensions of the ball array. The geometric parameters and mechanical properties are listed in Tables 1 and 2. Temperature-dependent elastic properties were used for all materials except for the eutectic solders. The property data of the 63Sn37Pb eutectic solder was directly measured in the CALCE EPSC laboratory and used for simulation. Table 3 and 4 are linear isotropic material properties and creep properties for 63Sn37Pb eutectic solder. The CTE of solder is 21 ppm/ °C at 25 °C. The nonlinear stress vs. plastic strain curves of eutectic solder is shown in Figure 4 [7].

Based on the information stated above, a 3-D finite element model has been developed to analyze this FBGA44 configuration using ANSYS, a general purpose non-linear finite element program. Due to geometrical symmetry of the FBGA, only a quarter model of the package was modeled as shown in Figure 5. Symmetry boundary conditions are

Qi, Simulation Model Development for Solder Joint Reliability … 20th IEEE SEMI-THERM Symposium

applied to the vertical planes parallel to x-y plane and vertical plane at the origin. All other surfaces are unconstrained. The fine mesh model consists of 37,792 elements and 65,584 nodes.

3. FEA model verification Since assumptions are made during FEA model

development, verification and validation of a model is the key for accurate reliability prediction. Generally there are two ways to verify the FEA models. First one is to run reliability tests and compare the time to failure of products with the predicted life using models [2-3]. This approach is normally expensive and time consuming due to accelerated tests. The second approach is to use deformation information. For example, ball grid array packages (BGA) can warp due to local and global mismatch of the coefficients of thermal expansion and the asymmetric package geometry. This will affect the solder joint reliability. Therefore, to compare the predicted deformation with the experimental results is another alternative way to verify the FEA models.

Measurement methods to assess the design and process factors influencing warpage include electronic speckle pattern interferometry (ESPI), shearography, and moiré. The disadvantages of these methods are generally time consuming and expensive.

A non-contact and material independent determination of deformation and strain using three-dimensional image correlation method and high-resolution digital CCD cameras has now been successfully implemented for measuring package warpage 8. This method significantly reduces the cost and time-consuming preparation necessary. For fast measurements, standard cameras can gather data at up to 20 fps as necessary, and for high-speed manufacturing tests, high-speed cameras can gather data at 485 fps to rates of up to 10M fps, or in real-time.

To implement, a pattern which can be random or regular with good contrast is applied to the surface of the test object. The pattern deforms along with the object. The deformation of the object under different load conditions is recorded by two CCD cameras (Figure 6) and evaluated using digital image processing.

An initial image prior to loading defines a set of unique correlation areas across the entire area of interest. These areas are known as macro-image facets, typically 5-20 pixels across. The center of each facet is a measurement point that can be thought of as an extensometer and strain rosette. The macro-image facets are tracked in each successive image during loading with sub-pixel accuracy.

Using photogrammetric principles and sensitivity of 1/30,000 field of view, the 3D coordinates of the surface of the specimen, can be calculated. The results are the 3D coordinates of every point in the area of interest, the contour of the component, the displacements during the test, as well as the plane strain tensor 9. Although, only two image sets are required to measure the change from

minimum to maximum loads, multiple image sets provide a progressive measurement of the deformations and strains.

Warpage of FBGA at room temperature of 26oC was measured first as no loading condition. After heated up to 150oC in the oven and waited until cool down to 102oC, the second measurement was taken for loading condition. Subtraction of deformation under no loading condition and rigid body displacement from second measurement gives package warpage due to temperature drop. In figure 7, color plots of out-of-plane displacements using a 3-D image correlation measurement system show generally symmetrical behavior, which is expected under the applied thermal loading conditions. Figure 8 plots out-of-plane displacements along the diagonal arrow line shown in Figure 7. FBGA in this measurement has 44 solder balls with 4 balls located in outermost location (see Figure 3) and size of 7.5mm by 8.5mm.

To simulate the experimental loading conditions, the FEA model built previously was subjected to the temperature profile of Figure 9. The shape of package warpage in Figure 7 shows the temperature distribution through the package was not uniform. But, in simulation, a constant temperature was applied to FBGA. The temperature profile has the maximum and minimum temperature at 101°C and 71°C, respectively. The dwell period at extreme temperatures is 5 seconds. The ramp rate between extreme temperatures is considered as constant, 10°C/min. The loading and unloading portions of one thermal cycle last for 370 seconds.

For stabilization purpose, the FEA model was run through three complete temperature cycles. The out-of-plane displacements in quarter model were shown in Figure 10. The outputs of the model to be compared with testing result are the out-of-plane displacements along the diagonal line from the center of package to the corner. The diagonal line is shown in Figure 10. Under cyclic temperature profile, the maximum out-of-plane displacement occurred at the center of package, 4.38 µm, and the minimum out-of-plane displacement occurred at the corner of package, 1.90 µm.

From Figure 10, we can see the displacement profiles from both simulations and experiments are similar qualitatively. The difference between simulation and experimental results at package center and corner are listed in Table 6. At the center of package (the origin point in Figure 10), the displacement difference between experiments and simulations is 0.96 µm. At the corner of package, the displacements from experiment and simulation are very close. The errors between simulation result and experimental result are from 21.9% to 6.8% depending on locations. Difference between experiment and simulation may be caused by variation of material properties and discrepancy in thermal loading conditions between experiment and simulation. Since the warpage in simulation shows that model gives too conservative result, the calibrated thermal loading conditions will be recommended during the estimation of life cycle and design optimization analysis.

Qi, Simulation Model Development for Solder Joint Reliability … 20th IEEE SEMI-THERM Symposium

4. Solder joint reliability Several finite element based analysis methodologies

have been proposed to predict solder joint fatigue life [10-20]. Of all these methodologies, Garofalo’s seems to be the most popular due to the ease in its implementation. Garofalo’s methodology links laboratory measurements of low-cycle fatigue crack initiation and crack growth rates to the inelastic work. It is a strain energy based approach, where the work term consists of time-dependent creep and time-independent plasticity. This inelastic behavior is captured in ANSYS using Garofalo’s constitutive model [21]. The modeling methodology utilizes finite element analysis to calculate the strain energy density accumulated per cycle during thermal or power cycling. The strain energy density is then utilized with crack growth data to calculate the number of cycles to initiate a crack, and the number of cycles for the crack to propagate across a solder joints diameter.

Garofalo’s methodology has been previously presented in the successful analyses of various electronic assemblies from multiple industry sources [22-25](e.g. Amagai [13]; Fusaro and Darveaux [14]; Dougherty et al. [15]; Johnson [16]; and Zahn [17]). Recently, Zahn [18] and Goetz and Zahn [19] extended the methodology to predict both solder ball and solder bump reliability of a multi-chip silicon based system-in-package. In many of these publications, the authors have presented reliability test data that validates the accuracy of the methodology within +/-2X, which is considered state of the art for this type of complex physical analysis.

In this study, Garofalo’s methodology were implemented with known constants (listed in Table 4. Under accelerated temperature cycle conditions, the inelastic strain energy, ∆W, that accumulates in the solder joints per temperature cycle was calculated with FEA model described previously. The calculation of strain energy density is restricted to a single layer of solder elements in contact with the pads of each joint due to previous observations on BGA failure modes [2-5]. The ∆W is then related to the thermal cycles to crack initiation and crack propagation rate using the Darveaux total energy damage constants [28] to calculate the Weibull characteristic life (α, 63.2% failure). The characteristic life of the package was defined as the characteristic life of the worst-case solder joint interface.

A typical accelerated temperature range to simulate for life prediction was chosen from –25 to 125 °C with total 30 minutes per cycle. The dwell times at maximum and minimum temperature are 2 minutes and 7 minutes, respectively. The ramp time from minimum temperature to maximum temperature is 13 minutes and 8 minutes in reverse. The model solution phase required about 14 hours on a well-equipped 2.8 GHz PC. Figure 11 shows the vonMises strain and ∆W distribution within the solder joint which had the maximum vonMises stress and strain. The prediction of Weibull characteristic life for the FBGA44 assembly in this study was 168 cycles (84 hours) based on this sold joint,

which was shown in Figure 12 as the outmost solder joint. If predicting the life of the FBGA44 based on the inelastic strain energy of the second outmost solder joint (shown in Figure 11), the characteristic life was extended to 462 cycles (231 hours), almost 3 times than in previous case. This simulation result reveals that inelastic strains accumulated in the outmost solder balls from the center during thermal cycling express high risk of problems such as crack between solder ball and substrates or delamination between package and solder ball.

5. Conclusions A 3-D FEA model was developed and used to analyze

the deformation, strain, and stress of a FBGA package in this study. This model was validated with out-of-plane displacement measured in experiments with a 3-D image correlation method. The results showed this FEA model is fairly accurate to simulate the physical phenomena of FBGA under thermal cycling loadings.

Under a specific temperature cycling environment, the characteristic life of a FBGA package was predicted using this FEA model. Simulation result reveals that the outmost solder ball from the center was the most suspicious damageable ball. The damage can be cracks between solder balls and substrate or delamination between package and solder balls. Therefore, this solder ball is recommended not as normal electrically functioning permanent interconnector but the buffer to absorb the extra strain energy.

Further study will be continued on design of experiments (DOE) or parametric study on material properties, package geometry, architecture, and applications to improve FBGA assemblies reliability and introduce design guidelines for reliable FBGA s.

References 1. Ross, Andrew C., “Stacked fine-pitch BGAs,”

Advanced Packaging, June 2002, pp. 27-30. 2. Burnette T., Johnson Z., Koschmieder T., and Oyler W.,

“Underfilled BGAs for Ceramic BGA Package and Board-Level Reliability”, Electronic Components and Technology Conference, 2000. Proceedings, 50th, May 21-24, 2000, pp. 1221 –1226.

3. Burnette T., Johnson Z., Koschmieder T., and Oyler W., “Underfilled BGA for A Variety of Plastic BGA Package Types and The Impact on Board-Level Reliability”, Electronic Components and Technology Conference, 2001. Proceedings, 51st, 2001, pp. 1045 –1051.

4. Guha R., Humphrey D., Prodromides K., Burnette T., Koschmieder T., Osterman M., Qi H., Kennedy J., and Veum J., “Plastic Ball-Grid-Arrays (PBGA): Are They Ready for Environmentally Harsh Aerospace Applications?” 02WAC-149, World Aviation Congress & Display (WAC), Phoenix, Arizona, November 5-7, 2002.

5. Baumann T., Burnette T., Humphrey D., Kennedy J., Koschmieder T., Osterman M., Qi H., Prodromides K., and Veum J., “Underfilled PBGA Packages and Their

Qi, Simulation Model Development for Solder Joint Reliability … 20th IEEE SEMI-THERM Symposium

Board Level Cycling and Vibration Performance”, SMTA International 2002, Donald Stephens Convention Center, Rosemont, Illinois, Sept. 24-26, 2002.

6. Mikyoung Lee and I. Jasiuk, “Asymptotic Expansions for the Thermal Stresses in Bonded Semi-infinite Bimaterial Strips,” ASME Journal of Electronic Packaging, Vol. 113, No. 2, June, 1991

7. Calce Internal Report, “Thermal-mechanical Fatigue Modeling Using ANSYS”, 2003.

8. Mikyoung Lee, M. Pecht, J. Tyson, and T. Schmidt, “3D Measurement System for Use in Microelectronics,” Advanced Packaging, November 2003, pp. 33-34.

9. Tyson, J., Schmidt, T., Galanulis, K., “Advanced Photogrammetry for Robust Deformation and Strain Measurement,” Proceedings of SEM 2002 Annual Conference, Milwaukee, WI, June 2002.

10. Engelmaier, W., “Functional Cycling and Surface Mounting Attachment Reliability,” ISHM Technical Monograph Series 6894-002, ISHM, 1984, pp. 87-114.

11. Shine, M.C. and Fox, L.R., Fatigue of Solder Joints in Surface Mount Devices,” ASTM STP 942, Low Cycle Fatigue, Philadelphia PA, 1988, pp. 588-610.

12. Wong, B., Helling, D.D., and Clark, R.W., “A Creep-Rupture Model for Two-Phase Eutectic Solders,” IEEE CHMT, Vol. 11, No. 3, September 1988, pp. 284-290.

13. Yamada, S.E., “A Fracture Mechanics Approach to Soldered Joint Cracking,” IEEE CHMT, Vol. 12, No. 1, March 1989, pp. 99-104.

14. Subrahmanyan, R., “A Damage Integral Approach for Low-Cycle Isothermal and Thermal Fatigue,” Ph.D. Thesis, Cornell University, 1991.

15. Dasgupta, A., Oyan, C., Barker, D., and Pecht, M., “Solder Creep-Fatigue Analysis by an Energy-Partitioning Approach,” ASME Journal of Electronic Packaging, Vol. 114, June 1992, pp. 152-160.

16. Pao, Y.H., “A Fracture Mechanics Approach to Thermal Fatigue Life Prediction of Solder Joints,” IEEE CHMT, Vol. 15, No. 4, 1992, pp. 559-570.

17. Clech, J.P., Manock, J.C., Noctor, D.M., Bader, F.E., and Augis, J.A., “A Comprehensive Surface Mount Reliability Model (CSMR) Covering Several Generations of Packaging and Assembly Technology,” Proceeding of, 43rd Electronic Components & Technology Conference, June 1993, pp. 62-71.

18. Syed, A.R., “Creep Crack Growth Prediction of Solder Joints During Temperature Cycling – An Engineering Approach,” Transactions of the ASME, Vol. 117, June 1995, pp. 116-122.

19. Darveaux, R., Banerji, K., Mawer, A., and Dody, G., “Reliability of Plastic Ball Grid Array Assembly,” Ball Grid ArrayTechnology, J. Lau, ed., McGraw-Hill, Inc. New York, 1995, pp. 379-442.

20. Darveaux, R., “Solder Joint Fatigue Life Model,” Proceedings of TMS Annual Meeting, Orlando FL, February 1997, pp. 213-218.

21. Anand, L., “Constituitive Equations for the Rate-dependent Deformation of Metals at Elevated Temperatures,” Trans. ASME J. Eng. Matl’s and Tech., Vol. 104, No. 1, pp. 12-17.

22. Amagai, M., “Chip Scale Package (CSP) Solder Joint Reliability and Modeling,” Proceedings of 36th International Reliability Physics Symposium, 1998, pp. 260-268.

23. Fusaro, J. and Darveaux, R., “Reliability of Copper Base-Plate High Current Power Modules,” Int. J. Microcircuits and Electronic Packaging, Vol. 20, No. 2, 1997, pp. 81-88.

24. Dougherty, D., Fusaro, J., and Culbertson, D., “Reliability Model for Micro-Miniature Electronic Packages,” Proceedings of International Symposium on Microelectronics, 1997, pp. 604-611.

25. Johnson, Z., “Implementation of and Extensions to Darveaux’s Approach to Finite- Element Simulation for BGA Solder Joint Reliability,” Proceedings of 49th Electronic Components & Technology Conference, June 1999, pp. 1190-1195.

26. Zahn, B.A., “Finite Element Based Solder Joint Fatigue Life Predictions for a Same Die Size, Stacked, Chip-Scale, Ball Grid Array Package,” Not Yet Published, October 2000.

27. Darveaux R., “Solder Joint Fatigue Life Model,” Proc. The Metallurgical Society Annual Meeting, Orlando, FL, February 1997, pp. 213-218.

28. Darveaux R., Banerji, K., Mawer A., and Dody G., “Reliability of Plastic Ball Grid Array Assembly,” Ball Grid Array Technology, Lau, J., ed., McGraw-Hill, Inc., New York, pp. 379-442, 1995.

29. Darveaux R., “Effect of Simulation Methodology on Solder Joint Crack Growth Correlations” Proceedings of 50th Electronic Component & Technology Conference, May 2000, pp. 1048-1058.

Qi, Simulation Model Development for Solder Joint Reliability … 20th IEEE SEMI-THERM Symposium

EMC

Die (Face-up)

Die Adhesive

PSR(Photo Sensitive Resist)

BT Core(BT resin +Fabric Glass Fibers)

Copper PatternPTH

Plated Through Hole

Solder Ball

Gold Wire

9.02mm8m

m9.02mm

8mm

Figure 1. Vertical Structure and Top View of a FBGA Package with 2-Layer PCB (Courtesy of Samsung)

Figure 1: Vertical Structure and Top View of a FBGA Package with 2-Layer PCB (Courtesy of Samsung)

Figure 2: Cross-sectional View of FBGA Soldered on a Printed Circuit Board

Name Dimension (mm)

44FBGA package dimension (x X y) 9 X 8

Overmold thickness 0.56

Substrate thickness 0.16

Pitch 0.5

Die dimension (x X y X z) 4.5 X 6 X 0.25

Board thickness 0.95

Solder ball collapse height 0.19

Solder joint width 0.38

Pad thickness 0.038 mm for package, 0.054mm for board

Ball layout Full array Table 1: Geometry Parameters used in the Analysis

Figure 3: Solder Ball Arrangement and Dimension of Ball Array

Qi, Simulation Model Development for Solder Joint Reliability … 20th IEEE SEMI-THERM Symposium

Material E (GPa) CTE (ppm) Poisson’s ration

Silicon 168.9 2.6 0.3615

FR4 (board) 22 18 0.28

KMC212-3 (overmold) 21 12.66 0.3

BT (substrate) 26 15 0.22

Cu (pad) 121 17 0.3 Table 2: Material Properties (Samsung)

T1 T2 T3 T4 T5 T6 T7 T8

Temp (K) 193 208 218 273 298 338 378 398

Elastic Modulus (MPa), x 54497 50994 48658 35812 29973 20629 12455 12450

Poisson Ratio, xy 0.35 0.35 0.35 0.35 0.35 0.35 0.35 0.35 Table 3: Linear Isotropic Material Properties for 63Sn37Pb Eutectic Solder

T1 T2

Temperature (K) 218 398

C1 1239.5 255.95

C2 0.053523 0.14197

C3 3.3 3.3

C4 6359.5 6359.5

Table 4: Creep Table--Generalized Garofalo (Secondary) [7]

0

20

40

60

80

100

120

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

Plastic Strain

Str

ess

(MP

a)

-80 °C

-65 °C

0°C

65 °C

105 °C

Figure 4: Nonlinear Stress vs. Plastic Strain Curve for Solder Material [7]

Qi, Simulation Model Development for Solder Joint Reliability … 20th IEEE SEMI-THERM Symposium

Figure 5: High-resolution Finite Element Model of a Quarter of the FBGA Assembly

Figure 6: A 3-D Image Correlation Measurement System (left) and Setup Shot Showing Pair of Cameras with 50 mm Lenses and Built-in Macro Rings Aimed at the Prepared FBGA Assembly (right).

Figure 7: Out of Plane Displacement of Package Surface including a Printed Circuit Board on Left Hand Side

Figure 8: Out of Plane Displacement in Diagonal Line (a. Arrow of Lower Right to Upper Left, b. Arrow of Lower Left to Upper Right)

Qi, Simulation Model Development for Solder Joint Reliability … 20th IEEE SEMI-THERM Symposium

0

20

40

60

80

100

120

0 200 400 600 800 1000 1200 1400 1600

Time, sec

Tem

p, C

Figure 9: Temperature Profile used in the Simulation

Experiments

(µm)

Simulation

(µm)

Error

(%)

Center 3.42 4.38 21.9

Corner 1.77 1.90 6.8 Table 6: Comparison Between Out-of-Plane Displacements from Simulations and Experiments

0 . 0 0 1 5

0 . 0 0 2

0 . 0 0 2 5

0 . 0 0 3

0 . 0 0 3 5

0 . 0 0 4

0 . 0 0 4 5

0 . 0 0 5

0 0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4 4 . 5 5D i a g o n a l L i n e ( m m )

Ou

t o

f P

lan

e D

isp

lacm

ent

(mm

)

E x p e r i m e n t a l S i m u l a t i o n

0 . 0 0 1 5

0 . 0 0 2

0 . 0 0 2 5

0 . 0 0 3

0 . 0 0 3 5

0 . 0 0 4

0 . 0 0 4 5

0 . 0 0 5

0 0 . 5 1 1 . 5 2 2 . 5 3 3 . 5 4 4 . 5 5D i a g o n a l L i n e ( m m )

Ou

t o

f P

lan

e D

isp

lacm

ent

(mm

)

E x p e r i m e n t a l S i m u l a t i o n

Figure 10: Out-of-Displacement in a FEA Quarter Model and along the Diagonal Line from the Center of Package to the Corner

Figure 11: vonMises Strain (left) and Inelastic Strain Energy Density Distribution (right) in Solder Joint (Outmost Ball from the Center in the Quarter Model), after Three Complete Temperature Cycles.

The outermost solder ball

The second outermost solder ball

The outermost solder ball

The second outermost solder ball

Figure 12: Location of Solder Joint Balls