Microelectronics Reliability

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1 Microelectronics Reliability 52 (2012) Contents lists available at SciVerse ScienceDirect Microelectronics Reliability journal homepage: Failure mechanism of FBGA solder joints in memory module subjected to harmonic excitation Yusuf Cinar a, Jinwoo Jang a, Gunhee Jang a,, Seonsik Kim b, Jaeseok Jang b, Jinkyu Chang b, Yonghyun Jun b a Department of Mechanical Engineering, Hanyang University, 17, Haengdang-dong, Seongdong-gu, Seoul , Republic of Korea b Memory Division, Samsung Electronics Co. Ltd., San #16, Banwol-Ri, Taean-Eup, Hwasung-City, Gyeonggi-Do , Republic of Korea article info abstract Article history: Received 7 October 2010 Received in revised form 2 November 2011 Accepted 23 November 2011 Available online 20 December 2011 This paper investigates the failure mechanism of Fine-pitch Ball Grid Array (FBGA) solder joints of memory modules due to harmonic excitation by the experiments and the finite element method. A finite element model of the memory module was developed, and the natural frequencies and modes were calculated and verified by experimental modal testing. Modal damping ratios are also obtained and used in the forced vibration analysis. The experimental setup was developed to monitor resistance variation of FBGA solder joints due to the harmonic excitation under Joint Electron Devices Engineering Council (JEDEC) standard service conditions. Experiments showed that the failure of the solder joints of the memory module under vibration mainly occurs due to resonance. Forced vibration analysis was performed to determine the solder joints having high stress concentration under harmonic excitation. It showed that failure occurs due to the relative displacement between PCB and package and solder joints are the most vulnerable part of the memory module under vibration. It also showed that cracked solder joints in the experiments match those in the simulations with the highest stress concentration. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Memory modules are composed of a number of packages or dynamic random access memory integrated circuits modules mounted on a printed circuit board (PCB), designed for use in personal computers, workstations, and servers. There are various types of memory modules, such as Single In-Line Memory Module (SIMM), Dual In-Line Memory Module (DIMM), and Small Outline Dual In-Line Memory Module (SO-DIMM). Packages are mounted on PCBs through solder balls called ball grid arrays (BGAs). Solder balls are further used to provide the electrical signals between package chips and the PCB. Memory modules are exposed to vibration over various frequency ranges [1], which may result in the malfunction of products. Reliability and performance of memory modules are standardized by the Joint Electron Devices Engineering Council (JEDEC). Performance and fatigue life time of the memory modules mostly depend on the solder joints. According to a report released by a semiconductor company, solder joint cracks reflected approximately 40% of total failure of memory modules. Therefore, it is important to understand the dynamic behavior of memory modules, as well as the behavior of solder joints subjected to vibration. Corresponding author. Address: PREM, Department of Mechanical Engineering, Hanyang University, 17, Haengdang-dong, Seongdong-gu, Seoul , Republic of Korea. Tel.: ; fax: address: ghjang@hanyang.ac.kr (G. Jang). Many researchers have investigated the fatigue life reliability of packages or memory modules due to thermal load by experiment and simulation [2 4]. Once vibration was realized to be one of the dominant failures for electronic components, the electronic packaging industry began to show interest in understanding its failure mechanism under vibration. Basaran and Chandaroy [5] presented a study based on a viscoplastic model to compute damage mechanics of a solder joint with Pb40/Sn60 solder alloys under different dynamic load conditions. They showed that fatigue life of solder alloys is greatly affected by dynamic loads and the frequency ranges of applied loads. Kim et al. [6] investigated the high cycle vibration fatigue life characteristics of Pb-free and Pb packages under various mixed mode stresses by experiment and FEM. They observed the failure process of a solder ball at low frequency during the fatigue test by using an optical microscope. Wu [7] utilized the global local modeling concept to calculate von Mises stress in solder joints of interest under random vibration loading. Then, she predicted the fatigue life of solder joints by using a damage model, called the Basquin power-law relation. Che and Pang [8] studied flip chip solder joints under out-of-plane sinusoidal vibration load with constant and varying amplitudes and they used Miner s cumulative damage law to estimate fatigue life of flip chip solder joints. Zhou et al. [9,10] investigated the vibration durability of Sn3.0Ag0.5Cu (SAC305) and Sn37Pb solder interconnects under narrow-band harmonic vibration and random vibration. They found that SAC305 interconnects have lower fatigue durability than Sn37Pb interconnects. However, prior researchers did not /$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi: /j.microrel

2 736 Y. Cinar et al. / Microelectronics Reliability 52 (2012) Fig. 1. A DDR3 SDRAM type memory module: (a) front view, (b) back view and (c) side view. Table 1 Dimensions of bare PCB and packages of a memory module. Components Number of components (EA) Dimensions in mm (L W H) Bare PCB Package Package register investigate failure mechanism of the solder joint in terms of resistance variation over high frequency ranges, as well as the most vulnerable solder joint of the memory module under vibration. This paper investigates the failure mechanism of Fine-pitch Ball Grid Array (FBGA) solder joints in daisy chains assembled memory module with double-sided packages due to harmonic excitation by using the experiments and the finite element method. The experimental setup was developed to monitor resistance variation of FBGA solder joints due to the harmonic excitation of the JEDEC standard service condition 1 [11]. A finite element model of the memory module was developed, and the natural frequencies and modes were calculated and verified by experimental modal testing. Forced vibration analysis was performed to correlate the cracked solder joints in experiments with the solder joint of high stress concentration in simulation. There are two types of solder balls used in the packages and the package register. The solder balls of the packages and the package register have diameters of 0.4 mm and 0.3 mm and heights of 2. Analysis model Fig. 1 shows a double-data-rate three synchronous dynamic random access memory (DDR3 SDRAM) type memory module used in this research. It is mainly composed of a PCB, packages and package register, and solder balls. The PCB is composed of 10 layers of copper conductor and FR-4, while the packages and package register are composed of many integrated circuits (ICs). This memory module has 36 packages and 1 package register. Table 1 shows the mechanical dimensions of PCB, package, and package register. Fig. 2. BGA patterns of package and package register: (a) package and (b) package register.

3 Y. Cinar et al. / Microelectronics Reliability 52 (2012) Fig. 3. Finite element model of a memory module: (a) front view, (b) back view and (c) side view. Table 2 Material properties of each component of a memory module. Component Material type Density (kg m 3 ) Young s modulus (MPa) Poisson s ratio ( ) Element type Element number Bare PCB FR , Brick 78,368 Package , Brick 77,184 Package register , Brick 6964 Solder ball Sn Ag Cu , Brick 46,337 Fig. 4. Experimental setup for modal testing mm and mm, respectively. Solder balls are modeled as an octagonal structure with sixteen elements. The BGAs of the package and package register have different patterns, as shown in Fig. 2. The red square 1 in package and package register in Fig. 2 shows their orientation in memory module in Fig For interpretation of color in Figs. 1 3,8,9, the reader is referred to the web version of this article. 3. Free vibration analysis A finite element model of the memory module was developed, as shown in Fig. 3. Table 2 shows the material type, properties, element type and number of each component. The PCB, packages, package register, and solder balls are modeled by the linear brick elements with eight nodes, and each node has three degrees of freedom. The total number of elements was 208,853. In the finite element model, packages, solder balls and PCB were connected

4 738 Y. Cinar et al. / Microelectronics Reliability 52 (2012) Table 3 Simulated, measured natural frequencies and damping ratios of the memory module. Mode number Simulation Experiment Error (%) Damping ratio (%) f (Hz) f (Hz) Mode Mode Mode Mode Mode Mode by node sharing. DDR type memory modules are usually inserted in a slot in a motherboard so that the finite element model is assumed to have a fixed boundary condition of two short sides and one long side and a free boundary condition of one long side. The natural frequencies and mode shapes were calculated by using ANSYS. Experimental modal testing was performed to verify the proposed simulated results. Fig. 4 shows the experimental setup for the modal testing. An impact hammer was used to excite a memory module. The responses on the memory module were measured at 18 points by using a laser Doppler vibrometer (sensor) and PULSE 3560C (signal analyzer). The mode shapes were determined by using the STAR modal system. Table 3 shows the comparison between the simulated and experimental natural frequencies of the memory module. Fig. 5 shows the comparison between the simulated and experimental mode shape of vibration modes 1, 2, and 3 of the memory module. Table 3 and Fig. 5 show that the simulated natural frequencies and the mode shapes match well with the experimental ones. 4. Experiment to detect failure mechanism of memory module Failure mechanism of the solder joints can be observed by monitoring the resistance increase of the memory module with daisy chain assembly, because failure of solder joints due to crack increase resistance. Except package register, every package on the memory module was assembled with daisy chains. The daisy chains of the packages in the front and back side of the memory module were connected in series. Total resistance of daisy chain of the memory module was measured to be around X. A failure of the solder joint is assumed to occur when total resistance reaches greater than 100 X [9]. An experimental setup was developed to detect failure mechanism of the memory module, as shown in Fig. 6. An EDS type electro-dynamic shaker was used to excite the memory module fixed on a jig. The variation in input acceleration profile was measured by an accelerometer on the top of the shaker and was controlled by VibrationVIEW software, shown on the right side in Fig. 6. A potential of 2.5 V was supplied to the memory module with daisy chain assembly from a power supply. Variations in acceleration on top of the shaker, current, and voltage were measured simultaneously in real time, and then acceleration and resistance (voltage/current) were obtained using an oscilloscope. Three sides of memory module were fixed to the jig mounted on the top of a shaker, shown in Fig. 6, to maintain the same boundary conditions as a DDR memory module. The memory module in this research was excited by the JEDEC service condition [11]. Fig. 7 shows the variation of input force in terms of acceleration. The harmonic excitation was applied to the memory module in a constant peak displacement (1.5 mm) from 20 Hz to 80 Hz (cross-over frequency). A constant acceleration level was then applied between 80 Hz and 2000 Hz. A complete sweep of the test frequency range, which goes from 20 Hz to Fig. 5. Comparison of simulated and experimental results of three modes (a) mode 1, (b) mode 2 and (c) mode Hz and back to 20 Hz, is repeated in a logarithmic manner every 4 min. The sweep rate is 1 decade/min. Harmonic excitation is applied in the out-of-plane direction (Z direction) because pretests showed that the solder joints are more vulnerable in the Z direction than in the X and Y directions. Fig. 8 shows the variation in resistance in the developed experiment. Red dotted lines in Fig. 8 show the first, second, and third natural frequencies of the memory module corresponding to 885, 1260, and 1890 Hz. Total resistance of the memory module with the daisy chain was measured to be around X before experiment is performed. Failure mechanism due to resistance variation under vibration is presented in Fig. 8. Fig. 8a shows that the resistance of the memory module did not change until 12 min. The resistance slightly increased around the 1st natural frequency of the memory module between 12 and 14 min, as shown in the left plot of Fig. 8b. While the sweep frequency was returning to 20 Hz from 2000 Hz, the resistance increased around its 1st natural frequency again between 14 and 16 min, as shown in the right plot of Fig. 8b. Fig. 8c shows that the resistance continuously increased around the 1st, 2nd, and 3rd natural frequencies of the memory

5 Y. Cinar et al. / Microelectronics Reliability 52 (2012) Fig. 6. Experimental setup to monitor failure mechanism. method and mode superposition. The damped global finite element equation of the memory module due to harmonic excitation is given by ½MŠf xðtþg þ ½CŠf_xðtÞg þ ½KŠfxðtÞg ¼ fpðtþg ð1þ where [M], [C], [K] and {P(t)} are the mass, damping, stiffness matrices and excitation force vector, and f xðtþg, f_xðtþg and {x(t)} are the nodal acceleration, velocity and displacement vectors. In the large sized problem, the direct integration technique to obtain the forced response is not the computationally efficient method [12]. The forced response, {x(t)}, can be approximated by using the mode superposition method as follows: Fig. 7. Variation of acceleration input of JEDEC service condition 1. module between 28 and 32 min. Finally, the resistance of the memory module at the 1st natural frequency increased bigger than 100 X between 44 and 48 min, as shown in Fig. 8d. Crack growth reduces stiffness of the memory module, which decreases natural frequency of the memory module. As the experiment goes by, frequency range at which resistance increases near natural frequencies gradually becomes wider as shown in Fig. 8. In this work, the same experiment was performed for six memory modules with daisy chains. Time to failure is approximately min, and packages near the fixed boundary shown in Fig. 5 were the frequently failed packages. One of the most frequently failed packages is the package 5 and 33 shown in Fig. 1. Crack propagation is also found in package 1. Fig. 9 shows the cross-sectional view of the solder ball of packages 1 and 5 of the memory module after the molding, cutting, and polishing process. The crack in the solder ball is located near the PCB side. 5. Finite element forced vibration analysis The dynamic response of a memory module subjected to harmonic excitation was investigated by using the finite element fxðtþg Xn f/ i gfz i ðtþg ð2þ i¼1 where {/ i } is the i-th mode vector which is the eigenvector calculated by the free vibration analysis. {z i (t)} and n are modal displacement and the number of modes to be used in calculation of the displacement vector, {x(t)}, respectively. Substituting Eqs. (2) into (1) and rearranging it after multiplying with {/ i } T, the decoupled n equations of motion are obtained as follows: ½UŠ T ½MŠ½UŠf zgþ½uš T ½CŠ½UŠf_zgþ½UŠ T ½KŠ½UŠfzg ¼½UŠ T fpg Then Eq. (3) can be written in terms of modal coordinates as follows: z i þ 2x i n i _z i þ x 2 i z i ¼ Q i ði ¼ 1...nÞ ð4þ where x i and n i donate natural frequency and damping ratio of i-th mode, respectively. Eq. (4) becomes a set of n decoupled differential equations for the damped system and the solution in terms of modal coordinate, z i, can be obtained by using the time integration. Then the displacement, {x(t)}, can be calculated with the Eq. (2). Frequency response of a memory module was obtained experimentally, and the half power method was used to calculate modal damping ratios of the memory module from Eq. (5) for its six modes shown in Table 3 [13]. They were inserted into Eq. (4). ð3þ

6 740 Y. Cinar et al. / Microelectronics Reliability 52 (2012) Fig. 8. Variation of the resistance of the memory module during the harmonic excitation of the JEDEC service condition 1: (a) from 0 to 12 min, (b) from 12 to 16 min, (c) from 28 to 32 min and (d) from 44 to 48 min. n i ¼ 1 Dx 2 x i ð5þ pðtþ ¼p _ ðtþ sin xðtþ ð6þ where Dx is the width of the frequency response curve at the half power level. The harmonic excitation force of JEDEC service condition 1 can be written as follows: In the case of logarithmic sweep, the variation of frequency with respect to time is given by xðtþ ¼x s 2 3:3219St=60 ð7þ

7 Y. Cinar et al. / Microelectronics Reliability 52 (2012) Fig. 11. Convergence of the displacement of the package 1 with the increase of mode number. Fig. 9. Crack of the solder ball: (a) N6 solder joint of package 5 and (b) A1 solder joint of package 1. where x s is the starting frequency and S is the sweep rate in decade per minute. The sweep rate in JEDEC service condition 1 is defined as 1 decade/min. Combining Eqs. (7) and (6) yields the force function for the logarithmic sweep as follows [14]: pðtþ ¼p _ 60x s ðtþ S lnð2þ 23:3219St=60 1 þ b ð8þ where p _ ðtþ ¼M n a 0 g and M n, a 0 and g are the nodal mass, applied acceleration corresponding to JEDEC service condition 1 which is 20G and gravitational acceleration, respectively and b represents phase. Fig. 10 shows the finite element model for the forced vibration analysis. Two short sides and one long side of the finite element model were rigidly coupled with a nodal mass, while the base excitation in Fig. 7 is applied. The solution z i of Eq. (4) is obtained by using numerical integration with an integration time step of s. Numerical integration was performed around the natural frequencies over 2 s with the starting frequency of 830 Hz, 1222 Hz and 1852 Hz for the first three modes of memory module, respectively. The accuracy of the mode superposition depends on how many modes are superposed to calculate the response, as shown in Eq. (2). Convergence of the proposed method was verified by increasing the superposed mode numbers. Displacement response on any location on the finite element model of memory module due to increasing number of modes can be observed to determine the number of modes to superpose in the calculation. The package 1 is selected to verify the accuracy of the proposed method. Fig. 11 shows the calculated axial displacement on the package 1 near the first natural frequency of the memory module, indicating that 40 or more than 40 modes are sufficient to guarantee the accuracy. But increasing number of superposed modes increases computation time, therefore 50 modes was selected to calculate the response under vibration. Package 5 is one of the most failed packages from the experiments. Therefore displacement variation on package 5 of the memory module is obtained and shown in Fig. 12 for comparison. As shown, the biggest displacement occurs at the first natural frequency for package 5. Displacement variation around modes 2 and 3 are relatively small, as shown in Fig. 12b and c. Under the harmonic excitation in Z direction, Fig. 13 shows displacement distribution and von Mises stress distribution at the first resonance frequency for the section view of package 5 (P5) and package 15 (P15) and their symmetric packages (package 33 (P33) and package 24 (P24)) on the back side of the memory module shown as yellow dotted border in Fig. 3a and b. Table 4 shows the absolute and relative Z displacements for P5-N6 (near fixed edge) and P15-J6 (near free edge) solder joints at the first resonance frequency. Though the absolute displacements at the free edge of memory module near P15 and P24 is bigger than that at the fixed edge as shown in Fig. 13a, the relative displacements at Fig. 10. Finite element model of a memory module for harmonic analysis.

8 742 Y. Cinar et al. / Microelectronics Reliability 52 (2012) Fig. 13. Displacement and stress distribution on the 1st natural frequency for the section view of yellow dotted border in Fig. 3a: (a) Distribution of Z directional displacement and (b) von Mises stress distribution. Table 4 Absolute and relative displacements of P5-N6 and P15-J6 solder joints. Solder joint Absolute displacement (mm) PCB Package P5-N P15-J Relative displacement (PCB-package) (mm) Fig. 12. Variations of displacement on package 5 with time: (a) the response around the 1st natural frequency, (b) the response around the 2nd natural frequency and (c) the response around the 3rd natural frequency. the fixed edge of memory module near P5 and P33 is bigger than that at the free edge as shown in Table 4. It results in bigger von Mises stress on the solder joints of packages (P5 and P33) near the fixed boundary than that near free edge as shown in Fig. 13b. The von Mises stress was calculated to determine the stress distribution on package 5 and the vulnerable solder balls where failure may occur. Stress distribution in the BGA for package 5 at the first natural frequency is represented in Fig. 14. The two solder balls with the highest stress concentration are N1 and N6 on package 5, while the cracked solder ball during the experiment was N6, whose location is shown with a red star (right corner of P5) in Fig. 1a. Fig. 15 shows the variation in the von Mises stress of solder balls P5-N1 and P5-N6, due to changes in excitation frequency. Stress level is averaged at the node between solder ball and PCB interface. As shown, their stress levels are similar and higher than the rest of the solder balls in BGA of package 5. Fig. 16 shows the stress distribution of the solder ball at P5-N6 and the adjacent PCB and package. As shown, the stress concentration occurs at the solder ball just over the PCB, which matches with the cracked location of the solder ball in Fig. 9a. 6. Conclusion Fig. 14. Stress distribution of BGA for package 5. This paper presents an investigation of failure mechanism of FBGA solder joints of memory modules due to harmonic excitation by using the experimental and finite element methods. The experimental setup was developed to monitor and to characterize the failure mechanism of FBGA solder joints due to the harmonic excitation of JEDEC standard service condition 1. Results showed that the crack of the solder joints of the memory module under vibration mainly occurs due to resonance. A finite element model of the memory module was developed, and the natural frequencies and modes were calculated and verified by experimental modal testing. Forced vibration analysis was performed to determine

9 Y. Cinar et al. / Microelectronics Reliability 52 (2012) the high-stress concentration solder joints under harmonic excitation. Modal damping ratios are also used in the forced vibration analysis. It showed that the failure occurs due to the relative displacement between PCB and package and solder joints are the most vulnerable part of the memory module under vibration. It also showed that cracked solder joints in the experiments match those in the simulations with the highest stress concentration. Furthermore, the most vulnerable part of the memory module under vibration was found to be the solder joint near the PCB. The proposed experiment and simulation will be applied to develop robust memory module designs and solder joint fatigue failure analysis will be studied in a future paper. Acknowledgments Fig. 15. Comparison of stress variation for N-1 and N-6 solder balls on package 5. Fig. 16. Stress distribution around N-6 solder joint of package 5. This research has been supported by Samsung Electronics, and Y. Cinar thanks the Korean National Institute for International Education (NIIED) for a scholarship supporting his Ph.D work. References [1] Steinberg DS. Vibration analysis for electronic equipment. 3rd ed. New York: Wiley; [2] Pidaparti RMV, Song X. Fatigue crack propagation life analysis of solder joints under thermal cyclic loading. Theor Appl Fract Mech 1996;24: [3] Che FX, Pang HLJ. Thermal fatigue reliability analysis for PBGA with Sn 3.8Ag 0.7Cu solder joints. In: Proc 6th electronics packaging technology conference, Singapore; December 8 10, p [4] Che FX, Pang HLJ, Zhu WH, Sun Wei, Sun Anthony YS, Wang CK, et al. Development and assessment of global local modeling technique used in advanced microelectronic packaging, EuroSime2007, London, UK; April 15 18, p [5] Basaran C, Chandaroy R. Mechanics of Pb40/Sn60 near eutectic solder alloy subjected to vibration. Appl Math Model 1998;22: [6] Kim YB, Noguchi H, Amagai M. Vibration fatigue reliability of BGA-IC package with Pb-free solder and Pb Sn solder. Microelectron Reliab 2006;46: [7] Wu ML. Vibration induced fatigue life estimation of ball grid array packaging. J Micromech Microeng 2009;19: [8] Che FX, Pang John HL. Vibration reliability test and finite element analysis for flip chip solder joints. Microelectron Reliab 2009;49: [9] Zhou Y, Al-Bassyiouni M, Dasgupta A. Vibration durability assessment of Sn3.0Ag0.5Cu and Sn37Pb solders under harmonic excitation. J Electron Pack 2009;131: [10] Zhou Y, Al-Bassyiouni M, Dasgupta A. Harmonic and random vibration durability of SAC305 and Sn37Pb solder alloys. IEEE Trans Compon Pack Technol 2010;33(2): [11] JEDEC standard for vibration: JESD22-B103B, JEDEC Solid State Technology Association; [12] Bathe KJ. Finite element procedures. Prentice-Hall; [13] de Silva CW. Vibration fundamentals and practice. CRC; [14] Gloth G, Sinapius M. Analysis of swept-sine runs during modal identification. Mech Sys Signal Process 2004;18: