Reliability of BGA solder bumps - Advanced Packaging
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Reliability of BGA solder bumps


Moiré interferometry evaluates shock and vibration performance


Illustration by Gregor Bernard
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BY CEMAL BASARAN, ALEXANDER CARTWRIGHT, YING ZHAO AND TERRY DISHONGH

Recent trends in reliability and fatigue life analysis of electronic devices have involved developing structural integrity models for predicting the operating lifetime under vibratory and thermal environmental exposure. Extensive research has been done on thermal behavior of solder joints, but dynamic loading effects on solder joint fatigue life have not been thoroughly investigated. The physics of solder joint failure under vibration is still not well understood. This article presents a test program that was performed to study inelastic behavior of solder joints of ball grid array (BGA) packages. It is found that at elevated temperature, vibration and shock can cause the accumulation of inelastic strains and damage in solder joints. In this article, contrary to the popular belief that all vibration-induced strains are elastic, it is shown that vibration can cause significant inelastic strains.

Previous Work
It has been generally recognized that thermal loading is one of the major causes of failure in electronic devices. However, many types of electronic devices have to endure severe working environments that involve not only thermal but also dynamic loading conditions. The U.S. Air Force estimates that vibration and shock cause 20 percent of the mechanical failures in airborne electronics. Empirical fatigue life predictions of components, component leads and joints have been performed based on the dynamic displacement of the printed circuit board (PCB).1 However, the nonlinear stress-strain behavior of solder joints under vibration is still not clear, and the role of vibration in the life of solder joints has not been studied sufficiently.


Figure 1. Cross-section of the BGA specimen.
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According to a literature survey, vibration fatigue analyses to date have been mainly modal testing, mode analysis and cyclic failure investigation. Only very few existing fatigue models are based on stress or strain analysis. Steinberg developed an empirical relation to estimate the fatigue life of components mounted on a PCB based on testing and experience.2 Lau et al. studied solder joint reliability under shock and vibration.3 They conducted in-plane random vibration testing, in-plane shock testing, out-of-plane vibration testing, and out-of-plane shock testing. Their testing was limited to determine the failure status after certain cycles of dynamic loading. Wong et al. reported experimental modal analysis and dynamic response prediction of PC boards under out-of-plane vibration.4

Ham and Lee also conducted reliability testing of electronic packaging under vibration.5 An experimental method was developed to measure the changes in electrical resistance in the lead, with the results being used to indicate fatigue life. They discussed a relationship between the loading forces and the high-cycle fatigue life


Figure 2. Optical set-up for Moiré interferometry.
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for the lead part of spider gull-wing surface mount components. Liguore and Followell also reported a test program to obtain structural fatigue data for surface mount solder joints exposed to an out-of-plane random vibration environment.6 The dynamic response characteristics of the PCBs were studied, and an empirical fatigue model for components, component leads and solder joints was developed. Sidharth and Barker reported vibration fatigue life estimation of corner leads of peripheral leaded components using linear elastic finite element procedures.7

This article presents the results of laboratory tests dedicated to the investigation of the dynamic behavior of 63Sn/37Pb eutectic solder joints in a high-performance BGA. The goal of the work is to understand the nonlinear inelastic response of the solder joints under vibration at certain temperatures, as well as the role of dynamic loading to the fatigue life of the solder joints.

Figure 1 shows the tested BGA package. In the shock tests, the force was applied at room temperature and in a temperature range ramping from 20°C to 130°C. In harmonic vibration tests, sine-wave vibrations were conducted at 20°C and 100°C. The plastic deformation in each solder joint after each shock and vibration test was measured by laser Moiré interferometry (MI).

Experimental Procedures
There are two relatively independent parts in the entire testing procedure: loading and optical measurement. For the first part, an environmental thermal chamber and an electrodynamic shaker are used simultaneously. For the second part, a laser MI device was designed and manufactured for deformation measurement at the resolution of half of the light wavelength.

A high-capacity environmental chamber was used to control the environmental temperature. It is capable of maintaining the temperature to ±1°C, and its fastest ramping rate is 30°C/min. The electrodynamic shaker system used consists of a power amplifier, electrodynamic shaker, DC field supply and remote-cooling blower. The shaker armature incorporates a multi-ribbed table for maximum stiffness and minimum weight, and it is ideal for applications requiring high-acceleration (up to 360 g shock and 100 g sine) testing involved in this project. A PC is equipped for the shaker system for function generating, test control, test monitoring and data acquisition, as well as to form a closed loop with the power amplifier, shaker, accelerometer and charge amplifier. It is capable of performing random vibration, sinusoidal vibration and shock.


Figure 3. Optical table used for Moiré interferometry.
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To combine the shaker and the chamber, the shaker is inserted underneath the chamber, and the slip table of the chamber is replaced by a special flexible thermal barrier. An aluminum round plate with the same configuration as the shaker armature table is made to hold the specimen fixture inside the chamber. The plate is connected to the armature table through the thermal barrier.

Moiré Interferometry
The MI and the imaging systems used for sub-micron displacement measurement have been described in detail.8 The major advantage of MI is its high sensitivity, high resolution and the ability to view the whole deformation distribution of the specimen surface. To describe the technique briefly, the optical diffraction grating is replicated on the specimen surface. The specimen grating diffracts the two coherent incident beams with certain incident angle, and in the direction normal to the specimen surface, two strong diffracted beams are obtained. When the specimen surface deforms, the optical diffraction grating deforms with the specimen, and the two diffracted beams in the normal direction generate an interferometry pattern that represents the in-plane displacement distribution. Figure 2 shows the one-dimensional setup of MI. This scheme applies to both the horizontal and vertical directions, so deformation in the two perpendicular directions can be obtained. The feature fringe pattern generated by the two vertical beams represents the vertical deformation field, and the fringe pattern generated by the horizontal two beams represents the horizontal deformation field. Figure 3 shows the optical table for the MI system designed for this project.

The fringe pattern can be related to in-plane deformation

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quantitatively9 where, U is the displacement in x direction;

V is the displacement in y direction; f = 2fs where fs is the frequency of specimen diffraction grating (in this study, fs=1,200 lines/mm); Nx is the horizontal fringe order, and Ny is the vertical fringe order. Thus, each fringe represents 1/f = 0.417 µm in this study.

Once the displacement data are available, the plastic strains are computed by differentiation of the displacement distributions with respect to the two basic directions: horizontal (x) and vertical (y). The strains are given by:

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Specimen and Fixture
The specimen was cut by a high-precision diamond wheel saw through the center of the solder joints of interest to expose the two rows of solder ball joints.8 There are 30 joints in the bottom row and 24 joints in the upper row. The exposed cross section was then polished flat, cleaned and dried thoroughly before a thin layer of epoxy was applied on the cut surface to transfer the diffraction grating pattern to the specimen surface.

A special fixture was designed to hold the specimen tightly at two ends of the middle FR-4 layer. This fixture is then mounted on the shaker armature table. The boundary condition and the loading direction are shown in Figure 4. The estimated natural frequency of the chip specimen under this configuration is well above 1,000 Hz.3,10,11,12

Shock Test Results
Two sets of testing were completed to examine the dynamic response of BGA solder joints. In the first set of tests, shock pulses were continuously applied at room temperature (20°C) for five minutes. In the other set of tests, shock pulses were applied with concurrent temperature ramping from 20°C to 130°C for five minutes, and then the specimen was air cooled to room temperature. The loading period is five minutes.


Figure 4. Boundary condition and dynamic loading.
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The measured deformation fields after five minutes of continuous shock at room temperature show that little change has occurred during the testing, which means that little inelastic deformation happened. Clearly, the dynamic response of the solder joints at room temperature in the shock testing is elastic.

The results in the second set of tests, shock pulses during temperature ramping, are shown in Figure 5. The bottom row of solder joints developed a significant amount of plastic deformation. Because the bottom row is between the FR-4 layer and the polymer layer, whose coefficients of thermal expansion (CTE) are very different, the thermal effect is very large, and the dynamic effect cannot be determined. However, the first row of solder balls seems to show some dynamic effect. This row of solder balls is between two identical material layers, and the fringe patterns indicate significant plastic deformation.

Harmonic Vibration Testing Results
Two sets of tests examined the vibration response and the effect of nonlinear response to solder joint fatigue life under harmonic vibration. One set of tests was conducted at temperature (20°C, which is 0.64 of the absolute melting temperature), and the other tests were conducted at 100°C (0.82 of the absolute melting temperature). Each test lasted 42 minutes. Different frequency values were applied during each set of tests. The applied vibration was a sine wave with peak acceleration of 30 g in all cases.


Figure 5. Interference fringes for concurrent thermal and shock testing: a) initial U-field (X-axis), b) U-field after test, c) initial V-field (Y-axis), and d) V-field after test.
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In the first set of tests, harmonic sine wave vibrations were applied with frequencies of 2,000 Hz, 500 Hz, and 50 Hz in sequence. The deformation responses indicate that the solder joints in this BGA package behave elastically under vibrations at room temperature. The fringe patterns did not change from the initial fields.

Because most semiconductor devices experience heat dissipation during service, studying the effect of vibrations at elevated temperatures is more realistic. For the second set of tests, the specimen was heated to 100°C and then was subjected to sine wave vibration at that temperature. Because the specimen is a multilayered structure, and the two ends of the middle board were fixed, the heating process introduces thermal stresses and thermal strains. Thermal strains must be subtracted to study the vibration-induced deformation. Therefore, the specimen was first heated from 20°C to 100°C in five minutes, and then was held at 100°C for another six minutes to allow full relaxation of thermal stresses. It was then air-cooled to room temperature for measurement of the initial deformation fields. Therefore, the initial fields actually recorded the plastic deformation caused by heating. Thermal strains were subtracted from elevated temperature vibration test strains to get the vibration induced plastic deformation.


Figure 6. Interference fringes for elevated temperature (100°C) vibration testing: a) Initial U-field, b) initial V-field, c) U-field after cycling at 50 Hz, d) V-field after cycling at 50 Hz, e) U-field after cycling at 1,000 Hz, and f) V-field after cycling at 1,000 Hz.
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The specimen was then heated to 100°C again for further testing. 50 Hz, 500 Hz, 1,000 Hz, and 700 Hz frequencies were applied in sequence. The deformation fringe patterns for some of the tests are shown in Figure 6. Clearly, the fringe patterns changed from the initial patterns. (Dense fringe patterns indicate larger deformation.) Figure 7 shows the calculated net shear strain induced in each vibration test at 100°C at different frequencies. It seems that beyond 1,000 Hz, the solder joints subjected to vibration behave elastically. However, at lower frequencies, significant plastic strain is developed under vibration. Specifically, smaller frequencies cause larger plastic deformation. However, the lower frequency limit of 700 Hz needs to be further explored. It should be pointed out that 700 Hz is very close to 750 Hz reported for the boundary between elastic and plastic behavior.13


Figure 7. Plastic shear strain in the solder balls as a function of frequency. The solder ball number increases moving from the periphery to the center of the die.
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In Figure 7, the strain is greatest at the edge of the structure. In such laminated configuration, outer solder joints experience larger shear stress and strain than the center parts.14 It has been shown that the modulus of elasticity increases with frequency and decreases with increased temperature.15 At higher temperature, solder material also softens, and the yield point is much lower than that at room temperature. Therefore, with the same frequency, solder material responds plastically at high temperature while it responds elastically at lower temperature.

Essentially the solder behaves elastically for higher frequencies and inelastically for lower frequencies. This observation is consistent with the experience reported previously,2 and this is because at lower frequencies the period of the loading is higher.13 The time-dependent inelastic deformations are directly related to the period of the load. When the load has a longer period, the material has more time to creep. Consequently for vibrations with small frequencies, creep dominates the response.

Conclusion
The test data shows that solder joints respond elastically at room temperature under either shock or sine vibration loading. Inelastic deformation may be still accumulating, albeit very slowly. At elevated temperature, both shock and vibration induce significant inelastic shear deformation in solder joints, thus shortening the fatigue life of solder joints. The solder joints suffer more severe damage at lower vibration frequencies than at higher ones. At 100°C, the test data shows that dynamic loading with frequencies higher than 1,000 Hz has little effect on the solder inelastic response.

Thus, it can be concluded that contrary to popular belief, the solder alloy does not remain in the elastic region. With a low melting point (183°C) and high visco-plastic characteristics, a eutectic solder alloy shows creep behavior during dynamic loading processes, which should not be underestimated in solder joint fatigue life prediction. AP


Acknowledgements
This research project was sponsored by the Department of Defense Office of Naval Research (ONR). Helpful discussions with Dr. George Campisi, program director at ONR Advanced Electrical Power Systems, are gratefully acknowledged.

References

  1. H.W. Markstein, "Designing Electronics for High Vibration and Shock," Electronic Packaging & Production, pp. 40-43, April 1987.
  2. D.S. Steinberg, Vibration Analysis for Electronic Equipment, 2nd ed., John Wiley & Sons, New York, 1988.
  3. J. Lau et al., "Solder Joint Reliability of Fine Pitch Surface Mount Technology Assemblies," IEEE Transactions on CHMT, 13(3) pp. 534-544, September 1990.
  4. T-L Wong, K.K Stevens, and G. Wang, "Experimental Model Analysis and Dynamic Response Prediction of PC Boards with Surface Mount Electronic Components," Journal of Electronic Packaging, 113, pp. 244-249, September 1991.
  5. S-J Ham and S-B Lee, "Experimental Study for Reliability of Electronic Packaging under Vibration, Experimental Mechanics, 36(4), pp. 339-344, Dec.1996.
  6. S. Liguore and D. Followell, "Vibration Fatigue of Surface Mount Technology (SMT) Solder Joints," 1995 Proceedings Annual Reliability and Maintainability Symposium, pp. 18-26, 1995.
  7. Sidharth and D.B. Barker, "Vibration Induced Fatigue Life Estimation of Corner Leads of Peripheral Leaded Components," 1995 ASME International Mechanical Engineering Congress & Exposition, November 1995.
  8. Y. Zhao, C. Basaran, C. Cartwright and T. Dishong, "An Experimental Observation of Thermomechanical Behavior Of BGA Solder Joints By Moiré Interferometry," J. Mechanical Behavior of Materials, 10(3), pp. 135-146, 1999. D Post, B. Han and P. Ifju, High Sensitivity Moiré, Springer-Verlag, 1994.

  1. R. Chandaroy, "Damage Mechanics of Microelectronic Packaging Under Combined Dynamic and Thermal Loading," Ph.D. dissertation, SUNY at Buffalo, September 1998.
  2. C. Basaran and R. Chadaroy, "Nonlinear Dynamic Analysis of Surface Mount Interconnects: Part I - Theory," ASME Journal of Electronic Packaging, 121(1) pp. 8-12, 1999(a).
  3. C. Basaran and R. Chadaroy, "Nonlinear Dynamic Analysis of Surface Mount Interconnects: Part II - Applications," ASME Journal of Electronic Packaging, 121(1), pp. 12-18, 1999(b).
  4. C. Basaran and R. Chadaroy, "Mechanics of Pb40/Sn60 Near Eutectic Solder Alloys Subjected To Vibrations," Applied Mathematical Modeling, 22, pp. 601-627, 1998.
  5. C. Basaran and Y. Zhao, "Closed Form vs. Finite Element Analysis of Laminated Stacks," International Journal of Finite Elements in Analysis & Design, accepted for publication, 32, pp. 163-179, 1999.
  6. John H. Lau and Donald W. Rice, "Solder Joint Fatigue in Surface Mount Technology: State of the Art," Solid State Technology, pp. 91-104, October 1985.

Cemal Basaran, associate professor and director, and Alexander Cartwright, associate professor and co-director, can be contacted at Electronic Packaging Laboratory, 212 Ketter Hall, University at Buffalo, SUNY, Buffalo, NY 14260-4300; Tel: 716-645-2114; Fax: 716-645-3733; E-mail: [email protected] and [email protected]. Ying Zhao is a senior reliability engineer at Analog Devices. Terry Dishongh is a pathfinding architect at Intel Corp.

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