The Study of Vibrational Performance on Different Doped Low Creep Lead Free Solder Paste and Solder Ball Grid Array Packages Sivasubramanian Thirugnanasambandam1, Thomas Sanders1, Anto Raj1, Derrick Stone2, Dr. John Evans1, Dr. George Flowers2, Dr. Jeff Suhling2 1 Department of Industrial & Systems Engineering, Auburn University, Auburn, Alabama 36849, USA 2 Department of Mechanical Engineering, Auburn University, Auburn, Alabama 36849, USA Email: [email protected]
Abstract Relatively little is known about the performance of the doped Ball Grid Array (BGA) packages used in semiconductor industries, even though newer products are widely being introduced to the market. This work experimentally investigates the doping effects of the BGA packages with SAC 305 alloys, caused by the vibration loading. This experiment focuses on the vibration fatigue life of 15 mm CABGA packages with 208 perimeter solder balls on a 0.8 mm pitch. The test boards were built to withstand JEDEC JESD22-B103B standards of high stress test in vibrational shaker table to assess the solder joint performance. The test boards are built with three different reflow profiles and two different stencil thicknesses 8 mil (6 mil with overprint) and 4 mil to study the differences in doping effect of the new doped alloys. The WLCSP assembly was subjected to accelerated life test of severe random vibration per board. The reliability of the component is determined by the ability of the components to withstand vibration as a result of motion produced by field operations. The deleterious effect of the mechanical loading of BGA’s on the characteristic fatigue lifetime is reported. The results show that the material characteristics has a direct impact on the total time to failure. The results show that the Time-ToFailure (TTF) of the solder joint decreases with doping. The effectiveness of this characteristics was demonstrated with promising results through vibration testing of different lead free low creep alloys. This paper concludes with discussion of the deterioration intensity aging has on SAC alloys and the change in reliability due to doping. Introduction A demand for using ball grid array (BGA) packages, with reduced substrate interconnect area, complexity, and cost of substrate assemblies, has significantly increased over the past few years. However, the failures in the BGA solder joints over vibration environments become a serious concern in electronic package industries. The reliability of these solder joints under vibrational study is considered vital to the success of BGA applications [1-2]. Vibration study in electronic packages for solder joint characteristic life is still in early stages of development, in comparison to other established reliability tests like temperature cycling, drop test and thermal shock tests. The continuous reduction in the reliability of BGA package and solder ball dimensions is increasing the risk of solder joint failure under vibrational and mechanical loading. The present industry standards for vibrational testing are conducted mainly on pass or fail functional test criterion. This is a 978-1-4799-5267-0/14/$31.00 ©2014 IEEE
limiting factor contributing towards the occurrence of failures. Also, there is no measurement metric available that could be effectively be used to monitor the fatigue of solder joints under vibration. In this paper, a study that can be successfully used to determine the characterize fatigue life of BGA solder joints under vibration loading is shown . The mechanical properties (and related failure behavior) of lead-free solder joints vary on different materials in electronic assemblies. Prior work by the Center for Advanced Vehicle and Extreme Environment Electronics (CAVE3) at Auburn University has shown rapid deterioration in the material behavior of SAC105, SAC305 and Innolot solder joints for varying elevated temperatures and aging durations. The rates of degradation demonstrated are concerning, as the behavior of such lead-free solder materials is specifically important to harsh-environment applications present in modern era of high performance computing, automotive, and aerospace applications. The material degradation observed is predicted to be even more pronounced at mechanical loading compared to higher temperatures. Despite its importance, however, relatively little comparable work has been reported in the literature concerning the mechanical reliability of lead-free solder joints which are doped with Bismuth and Antimony. Most of the work done to date has been focused on SAC 105 and SAC 305 alloys. Existing finite element models used to understand solder joint reliability during accelerated vibration testing are not able to account for the effects of material properties on doped alloys. Hence there will be significant deviations in the output of such models when comparing their reliability calculations to the actual performance of real-world solder joints subjected to the continuous mechanical effects that have been discovered. In this current work, a mixed model Test Vehicle containing 0.8 mm and 0.65 mm fine pitch components of CABGA 208 and QFN 20 of various sizes and solder materials (Innolot, SAC105 and SAC305) on Immersion Silver plating. The goal of this experiment was to determine the characteristic fatigue life of Innolot, SAC105 and SAC305 BGA solder joints aged at 25°C for 0 months. The test vehicles with SAC 105, SAC305 solder balls on SAC 305 solder paste are considered as a base-line for the characteristic life of the components. Post baseline experimental results, the doped solder joint materials with the effect of vibrational stress on the degraded solder-joint strengths are measured as the Time-To-Failure (TTF). Experimental setup test vehicle A test vehicle (TV) is designed and constructed as shown in Figure 1. All electronic packages soldered onto the printed circuit board assembly (PCBA). This TV includes four 14th IEEE ITHERM Conference
different materials of BGAs. They are: SAC 105 alloy on SAC 305 paste, SAC 305 alloys on SAC 305 paste, SAC 105,305 on Innolot paste. There are also 2512 resistors tin lead components. The Test Vehicle is built with the capability to house multiple packages of CABGA 208 to provide an opportunity to investigate the effect of material alloy, reflow profile and solder paste of components combined with solder paste volume differ from SAC 105 and SAC 305 alloy’s properties. The plating material used for the Test Vehicles is Immersion Silver. CABGA’s were the primary components chosen for the study.
Figure 1. Assembled Test Vehicle Test Description The boards are mounted on vertical aluminum fixtures and are fastened onto the shaker table for vibration test. The vibration axis is chosen to be Z axis. First, it is necessary to determine the natural frequencies of the boards to calibrate the test parameters of the equipment. In order to determine the natural frequency of the boards, a laser source and oscilloscope is used upon the boards to measure the natural frequencies of the test assembly. In this setup, the laser source records the frequencies induced with the highest distortions. Only the first natural frequencies which induces more damage to the components is measured in this test.
Test Method Measurements for failure detection were hand probed every sixty minutes and was based on resistance continuity measurements. Testing was stopped when the definition of failure which was taken as an intermittent event of value >300Ω, was met. It was observed that once a package suffered its initial open event, the remaining component failures occurred within a remaining few hours of testing, and continued to complete failure of the package. The failure data are analyzed and the reliability of the solder joints are determined from their characteristic life (η) and slope (β) from a two parameter weibul analysis. The effect on the isothermal aging of the solder joints is understood from the degradation trend of the characteristic life (η) of SAC 105, SAC 305 and Innolot alloys in the mechanical conditions. The effect on material properties of the solder joints is understood from the degradation trend of the characteristic life (η) of lead free alloys. Results and Discussions The failure life of electronic packages is characterized with a two-parameter (η, β) weibul distribution. The characteristic life (η) is the point (i.e., number of cycles) at which 63.21% of the population is expected to fail. The slope (β) of the weibul distribution distinguishes different classes of failure modes. The least squares method estimates the characteristic life and slope of the weibul distribution. The r2 value indicates the quality of the data fit .
Test Setup and Profile The setup test assemblies were mounted on an LDS LV217 electro-dynamic shaker table and subject to a 4.6 Grms stress vibration profile. The transfer functions are measured with input on shaker and output on PC boards. According to the results, one major natural frequency appears at between 350 to 400 HZ. The magnitudes are very consistent with close peak magnitudes.
Figure 3. Solder Paste comparison Figure 4-6 shows the weibull plots for the vibration loading on SAC 105 and SAC 305 15-mm BGA samples on SAC 305 paste and Innolot paste. A dramatic degradation in the reliability is observed for both SAC alloys on mechanical loading conditions at 4.6 Grms, 25°C loading. Contrastingly, the doped materials were minimally affected by performance over vibration loading test. The degradation of the alloys are tabulated for vibration test results.
Figure 2. Bode Plot
better understand the loading and stress distribution correlations to the solder joint performances on different lead free materials. Figure 7 gives a zone location visual map.
Figure 4. SAC 305 Paste Best Profile
Figure 8. Characteristic Life Comparison The characteristic life of the SAC 105 alloys are seen to be better than the SAC 305 alloys on the SAC 305 Paste at Best Profile and also the Innolot High Profile, whereas the SAC 305 alloys outperformed the 105 alloys on the Low Profile.
Figure 5. Innolot Paste Low Profile
Microstructure Characterization Failure Analysis To better understand the reduction of Characteristic life in SAC alloy electronic packages, failure analyses are made based on the microstructural evolution of IMCs in solder joints. The failure analysis of failed samples in vibration stress tests were performed using cross-section by a Scanning electron Microscope .This failure analysis is made by investigating the intermetallic component failure mode in the solder joint. The mode of failure is important to determine the reason for the better vibration loading performance of SAC ball grid array packages, compared to Innolot packages in the vibration tester. The analysis showed different failure modes for the SAC and Innolot components. The interfacial and bulk solder failures on the chip side and board side are the most common failure modes observed in package vibration stress testing .
Figure 6. Innolot Paste High Profile
Figure 7. Zone Location Map The failures are location based as the stress induced on the components are different in at various locations across the board. Thus the results are grouped into 4 different zones to
Figure 9. Microstructure Analysis Full Array
 S. Thirugnanasambandam, J. Evans, M. Perry, B. Lewis, D. Baldwin, K. Stahn, and M. Roy, “Component level reliability on different dimensions of lead free wafer level chip scale packages subjected to extreme temperatures,” in 13th InterSociety Conference on Thermal and Thermomechanical Phenomena in Electronic Systems, 2012, pp. 612–618.
Figure 10. Microstructure Analysis Solder SAC 305 Summary & Conclusion Package board level vibration stress tests and IMC analysis were performed for various fine pitch ball grid array chip scale packages with different material- solder volume composition of lead free SAC 105 and 305 interconnects on SAC 305 paste against Innolot alloy paste. The results showed significant difference in the characteristic life of SAC and Innolot alloys compared with the 6 mil stencil of solder paste printing at 25°C. All failures for both the lead free structures showed failure within the intermetallic layer. Acknowledgments The authors would like to thank Center for Advanced Vehicle and Extreme Environment Electronics at Auburn University for providing all of the testing equipment. References  T. E. Wong, F. W. Palmieri, B. A. Reed, H. S. Fenger, H. M. Cohen, and K. T. Teshiba, “Durability/reliability of BGA solder joints under vibration environment,” 50th Electronic Components and Technology Conference Proceedings (Cat. No.00CH37070), 2000, pp. 1083–1088.  S. Thirugnanasambandam, N. Vijayakumar, J. Zhang, J. Evans, F. Xie, D. F. Baldwin, and D. Ph, “Drop Reliability test on different dimensional Lead-free Wafer level Chip Scale packages”, SMTA International, 2012.  S. F. Wong, P. Malatkar, C. Rick, V. Kulkarni, and I. Chin, “Vibration Testing and Analysis of Ball Grid Array Package Solder Joints,” 57th Electronic Components and Technology Conference proceedings, 2007, pp. 373–380.  J. Zhang, S. Thirugnanasambandam, J. L. Evans, M. J. Bozack, and R. Sesek, “Impact of Isothermal Aging on the Long-Term Reliability of Fine-Pitch Ball Grid Array Packages With Different Sn-Ag-Cu Solder Joints,” IEEE Transactions on Components, Packaging and Manufacturing Technology, vol. 2, no. 8, pp. 1317–1328, Aug. 2012.  P. Lall, R. Lowe, and K. Goebel, “Prognostication of accrued damage in board assemblies under thermal and mechanical stresses,” Electronic Components and …, no. 1, pp. 1475–1487, May 2012.