vibration testing and analysis of ball grid array package solder joints

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Vibration testing and analysis of ball grid array package solder joints

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  • 1. Vibration Testing and Analysis of Ball Grid Array Package Solder Joints Shaw Fong Wong*, Pramod Malatkar, Canham Rick, Vijay Kulkarni and Ian Chin Intel Technology Sdn. Bhd. Lot 8, Jalan Hi-Tech 2/3, Kulim High Technology Park, 09000, Kulim, Kedah, Malaysia *Email: shaw.fong.wong@intel.com, Phone: +60-4-433-2941, Fax: +60-4-433-7581 Abstract A methodology to characterize and predict fatigue failure of BGA package solder joints under vibration loading is presented. The results show that the board strain versus number-of-cycles-to-failure (or E-N) curve has a linear trend with little scatter in data points, similar to that of a classical fatigue theory using cyclic stress versus number-of-cycles-to- failure (or S-N) curves. Using finite element analysis (FEA), the solder joint stress was shown to be linearly correlated to the board strain. Therefore, board strain can indeed be used as an optimum engineering metric to study the fatigue of ball grid array (BGA) solder joints. In addition, the E-N curve approach was shown to be applicable to cyclic bend and cyclic shock loading conditions as well. The E-N curves of lead-free and leaded solder systems also have been generated and compared to demonstrate that the lead-free system has a better high-cycle fatigue performance. In addition, a fatigue- life prediction methodology based on the Miners cumulative damage theory is proposed. The effectiveness of this methodology was demonstrated with promising results through random vibration testing of actual motherboards. Finally, a novel approach to study solder joint reliability (SJR) under vibration loading at the system level, using a fatigue curve generated at the component level is presented. Introduction Vibration testing and analysis of electronic packages for SJR characterization is still in early stages of development, in comparison to other established reliability stress conditions, like temperature cycling, bend, and shock. The continuous reduction in the BGA package and solder ball dimensions is increasing the risk of solder joint failure under vibration loading. Current industry standards for vibration testing rely mainly on pass or fail functional test criterion, with limited knowledge of factors contributing towards the failure. Also, there is no measurement metric available that could be effectively used to monitor the fatigue of solder joints under vibration. Therefore, understanding SJR under vibration loading is gaining momentum. In this paper, a metric that can be successfully used to characterize fatigue life of BGA solder joints under vibration loading is proposed. Various fatigue methodologies related to vibration have been presented in literature. Barker et al. proposed a generalized strain versus life approach in 1990. They combined the low- and high-cycle fatigue contributions from thermal and vibration stress conditions, respectively, to predict solder joint fatigue life by adopting Coffin-Manson equation and Miners damage rule [1]. Solomon used plastic strain range to plot various solder systems fatigue life from shear test results [2]. This was followed by the fatigue crack length method by Guo and Conrad. They used an optical microscope to measure directly the crack growth in the test specimen [3]. A fatigue approach based on deflection amplitude was suggested by Chuang et al. to quantify the vibration fracture life of lead-tin and lead-free (tin-zinc) eutectic systems. In their study, samples were clamped at one end and the other end was equipped with a deflection sensor to measure the total deflection amplitude under resonant frequency [4]. Shi et al. utilized the strain-life approach to evaluate fatigue life. Both total and plastic strains were measured using a dynamic extensometer [5]. Liu et al. applied Paris Law towards high-cycle vibration fatigue life prediction, and their approach was quite similar to that of Chuang et al. A stroboscope and optical sensors were used to monitor crack propagation at solder joint interfaces [6]. To date, the most common and popular approach used to characterize fatigue in metals/alloys is the S-N curve approach [7]. To predict fatigue failure, Miner proposed a cumulative damage index (CDI) to represent the fraction of fatigue life that was used up in every stress cycle back in 1945. For aluminum test specimen, he showed that the average value of CDI is around one at failure [8]. Under random vibration input, the study by Wong et al. indicated failure when CDI has a value greater than or equal to 0.5 [9]. However, Steinberg [10] and Pang et al. [11] assumed failure for CDI value greater than 0.7. Although various fatigue methodologies have been suggested, those approaches were skewed towards lab-scale validation on homogeneous samples. Furthermore, application on real electronic packaging products was not fully addressed. No direct measurable engineering metric can be referred to characterize the fatigue life of a fully assembled BGA solder joints. However, an S-N curve like approach and Miners rule application were benchmarked from literature learning. In this paper, a fatigue life prediction methodology based on Miners cumulative damage theory was developed and validated on fully loaded motherboards. Since it is impractical to measure the stress within the small BGA solder joint under vibration, board strain was used as the reference metric. The E-N curve was generated using a specific component-level vibration test setup, to aid in accurate estimation of strain and cycles to failure. This board also eliminated extraneous failure modes like pad crater and trace cracking. Data collected from the system-level random input test was used in conjunction with the E-N curve to validate the fatigue methodology. All of the tested boards went through failure analysis to confirm the failure modes and FEA was used to explain the failure trends. In addition, the effects of solder metallurgy on the fatigue life were validated. Finally, the universal applicability of the E-N curve approach to study fatigue failure under all mechanical loading conditions Vibration, Cyclic Bend and Cyclic Shock were demonstrated. A novel approach to study solder joint reliability (SJR) under vibration loading at the system 1-4244-0985-3/07/$25.00 2007 IEEE 373 2007 Electronic Components and Technology Conference

2. level, using a fatigue curve generated at the component level is presented as well. Setups and Methodology Component-Level Test Setup A Dynamic Test Board (DTB) was designed and tested with an electro-dynamic vibration shaker. The DTB, shown in Figure 1(a), is an approximately 1.57mm thick, 305mm2 square board with only a BGA package mounted on it in the center. It was made up of typical FR4 printed circuit board material. The holes on the board, arranged in a circular pattern with constant distance, were used to secure the board onto a support plate using standoffs. The multiple holes enable obtaining different bend modes using the same board, by just changing the mount locations appropriately. The fine pitch BGA components were designed with a full daisy chain array of either lead-free with tin-silver-copper (SnAgCu) or leaded (SnPb) solder balls. Figure 1(b) shows the test board assembly on an electro-dynamic vibration shaker. Figure 1: Vibration test setup: (a) a DTB mounted onto a fixture using eight standoffs, and (b) complete picture of the setup. The potential failure modes in vibration loading are: (i) solder joint crack on board and/or package side, (ii) PCB trace crack close to package outer corners (especially when trace thickness is relatively small compared to the pad diameter), and (iii) pad crater on board side (normally with metal-defined pads). In order to focus on solder joint failure only, the other two failure modes were eliminated by adding the following design features in the corner areas of the board: thicker traces, redundant traces, and solder-mask defined (SMD) pads to ensure only BGA fatigue failure in all of the test boards. More detail on the impact of board design can be found in Malatkar, et al [12]. Under harmonic input, the board response would typically contain significant contribution from one or more modes. These are due to nonlinearities inherent in the system. In order to obtain a clean sinusoidal response, it would become necessary to minimize the impact of nonlinearities. For inputs beyond 5g (gravity acceleration), the nonlinear response was prominent, and hence the input g-level did not exceed 5g. Also, to avoid interaction between modes through internal resonance, the location of the standoffs (used to mount the board onto the shaker) was varied until the ratio of the first and second/third natural frequencies was away from 1:2 and 1:3 [13]. As discussed earlier, it is not feasible to characterize fatigue failure of solder joints using the S-N curve approach due to difficulty in measuring the solder joint stress during vibration. Instead, the board strain which is indicative of the severity of the solder joint stressing was used. Therefore, a board strain versus cycles-to-failure plot (or E-N curve) was developed. A 45 strain gage rosette was placed on the secondary side of every test board, underneath the package corner, to measure the localized bending strain as shown in Figure 2. Due to slight variations in the natural frequency and therefore the amplification of individual boards, it was necessary to directly measure the strain for every board tested. Each DTB was subjected to a sinusoidal input till failure by having a fixed G-level at a resonant frequency. The number of cycles to failure was calculated through the time to failure (in seconds) multiply with the input frequency (in Hz). Input G-levels from 0.75g to 4.0g were used to produce a range of board strain values. To monitor the connectivity of the solder joints, resistance of daisy chain structure in the package and board was recorded. A 1

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