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Combining vibration test with finite element analysis for the fatigue
life estimation of PBGA components
Y.S. Chen *, C.S. Wang, Y.J. Yang
Department of Mechanical Engineering, Yuan Ze University, 135, Yuan-tung Road, Chung-li, Taoyuan, Taiwan
Received 8 November 2007
Available online 31 December 2007
Abstract
The study develops a methodology that combines the vibration failure test, finite element analysis (FEA), and theoretical formulation
for the calculation of the electronic component’s fatigue life under vibration loading.A specially designed plastic ball grid array (PBGA) component with built-in daisy chain circuits is mounted on a printed wiring board (PWB) as the test vehicle for the vibration test. It is then excited by a sinusoidal vibration whose frequency equals the fundamental frequency of the test vehicle and tested until the component fails. Because the solder balls are too small for direct measurement of their stresses, FEA is used for obtaining the stresses instead.Thus, the real displacements in the vibration test are then inputted to the FEA model when performing the stress analysis.
Consequently,the stress versus failure cycles (S–N) curve is constructed by correlating both the obtained stresses on the solder balls and the number of failure cycles in the vibration test. Furthermore, the Miner’s rule is applied in calculating the fatigue damage index for those test components when failed.Finally, a formula for the prediction of the component failure cycle is deduced from all these procedures studied. It is also examined later by firstly predicting the fatigue failure cycle of a component and then conducting a vibration test for the same component for the verification purposes. The field test results have proven to be consistent with predicted results.
It is then believed that the methodology is effective in predicting component’s life and may be applied further in improving the reliability of electronic systems.
_ 2007 Elsevier Ltd. All rights reserved.
1. Introduction
The ball grid array (BGA) package has become a majorpackaging type in recent years, due to its high capacity forthe input/output (I/O) counts. Connections with outside circuits for these packages are normally through either the solder balls or pins under the package. This results in reliability issues, since there is a higher overall risk of failure given the large number of solder balls and pins.
This problem has attracted much attention from researchers into the BGA component reliability in the past few years. The majority of research has focused on the thermal stress induced reliability issues because large quantities of heat are generated in such complicated high I/O circuit designs. This situation is uncontroversial for electronic devices used in motionless environments. However,for many real world applications, in addition to thermal stress, electronic systems are often subjected to dynamic loadings. The most familiar case is the vibration that is always encountered when the electronic product is transported from one place to another. However, for applications involving vehicles such as automobiles, ships, and aircrafts, vibration induced stresses are the dominant stresses and may not be ignored.
In general, long term vibration loadings typically will cause IC component failure, and will definitely impact the reliability of electronic systems. Much experience with tracing the root causes of failure has shown that the solder
joints are probably the most stressed area and are the major failure locations in components under such dynamic loadings. In BGA components with tens, hundreds, or even thousands of solder balls, a disastrous failure may occur even when only one of these solder joints fails. This kind of problem is not unusual from our perspective, such as electronic module failures that leads to catastrophic loss of life and property in the avionics industry. Assuring the reliability of these solder balls is thus a critical concern especially for electronic devices used in the dynamic environment.Most electronic systems used in vibration environments are subjected to random instead of harmonic excitations.As a result, quality assurance of electronic devices usually uses random vibration as the test specification for acceptance tests, screening tests, and reliability qualification tests. Generally, this kind of test can be conducted only after the prototype is manufactured. This is generally feasible only after a period of time has passed, and is often seen as uneconomic in today’s fast-paced electronic technology markets. Thus, the establishment of an accurate and effective methodology for estimating of the fatigue life of components under vibration loading has become an urgent
demand.Previous research has already attempted to establish such a methodology. Wang [1–3] applied Manson’s work [4] on solder material fatigue properties to investigate theBGA solder joint fatigue life in a random vibration environment.Wang’s results indicated that the validated model is effective in determining the integrity of the PBGA solder joints during random vibration loading. In addition to validating models, understanding failure mechanisms for the components under vibration loading is also crucial. This includes both finding the failure location and further improvement of weak areas in electronic components.Yang [5,6] used the out-of-plane sweep sinusoidal vibration test to assess the reliability of the PBGA assembly against vibration fatigue. Examination of cross-sections of the failed PBGA modules showed that fatigue failure always occurred at the corner solder balls of the PBGA module under the vibration loading. Wang [7] conducted a series of vibration fatigue tests both with a PBGA assembly
and an FCBGA assembly and then observed the differences in their failure modes.However, with the realistic loading, a vibration fatigue failure test will always take time to complete before the failure on the component is observed. In experimental studies,it is impractical to use such field vibration loadings for a long period of time. Therefore, to obtain the results within an acceptable period, the study utilized the most severe situation of vibration resonance loading in examining the fatigue life of all PBGA test components. Additionally, a widely used fatigue model, Miner’s rule, is also used to estimate the fatigue life of the PBGA components.In any examination of fatigue failure for solder balls,stress and cycles to failure data must be recorded. Unfortunately,most solder balls are too small for accurate measurement of their stresses during vibration tests. Instead, this data is obtained indirectly from finite element analysis (FEA) by taking the real displacements in the vibration test as the input for the analysis. To perform the reliability assessment, these analyzed stresses on the solder balls are
then correlated with the number of failure cycles in the vibration test.
2. Experimental set-ups
In order to trace when the component has been failed, a specially designed PBGA component with a built-in daisy chain circuit is used in the vibration test. The component and the corresponding daisy chain circuits are shown in
Fig. 1. The PBGA component, 35 mm 35 mm, is mounted with 0.6 mm diameter solder balls of eutectic solder in 1 mm pitch. The PCB is made of FR4 and is 203 mm in length, 63 mm in width, and has a thickness of 1.6 mm. The daisy chain circuit connects all the solder balls on the PBGA in series with a certain resulting resistance that is monitored constantly throughout the test.Once a crack is initiated in one of the solder balls during
the vibration test, the resistance will increase. The failure criterion as set in the study follows the IPC standard [8] by checking the daisy chain resistance when it exceeds the initial resistance by 20%, and occurring consecutively
five times. A data acquisition system is used to record and calculate the instantaneous daisy chain resistance.When the resistance exceeds the defined failure resistance,and five occurrences have been recorded consecutively, the
component is then considered as having failed and the test is stopped.
To perform the vibration fatigue life test, the PBGA component and PWB assembly is mounted on the shaker with one of the two opposite edges clamped while the other is kept free. It is then excited with a harmonic displacement of 131 Hz, that is, the first natural frequency of the test
vehicle. The set-up of the test component on the vibration shaker is shown in Fig. 2.
3. Stress analysis
As described previously, the vibration test is used primarily to check the time to failure for the component under a specified excitation. However, it is also necessary to check the stresses on the solder balls when conducting a fatigue life assessment of the components. In this study, FEA is used for the stress analysis of the solder balls on the PBGA components, with boundary condition settings identical to those used in the vibration test. The FEA model as presented in Fig. 3 is constructed with the commercial computer
software ANSYS 10.0. The symmetric FEA model of the test board is utilized because of the symmetry both in the geometry and the corresponding boundary conditions.Also, the boundary conditions for one of the two opposite edges are set as clamped and the other is left free to reflect the real edge conditions of the test vehicle. The material properties used in this FEA model, including those of the PWB, solder balls, substrate, chip and molding compound are listed in Table 1. It is also noted that the mesh density will have a strong impact on the FEA results.Consequently, variations in mesh densities are applied in the model in order to examine the convergence of the analyzed frequency results. Fig. 4 shows that the results
have already converged with a total mesh of 1152 elements on a single solder ball.For verification of the FEA model, the natural frequencies of the test vehicle are examined experimentally with the modal testing method and the results are then compared with those from the FEA. Fig. 5 shows the test set-up of the modal testing method where the test sample is fixed by its two opposite edges and its frequency response function is acquired with the attached accelerometer. Fig. 6 depicts the frequency response function (FRF) of the clamped test board as obtained through the modal testing.The first three peaks on the FRF indicate that the first three natural frequencies of the test vehicle are at 131 Hz,398 Hz, and 769 Hz, respectively. Table 2 gives the comparison of the natural frequencies as found both in the modal testing and FEA. As listed in the last column of the table for the error percentages relative to those of modal testing results, all the first three natural frequencies are all within 3%.
Once the FEA model is verified, further analysis with the model is then carried out to investigate the responses of the PBGA component under vibration excitation. As shown in Fig. 7 for the side view of the FEA model, the harmonic displacements as listed in Table 3 are imposed on both sides of the clamped edges with an exciting frequency of 131 Hz so that resonance will occur. This will accelerate the occurrence of component failure and save
time on the test.The corresponding modal shape of the first mode is shown in Fig.7.
4. Discussions
4.1. Developing the S–N curve
In order to build the stress versus fatigue failure cycles curve (S–N curve) for the eutectic solder ball, the vibration test was conducted for a total of six different exciting specifications by varying the excitation displacement each time. All the test components are tested until their daisy chain circuits have been failed, and the resulting failure cycles are recorded. The corresponding stresses on the failed solder balls are then calculated through the harmonic excitation analysis in FEA.
Table 3 lists the number of experimental failure cycles and the corresponding maximum stresses on the solder balls. The relating accelerations and equivalent displacements inputted to the shaker are also listed in the table.The S–N curve as listed in Eq. (3) can be worked out
though the curve fitting of these experimental data. Eqs.(1) and (2) are the S–N curves of the eutectic solder as offered by Manson [4] and Steinberg [9], respectively.
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