The relationship of life prediction between cyclic bending and thermal cycle testing on CSP package

With more reliability requirement increasing in the IC packaging for mobile product, the industry has been searching for a more cost-effective and time-effective way to achieve the evaluation task. In light of the long cycle time of thermal cycle testing, we investigated the mechanical fatigue test instead of thermal cycle testing for shortening test time. The 4-point cyclic bending test is considered a good candidate in this study. The 4-point cyclic bending condition including different bending frequency and displacement to simulate actual service condition is defined in JEDEC22-B113. The thermal cycle test and 4-point cyclic bending test both apply in the evaluation of solder joint for fatigue effect. The study in the past has shown that the damage mechanism of these two testing methods is similar. For these reasons, the correlation between the mentioned two testing methods by life prediction is discussed in this paper. The fatigue life and strain data are used to investigate the correlation to correspond to their failure modes. In this paper, the effect of strain variations on the solder joint durability of 0.5 mm pitch lead free TFBGA packaging will be investigated. This investigation is useful to predict the fatigue life of a practical surface solder joint. The micro-structure observation of failure mechanism through Scanning Electron Microscope (SEM) and Focused Ion Beam (FIB) plays a major role in the metallurgical analysis. The strain-life method is used to fatigue life predictions under the thermal cycle test and the 4-point bending test in various conditions. This method used to estimate SnAgCu solder joints in accordance with the proposed Coffm-Manson equation. The curves are used to investigate the co-relation between strain amplitude and fatigue life for two test methods. The behavior of the components on the boards in various conditions can be regarded as the foundation to simulate the strain deformation and fatigue life in the cyclic fatigue test (i.e. thermal cycle test or cyclic bending test) so as to optimize the experimental design intended for solder joint reliability evaluation. The plastic shear strain range of the solder after cyclic fatigue test is calculated by Coffin-Manson equation. According to Coffin-Manson, the number 2N
_f
of cycles-to-failure in the low-strain/high-cycle flexing is related to the amplitude of the applied cyclic plastic deformation Δϵ
_p
/2 Fatigue-ductility testing involves the relative strain over one cycle is represented by Δϵ
_p
/2. The famous empirical relation named Δϵ
_p
/2 = ϵ(2N
_f
)
^c
. Here, we propose a theory of the Coffin-Manson law which is based on a model for the creep-fatigue life prediction in terms of a new damage function. Using this damage function, one may realize that all the Coffin-Manson plots at the various levels of life and strain range under creep-fatigue tests can be normalized to make the master curve. Finally, we can get the relationship of cyclic bending and thermal cycling testing from the curve of fatigue life and strain range. The strain range of the thermal cyclic -40 ~ 125°C in the backside corner of test samples are 1000 ~ 1100 μ strain for HF (Halogen Free) PCB and 750 ~ 850 μ strain for FR4(Halogen) PCB. We can get the starting strain condition of cyclic bending with the 1050 ± 50 μ and 800 ± 50 μ strain of delta strain to simulate the life of thermal cycle -40 ~ 125°C. Eventually, the objective of find a new effective testing method can be achieved.