http://ws.binghamton.edu/park/publications/2011-1_Da_Yu_High-cycle_fatigue.pdf

High-cycle fatigue life prediction for Pb-free BGA under random vibration loading

Relative damage-index based on the leadfree interconnect transient strain history from digital image correlation, explicit finite-elements, cohesive-zone elements, and component's survivability envelope has been developed for life-prediction of two-leadfree electronic alloy systems. Life prediction of pristine and thermally-aged assemblies, have been investigated. Solder alloy system studied include Sn1Ag0.5Cu, and 96.5Sn3.5Ag. Transient strains during the shock-impact have been measured using digital image correlation in conjunction with high-speed cameras operating at 50,000 fps. Both the board strains and the package strains have been measured in a variety of drop orientations including JEDEC horizontal drop orientation, vertical drop orientation and intermediate drop orientations. In addition the effect of sequential stresses of thermal aging and shock-impact on the failure mechanisms has also been studied. The thermal aging condition used for the study includes 125°C for 100hrs. The presented methodology addresses the need for life prediction of new lead-free alloy-systems under shock and vibration, which is largely beyond the state of art. Three failure modes have been predicted including interfacial failure at the copper-solder interface, solder-PCB interface, and the solder joint failure. Explicit non-linear finite element models with cohesive-zone elements have been developed and correlated with experimental results. Velocity data from digital image correlation has been used to drive the attachment degrees of freedom of the submodel and extract transient interconnect strain histories. Explicit finite-element sub-modeling has been correlated with the full-field strain in various locations, orientations, on both the package and the board-side. The survivability of the leadfree interconnections under sequential loading (thermal aging and shock-impact) from simulation has been compared with pristine circuit assemblies subjected to shock-impact. Sequential loading changes the failure modes and decreases the drop reliability as compared to the room temperature experimental results. Damage index based survivability envelope is intended for component integration to ensure reliability in harsh environments.

Explicit submodeling and digital image correlation based life-prediction of leadfree electronics under shock-impact

Random vibration reliability of BGA lead-free solder joint

http://ws2.binghamton.edu/park/publications/2010-1_Da_Yu_Free_Drop_Test.pdf

Dynamic responses of PCB under product level free drop impact

Numerical simulation and fatigue life estimation of BGA packages under random vibration loading

In this work, free drop impact responses of printed circuit boards (PCBs) mounted in cellular phones has been investigated to assess their dynamic responses and investigate the effects of fastening methods. The digital image correction (DIC) method was used in order to measure a full-field deformation of PCBs during drop from a certain height. Three different fastening techniques which have point or edge contact between the PCBs and the casings were considered. Along with the drop impact experiments, the impact response analysis using ANSYS/LS-DYNA has been performed. To validate the numerical model, the impact response has been compared with the experimental one extracted from the DIC of the PCB. The deformations of the numerical model are well matched with the experimental ones. The effects of assembly method are investigated to assess reliability of PCBs.

Influence of fastening methods on the dynamic response and reliability assessment of PCBS in cellular phones under free drop

This work develops an assessment methodology based on experiments and finite element analysis (FEA) to determine the solder joint fatigue life of electronic components under random vibration loading. Specially designed PCB with Ball Grid Array (BGA) packages attached was mounted to the Electro dynamic shaker and was applied to different random vibration excitations at the supports. Meanwhile, an event detector monitored the resistance of the daisy chained circuits and recorded the failure time of the electronic components. In addition accelerometers and dynamic signal analyzer were utilized to record the time history data of both the shaker input and the PCB's response, and to obtain the transmissibility function of the test vehicles. This finite element based fatigue life prediction approach consists of two steps: The first step aims at characterizing fatigue properties of the solder joint by generating its own S-N (stress-life) curve. A sinusoidal vibration over a limited frequency band centered at the test vehicle's 1st natural frequency was applied and the time to failure was recorded. The resulting stress was obtained from the FE model through harmonic analysis in ANSYS. Spectrum analysis specified for random vibration, as the second step, was performed numerically in ANSYS to obtain the response Power Spectral Density (PSD) of the critical solder ball. The volume averaged Von Mises stress PSD was calculated out of the FEA results and then was transformed into time history data through inverse Fourier transform. Rainflow cycle counting was used to estimate cumulative damage of the critical solder joint. The calculated fatigue life based on the Rainflow cycle counting results, the S-N curve, and the modified Miner's rule agreed with actual testing results.

Finite element based fatigue life prediction for electronic components under random vibration loading

In this study both finite element modeling and experimental techniques are employed to study the reliability of electronics components under random vibration loading conditions. A fatigue life estimation procedure, that would help analyst to make relatively accurate prediction of induced fatigue life, Finite Element model of the test vehicle is built using ANSYS software. The model is first validated by correlating the natural frequencies, mode shapes and transmissibility functions from simulation with experimentally measured ones. The model is then used to simulate both sinusoidal and random vibration tests to obtain the stress and the response spectrum at critical solder interconnects respectively. Results show that the corner solder ball experience the highest stress level under both tests, hence averaged stress at this solder ball was used the construct the S-N curve of the solder material and later to calculate the fatigue life using Steinberg method. A comparison between simulation and experimental results is conducted at the end and a good correlation is obtained.

Random vibration test for electronic assemblies fatigue life estimation

The dynamic response of printed circuit board (PCB) is a major concern to electronic manufacturers when it is subjected to drop impact. In this work, more realistic drop condition is achieved through product-level free drop test. A repeatable free-drop system is developed with an adjustable pair of forks to control the impact orientation during the guided free drop. Digital Image Correlation (DIC) technique is applied to measure and produce the full-field dynamic responses of PCB. Air cushion effect between mobile phone and impact surface is found experimentally, which may be considered significant for the product-level free drop test. The rebound test has been performed to measure the actual impact velocity and to provide a better insight into the drop impact event. The effect of impact velocity is investigated to assess dynamic responses of PCB. Along with the drop impact experiments, the 3D FEA models are analyzed using ANSYS/LS-DYNA. The energy loss from the damping is considered by including the Rayleigh damping in this FEA model. Vibration analysis is performed experimentally and numerically to choose the proper damping parameters.

Abdullah Al-Yafawi

Papers

High-cycle fatigue life prediction for Pb-free BGA under random vibration loading

Influence of fastening methods on the dynamic response and reliability assessment of PCBS in cellular phones under free drop

Finite element based fatigue life prediction for electronic components under random vibration loading

Random vibration test for electronic assemblies fatigue life estimation

Effect of damping and air cushion on dynamic responses of PCB under product level free drop impact