Guoyuan Li

Study of thermal fatigue lifetime of fan-in package on package (FiPoP) by finite element analysis

Thermal fatigue lifetime of the Fan-in Package on Package (FiPoP) was analyzed based on the plastic strain model by finite element analysis (FEA). Both the stress and strain of FBGA and PBGA solder joints were studied under thermal cycling. It was found that the outmost solder joint on the PCB was the most dangerous position. The fatigue lifetime for the key solder joint on the PCB was estimated using Engelmaier modified Coffin-Manson model. Results showed that the fatigue lifetime of the solder joint of the FiPoP was up to 2073 temperature cycles. The effects of different Ag contents of the lead-free solder paste used for board level assembly on the fatigue lifetime of solder joints of the FiPoP were studied. Results revealed that Sn-4.0Ag-0.5Cu solder had a longest fatigue lifetime of 2441 cycles, the Sn-1.0Ag-0.5Cu had a shortest fatigue lifetime of 1830cycles, and the Sn-3.0Ag-0.5Cu which was used in previous work showed a middle lifetime. The effects of thermal cycling temperature profile on the fatigue lifetime were also investigated. Results showed that the faster temperature ramp rate decreased fatigue lifetime significantly. When the ramp rate increased 16.5°C/min to 33°C/min, the fatigue lifetime dropped to 1686 cycles. On the other hand, the shorter the temperature dwell time, the longer the lifetime. When the dwell time reduced to 10mins, the fatigue lifetime was improved to 2572cycles. It meant that the creep suffered by solder joints had a contribution to the lifetime of the solder joints as well.

Fatigue life prediction of Package-on-Package stacking assembly under random vibration loading

Abstract This paper presents a general methodology to predict the fatigue life of the Package-on-Package (PoP) under random vibration loading by means of vibration tests and finite element (FE) simulation. The behavior of the critical solder joints of the PoP under vibration loading was accurately described by FE model using ANSYS software and confirmed by modal analysis and linear sweep tests. The stress-life (S-N) curve of the PoP solder joints was obtained by the sinusoidal fatigue vibration tests and FE simulations to characterize the fatigue properties of the PoP under vibration. The comparison of the S-N curves of the SAC305 solder joints in different structures indicates the S-N curve of the SAC305 solder joints depends on the package structure. With the same fatigue cycles, the stress levels for the current 3D package is the smallest compared with the other 2D package due to the stacked structure of the PoP. Spectrum analysis for the random vibration tests was performed by the FE simulations to acquire the stress response power spectral density (PSD) of the critical solder joint. A specific frequency domain approach based on the Palmgren-Miners rule was established to predict the fatigue life of the PoP under random vibration. Results show that simulated fatigue life matches well with the random vibration test results, which implies that this approach could be a potential method for the predication of fatigue life of the 3D packaging under random vibration.

Effects of dopants on wettability and microstructure evolution of Lead-Free solder joints

In this study, the synergy effects of Sb and rare earth (RE) element addition on the wettability and microstructure evolution of Lead-Free solder joints are investigated. Experiment results indicate that Sb and RE element possibly had synergy effect on improving the wettability of lead-free solder. It is found that Sn-3.0Ag-0.5Cu-0.8Sb-XY, with X ranging between 0.05 and 0.1, has the best wettability on Cu substrate. The thickness of IMC increases as the amount of yttrium increase from 0wt% to 0.25wt% in the SAC-0.8Sb-Xwt%Y/Cu system.When the yttrium concentration is increased from 0.25wt% to 0.5wt%, the IMC thickness decreases. Further increase in the RE concentration just results in a slightly increasement of the IMC thickness. Considering the wettability and IMC growth, it is found that the optimal composition of the Y is about 0.5wt%. Results in this study just reveal that simutenuasly adding 0.8wt% of Sb and some Y does not have obvious effect on suppression of the IMC growth of the SAC-0.8Sb-Xwt%Y/Cu solder joints.

Effects of surface finishes on the intermetallic growth and micro-structure evolution of the Sn3.5Ag0.7Cu lead-free solder joints

The growth rate and kinetics of the intermetallic formation, intermetallic grain morphology, and micro-structural evolution for the solder with I-Ag, OSP and ENIG finish were investigated. Comparing the intermetallic thickness growth rate in Sn3.5Ag0.7Cu solder joint on PCB Cu metallizations with I-Ag, OSP and ENIG finish, the OSP had the highest intermetallic growth rate followed by I-Ag and ENIG. The growth rate for I-Ag was a little bit lower than OSP finish. ENIG served as a good barrier to block the IMC formation. These results suggested that the PCB finishes had a big impact on the intermetallic growth rate. The activation energy was determined to be 30.83kJ/mol and 32.67 kJ/mol for I-Ag and OSP finishes respectively. The growth mechanism of the intermetallic layer was compared in terms of IMC thickness and grain size evolution during isothermal ageing. Results reveal that the selection of solder alloys and PCB finishes played an important role in the morphology, microstructure evolution, growth of the intermetallics formed in the solder joint interface.

Effect of fixation method on solder joint vibration fatigue reliability of high density PCB assembly

In order to explore the influence of various fixation method on vibration fatigue reliability of high density PCB assembly, a three-dimensional simulation model which was almost completely in keeping with test samples was created, and the modal parameters were obtained through modal test and finite element method respectively. Based on the verification of FEM Model correctness, through designed different fixation methods, random vibration finite element simulation was being done, and vibration parameters were picked up and analyzed. The result showed that more PCB fixations could increase the first frequency, decrease the deformation of PCB. Under the same conditions, the order of the first frequency of the PCB Assembly with different fixation methods is: one end fixation <two screws fixation< two ends fixation<four screws fixation<six screws fixation. Consequently, more PCB fixations will decrease stress of key positions of PCB assembly, and BGA (ball grid array) devices should be placed near the region which has more fixation positions. Maximum stress of solder joint is located in the outermost corner of central PBGA module, and the accurate interface of maximum stress is located on the BGA side. And stress increases with the distance between solder and the center of BGA. So, the dimensional structure of PBGA assembly, fixation method of PCB and chip position should be considered comprehensively when vibration reliability of PBGA assembly was analysized. Finally, it can make some references for guiding BGA placement, selecting the appropriate size of BGA and the best fixation method for predicting and optimizing random vibration fatigue life of solder joints.

Reliability of package on package (PoP) subjected to thermal and power loadings

Traditional thermal loading test neglects the influence of power applied to PoP. To address this challenge, board level reliability of PoP subjected coupled thermal and power loadings were investigated in this study. In order to provide the coupled thermal and power loadings, high temperature storage (HTS) and constant current source were chosen for the accelerated test. Five test conditions (0A, 0.5A, 0.75A, 1.0A, 1.5A at 120°C) were adopted to evaluate the board level reliability of PoP. Experiment results revealed that the thermal and especially the power loadings had critical impact on the resistance of solder joints of PoP. The larger the current applied, the more the voids produced. Most of the voids were located on the inner ring of the bottom package. Besides, when solder joints suffered a large current, the generated Joule heat could melt down the solder balls in a short time. Current applied to PoP should be carefully considered and the critical spots located on the inner ring of the bottom package need to be specially tackled to improve the reliability of PoP.

Effect of Dopant on Microstructure and Mechanical Properties of Pb-Free Solder Joints

The effect of Sb addition on IMC growth in Sn-3.5 Ag-0.7 Cu solder joints has been studied in this work. IMC morphologies of Sn-3.8 Ag-0.7 Cu solder joints have been changed as a result of the Sb addition. The IMC layer becomes thinner with the increase in Sb proportion. It is observed that the fine particles precipitating increase in IMC layer with the increase in Sb proportion during reflow. Sb addition can increase the activation energy of Sn-3.8 Ag-0.7 Cu solder alloy system, and reduce the atomic diffusion rate, so as to inhibit the excessive growth of the IMC. It is found that the solder alloy with about 0.8 wt% Sb has the highest activation energy and lowest interdiffusion coefficient for the formation of the IMCs. The tensile strength can be slightly improved by Sb addition and is significantly affected by the IMC formation in solder joints.

Effect of Pb content on thermal fatigue life of mixed SnAgCu-SnPb solder joints

The effect of Pb content on fatigue life of mixed SnAgCu-SnPb solder joints was investigated by thermal cycling with temperature ranging from −55°C to 125°C (with 10 min dwell times and 10°C/min ramp rate). The cross-sections of mixed solder joints were characterized by scanning electron microscopy equipped with energy dispersive spectrometry. The results showed that the thermal fatigue life of Sn3.0Ag0.5Cu was totally outclassed the Sn63Pb37, minor Pb additions would cause the fatigue life significantly decreasing. With the Pb content increasing, the fatigue life of mixed solder joints decreased gradually. Moreover, the fatigue life was negative linear relationship with the Pb content increasing from 9.22 wt% to 26.49 wt%, when the Pb content was higher than 26.49 wt%, the thermal fatigue life dropped sharply and then leveled off. The thermal fatigue failure of mixed solder joints was not only relative to the evolution of microstructure, but also influenced by creep and plastic deformation coupling effects. However, the influence of Sn-Ag-Pb on the thermal fatigue reliability is not visible.

Analysis of board level vibration reliability of PoP structure with underfill material

The board level vibration reliability of the Package-on-Package (PoP) structure with different underfill types was investigated by finite element method (FEM). Underfill methods used in this study were the full-filled method, the corer-bonded method and the edge-bonded method. Results show that all of them can obviously improve the reliability of PoP structure in random vibration environment. The stress level of the solder joint significantly decreases by the application of underfill materials in the bottom package body of PoP structure. In addition, the location of critical solder joint is different with different underfill types.

Effect of nano-dopant on intermetallic compounds growth in Sn-3.0Ag-0.5Cu-xBi solder joints during aging process

The effect of nano-Bi particles on the growth of intermetallic compound (IMC) between Sn-3.0Ag-0.5Cu-xBi (x=0.0, 0.8, 1.5, 2.5, 3.5, and 4.5 wt.%) solder and Cu substrate during aging process at temperatures of 120, 150, and 190°C has been investigated in this study. Scanning electron microscopy (SEM) was used to observe the microstructural evolution of the solder joints and measure the thickness of IMC layer. Energy dispersive X-ray (EDX) was adopted to identify the composite of the IMC phase. The activation energies and growth rates of the IMC layer were determined. Results show that adding nano-Bi into Sn-3.0Ag-0.5Cu solder can increase activation energy and thus reduce the atomic diffusion rate, so as to suppress the excessive growth of the IMC layer. The solder joints containing about 0.8wt.% nano-Bi has the highest activation energy and the lowest growth rate. SEM images reveal that with an increase in nano-Bi to 0.8 wt.%, the number of small particles precipitated along grain boundary reaches maximum. Based on the observation of the microstructural evolution of the solder joints, a grain boundary pinning mechanism for inhibition of the IMC grow due to nano-Bi addition is proposed.