Takahiro Kinoshita

Thermal Stresses of Through Silicon Vias and Si Chips in Three Dimensional System in Package

Rbased on finite element method (FEM) was used to simulate the effects of voids formed inside Cu TSVs on the thermal conduction and mechanical stresses in the TSV structure. The thermal performance that was required in 3D SiP was estimated to ensure the reliability. Simulations for thermal stresses in the TSV structure in 3D SiP were carried out under thermal condition due to power ON/ OFF of device. In case that void was not present inside the TSV, the stresses in TSV were close to the hydrostatic pressure and the magnitude of the equivalent stress was lower than the yield stress of copper. Maximum principal stress of the Si chip in the TSV structure for the case without voids was lower than that of the bending strength of silicon. However, the level of the stresses in the Si chips should not be negligible for damages to Si chips. In case that void was present inside the TSV, stress concentration was occurred around the void in the TSV. The magnitude of the equivalent stress in the TSV was lower than the yield stress of copper. The magnitude of the maximum principal stress of the Si chip was lower than that of the bending strength of silicon. However, its level should not be negligible for damages to TSVs and Si chips. The stress on inner surfaces of Si chip was slightly reduced due to the presence of a void in the TSV. [DOI: 10.1115/1.4006515]

Anomalous elastic–plastic transition of MgO under shock compression

The particle velocity profiles of an MgO single crystal under shock compression were measured up to a pressure of 35 GPa by a velocity interferometer system for any reflector using a LiF window combined with a powder gun. The Hugoniot-elastic limit (HEL) of MgO along the ⟨100⟩ direction was 2.9–4.3 GPa and the initial elastic waves contain some oscillations. Along the ⟨110⟩ direction, the HEL was higher than along the ⟨100⟩ direction and anomalous, two-step-structure elastic waves were observed. These characteristics are discussed based on the slip system of the MgO crystal.

Thermal stress analysis of die stacks with fine-pitch IMC interconnections for 3D integration

The thermo-mechanical reliability of stacked die structures is a critical issue in 3D packaging. The assessment of the stress and the warpage of silicon dies in 3D stacked structures become important in achieving low-stress and low-warpage 3D packaging. However the parametric analyses of thermal stress and die-warpage by rigorous finite element analysis can be time consuming for 3D systems, since it involves many layers of materials such as silicon dies and organic layers. In this paper, we used the finite element method (FEM) with a simple 2D model to analyze the stress under thermal cycling condition on the die stack system and applied the 1D multilayered beam theory to perform parametric analyses of the die-warpage for the thermal stress condition. We used a 3D slice model to analyze the stress in the intermetallic compound (IMC) joints. The die-warpage values and the high stress sites in stacked structures obtained by these analyses were consistent with the measured data and experimental observations from the thermal cycle tests on full-area-array 40 μm bump pitch stacked die test vehicles with intermetallic compound joints.

Stresses in 3D SiP with TSV under unsteady thermal loads

Recently, high density 3D packaging technology has been developed to reduce the size and improve the performance of semiconductor devices [1–10]. Through silicon vias (TSV) technique enabled downsizing of electronic devices and faster signal communication between semiconductor chips. The delay time of signal, which depended on circuit length, was reduced by direct communication with TSV as compared to the existing 2D structure by wire bonding method. Due to above advantages, 3D SiP (Three Dimensional System in Package) with TSV structure has been investigated in actively. However, much heat generation was induced by concentrated heat sources in 3D SiP. Strength of Si and Si wafer was evaluated by experimental methods and numerical simulations [11–13]. Several hundred MPa was reported at global area of Si and several GPa was reported at local area of Si. Large difference of strength was reported between at global area and at local area of Si. In cases of power ON/OFF of electronics devices and hot spot which was steep temperature increment in local area in devised, unsteady thermal loads should be considered as a potential for damages to Si chips, TSV and materials on 3D SiP.

Thermal stresses around void in Through Silicon Via in 3D SiP

Thermal stresses around void in TSV (Through Silicon Via) structure in 3D SiP were discussed under the conditions of device operation and reflow process by using FEM (Finite Element Method). In case of the condition of device operation, equivalent stress around void inside Cu TSV was estimated at around 100 MPa. It showed the low possibility for low cycle fatigue of Cu TSV under device operation because that the stress was lower than yield stress of copper, 210MPa. Maximum principal stress of Si was estimated around 100 MPa. It was lower than bending stress of Si. In case of the condition of reflow process, the equivalent stress of TSV with void was higher than yield stress of Cu. However temperature elevation due to reflow process was once or twice during the process. It showed the low possibility for fracture by low cycle fatigue under reflow process. In case without void, maximum principal stress of Si was estimated around 400 MPa. It was almost similar to the bending strength of Si. Stress concentrations were occurred at parts of corner and interface of materials. It has possibility that singular stress field was formed at the parts, and we should discuss fracture induced by singular stress field.

Actual stresses around TSV in whole 3D-SiP under reflow or power ON/OFF thermal load

In this study, simulations on the reflow process or the power ON/OFF thermal loads were performed with a large scale simulator based on FEM (Finite Element Method), ADVENTURECluster® to discuss actual stresses around TSV and brittle Si chip and to ensure the reliability of 3D SiP. Elasto-plastic condition was considered to Cu materials of TSVs, micro bumps and heat sink.

Effect of fiber cloths structure on thermal fatigue strength of through hole in printed circuit boards

In this study, notch-shaped irregularities which could be observed at actual through hole were modeled. Inelastic thermal stress simulation was performed to discuss effect of fiber cloths structures on thermal fatigue strength around through hole in PCB by using a large scale simulator ADVENTURECluster (ver. 4. 8) [12], which was based on finite element method (FEM).

Inelastic analysis for singular stresses around TSV under reflow or power ON/OFF thermal load

In this study, the required heat transfer coefficient of heat sink is quantitatively shown by steady heat conduction simulation. Maximum principal stress of silicon and equivalent stress of the TSV are obtained from thermal stress simulation.

Fatigue life of lead free solder material with very sharp notch

Lead free solder materials have been wildly used on printed circuit boards in electronic apparatus. The solder joints sometimes are opened under thermal cyclic loads. The fatigue crack is initiated usually around the edge of the interface where the strain concentrates severely. In this study, Sn-3.0Ag-0.5Cu test pieces with V shape notch were supplied to fatigue tests, and inelastic simulation was performed to obtain the strain distribution around the notch. The low cycle fatigue test result with or without a notch and the simulated result were compared, and discussed the prediction method for fatigue life of lead free solder joint.

Deformation and behaviour of membrane structure by large deformation and contact simulation

Large deformation and contact simulations were performed to study deformation behaviour and stresses of membrane structure at conditions of snow weight by using a large scale simulator ADVENTURECluster Ⓡ which was based on Finite Element Method (FEM). The initial state of the membrane structure was reproduced by loading gravity on membrane. Pressure was applied on surface of membrane as snow weight. In this study, it is suggested that applying gravity on membrane is reasonable method for reproducing initial state of membrane structure. Deformation, stresses and behaviour of membrane were quantitatively discussed at condition of snow weight.