Analytical investigation on thermal-induced warpage behavior of ultrathin chip-on-flex (UTCOF) assembly

The serious warpage issues of ultrathin chip-on-flex (UTCOF) assembly induced by mismatched thermal stresses have greatly affected the mechanical stability and reliability of emerging ultrathin chip packaging technology. Currently, a theoretical prediction as a convenient and straightforward approach is still lacked for describing effectively the thermal-mechanical behavior of UTCOF during the adhesive curing and cooling process. In consideration of the adhesive thickness approximating to ultrathin chip and flexible substrate thickness, we develop a layerwise-model of ultrathin chip-adhesive-flex structure under plain strain condition, where the behavior of thick adhesive bonding can be described precisely through increasing the subdivided mathematical plies. Further, the analytical results show that the concave and convex forms of ultrathin chip warpage yield at the end of the curing and cooling process respectively. Meanwhile, the effects of its structure dimensions and material properties are also revealed for discussing a way to relieve the extent of ultrathin chip warpage. Additionally, in order to verify the validity of the theoretical prediction, we also introduce the corresponding numerical technique and experimental method. These results suggest that a kind of rigid and ultrathin flexible substrate such as metal foil should be adopted for small warpage of ultrathin assembly.     

神经网络-标称系统混合模型的离散变结构控制

提出将神经网络和标称系统混合建模方法引入到柔性结构主动控制当中，在混合模型的基础上，利用离散变结构控制（VSC）对柔性结构振动进行控制．离散变结构控制的滑模面是以标称系统为基础，由最优化二次型价值函数确定，并通过黎卡提方程求解．利用标称模型和神经网络混合建模方法来减小系统的不确定性，达到减弱变结构控制在实际控制系统中的抖动问题．神经网络采用多层前馈网络（MFNN），来对不确定部分进行建模．仿真结果表明系统振动受到了有效的控制，说明提出的神经网络变结构控制（NNVSC）方法非常有效。     

Experimental study of laser lift-off of ultra-thin polyimide film for flexible electronics

It is increasingly crucial for flexible electronics to efficiently and reliably peel large-area, ultra-thin flexible films off from rigid substrate serving as substrates of flexible electronics device, especially in industrial production. This paper experimentally investigated the mechanism and technologic characteristics of laser lift-off(LLO) process of ultra-thin(～ 2 μm) polyimide(PI)film. It was found increasingly difficult to obtain desirable ultra-thin PI film by LLO with the decrease of the film thickness. The optimal process parameters were achieved considering laser fluence and accumulated irradiation times(AIT), which were found to be strongly correlative to the thickness of PI film. The process mechanism of LLO of PI film was disclosed that laser ablation of interfacial PI will result in the formation of gas products between the PI and glass substrate, enabling the change of interface microstructures to reduce the interface bond strength. The amount of gas products mainly determines the result of LLO process for ultra-thin PI film, from residual adhesion to wrinkles or cracking. The strategy of multi-scanning based on multiple irradiations of low-energy laser pulses was presented to effectively achieve a reliable LLO process of ultra-thin PI film. This study provides an attractive route to optimize the LLO process for large-scale production of ultra-thin flexible electronics.     

Precipitation of Al3Zr Dispersoids under Various Homogenization and Its Effect on Recrystallization in 2198 Al–Cu–Li Alloy

Herein, the precipitation of Al₃Zr during one- and two-step homogenization processes is studied by the transmission electron microscopy method, and its effect on recrystallization is investigated by the electron backscattering diffraction technique after the same hot compression and solution treatment, respectively. The results show that when the temperature reaches 430 °C, just some small Al₃Zr dispersoids are visible in samples that undergo one-step treatment. As the first-step homogenization temperature increases from 365 to 430 °C, Al₃Zr radius increases from 9.4 to 19.1 nm while the number density decreases rapidly from 1201 to 183 μm⁻³. In addition, the average radius of Al₃Zr increases to 22.1 nm while the number density decreases to 76 μm⁻³ when the sample is homogenized at 500 °C/24 h. After a double-step homogenization with decreased first-step temperature, lower recrystallization fraction is obtained. Therefore, the optimal homogenization treatment is determined to be 365 °C/16 h + 500 °C/24 h, after which the most desirable Al₃Zr distribution and the smallest recrystallization fraction are obtained during thermomechanical process. This result can provide a more appropriate parameter for homogenization treatment in order to achieve a more dispersed distribution, which will result in stronger pinning force during the thermomechanical process.     

Laser projection proximity transfer for deterministic assembly of microchip arrays at scale

Science China Technological Sciences - Deterministic assembly techniques that enable programmatic and massively parallel integration of chips are essential for the development of novel electronic...     

Real-time adaptive particle image velocimetry for accurate unsteady flow field measurements

Almost all conventional open-loop particle image velocimetry (PIV) methods employ fixed-interval-time optical imaging technology and the time-consuming cross-correlation-based PIV measurement algorithm to calculate the velocity field. In this study, a novel real-time adaptive particle image velocity (RTA-PIV) method is proposed to accurately measure the instantaneous velocity field of an unsteady flow field. In the proposed closed-loop RTA-PIV method, a new correlation-filter-based PIV measurement algorithm is introduced to calculate the velocity field in real time. Then, a Kalman predictor model is established to predict the velocity of the next time instant and a suitable interval time can be determined. To adaptively adjust the interval time for capturing two particle images, a new high-speed frame-straddling vision system is developed for the proposed RTA-PIV method. To fully analyze the performance of the RTA-PIV method, we conducted a series of numerical experiments on ground-truth image pairs and on real-world image sequences.

     

Flexible hybrid electronics: Enabling integration techniques and applications

The conventional electronic systems enabled by rigid electronic are prone to malfunction under deformation, greatly limiting their application prospects. As an emerging platform for applications in healthcare monitoring and human-machine interface (HMI), flexible electronics have attracted growing attention due to its remarkable advantages, such as stretchability, flexibility, conformability, and wearing comfort. However, to realize the overall electronic systems, rigid components are also required for functions such as signal acquisition and transmission. Therefore, flexible hybrid electronics (FHE), which simultaneously possesses the desirable flexibility and enables the integration of rigid components for functionality, has been emerging as a promising strategy. This paper reviews the enabling integration techniques for FHE, including technologies for two-dimensional/three-dimensional (2D/3D) interconnects, bonding of rigid integrated circuit (IC) chips to soft substrate, stress-isolation structures, and representative applications of FHE. In addition, future challenges and opportunities involved in FHE-based systems are also discussed.