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Flexible electronics have attracted rapidly growing interest owing to their great potential utility in numerous fundamental and emerging fields. However, there are urgent issues that remain as pending challenges in the interfacial stress and resulting failures of flexible electronics, especially for heterogeneous laminates of hard films adhered to soft polymer substrates under thermal and mechanical loads. This study focuses on the interfacial stress of a representative laminated structure, that is, the Si film is adhesively bonded to soft polydimethylsiloxane with a plastic polyethylene terephthalate substrate. An novel thermal-mechanical coupling model for this flexible structure is established in this paper, which presents the essential characteristics of interfacial shear stress. In addition, under thermal and mechanical loads, a typical case is investigated by combining an analytical solution with numerical results using the differential quadrature method. Furthermore, thermal and mechanical loads, material and geometry parameters are quantitatively explored for their influences on the interfacial shear stress. Targeted strategies for decreasing stress are also suggested. In conclusion, the thermal-mechanical model and application case analyses contribute to enhancing the design of interfacial reliability for flexible laminated structures.

The ultrasonic vibration-assisted drilling process and the surface micro characteristics of Hotan jade were evaluated in detail. Because of the special physical properties of Hotan jade, traditional machining methods have great limitations. In consequence, it is difficult to achieve the required precision and surface quality. In this study, the physical properties of Hotan jade were considered and its unique cutting and machinability characteristics were evaluated for comparison purposes. The design principles of UV drilling and the important components of such a system were examined. The material removal mechanism of UV drilling was evaluated in detail. Finally, comparative tests using UV drilling and traditional drilling were carried on Hotan jade. The test results were evident from the degree of tool wear, degree of reaming, the extent of damage at the entrance to the drilled hole, the outlet burr and the roughness of the hole surface. The results of the machining trials confirmed that UV drilling not only was an easier process, but the straightness of the hole could be guaranteed, and the machined quality and drilling efficiency usually were better than was the case using traditional machining.

Delamination of polymer/metal interfaces has a destructive effect on plastic packaged devices containing thin layered structures. The physics mechanisms of the delamination inside the microstructure devices are not well understood and difficult to reveal. In order to get a deeper understanding of strain variations at the interface between the viscoelastic polymer and other elastic metal bases under a thermal-hygro environment, the following researches are conducted. Firstly, by combining the generalized Maxwell model and the hygro-mechanical strain equation of bi-layer plate, the equation set for hygro-mechanical strain at the interface of bi-layer plate is established considering the viscoelasticity of epoxy resins. Secondly, two bi-layer plate test pieces are made using elastic copper plates and two different types of viscoelastic epoxy resins (diglycidyl ether of bisphenol F (DGEBF) and cycloaliphatic epoxy resins), and the normal strains at the interface of the two bi-layer plates under a thermal-hygro environment are measured by strain gauges sealed between the epoxy resin layer and the copper plate. Thirdly, by substituting the thermal strain results into the polynomial equations for thermo-mechanical strain at the bi-layer plate interface, the coefficients of thermal expansion of the two epoxy resins at different temperatures are obtained. Finally, multi-parameter fitting of the interface hygro-mechanical strain curves are drawn through programming in MATLAB software based on the genetic algorithm, and the shear modulus and relaxation time of the two viscoelastic epoxy resins are obtained. This paper's experiment and calculation results reveal the relationship between time-dependent strain and delamination of plastic packaged devices at polymer/copper interface under a thermal-hygro environment, and provide a valuable reference to improve the reliability of plastic packaged devices.

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