Quayle Chen

Reliability study of flexible display module by experiments

Flexible display module reliability were investigated herein with experiments, such as bending, twisting and ball drop. The pretests of all the three experiments were carried out firstly to primarily understand the flexibility and mechanical behavior of the display. Based on the pretest results, the corresponding fatigue test setup method and process were put forward. Then, the fatigue tests were executed. At last, through the failure analysis, the flexibility and reliability of the flexible display in different use cases were evaluated. Suggestions about how to use the display and improve the reliability through change the design were given also.

Rigid-Flexible Printed Circuit structure optimization by simulations

Rigid-Flexible Printed Circuit Boards (RFPCB) has the advantages of providing not only the flexibility and bendability required by many electronic products, but also the reliability required by the key electronic components. In this study, based on one kind of RFPCB which would be used for a wearable device prototype, the structure optimization was performed by simulations. Through the structural analysis, it is found that there are some risk places for potential failures, such as the corners of the connecting part of the RFPCB. Normally, the stress concentrations would locate on these places if the circuit boards encounter the bending, twisting or tension conditions during the assembly process or usages. Hence the three load conditions are simultaneously considered in the optimization. To reduce the stress concentration in the corners of connecting part, the structure of the FPC cross section were optimized. The various optimization proposals are presented firstly. Then the simulations were executed. At last, by comparing the maximum Von Mises stress of different simulation cases, the optimized structure was attained. Appending substrate layers on top and bottom layer of FPC and setting air gap between the appending substrate and FPC is suggested using tension and twisting cases. The original design is better used under the loading condition of bending.

Failure evaluation of flexible-rigid PCBs by thermo-mechanical simulation

In this present work the finite element method has been used for the simulation models in order to develop tools for the early stages of product design. The objective is to develop simulation models for Flexible Printed Circuits Boards (PCBs) in a flex-rigid concept in the shape of a wrist device to evaluate its critical stress and strain when this device is submitted to thermal loading considering FR4 and Polyimide substrates as the constitutive materials of the board with BGA attached components. The critical points are the mismatch of the coefficient of thermal expansion of different materials as well as the thermoset viscoelastic nature of the polyimide. In the end of this present study it will be shown that additional research should be done to the final product. In this paper we will show the preliminary results of strain and stress distribution induced by thermo loading using a commercial finite element package.

Reliability of Flexible Display by Simulation and Strain Gauge Test

With the requirements of miniaturization of the portable consumer electronic products, more and more new materials, fabricating process and technologies are used to make the electronic components flexible, bendable and wearable. Flexible display, as one of the most important components, recently attracts an enormous research interest in industry and university. With the application of these technologies, the reliability issue has been coming forth with the different designs, fabrication processes and substrate material applications. In this paper, simulations including three points bending and twisting in different ways are investigated. In order to verify the simulation model, a special simulation model that contains the strain gauge is built. Through the comparison of the simulation results and strain gauge test results, the initial right model was given. Based on this model, the impact of different bending setup parameters to the display flexibility were studied, such as the bending roller radius, the span of support roller, bending height and etc. And the flexibility and reliability of the display were evaluated by the comparisons.

Prediction of drop impact reliability of BGA solder joints on FPC

Recently, Flexible Printed Circuit (FPC) is extensively used in portable electronic products because of its excellent flexibility and twistability. Compared with rigid boards, FPC usually experiences much larger deformation in its usages. However, most available packaging-components at the moment are rigid, FPC-contained products may cause failures under accidental mechanical conditions, such as drop, bending and twisting. Many literatures have been published on the reliability of rigid boards, while much less results were reported for the research of drop performance of FPCs [1,2,3]. In this paper, the mechanical reliability of a BGA mounted on FPC under drop impact has been studied by simulations. To make the model more close to the practical products, one plastic cuboid with the size of 89mm×69.5mm×8mm taken as the housing is involved in the simulation model. The FPC mounted with a BGA is fixed in the cuboid through four screw bars and dropped from 1.5 meters height to the concrete ground. Drop direction, BGA layout location and solder joint shape and size, as the three main factors are used in the reliability study. In practical drop reliability tests, six orthogonal drop tests are taken. As well as considered that the simulation sample is rectangle, the simulations of different drop directions are divided into two groups according to if the long housing edge or short housing edge impact with ground firstly. To simulate the different drop impact angle based on the orthogonal drop, several drop cases were built by setting the angle from 0º to 90º between the ground and model. In all of above drop cases, the single difference is the drop direction. To study the impaction of BGA location, simulation cases with the BGA moved from edge to center had been simulated also. The height and the diameter of solder joint is another important factor to the connection reliability. To predict its impaction, solder joint height varied from 0.15mm to 0.23mm and diameter varied from 0.25mm to 0.35mm were considered in the models. Through the corresponding simulations, analysis and comparison, it is found that the strain rate could be up to a peak of 105/s, but most of the time, the strain rate locates in the range of 102/s. The max stress decreases with the increasing of the solder joint height. Drop reliability can be improved by increasing the height of solder joints. The max stress increases when the diameter of the solder joint increased from 0.25mm to 0.3mm, while decreases when the diameter increased from 0.3mm to 0.35mm. Drop reliability can be only improved when the diameter is larger than 0.3mm.

Thermal analysis based on the environmental tests of STN display

Environmental test is widely used to verify the adaptability and reliability of various electronic products in the last decades. The sharp change of temperature and humidity in certain situations could pose a serious reliability issue on the electronic components. This paper presents an experimental study of one kind of plastic substrate based flexible display called STN(Super-Twisted Nematic). The study focuses on the reliability of display under various temperature and humidity conditions. Considering the range of temperature and humidity changed in applications, the experiments are divided into four subgroups: cold test, cyclic damp heat test, the steady state damp heat test and dry heat test. Through all the tests, the STN display reliability was evaluated and the failure modes were obtained. Based on these tests, the thermal-structural simulation model was built. By the simulation, part of the experiment phenomenon were explained. It provides a solid foundation for further reliability study of the STN display in the practical applications.

Prediction of bending reliability of BGA solder joints on Flexible Printed Circuit (FPC)

In this paper, the mechanical performance of solder joints of BGA mounted on flexible printed circuit (FPC) was studied using finite element method (FEM). To optimize the electric components layout during the design, it is necessary to predict the bending reliability of components on FPC. Traditionally, 3-point or 4-point bending are common used to study if the substrate is rigid. In order to simulate the free bending condition with soft substrate, a new bending method is taken. Thats to fix the FPC at one side and move the other side to bend the FPC. Based on this novel model, the length of the FPC, the height and diameter of the solder joints are respectively taken as the optimized variables. With the variable change, there are many different cases. For one general case, the bending process is divided into two steps. At first, to rotate the FPC about 180° at one side and make the two fixtures parallel; then move the fixture to control the bending. It is found that the stress of the solder joints in first step linearly proportioned increase with the bending. While in the second step, the stress reaches the peak and declines gradually with the two ends of the FPC get closer. Simulation results indicate that by using the longer FPC can improve the reliability significantly. Normally, reliability can be also improved by increasing the height and diameter of the solder joints. While in the translating-induced bending, increasing the diameter will worsen the solder reliability unless the diameter is larger than 0.3mm.

Cold test and cyclic damp heat test of plastic substrate based displays

The plastic substrate based displays have attracted a lot of interest in recent years because of their potential applications in portable electronic devices. Many of them have high brightness, high contrast, and large angle of view, with the flexibility for accommodating different form factors. However, their reliability in practical applications has not been systematically studied. For many plastic materials, the temperature and humidity reliability are among the biggest concerns. This paper presents an experimental study of the plastic substrate based liquid crystal display with the focus on its reliability under varying temperature and humidity conditions. In this study, the cold test and the cyclic damp heat tests were conducted with visual inspections and electro-optical properties measurement during each test procedure. These provide a solid foundation for further studies on the reliability of displays based on plastic substrate technology in practical applications.

The steady state damp heat test and dry heat test of plastic substrate based displays

The plastic substrate based display has many advantages in practical applications of electronics, such as portable, flexible, super thin etc. However, its reliability may significantly impact its performance under the specific environmental conditions. Among the environmental factors, the temperature and humidity are two of the most important factors which may affect the performance of the displays dramatically. In this study, to evaluate the reliability and adaptability under the high temperature and the high humidity, the new environmental testing process for the plastic substrate based displays is presented. It consists of the damp heat under steady state test and the dry heat test, together with visual inspections and electro-optical property measurement. Based on the test, the analysis and assessment of the reliability and adaptability of the plastic substrate based displays under high temperature and high humidity were further performed.

Reliability study of stretchable electronics interconnect by simulation

In this paper, the stretchable interconnect is taken as the research object and its reliability has been analyzed by simulations. Polydimethylsiloxanes (PDMS), as a hyper elastic material, is used here as the stretchable interconnect substrate because of their high elastic property and high stretchability. Traditionally, metals are still the best options to realize the interconnections due to good electronic conductivity and mechanical elongation [1]. Based on that, gold, copper and silver are taken in the simulations for evaluating their performance. Different with other researchers to take curved routing profile shape, “dog bone” shape has been used in this paper due to considering the electro-magnetic confliction and lower requirements of stretchability in our practical application [2]. In order to optimize the designs and metal foil layout, different layout patterns have been considered as the optimized parameters, through the simulations with load condition of elongation and bending, the equivalent principal strain and Mises stress are taken from the different simulation cases to have comparison. Based on that, the reliability of different designs has been evaluated. The pattern of metal foil in plane has been evaluated, as well as the substrate thickness, different materials etc. Without loss of generality, with or without the two rigid components are involved in the model are taken to evaluate the metal foil reliability under the different load condition. The metal foil can be fabricated with different width and thickness, and as well as different methods, all these cases had also been simulated. By all these simulations, all the stretchable interconnect designs and combinations had been evaluated. The design suggestions have been made according to the simulation results analysis.