The stress measurement system for the JUNO Central Detector acrylic panels

The Jiangmen Underground Neutrino Observatory (JUNO) Central Detector (CD) is a huge acrylic spherical vessel containing 20,000 tons of liquid scintillator; the sphere is composed of 263 pieces of acrylic spherical panels bonded by the mass polymerization. The operation life time of the JUNO CD is 20 years. To ensure the structural safety during the JUNO CD life time, the acrylic stress of CD is required not to be greater than 3.5 MPa. The stresses of acrylic spherical panels are required to be measured during the installation on-site; unfortunately there is no suitable commercial measurement equipment that can meet JUNO's requirements. Therefore, a measurement setup based on photo-elastic principle and spectrometric methods was designed, developed and tested for on-site measurements. The measurement system performs accurate calibration of stress-optical coefficient of acrylic in JUNO, and gives reliable results of acrylic stresses. The measurement system has been tested in the Taixing Donchamp Acrylic Ltd mechanical workshop, and the achieved results meet the JUNO's requirements. The measurement principle, the system components, and the tooling design are introduced in the paper. Moreover, the calibration of stress-optical coefficient of the acrylic and measurements results on JUNO acrylic spherical panels are discussed in the following.

Using Element Birth and Death Technique in Modeling Cumulative Molded Substrate Expansion before Singulation

Semiconductor packages are commonly assembled and molded in array format on a substrate strip before they are singulated into individual units. However, cumulative substrate expansion causes problems such as machine vacuum error or misaligned cut during singulation if the substrate expansion is not factored in. This study uses element birth and death technique in modeling the overall expansion of the molded substrate strip so that the predicted expansion could be considered in the singulation tooling design offsets. The expansion of the substrate was modeled with the different package assembly processes and thermal conditions. Modeling results showed that there is a cumulative increase in the length of the substrate as it passes through the different processes. The results are in agreement with actual substrate expansion prior to package singulation. This would not be captured when simulation is done only for the molded substrate without considering the cumulative contribution of the preceding processes. With the element birth and death technique in process-based thermomechanical modeling, substrate expansion could already be forecasted, and package assembly problems avoided.

Assembly Wirebond Process Solution for Mitigating Leadframe Bouncing on Multi-Die QFN Device

Modification and improvement of an existing tooling design in semiconductor packaging industry has been a usual practice, to enhance the current setup and to provide a solution to a specific assembly problem. This paper discusses the solution in eliminating the smashed ball defect occurrence observed after wirebond process. Smashed ball is usually encountered if the unit is unstable and creates a bouncing effect during wirebond process. It is therefore important to mitigate this micro-bouncing effect by analyzing the package design and the window clamp and top plate (WCTP). The objective is to increase the stability of the unit during wirebonding, especially for quad-flat no-leads (QFN) package with no tape. To achieve this, the solution is to alter the vacuum hole design of the top plate from single hole per unit to multiple holes of varied sizes per unit. Ultimately, after changing the design of the top plate, the micro-bouncing encountered during wirebond process was significantly reduced. This in turn created a consistent ball formation in all bonded wires. The comparative data presented in this paper confirmed the effectivity of the redesigned WCTP. For future works and studies, the improvement and learnings could be used on devices with comparable configuration.

Expansion of oval tubes: prediction and experiment

The manufacturing of oval tubes for automotive components from sheets consists of several steps, from the flat sheet to a tube with expanded ends. It involves roll-bending of tubes, welding and several expansion processes with segmented tools. Forming steps in this process are subject to springback after the release of tools. Finite-element-simulations offer an efficient method to predict the springback behavior. For the industrial application it is important to identify the processes which contribute significantly to springback. At first glance one might expect that the consideration of the whole process chain is required to predict the final shape of such tubes. It turns out, that springback is related to the later stages of the process. The difference in springback behavior of circular and oval tubes is investigated. A simulation model is validated on the basis of experiments for circular tubes and applied to predict the final shape of oval tubes. This offers the perspective to adjust the tooling design at an earlier design stage to respect all the influences in the process on the final geometry and therefore meet tighter tolerances.