Luminescent Cavity Design for High Ambient Contrast Ratio, High Efficiency Displays

A key display characteristic is its efficiency (emitted light power divided by input power). While display efficiencies are being improved through emissive (e.g., quantum dot and organic light emitting display (OLED) designs1,2, which remove the highly inefficient color filters found in traditional liquid crystal displays (LCDs)3,4, polarization filters, which block about 50% of the light, remain required to inhibit ambient light reflection. We introduce a luminescent cavity design to replace both the color and polarization filters. Narrow-band, large Stokes shift, CdSe/CdS quantum dot emitters are embedded in a reflective cavity pixel element with a small top aperture. The remainder of the top surface is coated black reducing ambient light reflection. A single pixel demonstrates an extraction efficiency of 40.9% from a cavity with an 11% aperture opening. A simple proof-of-concept multi-pixel array is demonstrated.

Optimisation of a composite difference metric for prompt error detection in real-time portal dosimetry of simulated volumetric modulated arc therapy

Objectives: In real-time portal dosimetry, thresholds are set for several measures of difference between predicted and measured images, and signals larger than those thresholds signify an error. The aim of this work is to investigate the use of an additional composite difference metric (CDM) for earlier detection of errors. Methods: Portal images were predicted for the volumetric modulated arc therapy plans of six prostate patients. Errors in monitor units, aperture opening, aperture position and path length were deliberately introduced into all 180 segments of the treatment plans, and these plans were delivered to a water-equivalent phantom. Four different metrics, consisting of central axis signal, mean image value and two image difference measures, were used to identify errors, and a CDM was added, consisting of a weighted power sum of the individual metrics. To optimise the weights of the CDM and to evaluate the resulting timeliness of error detection, a leave-pair-out strategy was used. For each combination of four patients, the weights of the CDM were determined by an exhaustive search, and the result was evaluated on the remaining two patients. Results: The median segment index at which the errors were identified was 87 (range 40–130) when using all of the individual metrics separately. Using a CDM as well as multiple separate metrics reduced this to 73 (35–95). The median weighting factors of the four metrics constituting the composite were (0.15, 0.10, 0.15, 0.00). Due to selection of suitable threshold levels, there was only one false positive result in the six patients. Conclusion: This study shows that, in conjunction with appropriate error thresholds, use of a CDM is able to identify increased image differences around 20% earlier than the separate measures. Advances in knowledge: This study shows the value of combining difference metrics to allow earlier detection of errors during real-time portal dosimetry for volumetric modulated arc therapy treatment.

Understanding the Effects of Process Parameters to Compensate for Substrate Warpage in Chip on Flex (CoF) Assembly Using Conventional Reflow

Mounting rigid silicon (Si) die on a flex substrate presents a unique set of challenges. Substrate surface flatness and warpage of the flex substrate before and during thermal processing being two of the biggest issues. Various processes such as thermocompression bonding using anisotropic conductive films and thermosonic bonding using non-conductive adhesives have been shown to be effective for chip on flex (CoF) applications. However, these processes sacrifice process throughput and are less cost effective. Hence these methods may not be suitable for high volume manufacturing (HVM). For this reason, a process that is suitable for HVM and uses standard reflow processes and equipment is highly desirable. This works presents preliminary results for such a CoF process that uses a simple fixture and standard processes, materials and equipment. CoF process for Si die using two types of interconnects, solder balls or Cu pillars, was developed. Solder paste was stencil printed before pick and place of the Si die to offset the lack of flex substrate flatness and substrate warpage. The effect of stencil aperture diameter and placement force on process yield and stand-off height was studied. It was demonstrated that high yield can be obtained with a process that uses wider stencil aperture opening for both type of interconnects investigated. It was also observed that the placement force did not have any effect on process yield or stand-off height. The process can also be easily scaled up for HVM.

Improvement in Paretic Arm Reach-to-Grasp following Low Frequency Repetitive Transcranial Magnetic Stimulation Depends on Object Size: A Pilot Study

Introduction. Low frequency repetitive transcranial magnetic stimulation (LF-rTMS) delivered to the nonlesioned hemisphere has been shown to improve limited function of the paretic upper extremity (UE) following stroke. The outcome measures have largely included clinical assessments with little investigation on changes in kinematics and coordination. To date, there is no study investigating how the effects of LF-rTMS are modulated by the sizes of an object to be grasped.Objective. To investigate the effect of LF-rTMS on kinematics and coordination of the paretic hand reach-to-grasp (RTG) for two object sizes in chronic stroke.Methods. Nine participants received two TMS conditions: real rTMS and sham rTMS conditions. Before and after the rTMS conditions, cortico-motor excitability (CE) of the nonlesioned hemisphere, RTG kinematics, and coordination was evaluated. Object sizes were 1.2 and 7.2 cm in diameter.Results. Compared to sham rTMS, real rTMS significantly reduced CE of the non-lesioned M1. While rTMS had no effect on RTG action for the larger object, real rTMS significantly improved movement time, aperture opening, and RTG coordination for the smaller object.Conclusions. LF-rTMS improves RTG action for only the smaller object in chronic stroke. The findings suggest a dissociation between effects of rTMS on M1 and task difficulty for this complex skill.

Characterizing and Minimizing Voids in Quad Flat No-Lead Pack (QFN) Device Assembly Using Pb-Free Solder Alloys

Quad Flatpack No lead (QFN) packages have become a popular choice in electronics packaging due to its small form factor. They are also gaining rapid industry acceptance because of its excellent thermal and electrical performance. The bottom side of the QFN package has a large thermal pad. This exposed die attach pad effectively conducts heat to the PCB and also provides a stable ground connection. Effective soldering of this surface to the pad on the PCB is required for good thermal dissipation and component functionality. The exposed thermal pad presents various challenges during the surface mount assembly process. One major challenge is solder void formation. Voids are primarily formed due to the entrapment of volatiles in flux outgassing during the reflow process. The primary objective of this study is to determine optimal parameters to minimize void formation in QFN packages (QFN16, QFN20, QFN28 and QFN32), specifically the reflow profile, lead-free solder paste and stencil aperture opening for the thermal pad. A systematic DOE based approach was used to arrive at conclusions, using the ratio of void volume on the thermal pad to the actual volume of solder paste printed as the response variable. Various graphs are presented to understand the impact of different parameters. Interaction graphs are used to determine optimal settings for each parameter.