Low-profile Applications Research Articles

Nonplanar Dipole Antennas for Low-Profile Ultrawideband Applications: Design, Modeling, and Implementation

This letter presents a printed circuit board (PCB)-based nonplanar dipole antenna with a flat reflector for low-profile ultrawideband applications. Physical fundamentals of the proposed antennas are summarized and discussed. Design considerations, modeling, and implementation methods are introduced in detail. A prototype antenna was fabricated and tested to verify the presented results. The prototype antenna has a measured impedance bandwidth of 7.2 GHz from 2.8 to 10 GHz, and it also possesses stable unidirectional radiation patterns up to 6.0 GHz, which is suitable for many broadband applications.

Commercial-off-the-Shelf (COTS) 3D Integration using Low Temperature Wafer Bonding

This paper reports on a successful 3D integration (3DI) of multi-purpose signal processor (MSP) chips with memory chips using die-to-wafer (D2W) and wafer-to-wafer (W2W) bonding technologies. 3D integration enables compact systems of commercial-off-the-shelf (COTS) parts with high functionality using a wafer-level process for better thinning process uniformity and high yield throughput. The3D system is comprised of commercial Flash memory bare die and MSP bare die. The bare die are face-down aligned to a 150mm diameter silicon handle wafer with alignment marks polished silicon surface. Unique features on the commercial die are detected and used for die registration using a flip-chip bonder with vision automation. An adhesive film between the die and silicon handle wafer are used for temporary bonding. After the die-to-wafer population and bonding, the die substrates are thinned at the wafer-level to a target of 60 microns for the memory die and 25 microns for the MSP die, respectively. The thinned memory die set is permanently transferred onto a 150mm diameter silicon carrier wafer using a low temperature silicon covalent wafer bonding. Following bonding, an adhesive film release process is used to separate the memory die set from the temporary handle wafer. The thinned MSP die on a second handle wafer are then aligned to the thinned memory die set using a wafer-to-wafer alignment tool, and bonded with thin-film polyimide in a high-yield, low temperature wafer bonding process, followed by the release process to separate the MSP die set from the handle wafer. Finally, the MSP/memory stack are electrically connected using a via-last through-silicon-via (TSV) process. One of the key considerations for COTS 3DI is to meet the back-end-of-line (BEOL) thermal budgets of 350–400 Celsius. Plasma-assisted preparation facilitates the reduction in thermal budget for silicon covalent bonding and is performed at 150 Celsius, followed by a long-term annealing process at 175 Celsius. Stacking of thinned die relies on low temperature polyimide bonding that is performed at 200 Celsius. Fluorine and oxygen based plasma surface activation process and CTE-matched polyimide bonding play a critical role in enabling the low temperature bonding for this 3D MSP/memory integration. The thinning and bonding processing details that are presented in this paper are essential for COTS 3DI but can also be applied to several low-profile multi-chip module and packaging applications.

Thermal Analysis of Miniature Low Profile Heat Sinks With and Without Fins

Reliable and efficient cooling solutions for portable electronic devices are now at the forefront of research due to consumer demand for manufacturers to downscale existing technologies. To achieve this, the power consumed has to be dissipated over smaller areas resulting in elevated heat fluxes. With regard to cooling such devices, the most popular choice is to integrate a fan driven heat sink, which for portable electronic devices must have a low profile. This paper presents an experimental investigation into such low profile cooling solutions, which incorporate one of the smallest commercially available fans in series with two different heat sink designs. The first of these is the conventionally used finned heat sink design, which was specifically optimized and custom manufactured in the current study to complement the driving fan. While the second design proposed is a novel “finless” type heat sink suitable for use in low profile applications. Together the driving fan and heat sinks combined were constrained to have a total footprint area of 465 mm2 and a profile height of only 5 mm, making them ideal for use in portable electronics. The objective was to evaluate the performance of the proposed finless heat sink design against a conventional finned heat sink, and this was achieved by means of thermal resistance and overall heat transfer coefficient measurements. It was found that the proposed finless design proved to be the superior cooling solution when operating at low fan speeds, while at the maximum fan speed tested of 8000 rpm both provided similar performance. Particle image velocimetry measurements were used to detail the flow structures within each heat sink and highlighted methods, which could further optimize their performance. Also, these measurements along with corresponding global volume flow rate measurements were used to elucidate the enhanced heat transfer characteristics observed for the finless design. Overall, it is shown that the proposed finless type heat sink can provide superior performance compared with conventional finned designs when used in low profile applications. In addition a number of secondary benefits associated with such a design are highlighted including lower cost, lower mass, lower acoustics, and reduced fouling issues.

An Experimental Study on the Design of Miniature Heat Sinks for Forced Convection Air Cooling

An experimental study is performed on one of the smallest commercially available miniature fans, suitable for cooling portable electronic devices, used in conjunction with both finned and finless heat sinks of equal exterior dimensions. The maximum overall footprint area of the cooling solution is 534mm2 with a profile height of 5 mm. Previous analysis has shown that due to fan exit angle, flow does not enter the heat sinks parallel to the fins or bounding walls. This results in a nonuniform flow rate within the channels of the finned and finless heat sinks along with impingement of the flow at the entrance giving rise to large entrance pressure losses. In this paper straightening diffusers were attached at the exit of the fan, which resulted in aligning the flow entering the heat sinks with the fins and channel walls. Detailed velocity measurements were obtained using particle image velocimetry, which provided a further insight into the physics of the flow in such miniature geometries and in designing the straightening diffusers. The thermal analysis results indicate that the cooling power of the solution is increased by up to 20% through the introduction of a diffuser, hence demonstrating the need for integrated fan and heat sink design of low profile applications.

Impedance Matched Ferrite Layers as Ground Plane Treatments to Improve Antenna Wide-Band Performance

In low-profile applications, ground plane backing significantly degrades antenna performance when the electrical height is a small fraction of a wavelength. An approach to suppress the destructive interference from the ground plane reflections is to employ absorbing layers of ferrite materials. In this paper, we introduce a means to also control and utilize the mode excited in these finite ferrite layers to recover antenna performance at ultra/very high frequencies. Experimental verification is provided using commercial ferrite materials.

UWB magneto-dielectric ground plane for low-profile antenna applications

In low-profile applications, the antenna is often positioned very closely to an electrically conducting surface, or ground plane. Such a ground-plane backing significantly degrades antenna performance when the electrical height is less than A115. Existing approaches, such as lossless electromagnetic bandgap (EBG) structures and artificial magnetic conductors (AMC), tend to operate over a small bandwidth. This paper discusses two alternative ultra-wideband (UWB) approaches that laminate the ground plane with 'special magneto-dielectric (or ferrite) layers. The first approach employs a ferrite layer with a high r/rratio to produce an in-phase reflection, similar to that caused by a perfect magnetic conductor (PMC). The second approach adapts a lossy ferrite with similar values of relative permeability, uir, and relative permittivity, 6r. Radiation enhancements achieved via these ground-plane treatments will be demonstrated by simulated and measured examples. Design guidelines based on a parametric study will also be discussed.

Low profile fan and heat sink thermal management solution for portable applications

The increasing heat flux densities from portable electronics are leading to new methodologies being implemented to provide thermal management within such devices. Many technologies are under development to transport heat within electronic equipment to allow it to be dissipated into the surroundings via conduction, natural convection and radiation. Few have considered the approach of implementing a forced convection cooling solution in such devices. This work addresses the potential of low profile integrated fan and heat sink solutions to electronics thermal management issues of the future, particularly focusing upon possible solutions in low profile portable electronics. We investigate two heat sink designs with mini-channel features, applicable to low profile applications. The thermal performance of the integrated fan and heat sinks is seen to differ by approximately 40% and highlights the importance of designing an integrated thermal management solution at this scale rather than fan or heat sink in isolation.

Modeling of losses in a sandwiched-winding matrix transformer

This paper assesses the suitability of the sandwiched-winding matrix HF power transformer for low-profile applications from the loss standpoint. Finite-element simulations and experiments suggest that the core loss can be adequately characterized by approximating the matrix transformer as a collection of identical uncoupled elements. Thus, a matrix transformer with a large number of elements may have higher core loss than a conventional (tall) transformer, and more core loss than winding loss. Eddy-current analysis reveals that the interconnects which parallel the elements incur a significant fraction of the winding loss. Thus, load distribution is recommended to eliminate the paralleling interconnects in matrix transformers.< <ETX xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">&gt;</ETX>