Showing papers by "Fred Barlow published in 2000"

Interfacial thermal resistance and temperature dependence of three adhesives for electronic packaging

Abstract: The thermal resistance and its temperature dependence was measured for three industrial adhesives used for electronic packaging. Measurements were made by the laser-flash method from room temperature to 300/spl deg/C. The samples were in the form of sandwiches consisting of two platelets of silicon carbide-reinforced aluminum (AlSiC) bonded together with the adhesives. The total thermal resistance of the bond (the sum of the bulk thermal resistance of the adhesive and the resistances at the two interfaces) was calculated from the thermal response of the sandwich subjected on one side to a single laser-flash. The total thermal resistance was found to decrease with increasing temperature. The bulk thermal resistance of the adhesive, calculated from its thickness and independently determined thermal conductivity, was found to be relatively independent of temperature. The interfacial resistance at the AlSiC interfaces, depending on the adhesive, ranged from about 60 to 80% of the total resistance decreasing to about 50% of the total interfacial resistance at 300/spl deg/C. For two of the adhesives considered in this study, the interfacial thermal resistances for the AlSiC/adhesive interfaces were found to be considerably higher than those found in an earlier study of Si/adhesive interfaces.

Polymer thick film (PTF) and flex technologies for low cost power electronics packaging

Abstract: Electronic power converters have been designed, produced, and disseminated in mass quantities using a number of fabrication techniques, from standard PCB technologies for low cost applications, to thick film on ceramic, to direct bond copper (DBC) for high power, higher cost applications. Each approach holds a share of the power packaging market, but they all restrict, for the most part, circuit and package designs to 2D boards. One potential pathway into the third dimension is by the use of multilayers, an approach which becomes increasingly difficult with each additional layer added beyond the first, and with the exception of high performance solutions is typically cost prohibitive for the majority of applications. This paper demonstrates the feasibility and viability of an alternative low cost power packaging option which uses familiar industry technologies in a unique manner: flexible polymer substrates. Flex technology uses industry standard PCB and/or thick film processes, offers the lower cost, higher performance solutions inherent with the majority of polymer plastics, and as a final bonus, frees the designer to more efficiently use all three dimensions. The researchers have shown the feasibility of this low cost alternative solution through the fabrication and testing of integrated power modules (IPMs) which use flex polymer substrates in conjunction with both surface mount and bare dice. These DC/DC power converters transform 120 V/240 V inputs to 9V, 7 W outputs, and illustrate the miniaturization advantages of fully utilizing the 3D space offered by flex circuitry.

Getting aggressive with passive devices

Abstract: The circuit boards of many mixed-signal and digital systems are now dominated by individually placed discrete passive (DP) components. This article looks at thin-film integrated passives (IPs) as an alternative to DPs in the effort to save board space and improve electrical performance and system reliability. Integrated passive components have been utilized successfully with ceramic substrate technology for over 50 years in the form of thick-film resistive and dielectric firable pastes. However, this considerable infrastructure cannot be transferred to FR4 and flex substrates due to the high firing temperatures required, and these board materials make up the vast majority of interconnect substrates, in consumer and commercial systems. Mmat has been lacking is thin-film IP materials and fabrication processes that are compatible with organic boards.

Z-axis interconnections for high density processors, Part 1

Abstract: Three-dimensional packaging has been considered by many the best path towards smaller, lighter, faster and less expensive electronics; however, interconnections and efficient thermal transfer have been stumbling blocks towards achieving that goal. Focusing on one of these challenges, this paper examines commercially available z-axis interconnection media, with advantages, disadvantages and selection criteria for the intended system. After an initial down selection based on published data, several connector styles will be tested for life cycle integrity and overall effectiveness for system integration. The primary specifications for the z-axis interconnections are 0.5-mm pitch, area array, high reliability in harsh military environments, and minimal contact force to simplify the external compression structure. This paper also describes the critical nature of the overall structural design and tolerance control, for reliable assemblies and continuous operation; with over two thousand interconnects between each of the five layers in the 3D stack-up.

Investigation of wirebonds on insulated-metal substrate for multichip power module applications

Abstract: One of the most common reliability problems for multichip power modules is wirebond lifting or wirebond failure. Thus, it is important to investigate the bonding of aluminum bond wire to copper, nickel-plated, and gold/nickel-plated surfaces on insulated-metal substrates (IMS) for multichip power module applications. Due to the high power requirements in multichip power modules, the aluminum bond wires usually carry high currents. As such, it is important to understand the high current effects on these aluminum wirebonds. Wirebonding of aluminum bond wire is an inherent destructive process to its structural properties due to the ultrasonic energy required to form the wedge bonds. It was found that the bond strength of the aluminum bond wire to copper, nickel-plated, and gold/nickel-plated on the insulated-metal substrate depends on the current carrying conditions and the thermal annealing of the aluminum bond wires. In general the bond pull strength of the aluminum bond wires on copper and nickel-plated IMS boards increases upon carrying a current large enough to cause a significant thermal annealing. Low temperature thermal annealing is found to improve the wire strength itself but degrade the bond strength slightly probably due to interfacial change or intermetallic formation at the bonding interface. Wire strength was measured based on elongation and tensile strengths. The paper will present data on the bond strength and interfacial change of the aluminum bond wire on IMS boards after carrying high currents and upon low temperature thermal annealing. These wirebonds were subjected to shear test and pull test according to MIL-STD-883E to evaluate the integrity of the wirebonds. The paper will provide measurements data of bond strength for copper, nickel, and gold surfaces on IMS boards, after current and annealing testing, and a comparison of these different surfaces. The paper will also address the possibility of interfacial change or intermetallic formation due to the passing of high current through the wirebonds.

Curriculum restructure to answer critical needs in packaging for energy efficiency/renewable energy systems, wireless, and mixed-signal systems areas

Abstract: The Electrical Engineering Department at University of Arkansas has been building considerable strength in Energy Efficiency/Renewable Energy Systems, Mixed-Signal, and Wireless Packaging areas. This effort is in coordination with critical other Departments within the College of Engineering; specifically Industrial Engineering and Mechanical Engineering Departments, in addition to the Physics Department within the College of Arts and Science. The High Density Electronics Center (HiDEC), established in 1992 with DARPA funds to conduct research on advanced electronic packaging technologies, enables the educators to interact within the various disciplines to achieve the set objectives of packaging in these areas. The paper will outline the mission of each area, the vision and objectives of the administration, the technical issues to be addressed, the technological challenges and barriers for the Department to face and overcome to make this vision a true reality, and the curriculum restructure. The paper will also outline how critical these strategic areas are for a national academic institution recognition and fulfillment of critical needs for our nation's global competitiveness.