Alexander J. Yerman

Hot Spots Caused by Voids and Cracks in the Chip Mountdown Medium in Power Semiconductor Packaging

The temperature increase under power dissipation in large semiconductor Chips mounted down with solder containing voids or cracks is explained in terms of two void types. They are distinguished on the basis of transient and steady-state thermal resistance measurements, and differ from each other in that for one the increased thermal resistance is mitigated by a reduced current density, but not for the other. They have been verified experimentally, and explain a large range of thermal and electrical characteristics, Such as occur in fatigue cycling.

Fabrication method for a winding assembly with a large number of planar layers

Z-folding of flex circuits is presented as a method of fabricating a transformer winding assembly having a large number of conductive and insulating layers. Flex-circuit patterns. for practical winding configurations are described along with a synthesis procedure. The steps to assemble interleaved planar windings from flex circuits are described. Equations relating winding resistance to geometrical parameters are derived for design purpose. >

Design issues for the transformer in a low-voltage power supply with high efficiency and high power density

Circuit model, design feasibility, and design tradeoffs are investigated for the transformer in 1.5-5 V power supplies with high efficiency and high power density. The transformer is constructed from a single or a matrix of pot cores and from interleaved planar windings. It has been determined theoretically and verified experimentally that such a transformer is realizable as long as the loss constraint is not severe (e.g. less than 0.5 W transformer loss per 100 W output). The primary source of loss is the winding, not the core, in 1.5 V/turn design. Measures to reduce the transformer height tend to increase transformer loss or volume. >

Evaluation of trade-offs in transformer design for very-low-voltage power supply with very high efficiency and power density

Design methodology and trade-offs have been presented for a transformer in low-voltage (1.5-5 V) power supplies intended to meet 95% efficiency and 100 W/in/sup 3/ power density. The design is based on the pot core geometry, which has low core loss, and the planar toroidal winding geometry, whose mechanical and electrical parameters are highly controllable and reproducible by modern fabrication processes. To achieve high power density in the presence of skin and proximity effects, the primary and secondary winding layers are interleaved to increase the effective thickness. Leakage inductance is reduced, and winding capacitance is increased as a result. The design equations have been coded into a computer program that selects the transformer with the lowest volume over specified ranges of optimizing parameters, such as power loss and flux density.<<ETX>>

Voids, Cracks, and Hot Spots in Die Attach

The operational life of high power device packages is often limited by metallurgical fatigue of soft solder layers used as joining materials in the package construction. During package fabrication, the solder layers may have voids or cracks which can spread during subsequent operational temperature cycling. To explain the observed range of thermal and electrical resistances after die attach and operation, two types of voids (cracks) are postulated. For one type in which thermal but not electrical conduction is impeded by the void, hot temperature spots form above the void. For the other type in which both thermal and electrical conduction are impeded above the void, there are no hot spots, but the current conducting area is limited, increasing the voltage drops. Experiments to verify these idealized void types are described.

Megahertz transformers for high density power conversion

Transformer designs that achieve efficiencies greater than 99% operating at 2 MHz at power densities in excess of 400 W/in/sup 3/ for the power range of 50-100 W with output voltages of 1.5 V are discussed. To achieve these results, copper density must be increased beyond what is possible with more conventional Litz wire constructions. Instead, multilayer windings are made of copper foil using flex circuit technology. The elimination of external connections between the winding layers by incorporating the interconnections as an integral part of the winding design is described. This results in Z-folded winding structures that can be ultimately interleaved to achieve high copper density, low copper loss, and very low leakage inductance. >

MHz transformers for high density power conversion

The authors discuss transformer designs that achieve efficiencies greater than 99% operating at 2 MHz at power densities in excess of 400 W/in/sup 3/ for the power range of 50 to 100 W with output voltages of 1.5 V. To achieve these results copper density must be increased beyond that possible with more conventional Litz wire constructions. Instead, multilayer windings are made of copper foil using flex-circuit technology. A particular innovation described is the elimination of external connections between the winding layers by incorporating the interconnections as an integral part of the winding design. This results in Z-folded winding structures that can be intimately interleaved to achieve high copper density, low copper loss, and very low leakage inductance.<<ETX>>

A 1 MHz, 100 W commercial, high-density point-of-load power supply using direct-bond copper and surface mount technologies

A point-of-load power supply is designed for use in a distributed power system. In order to be commercially viable this supply requires low-cost components and packaging, while maintaining a small footprint and a half-inch height for interboard spacing requirements. The unique combination of direct-bond copper, surface mount, and advanced magnetic technologies permits an estimated maximum hybrid power density of 40 W/in/sup 3/ while using low-cost, readily available commercial components. Packaging details of a 100 W prototype operating at 1 MHz with 34 W/in/sup 3/ hybrid power density are described, including packaging constraints caused by the electrical requirements of the resonant topology utilized. In addition, thermal considerations which must be met in order to reach the maximum hybrid power density are discussed.<<ETX>>

Corrosion of Aluminum Metallization Through Flawed Polymer Passivation Layers; In-Situ Microscopy

Aluminum corrosion in a polymer-coated circuit model was studied using in-situ microscopy coupled with time-lapse video recording. Pinholes in the polymer coating and exposure to water with bias voltage of 40V applied between adjacent aluminum tracks resulted in fast anode corrosion, accompanied by gas evolution. A mathematical model is developed that is in good agreement with the observed current-time relationship. The rate of corrosion is controlled by the resistance of a 6000A thick water film occupying the space of the corroded Al film between wafer and polymer coating. A tentative corrosion mechanism is proposed.