Stefan Kiefl

Some Integer Programs Arising in the Design of Main Frame Computers

In this paper we describe and discuss a problem that arises in the (global) design of a main frame computer. The task is to assign certain functional units to a given number of so called multi chip modules or printed circuit boards taking into account many technical constraints and minimizing a complex objective function. We describe the real world problem. A thorough mathematical modelling of all aspects of this problem results in a rather complicated integer program that seems to be hopelessly difficult — at least for the present state of integer programming technology. We introduce several relaxations of the general model, which are alsoNP-hard, but seem to be more easily accessible. The mathematical relations between the relaxations and the exact formulation of the problem are discussed as well.

3.5mW 1MHz AM detector and digitally-controlled tuner in a-IGZO TFT for wireless communications in a fully integrated flexible system for audio bag

We developed a fully flexible AM (amplitude modulation) radio receiver suitable for integration in an “audio bag”, by exploiting the heterogeneous integration of several fully flexible technologies. In this paper, we present a 2.9 mW 2-bit digitally-controlled tuner with a 576 kHz tuning range, a 3.5 mW 1 MHz AM detector and their integration in such a fully-flexible system. Their optimized power consumptions are essential because thin flexible batteries and organic solar cells serve as power supply. The circuits are fabricated in a low-temperature amorphous indium gallium zinc oxide (a-IGZO) technology. For the system integration textile techniques as well as flexible inkjet-printed packages and printed circuit boards (IPCBs) were used.

FE analyses and power cycling tests on the thermo-mechanical performance of silver sintered power semiconductors with different interconnection technologies

The paper reports on thermo-mechanical performance analyses of power semiconductors. Realistic transient temperature loadings as well as mechanical stresses were simulated by fully coupled electro-thermal-mechanical finite element analyses for power cycling loads. Power cycling tests were run in parallel to the theoretical investigations. The failure modes observed by testing were analyzed and adjusted to FE results. Some of the failures need a sophisticated evaluation strategy, as failure initiates at bi-material free edges, which obey a mechanical stress singularity. Damage mechanical modelling by means of the cohesive zone method (CZM) was adopted along with the coupled finite element analysis (FEA) in those cases. Applications of the methodology are presented for a SiC Mosfet testing sample operating at medium power and a high voltage inverter module with insulated gate bipolar transistors (IGBTs) and diodes, operating at high power. Both modules use silver sintering technology on directly bonded copper (DBC) substrates. Top interconnects are made by wire bonding for the Mosfet test sample but by an electroplating based planar technology for the inverter. Considering electro-thermal results it was calculated that stacks with planar copper interconnects outperform the wire bonded versions by 15–30% dependent on layout and current concerning thermal performance. For the die bonds, networks of cracks in the DCB copper and the silver layer replace the creep-ratchetting mechanism dominant for soft-soldered dies. This failure mode could be attributed to high cyclic in-plane normal stresses leading to subcritical crack growth at high power cycle numbers. The failure mode wire bond lift-off, characteristic for heavy Al wires, was investigated by CZM. The CZM methodology was also adopted to evaluate planar metallization delamination. For the latter, a parametric study has been made to optimize the materials choice and the layout of the metallization.

Electro-thermal-mechanical analyses on stress in silver sintered power modules with different copper interconnection technologies

Increasing demands for higher energy efficiency and operating at harsh environments lead to the development of new compact power electronics, which is complemented by new interconnection technologies. Investigations were made on a planar copper interconnection technology. The characteristic difference to other technologies can be seen in the replacement of bonding wires by planar copper interconnects and the high voltage applicability of the resulting modules. A high voltage and temperature resistant polymeric foil provides the insulation. Electrical connection is made by structured electrodeposited copper structures, which allow for additional heat spreading from top of the dies. Investigations on the thermo-mechanical behavior of prototype inverter modules, which use silver sintering and copper wire bonding technology or, alternatively, planar copper interconnection technology are reported. Fully coupled electrical-thermal-mechanical finite element (FE-) simulations were used to get realistic transient temperature loadings as well as mechanical stresses, also including wire heating or heating of the planar metallization, respectively. Improved thermal performance of the planar technology could be shown. A parametric FE-study was made to minimize delamination failure risks of planar structures based on cohesive zone modeling. Studies on processing dependent properties of the key materials sintered silver, electroplated copper, and dielectric foils are reported, which are indispensable for simulation input.

Bendable energy-harvesting module with organic photovoltaic, rechargeable battery, and a-IGZO TFT charging electronics

This work presents an innovative bendable module for solar-energy harvesting. The module consists of a mechanically flexible organic photovoltaic device (OPV), a rechargeable battery, and an a-IGZO TFT charge-control circuit. The total thickness of the module is 1.1 mm. We present measurements of hardware implementations and simulations. On this basis, voltage converter schemes based on a charge pump in the flexible a-IGZO TFT technology are explored, to enhance the range of operation of the module, particularly under low-light conditions. All investigations and data are presented with focus on two variants of the energy harvesting module that differ in nominal output voltage and capacity: a 6 V variant with capacity of 14.4 mAh and a 24 V variant with capacity of 5.5 mAh. Under exposure to 1 sun, both can be charged in less than 4 hours.