A detailed investigation of flip-chip interconnects up to W-band frequencies is presented in this paper. In a coplanar 50-/spl Omega/ environment, different test structures were fabricated and measured to determine the electromagnetic characteristics of flip-chip multichip modules, such as detuning, reflection at the interconnect, and parasitic coupling. Electromagnetic simulation is used to explain the details behind the measured results. Key to high return loss at the interconnect is a small bump-pad area. Applying simple compensation structures, the frequency range of operation can be further extended. It is shown that a return loss beyond 20 dB in the frequency range up to 80 GHz is achievable along with excellent reproducibility. Measurements on detuning and isolation are also presented.

Theory and measurements of flip-chip interconnects for frequencies up to 100 GHz

Low temperature co-fired ceramic (LTCC) technology, widely used in the automotive industry, is now being employed in microwave applications. Several commercial materials with low dielectric losses at microwave frequencies and adequate thermomechanical properties have been introduced. Computer-aided design of three-dimensional circuits has also become available. These advances together with high-quality manufacturing technology have placed LTCCs at the forefront in the development of miniature microwave devices. The paper outlines LTCC technology placing emphasis on those essentials of the materials and processing technologies about which the microwave circuit designer needs to be aware. The discussion is illustrated by examples.The crucial issue of component reliability is also addressed. Although the integration of passive components into the structure improves reliability, the joints between the LTCC module and PCB remain as significant ‘weak link’. Therefore, thermomechanical and structural design is a key to reliable LTCC assemblies.Finally, some future trends the LTCC technology for microwave applications are outlined.

Design aspects of microwave components with LTCC technique

In this paper, parasitic modes, such as slotline, parallel plane, and surface wave (SW) modes, commonly found on printed circuit boards (PCBs) are analyzed and their effects on electromagnetic compatibility (EMC) and signal integrity are discussed. The analysis is based on numerical simulations using the finite difference time domain (FDTD) method which is shown to be very well suited for rigorous modeling of parasitic mode effects. The EMC and signal integrity problems discussed include power loss, crosstalk, ground bounce, and free space radiation. Design guidelines for improved EMC and signal integrity are derived from the results obtained. Comprehensive simulation and characterization of SWs using FDTD is presented for the first time.

Parasitic modes on printed circuit boards and their effects on EMC and signal integrity

Single-chip 60 GHz transmitter (TX) and receiver (RX) MMICs have been designed and characterized in a 0.15mum (fT~ 120 GHz/f MAX> 200 GHz) GaAs mHEMT MMIC process. This paper describes the second generation of single-chip TX and RX MMICs together with work on packaging (e.g., flip-chip) and system measurements. Compared to the first generation of the designs in a commercial pHEMT technology, the MMICs presented in this paper show the same high level of integration but occupy smaller chip area and have higher gain and output power at only half the DC power consumption. The system operates with a LO signal in the range of 7-8 GHz. This LO signal is multiplied in an integrated multiply-by-eight (X8) LO multiplier chain, resulting in an IF center frequency of 2.5 GHz. Packaging and interconnects are discussed and as an alternative to wire bonding, flip-chip assembly tests are presented and discussed. System measurements are also described where bit error rate (BER) and eye diagrams are measured when the presented TX and RX MMICs transmits and receives a modulated signal. A data rate of 1.5 Gb/s with simple ASK modulation was achieved, restricted by the measurement setup rather than the TX and RX MMICs. These tests indicate that the presented MMICs are especially well suited for transmission and reception of wireless signals at data rates of several Gb/s

60 GHz Single-Chip Front-End MMICs and Systems for Multi-Gb/s Wireless Communication

A particularly critical issue in this regard is module packaging, i.e., the way to assemble and connect several monolithic microwave integrated circuits (MMICs) in a multichip environment in order to build the millimeter-wave front-end. The requirements on the packaging scheme are obvious: the interconnects should provide good millimeter-wave performance (primarily meaning: low reflections and low insertion loss) and, at the same time, they should allow for low-cost fabrication. Among the multichip packaging techniques available, the flip-chip approach is considered to be the most promising candidate to meet these requirements.

The flip-chip approach for millimeter wave packaging

Electromagnetic simulation and measurement data of flip-chip transitions are presented. First-order effects are identified and design criteria for millimeter-wave multichip interconnects are derived. Results cover chip detuning and bump geometry as well as simplified modeling. In a coplanar environment, the flip-chip scheme provides interconnects with excellent low-reflective properties. For conductor-backed structures, parasitic modes occur leading to unwanted crosstalk. These effects dominate the behavior so that overall performance of the flip-chip scheme can be evaluated properly only in conjunction with the actual motherboard packaging setup.

Millimeter-wave characteristics of flip-chip interconnects for multichip modules

Electromagnetic simulations and measurement data of flip-chip transitions are presented. First-order effects are identified and design criteria for mm-wave multi-chip interconnects are derived. In coplanar environment, the flip-chip scheme provides interconnects with excellent low-reflective properties. For conductor-backed structures, the suppression of parasitic modes represents the key issue.

Millimeterwave characteristics of flip-chip interconnects for multi-chip modules

LTCC multilayer substrates offer a cost-effective MCM solution for frequencies around 20 GHz and beyond. This paper reports on LTCC-specific transmission-line structures using a commercially available process. For the chip interconnect, a flip-chip approach contacting the backside is chosen, which, in a first step, is realized on conventional ceramics.

LTCC as MCM substrate: design of strip-line structures and flip-chip interconnects

A hot-via flip-chip interconnect for the 40 GHz band is presented. The chip in-out cell includes on-wafer probing pads and is minimized with regard to size. An optimized design shows excellent performance with 10 GHz of bandwidth and -40 dB isolation. This demonstrates the potential of the hot-via approach in mm-wave applications.

40 GHz hot-via flip-chip interconnects

A flip-chip approach is presented using a thin-film microstrip line (TFMSL) on silicon as the carrier substrate. Thin-film technology allows one to employ cheap low-resistivity Si wafers without degrading MM-wave performance. By means of 3D EM simulation, an optimized interconnect is designed for the 77 GHz band. Measurements of test structures up to 100 GHz demonstrate the potential of this approach.

A. Jentzsch

Papers

Millimeter-wave characteristics of flip-chip interconnects for multichip modules

Millimeterwave characteristics of flip-chip interconnects for multi-chip modules

LTCC as MCM substrate: design of strip-line structures and flip-chip interconnects

40 GHz hot-via flip-chip interconnects

W-band flip-chip interconnects on thin-film substrate