Henri Seppänen

Invited Article: Electric solar wind sail: Toward test missions

The electric solar wind sail (E-sail) is a space propulsion concept that uses the natural solar wind dynamic pressure for producing spacecraft thrust. In its baseline form, the E-sail consists of a number of long, thin, conducting, and centrifugally stretched tethers, which are kept in a high positive potential by an onboard electron gun. The concept gains its efficiency from the fact that the effective sail area, i.e., the potential structure of the tethers, can be millions of times larger than the physical area of the thin tethers wires, which offsets the fact that the dynamic pressure of the solar wind is very weak. Indeed, according to the most recent published estimates, an E-sail of 1 N thrust and 100 kg mass could be built in the rather near future, providing a revolutionary level of propulsive performance (specific acceleration) for travel in the solar system. Here we give a review of the ongoing technical development work of the E-sail, covering tether construction, overall mechanical design alternatives, guidance and navigation strategies, and dynamical and orbital simulations.

One kilometer (1 km) electric solar wind sail tether produced automatically

We produced a 1 km continuous piece of multifilament electric solar wind sail tether of μm-diameter aluminum wires using a custom made automatic tether factory. The tether comprising 90,704 bonds between 25 and 50 μm diameter wires is reeled onto a metal reel. The total mass of 1 km tether is 10 g. We reached a production rate of 70 m/24 h and a quality level of 1‰ loose bonds and 2‰ rebonded ones. We thus demonstrated that production of long electric solar wind sail tethers is possible and practical.

P1E-5 Understanding Ultrasound-Induced Aluminum Oxide Breakage During Wirebonding

A few models have earlier been proposed to explain Al oxide breakage during US wirebonding as presented by L. Levine, but no widely accepted theory exists. We propose a model to describe the AlOx breakage mechanism during tangential US excitation at constant pressure in a wirebonder. The model is based on theoretical estimations and experimental measurements. We measure with a laser Doppler vibrometer the relative wire-base displacement and propose that stick-slip and micro-slip behavior is prevalent during ultrasonic bonding. Displacement was measured at rim of the rectangular 14 mum thick and 80 mum width Al wire and at the silicon microchip base. A rectangular shaped wire was used to have probe light good reflection. We combined displacement measurements, detailed SEM analysis of contact interfaces and FEM of the bond structure during the bonding process. We also made a synthesis of the current bonding process knowledge. As a synthesis we propose a two-step model, including early stage scrubbing and later microweld expansion. Validation of the proposed model is discussed. This work and the obtained results are steps towards a fundamental quantitative US bonding theory that is necessary to develop reliable bonding technologies towards finer-pitch and more reliable interconnections.

Alice Silicon Strip Detector Module Assembly with Single-Point TAB Interconnections

The silicon strip detector (SSD) modules cover the two outermost layers of Inner Tracking System of ALICE. The SSD collaboration performs the module assembly in several locations in Europe. Mass production of the SSD modules was launched during autumn 2004 and presently all the sites are producing the SSD modules successfully. The bonding yield of spTAB interconnections is approaching close to 100%. This paper describes the assembly phases and bond process development for SSD modules, including discussion on the most probable rootcauses of failures and the long-term reliability of the interconnections.

Determining the quality of space tether in a nondestructive manner

We propose a nondestructive method to determine the quality of each bond in an electric solar wind sail (E-sail) tether. The method is verified by a method similar to the standard destructive pull test [1]. The setup that we built for the proposed in-line tether quality measurement comprises a custom-built ultrasonic bonder, a laser doppler vibrometer, an ultrasonic generator, contact resistance measurement electronics, and a laser-ultrasonic device. During the bonding process the setup continuously measures voltage and current driving the ultrasonic transducer, the bonding lower wedge displacement, the contact resistance of the bond interface and a laser-induced high frequency pulse transmission through the bond interface. The post-production analysis results are correlated with the destructive bond pull test results to identify a target signal path. Staying on this path should ensure strong bonds. This work is part of our efforts towards the produce to specification concept in the ESAIL programme.

Space tether produced to strength specification

A non-contact method to determine the quality of an ultrasonic weld was developed for wire-to-wire bonding purposes. Relative wire movement during the bonding process was monitored with a laser-doppler-vibrometer. This movement provides information about the bond development. Such information permits predicting the final quality of the bond. Signals from N=5000 bonds were analyzed to find tell tale characteristics that predict the final strength of the bond. A bond that eventually will end up failed (un-attached) can be recognized as early as 2 ms into the bonding process. A distinction between poor quality (maximum sustainable pull-force below 8 g) and good quality (pull-force above 10 g) bonds can be recognized within 6 ms from the bonding start. A real-time feedback system was implemented to reduce the uncertainty in final bond quality and to actively augment the quality during the bonding process. The purpose of this study was to confirm that real-time quality assurance (produce to specification) is feasible in wire-to-wire bonding. This work is part of our efforts towards the produce to specification concept in the ESAIL programme.

Transient IR imaging of light and flexible microelectronic devices

An advanced method for the quality assessment of microelectronic assemblies has been developed by combining IR thermography and several techniques for stimulation by transient temperature fields. The method exploits singularities in materials and interconnections by the observation of perturbations in transient heat flow phenomena. For very light microelectronic systems like chip-on-flex assemblies a method was developed taking advantage of short stimulations by photoflash. Such a method provided possibilities for detecting defects on the level of a single interconnection with a pitch of 80 μm. In addition, a programmable array of thermo-electric converters, prepared for the testing of a large variety of microelectronic assemblies, was also used to perform transient IR imaging for chip-on-flex assemblies.

Quality control of ultrasonic bonding tools using a Scanning White Light Interferometer

Working surface of the ultrasonic bonding wedge must adhere to tight specifications to permit strong and repeatable ultrasonic wire bonding. Even minute changes in the wedge surface reduce the yield, especially when bonding to fragile materials. Consequently the ability to determine the profile of the bonding surface of the wedge could contribute to improved yield. We propose to use Scanning White Light Interferometry (SWLI) to measure the working surface topology of these wedges. SWLI measures complex surfaces with nanometer height resolution.

REAL‐TIME NONDESTRUCTIVE CONTACT RESISTANCE METHOD TO ESTIMATE WIRE BOND PULL FORCE

We estimate microelectronic wire bond quality nondestructively by measuring the contact resistance (CR) of the bond in situ during the bonding process. This measurement employs a Kelvin cross setup contacting the wedge, 25 um Al wire and an Au substrate. The results verify that the method can identify the bond process phases and predict whether the bonding was successful (94% classification accuracy). The method can be used for process control and optimization to create stronger bonds and higher yield.

Scanning white light interferometry in quality control of single-point tape automated bonding

We report on using a Scanning White Light Interferometer (SWLI) for quality control of aluminum lead single-point Tape Automated Bonding (spTAB). A spTAB process was used to connect 14 μm thick, 42 μm wide aluminum leads on a 12 μm thick polyimide layer to a micro chip. Three different bonding process parameters were varied in order to maximize the pull force: bond force, ultrasonic power, and ultrasonic time. A custom built SWLI was used to measure the topography of the bonds in order to find features that correlate with the tensile bond force. This force was obtained in a destructive way by a pull test. By keeping the bond height within 3±1.5 μm, bonds with acceptable tensile forces in excess of 54 mN were obtained. This was verified by a separate validation measurement where the pull force of bonds complying with the height requirement was recorded.