Studies of IBL wire bonds operation in an ATLAS-like magnetic field and evaluation of different protection strategies

At the Large Hadron Collider (LHC) experiments, most of silicon detectors use wire bonds to connect front-end chips and sensors to circuit boards for the data and service transmissions. These wire bonds are operated in strong magnetic field environments and if time-varying currents pass through them with frequencies close to their mechanical resonance frequency, strong resonant oscillations may occur. Under certain conditions, this effect can lead to fatigue stress and eventually breakage of wire bonds. Systematic studies have been conducted to analyse the effects of resonance vibration on wire bonds. In particular, the case of the Insertable B-Layer (IBL) detector, the new innermost layer of the ATLAS Pixel Detector, has been reviewed. An experimental set-up has been built to simulate as much as possible the operation conditions of IBL wire bonds in the ATLAS magnetic field. The results provide useful information for the comprehension of the IBL wire bonds behavior. The dangerous resonance frequencies have been identified experimentally for different wire bond lengths. The resonance frequency amplitudes have been characterized in terms of several parameters, like wire length, wire orientation angle with respect to B-field and current amplitude. Several fatigue studies have been performed with simulations and laboratory tests. It has been demonstrated that in well-defined conditions, as for example with high currents, the wires can get irreparably damaged after few oscillation cycles and they can break. Two types of wire bond protections have been considered: the classical encapsulation of the wire feet and the coating of the whole wire. The results reveal that these methods minimize the oscillation amplitude reducing the possibility of damaging or breaking the wire bonds. For the IBL detector a Fixed Frequency Trigger Veto has been implemented for excluding the potentially dangerous frequencies identified in these studies.