Muon drift chamber wire bond failure analysis

We report here on the detailed analysis of a gold-coated tungsten wire of the type used in many multi-wire proportional chambers and drift chambers. The wire was attached under tension to a printed circuit board bond pad using standard tin/lead9 solder and was observed to pull out of the solder under uniform tensile loading. Examinations of the failed wire end were performed using electron beam microprobes to study the elemental composition of the residual material on the wire. It is found that the thin gold coating is removed from the surface of the tungsten and alloyed with tin from the solder, forming a nonuniform coating on the wire surface. A surroundung lead-rich region is inferred from this result, leading to the wire failure because of the low shear strength of lead. Discussions of previous work on wire failure and wire aging in high energy physics experiments and similar experiences in the electronics industry are presented.