Investigation of Sn-Pb solder bumps of prototype photo detectors for the LHCb experiment

The Large Hadron Collider (LHC) is now under construction at the European Organization for Nuclear Research (CERN). LHCb is one of the dedicated LHC experiments, allowing high energy proton-proton collisions to be exploited. This paper presents the results of the metallurgic studies carried out on Sn-Pb solder bumps of prototype vacuum photo detectors under development for LHCb, and in particular for the ring imaging Cherenkov-hybrid photo diode (RICH-HPD) project. These detectors encapsulate, in a vacuum tube, an assembly made of two silicon chips bonded together by a matrix of solder bumps. Each bump lies on a suitable system of under-bump metallic layers ensuring mechanical and electrical transition between the chip pad and the solder alloy. During manufacturing of the detector, bump-bonded (BB) assemblies are exposed to severe heat cycles up to 400 degree C inducing, in the present fabrication process, a clear degradation of electrical connectivity. Several investigations such as microstructural observations and analyses, as well as pull tests at room temperature, were performed. First results show severe dissolution of under-bump nickel and copper layers with formation of tin intermetallic compounds and segregation of lead phase. This microstructure, developing after the heat cycles, is at the origin of the observed poor strength and adherence of bumps. The present study provides a better understanding of the mechanical and microstructural properties of the bump-bonded assemblies in view of improving their fabrication.