Thermal stability of various ball-limited-metal systems under solder bumps

Thermal stability is discussed of ball-limited-metal (BLM) layers formed under controlled-collapse solder (lead-5 wt.% tin) bumps for flip-chip interconnections. All BLM systems used here consist of a triple-layer deposit of Cr or Ti as an adhesive; Ni, Mo, Pd, or Pt as a barrier; and Au as a surface metal. Using test chips with these BLM systems, mechanical pull-strength values of solder-bump joints are obtained. These values are associated with thermal phenomena at the interface between the BLM and solder layers. The chips with Ti adhesive layers show generally superior solder-joint strength to those with Cr layers, independently of the barrier metal used. This is due to the presence of a joint fracture mode closely related to the kind of adhesive metal. Among the BLM systems examined, Ti/Ni/Au is found to provide the most reliable solder joints. This is successfully utilized as a BLM system for GaAs devices.<
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