Ti/Au Die Backside Metallization for Flip Chip Heat Spreader Attachment

In this paper, a heat spreader attachment with indium solder for high-power flip chip-in-package application was investigated. The Cu heat spreader was metallized with Ni/Au and the flip chip die backside metallization was Ti/Au. A low voiding attachment process was achieved with vacuum soldering. The Au thin film was converted into AuIn₂ completely after initial soldering, but no intermetallic compund (IMC) formation between Ti and In was observed. The attachment had good mechanical strength as measured by shear testing. The shear strength was not degraded significantly after multiple lead free solder reflows or with thermal aging at 120°C. For thermal shock cycle test (-40°C to 85°C), die with Ti/Au (2000 A¿ of Au) metallization had early partial delamination. The effect of Au thickness on mechanical strength was further evaluated. Assemblies (Cu on Si) with Ti/Au (2000 A¿) die had lower shear strength compared with Ti/Au (3000 A¿) and Ti/Au (4000 A¿ ) die. The pull strength (Si on Si) increased with increasing gold thickness. Thermal shock testing (-40°C to 85°C) for assemblies with Ti/Au 3000 A¿ along with Ti/Ni/Au (control) did not show early delamination and had similar performance after 2000 cycles.