Title :
Pb-free solder paste reflow window study for flip chip wafer bumping
Author :
Li, Li ; Rao, Yang ; Lin, Jong-Kai
Author_Institution :
Interconnect Syst. Lab., Motorola Inc., Tempe, AZ, USA
Abstract :
A 2×3 full factorial experimental design was used to study the effects of peak reflow temperature (235°C-265°C) and time-above-liquidus (40-60 s) on bump characteristics. Ultra-fine mesh (type 5, -500/+650) Pb-free solder pastes, including Sn96.5Ag3.5, Sn95.5Ag3.8Cu0.7, and Sn99.3Cu0.7 were used on electroless NiP/Au under bump metallurgy (UBM). Belt speed and zone temperature settings of a reflow furnace were used to obtain the desired profiles. A linear ramp profile, with ramp rate of 1.5°C/sec was selected after initial profile screening experiments. The reflowed solder bumps were characterized in several categories to obtain an optimal reflow window. These categories included wetting characteristics, solder bump shear strength, shear failure mode, flux residue cleanliness, solder void population, and void size. The lowest peak reflow temperature and time-above-liquidus were established for each solder alloy. A low peak temperature reflow (cooler profile) resulted in fewer and smaller voids in the solder bump than a high temperature, long time-above-liquidus reflow condition. Bump shear strength was consistent for a wide range of reflow conditions. All three Pb-free solders have a reflow process window to bumps with good solder/UBM integrity and uniform bump geometry
Keywords :
copper alloys; design of experiments; environmental factors; failure analysis; flip-chip devices; integrated circuit interconnections; integrated circuit packaging; microassembling; optimisation; reflow soldering; shear strength; silver alloys; surface contamination; tin alloys; voids (solid); wetting; 235 to 265 C; 40 to 60 s; NiP-Au; Pb-free solder paste reflow window; Pb-free solders; Sn-Ag solder paste; Sn-Ag-Cu solder paste; Sn-Cu solder paste; SnAg; SnAgCu; SnCu; belt speed; electroless NiP/Au under bump metallurgy; flip chip wafer bumping; flux residue cleanliness; full factorial experimental design; linear ramp profile; optimal reflow window; peak reflow temperature; profile screening experiments; ramp rate; reflow conditions; reflow furnace; reflow process window; reflowed solder bumps; shear failure mode; solder bump; solder bump characteristics; solder bump shear strength; solder void population; solder/UBM integrity; time-above-liquidus; ultra-fine mesh Pb-free solder pastes; uniform bump geometry; void size; wetting characteristics; zone temperature settings; Assembly; Flip chip; Integrated circuit interconnections; Laboratories; Lead; Materials science and technology; Mechanical factors; Printed circuits; Printing; Temperature;
Conference_Titel :
Advanced Packaging Materials: Processes, Properties and Interfaces, 2001. Proceedings. International Symposium on
Conference_Location :
Braselton, GA
Print_ISBN :
0-930815-64-5
DOI :
10.1109/ISAOM.2001.916559
