LONG TERM AGING EFFECTS ON THE RELIABILITY OF LEAD FREE SOLDER JOINTS IN BALL GRID ARRAY PACKAGES WITH VARIOUS PITCH SIZES AND BALL ALIGNMENTS

摘要

This paper investigates the impact of isothermal aging on the long-term reliability of lead-free solder joints. The full experimental matrix contains SAC105 and SAC305 solder alloys assembled in ball grid arrays (BGAs) with package sizes ranging from 19mm, 0.8mm pitch to 5mm, 0.4mm pitch, and three surface platings (ImAg, ENIG, and ENEPIG). The test specimens were subjected to isothermal aging at temperatures 25°C, 55°C, 85°C to 125°C with aging times of 0, 6 months, 12 months and 24 months, followed by accelerated thermal cycling from -40°C to 125°C. A two-parameter Weibull plot shows the reliability degrades up to 70% after two years of aging at elevated temperature. The degradation rate slows with aging time. The degradation is greater with smaller solder joints and perimeter ball alignments when compared to large-diameter solder balls and full array ball alignments. The thickness of the Cu-Sn intermetallic layer (IMC) layer during the testing grows with an approximate ~ t05 dependence (diffusion-controlled reaction) during isothermal aging + thermal cycling.