Growth prediction of tin/copper intermetallics formed between 63/37 Sn/Pb and OSP coated copper solder pads for a flip chip application

This study quantifies the effect of temperature and time on the growth of Cu-Sn intermetallics, specifically for flip chip/ball grid array packaging technology. The activation energy and the growth rates were determined for solid state diffusion, after the initial assembly reflow(s). Three different types of solder joints were investigated. 1) BGA 63/37 solder joints which were formed by a standard convection oven attach of 30 mil (760 μm)diameter solder spheres to OSP protected, Cu plated ball pads of an organic flip chip substrate. The ball pads are solder mask defined and of 0.635 mm nominal diameter. 2) Flip chip bump pad solder joint consisting of 63/37 eutectic solder bumped die attached to a nonsolder mask defined, OSP protected, Cu plated pad of the flip chip substrate. The flip chip bumps on the die are created by screen printing solder paste on the die pads and subsequent reflow attach, by a standard convection oven to the die under bump metallurgy (UBM). The nominal die UBM pad diameter is 0.085 mm. 3) Solder joint formed on a coupon which involved the reflow of the balls randomly placed on a Cu plated layer with no solder mask coating. The investigation was performed by first establishing the intermetallic growth rate at six different temperatures, ranging from 85°C to 150°C. The relationship between intermetallic growth and time was shown to essentially follow the common parabolic diffusion relationship to temperature especially above 100°C. The activation energy (E_a) and the growth constant (k₀) were then calculated from this data. The results showed that the E. for the total intermetallic thickness was essentially similar for the three solder joint configurations of the ball, bump and the coupon described above. E. varied from 0.31 eV to 0.32 eV, while the k₀ varied from 18.0 μm/s¹2 / to 24.2 μm/s¹2 /.