Electromigration failure with thermal gradient effect in SnAgCu solder joints with various UBM

In flip chip package, the majority of heat is from the die side and a temperature gradient builds up from the chip to substrate through the solder joint and underfill. This will induces biased atomic flux from hot end to cold end in the solder. Even in electromigration test vehicle, the joule heating from the Al trace at chip side is also higher than substrate side. Any small temperature difference can generate a large temperature gradient given the small dimension of the solder bump. The combined effect of electromigration and theromigration may induce earlier failure. In the current work, the electromigration reliability of Pb-free solder bump with thick copper UBM at the chip side is studied. The effect of temperature gradient was considered and characterized. We compared the electromigration reliability of SnAgCu flip chip solder joints with and without Cu columns. The solder joint size is about 90 micron and the Cu column has a height of 60 mum. For the copper column solder joint itself, we introduced two types of copper column joint structures: (1) The solder contacts the Cu column at bottom and side walls and (2) Only the bottom surface of the copper column joints with solder. In the solder bump where electrons from chip side to board side, we saw much more IMC formation in the solder joint. The solder joint can even be totally converted to IMC. We propose that when the copper column was converted to IMC, there is a redistribution of current density. To discuss the electromgration reliability of an intermetallic compound joint, we take Cu₆Sn₅ as an example to calculate the ldquocritical productrdquo and found that the current density needed to cause electromigration damage for Cu₆Sn₅ IMC is at least one order of magnitude bigger than Sn-based solder.