Effect of electoless Ni/Au UBM thickness on mechanical and electromigration relibility of Pb-free solder joints

Electroless Ni-P plating is cost effective process to fabricate UBM for solder flip chip. Thickness of electroless Ni-P UBM is important factor for mechanical and electromigration reliability. Pb-free solder flip chips with different UBM thicknesses of 3, 5 and 10 mum were prepared. First, the effect of UBM thickness on mechanical reliability was investigated. Before underfilling, flip chips using Pb-free solders and electroless Ni-P UBMs on organic substrates were prone to a brittle failure. As lowering the thickness of electroless Ni-P UBM, failure type of solder joints was changed from brittle to fatigue. Thin electroless Ni-P UBM was more effective to resist the thermal deformation during solder reflowing than thicker UBMs. Second, the effect of UBM thickness on electromogration reliability was investigated. As results of electromigration test, time to failure of the solder joint with 10 mum UBM increased about 3 times than that of the solder joint with 5 mum UBM. Time to failure increased due to the lowered current crowding effect at UBM/solder interface in the solder joint with thicker UBM.