Assembly yield enhancement of electroless Ni-P UBM/Pb-free solder joints

Flip chips using Pb-free solders and electroless Ni-P UBMs on organic substrates are prone to a brittle failure before underfilling resulting in a low assembly yield. To investigate the reliability of Pb-free solder flip chip joint before underfilling, thermal cycling test and failure analysis were performed with SnAgCu solder/electroless Ni-P UBM flip chip. As a result, 15 percents of solder joints were failed after 4 cycles, because stress concentration at the corner bump caused brittle failures. To enhancing the reliability before underfilling, the effect of electroless Ni-P UBM thickness was investigated. 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. Because thermal stresses on a solder joint is caused by the warpage due to shrinkage of an organic substrate, effects of cooling processes on joint reliability and warpage were also investigated for enhancing the reliability before underfilling. Additional aging treatment during cooling stage of flip chip solder assembly increased about 30 percent assembly yield compared with direct air cooled flip chip joints. And longer aging time produced better reliability in a thermal cycling test before underfilling, Although Si chip warpages did not change by any additional thermal aging, the warpage of aged substrates was 5 mum smaller than that of direct air-cooled substrates. Smaller warpages of aged flip chip joints meant less thermal stresses and better reliability than direct air cooled flip chip joints. From these results, reliability of electroless Ni-P UBM/Pb-free solder joint can be enhanced by lower thickness and additional aging treatment during cooling process