Cryorolling of SAC305 solder : Microstructure analysis and shear strength of solder joint

Abstract

Miniaturization of devices along with demand of higher performance in electronic packaging industries led to development of higher reliability solder alloys. Solder material must meet the requirements of high solder joint strength, better wettability, high fatigue and creep resistance in order to provide highly reliable solder joint. This work attempts to evaluate effectiveness of cryorolling in providing ultrafine grains which could lead to higher solder strength. The microstructure of bulk solder and IMC layer at interface between solder and Cu substrate, and lap joint shear strength of cryorolled commercial SAC305 solder alloy are presented here. Solder alloys were dipped in liquid nitrogen at different dipping times (0, 10, 30 and 60 minutes) prior to rolling. Cryorolling was observed to encourage grain refinement in solder with 30 min dipping gave lowest grain size of 5.88 µm, compared to 11 µm for as-cast solder. Longer dipping up to 60 minutes resulted in larger grain size and tend to be brittle with small cracks started to appear during rolling. Shear strength test showed an improved and stronger joint for SAC305 dipped in liquid nitrogen for 30 minutes compared to sample without dipping in liquid nitrogen. As a conclusion, cryorolled solder alloys had smaller grain size and this led to an improved solder joint shear strength.