NiFe-based Ball-limiting-metallurgy (BLM) for microbumps at 50μm pitch in 3D chip stacks

In this work, Pb-free microbumps at 50μm pitch with NiFe-based Ball-limiting-metallurgy (BLM) are fabricated and tested. Detailed microstructural analysis has been performed, which shows a uniform thin layer (~0.2μm) of FeSn₂ Intermetallic Compound (IMC) is formed between the Pb-free solder and NiFe BLM after the first reflow. In comparison, the NiCuSn IMC can grow more than 2μm in microbumps with the conventional Ni BLM or solder-caped Cu pillar after the first reflow. An excessive lateral “thermal-undercut” has been discovered in NiFe BLM structures during solder reflows due to good edge wettability of NiFe. A dual-layer BLM structure is proposed and demonstrated to mitigate the “thermal-undercut”. Moreover, addition of NiFe layer on a micro-Cu pillar structures have been demonstrated and characterized.