Nickel Palladium-Based Pads for Higher Reliability in Au Wire Process

The necessity of high reliability in wire bonding processes is a well known factor, especially in Ultra Fine Pitch (UFP). High Temperature Storage (HTS) tests have become a crucial part in success criteria among others aspects in process qualification. The common Aluminum (Al) pad top stratum enables the gold to diffuse into the pad. HTS tests accelerate the growth of the Inter-metallic (IM) Inter- metallic until a certain point which the Au bond severs from the pad, either by itself (lifted bond) or during wire pull test. The last is characterized by low pull values. An alternative method of creating compatible interconnects is to change the Al top surface of the pad to a noble metal after it has left the IC fabrication clean room. One such method is depositing a layer of Nickel and Palladium (NiPd) using a known wet chemistry process. Unlike Al based pads, NiPd based pad

prevent the IM growth, substantially reducing HTS failure main root cause. In this paper the Interconnect between Au wire 25um 99.99% (4N), and NiPd pads has shown easy implementation in terms of bond ability (same process window as used for Au on Al pad) and extremely stability over time with zero failures up to 5000hrs of 175ºc Storage, while reference Al pad shown high level of lifted ball failure.