Journal of SMT Article

Understanding the Higher Wire Bond Strength Advantage of Au/Pd/Au (IGEPIG) as Compared to Alternate Immersion Gold Finishes, Before and After Heat Treatment with Different Types of Wires

Authors: Tatsushi Someya, Tetsuya Sasamura, Ph.D., Eriko Furuya, Katsuhisa Tanabe, and Shigeo Hashimoto
Company: C.Uyemura & Co., Ltd
Date Published: 6/30/2018   Volume: 31-2

Abstract: The wire bonding reliability was evaluated from wire bond pull strength and the failure mode after wire pull test for each deposit, which are electroless nickel (6-7 µm)/electroless palladium/Immersion gold (normal Ni ENEPIG), electroless nickel (0.15 µm)/electroless palladium/immersion gold (thin Ni ENEPIG), electroless palladium/immersion gold (EPIG), direct immersion gold on copper (DIG) and immersion gold/electroless palladium/immersion gold (IGEPIG). Au wire, bare Cu wire, Pd coated Cu (PCC) wire, and Ag alloy wire were used as the bonding wire.

IGEPIG deposit had high wire bonding reliability for Au wire, bare Cu wire, PCC wire, and Ag alloy wire, compared with the other deposits. From the result of auger electron spectroscopy (AES) analysis after heat treatment for 16 hours at 175C, Pd and Cu could be hardly detected at the top surface of Au, which indicated that IGEPIG deposit prevented Cu and Pd diffusion.

It was revealed from the cross-section observation by transmission electron microscope (TEM) that the initial Pd layer of IGEPIG deposit was formed with a continuous layered structure. On the other hand, initial Pd layer of EPIG deposit was formed with granular structure and there was some space between Pd grains and Cu. It was confirmed that IGEPIG deposit prevented Cu diffusion compared to EPIG deposit because IGEPIG deposit had better Pd coverage than EPIG. In addition, it was observed by energy dispersive x-ray spectrometry (EDS) analysis that Au of EPIG deposit diffused along the grain boundary into Pd layer and Pd of EPIG deposit formed solid solution phase in Au layer after heat treatment. On the other hand, Au layer and Pd layer of IGEPIG deposit did not diffuse into each other, even if after heat treatment.

Keywords: 

wire bonding, IGEPIG, EPIG, Pd diffusion, Cu diffusion



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