NDLI: Under bump metallisation of fine pitch flip-chip using electroless nickel deposition

Content Provider	IEEE Xplore Digital Library
Author	Liu, C. Hutt, D.A. Whalley, D.C. Conway, P.P. Mannan, S.H.
Copyright Year	2000
Description	Author affiliation: Interconnection Group, Loughborough Univ., UK (Liu, C.)
Abstract	For solder based flip-chip assembly, under-bump metallisation (UBM) layer deposition on the Al die bondpads is the first step in the wafer bumping process. The UBM is necessary, as the fragile Al pad has a tough oxide layer that cannot be soldered without the use of strong flux and a barrier layer is required to prevent dissolution of the bondpad into the solder during reflow. The UBM requirements are therefore to provide a solder wettable surface and to protect the underlying Al bondpad during and after assembly. In addition, the UBM deposition process itself must remove any oxide layers on the bondpads to ensure a low resistance interface between pad and UBM. This paper reports a study of the electroless nickel deposition process for the UBM of wafers that are subsequently to be bumped using solder paste printing. This work has extended the process from previous trials on 225 /spl mu/m pitch devices to wafers including die with sub-100 /spl mu/m pitch bondpads. The effect of the various pre-treatment etch processes and zincate activation on the quality of the final electroless Ni bump has been investigated. SEM examination of samples at each stage of the bumping process has been used to aid detailed understanding of the activation mechanisms and to determine their effects on the electroless Ni bump morphology. In addition, bump shear testing has been used to determine the best pre-treatment regime to ensure good adhesion of the electroless Ni to the bondpad. Finally, bumped die electrical resistance measurements have been used to confirm that the pre-treatment procedures are producing a low resistance interface between the Al and electroless Ni.
Starting Page	64
Ending Page	71
File Size	1061335
Page Count	8
File Format	PDF
ISBN	0780366549
DOI	10.1109/EMAP.2000.904134
Language	English
Publisher	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Publisher Date	2000-11-30
Publisher Place	China
Access Restriction	Subscribed
Rights Holder	Institute of Electrical and Electronics Engineers, Inc. (IEEE)
Subject Keyword	Metallization Wafer bonding Nickel Assembly Surface morphology Surface resistance Protection Printing Etching Testing
Content Type	Text
Resource Type	Article

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