These under bump metallization steps
produce optimum results on aluminum
and aluminum alloys:

PRETREATMENT

• Cleaner – Epithas MCL-16 alkaline soak
  cleaner for silicon wafers is designed for the
  pretreatment of aluminum prior to electroless
  nickel plating. (Also available: MCL-067
  CMOS-compatible cleaner.)

• Micro etch – Epithas LEC-18 micro etch for
  aluminum removes oxide surface film from
  aluminum and aluminum alloys. An exceptional
  pretreatment for plating electroless nickel on
  aluminum for silicon wafers. (Also available:
  LEC-19 CMOS-compatible micro etch.)

• HNO3 acid rinse forms a thin oxide layer on
  the aluminum IC pad.

• First zincate – Epithas MCT-17 zincate solution
  for silicon wafers is a dense, fine-grained,
  immersion zinc deposit for pretreating aluminum
  alloys for silicon wafers.  Reduces the dissolution
  of the aluminum pad, deposits a thin, uniform
  surface for the electroless nickel. 

• HNO3 zinc strip

• Second zincate – Epithas MCT-17
  zincate solution

PLATING

• Electroless nickel – Epithas NPR-18 mildly acidic electroless nickel plating bath for fine line circuitry in silicon wafer plating.  Stable, easy-to-use bath allows bridge-free deposit of electroless nickel.  Nickel layer forms an adherent coating to the IC and functions as a diffusion barrier. Optimized for automatic solution control using Uyemura Starline NI controller. (Also available: NPR-18CM
  CMOS-compatible electroless nickel.)

• Immersion gold – Epithas TDS-20 neutral immersion gold bath for silicon wafers is designed for surface mount applications and wafer plating.  TDS neutral immersion gold won’t corrode the underlying nickel surface, so it provides excellent solder joint reliability, including small solder balls.
  Cyanide-free.
  
  We recommend that Epithas NPR-18 electroless nickel be used for deposition of the Ni-P substrate.  Epithas TDS-20 is used as the initial layer over nickel for the subsequent deposition of the thicker electroless gold, Epithas TMX-15 needed for wire bonding applications.

• Auto-catalytic gold – Epithas TMX-15 electroless gold for silicon wafers is cyanide-free, with a neutral pH.  Ideal for surface mount and wafer plating.  Will not corrode the underlying nickel surface, thus it provides excellent solder joint reliability. 

We recommend TDS-20 immersion gold as the
under-layer, and NPR-18 electroless nickel with
TMX-15 electroless gold.

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Read Under Bump Metallization: Some Economical Alternatives by Mario Orduz, published in US Tech.