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

PRETREATMENT

• Cleaner – Epithas MCL-10 acid soak cleaner
  removes surface soils and contamination and
  wets the surface to enhance solution transfer.
  (Also available: MCL-067 CMOS-compatible,
  low-foaming acid soak cleaner.)

• Micro-etch – removes copper surface oxide
  and roughens the copper, which enhances
  nickel adhesion.

• Acid rinse – ensures that the copper surface is
  oxide-free and residual copper has been removed.

• Pre-dip – minimizes acid dilution of the
  subsequent activator bath.

• Activator – MCT-14 is an extremely stable
  activator for copper and copper alloys.  It is
  well-suited for activating isolated copper pads
  for subsequent nickel deposition.  The MCT-14
  immersion reaction bath displaces copper and
  seeds it with palladium. The palladium initiates
  the subsequent autocatalytic nickel plating step,
  electroless nickel. The electroless nickel is an
  autocatalytic process. The nickel layer acts as a
  diffusion barrier that is solderable or aluminum
  wire-bondable.

PLATING

• Electroless nickel – NPR-18 electroless nickel
  is mildly acidic plating bath formulated for fine
  line circuitry encountered in silicon wafer plating.
  Bath formulation allows deposition of electroless
  nickel without bridging.  Highly stable, easy to
  use. 6-8% phosphorus content as plated.

• Immersion gold – TDS-20 cyanide-free
  immersion gold bath was engineered specifically
  for surface mount applications and wafer plating.
  Neutral pH; will not corrode the underlying nickel
  surface.  Provides excellent solder joint reliability,
  including small solder balls.  The thin gold coating
  maintains long-term solderability and prevents
  nickel oxidization, and passivation.

• Auto-catalytic gold – TMX-15 neutral pH
  autolytic gold bath is cyanide-free, and was
  designed specifically for surface mount
  applications 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.