Electroless Ni/Au Plating Overview

For copper based semiconductors, the nickel and gold plating baths are the same as those for aluminum based semiconductors. Several acid based cleaning steps are typically used to clean off contaminate and to remove copper oxide from the surface of the I/O pads. The activation step for copper is similar to that used in the laminate board plating industry, and usually uses a palladium based catalyst. The know-how for plating Cu semiconductors is the ability to selectively catalyze the copper I/O pads without activating the surrounding passivation.

This electroless plating processes are inherently low cost, and can be used for a variety of different applications in addition to Flip Chip and WLCSP Bumping, including:

  • Polymer flip chip (1-5um of Ni/Au + conductive epoxies)
  • Anisotropic conductive adhesives (10-25um of tall Ni/Au + ACF or ACA materials)
  • Pad resurfacing of copper and aluminum for wire bonding (2-5um of Ni/Au, Ni/Pd, or Ni/Pd/Au)
  • Pad resurfacing of copper pads for probe testing (2-5 um Ni/Au, Ni/Pd, or Ni/Pd/Au)

High throughput, and consequently low cost, is accomplished by batch processing cassettes of wafer through an automated electroless plating line. The fact that the nickel plating process is highly selective, and will only plate on the exposed metal surfaces (aluminum or copper), translates into a major cost advantage for this UBM deposition technique. Compared to conventional techniques for depositing the UBM, the use of electroless nickel has the following advantages:

  • There are no processing steps necessary to define the solderable area (such as vacuum metal deposition, photolithography, and mask etching).
  • One system handles all wafer sizes without change over (3″ to 12″).
  • The capital investment for plating technologies is relatively small.
  • The operational costs (labor and overhead) are reduced.

Electroless plating on integrated circuits can, however, be challenging because of the fab-specific variations in materials and processes involved in creating the circuits. Aluminum (or copper) alloy composition, sub-structures under the pad metal, passivation material and quality, pad electrical potential, and energy sensitivity (radiation and grounding effects) all play a role in the plating rates, uniformity, and adhesion of the nickel.

Because the process details (inherent tricks of the trade) are not generally regarded as patentable, developers treat their processes as proprietary. Hence, particulars of electroless nickel plating are not readily available.

The first three steps in the process are critical in determining the overall selectivity of the plating process, nickel morphology, and the adhesion of the nickel to the aluminum (or copper) pad. In general, a process that produces fine grained, uniform, thin layers of the catalyst (zinc or palladium) will produce the best nickel plated structures. The specific chemistries and absolute component ratios are critical in producing this desired structure. In addition to selecting the appropriate plating chemistries, one must also consider availability, place of origin, price, toxicology, bath life, waste treatment/disposal, and environmental issues related to the chemicals when implementing a process in a manufacturing setting.