From Electronics Manufacturing: With Lead-Free, Halogen-Free, and Conductive-Adhesive Materials

14.6 IMMERSION Bi

Immersion Bi was introduced in 1996 as a nonprecious metal surface finish intended to address the coplanarity problem experienced by HASL in fine-pitch applications.59

Fabrication Process. The immersion bismuth process features three steps:

  1. An acidic cleaner that removes surface oils and solder mask residue.

  2. Microetch step that prepares the copper substrate topography for the deposit of immersion bismuth. For the bismuth to deposit on copper, the bath is highly acidic.

  3. Bi plating step.

The immersion bismuth bath is operated at 50 C, and a typical contact time of 1 to 2 min to produce a deposit of pure metallic bismuth onto the copper surface. Similar to other immersion processes, the reaction is complete when the copper can no longer be released and the surface is completely covered.

Performance. The plated Bi metal is uniform dark gray, and easily distinguished from the substrate copper. During aging or thermal excursions, the Bi finish becomes more copperlike in appearance. This is believed to be caused by diffusion of the bismuth into the underlying copper. Beigle considered this to be a cosmetic effect and has no major impact on solderability under normal conditions.59

The solderability of immersion Bi was found to be better than Pd in a throughhole filling test. For as-plated finishes, Bi exhibited 99 percent hole fill, while Pd only displayed 60 percent hole fill.59 In another experiment comparing Bi with OSP, PCBs with through-holes varying in hole size from 0.062 to 0.022 in were used...


Products & Services
Architectural Painting and Finishing Services
Architectural painting and finishing services coat, finish and paint interior and exterior surfaces and components in buildings and plants.
Electrolytic Cleaning Equipment
Electrolytic cleaning equipment uses electrolytes and applied current to electrochemically clean welds and other metal surfaces. Electrolytic cleaners are also known as electrocleaners, weld cleaners and electrochemical cleaning systems.
Passivates
Passivates are chemicals used to reduce surface reactivity to protect a metal against corrosion. Depositing an inert coating or forming a coherent oxide layer can also passivate a surface.
Fusible Alloys

Fusible alloys have low melting points, usually below 300º F. Many fusible alloys have eutectic compositions. Fusible alloys are used as work holders, electroforming mandrels, tube bending aids, fire sprinkler parts, steam boiler relief plug, resin casting or potting molds, fuse parts, thermostat components, switch parts, thermal management products, lens blocking alloys, and wax pattern injection dies.


Topics of Interest

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George Milad Shipley Ronal, Inc., Marlborough, Massachusetts 32.1 INTRODUCTION Surface finish is about connectivity. It is at the surface where a connection from the board to a device is to occur.