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


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
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.
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Architectural painting and finishing services coat, finish and paint interior and exterior surfaces and components in buildings and plants.
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Parts washing services remove grease, oil, dirt, soot and other contaminants from surfaces. They perform processes such as blasting, thermal cleaning, degreasing, oxygen cleaning, pickling, spray washing, immersion washing, mass finishing, stripping, and ultrasonic cleaning.
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Industrial painting services apply paint to substrates for protection and/or decoration. They perform processes such as coil or roll coating, dip or immersion coating, dry lubricant coating, powder coating, and web coating.

Topics of Interest

14.7 Pd Palladium (Pd) is an attractive, cost-effective alternative finish for printed wiring boards (PWBs) that is both solderable and wire bondable. Similar to gold, Pd is a noble metal; therefore,...

14.2 OPTIONS FOR PCB LEAD-FREE SURFACE FINISHES Table 14.1 lists the options of lead-free surface finishes for PCBs. The system is categorized by the key element used. Each category is further...

14.8 ELECTROLESS Ni/Pd/(Au FLASH) Electroless Ni/Pd with or without an overcoat of immersion Au is a cheaper alternative to ENIG (Ni/Au). A thin layer of Pd on copper is sufficient for delivering a...

Chapter W: W Wulfenite W W Symbol for the element tungsten (wolfram). Wagner's alloy An alloy containing 85% tin, 10% antimony, 3% zinc, l% copper, and 1% bismuth. W5 Alloy Alloy containing 95.7%...

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.

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