TABLE OF CONTENTS 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:
An acidic cleaner that removes surface oils and solder mask residue.
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.
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...
