Tin whisker formation on electroless tin films deposited on lead-frame alloys

Electroless Sn films have great potential in the lead-free age such as for high-density, fine-pitch, narrow soldering pad and bump interconnection applications. In the present work, electroless Sn were deposited onto lead-frame alloys (C194 and FeNi42). The microstructures of the electroless Sn films and tin whisker growth in thermal/humiditive chamber were investigated with scanning electron microscope and optical metallography. It was found that, in comparison with the FeNi42 substrate, the C194 substrate was more favorable for the deposition of electroless Sn coatings. The grain size increased with increase of the thickness of the Sn deposits. Within 45 min, autocatalytic activity still exists, which could keep the autocatalytic deposition of electroless tin on the C194 substrate. Tin whiskers growth was found only after 1000-hour exposure of the samples to the thermal-humiditive environment.