Mitigation of Sn Whisker Growth by Small Bi Additions

Abstract

In this study, the morphological development of electroplated matte Sn and Sn-xBi (x = 0.5 wt.%, 1.0 wt.%, 2.0 wt.%) film surfaces was investigated under diverse testing conditions: 1-year room-temperature storage, high temperature and humidity (HTH), mechanical loading by indentation, and thermal cycling. These small Bi additions prevented Sn whisker formation; no whisker growth was observed on any Sn-xBi surface during either the room-temperature storage or HTH testing. In the indentation loading and thermal cycling tests, short (<5 lm) surface extrusions were occasionally observed, but only on x = 0.5 wt.% and 1.0 wt.% plated samples. In all test cases, Sn-2Bi plated samples exhibited excellent whisker mitigation, while pure Sn samples always generated many whiskers on the surface. We confirmed that the addition of Bi into Sn refined the grain size of the as-plated films and altered the columnar structure to form equiaxed grains. The storage conditions allowed the formation of intermetallic compounds between the plated layer and the substrate regardless of the Bi addition. However, the growth patterns became more uniform with increasing amounts of Bi. These microstructural improvements with Bi addition effectively released the internal stress from Sn plating, thus mitigating whisker formation on the surface under various environments.

Effect of Electromigration and Thermal Ageing on the Tin Whiskers’ Formation in Thin Sn–0.7Cu–0.05Ga Lead (Pb)-Free Solder Joints

“…The increasing temperature and the alloying element both reduce the length and slow down the whiskers' growth. This result is in agreement with a study which stated that alloying with Fe and Bi on Sn-0.7Cu and ageing at high temperature result in fewer and shorter Sn whiskers [36].…”

Sn whisker growth mitigation by using NiO sublayers

“…However, these theories fail to explain why whiskers eventually still form when a Ni sublayer is used. Other studies on whisker mitigation methods involving Bi or In were reported very recently [23][24][25].…”