Effects of annealing on Sn whisker formation: Role of Cu alloy seed layer

To cope with the miniaturization trend in the electronic packaging, the parts of components are getting smaller and closer, so that the risk of shorting by Sn whiskers is becoming much greater. Many works have been done on the investigation of Sn whisker growth as well as on its mitigations. Intermetallic compound (IMC) formation is believed to be one of the major driving forces. One might consider that the grain size of the Cu as a diffusion species plays a role in IMC formation. Therefore, this study investigates the effects of the Cu alloy thin film as a seed layer and annealing on Sn whisker formation. In order to study those effects, Cu thin films, added with minor concentration of Ru with various thicknesses prepared by radio magnetron sputtering, were introduced as the seed layers. Pure Cu thin films with the same thicknesses were also introduced for comparisons. Annealing of the samples was also carried out prior to Sn electroplating. To accelerate Sn whisker growth, all of samples were aged at 60°C for 20 hr. Their effects in term of whisker density have been studied.