Whisker formation is a potential problem in tin-plating of copper wires. The term “whisker“ describes very fine tin mono-crystals with a diameter of up to 1mm and a length of sometimes up to several mm, which are spontaneously formed on the surface. Growth of these mono-crystals is diffusion-controlled and can sometimes take place extremely slowly, i.e., the crystals are formed over several years. Even relatively small whiskers can cause short-circuits, particularly due to the increasingly small distances between contacts in electrical engineering. 

The topic of whisker formation has already been dealt with in several scientific essays. These showed that bright, matte and fused tin layers demonstrate a different extent of whisker formation. Furthermore, formation of these mono-crystals is influenced by the composition of the electrolytes used for plating. In this process, the risk of whisker formation is lowest in the case of pure tin layers that are galvanically plated and then fused, still low in case of matte layers that are galvanically plated and highest in case of bright layers.

In contrast to these findings, the electronic industry requires abrasion-resistant layers with a low tendency for the need of cold welding. In addition, edge loss in fused connectors is not always accepted due to possible corrosion problems. In this area, particularly in recent years, there has been increasing demand for bright tin layers, which is probably also due to decorative aspects.

As a result of their high-quality look, bright pure tin layers are becoming more and more important, particularly in the area of connectors. In this context, the risk of whisker formation is either accepted or the problem is regarded as controllable.

Whiskers are formed under the influence of mechanical powers within the layer and should not be confused with surface phenomena or consequences of electro migration, the influence of humidity or ionic contamination.

Whiskers can take a range of forms. They exist in straight, bent or irregular knot shapes. As the mechanism of whisker formation has now been identified, successful countermeasures can be determined and implemented. Whisker formation and growth is based on various proven principles and can be understood as follows:

• The driving force for whisker formation are tensions within the tin layer
• Matte surfaces are only very rarely subject to these kind of tensions
• The development of tensions depends on the irregular formation of intermetallic phases at the copper-tin interface. This also includes the interface between tin and copper alloys such as bronze or brass.
  

Countermeasures

Based on these principles, several countermeasures can be taken to effectively prevent whisker formation:

• Application of dense barrier layers as a diffusion barrier for the base material made from nickel or silver
• Tempering the material to form an intermetallic barrier layer (IMC)
• Application of very thick tin layers, which reduces the tensions in the layer to an acceptable level

For instance, Infineon recommends applying either a silver barrier layer of at least 2 µm, tempering the material at 150 °C for at least one hour after application of the layer, or applying a plating layer of matte tin of at least 7.5 µm.

As a rule, whiskers of a maximum length of 50 µm after two years of storage are considered technically acceptable. The general conclusion following the results of an extensive series of test is that neither temperature nor relative humidity during storage influence whisker growth.