Tin whisker phenomenon in electronic packaging and its control strategy

Tin whiskers are elongated, needle-like single crystals of tin that form spontaneously on the surface of pure tin (Sn) or tin-containing alloys. These whiskers are usually only a few microns in diameter, but can be several millimetres or even more than 10 millimetres long, which can cause serious reliability problems. The following is a comprehensive explanation of the mechanism of tin whisker growth, the factors affecting it, and the measures taken to suppress it:

Tin whisker growth mechanism

1. Driving force: The driving force for the growth of tin whiskers mainly comes from the reaction between Sn and Cu at room temperature to produce Cu₆Sn₅ intermetallic compounds. This reaction generates compressive stresses within Sn.

2. Compressive stress release: due to the high room temperature homogenisation temperature of Sn, Sn atoms diffuse faster along the grain boundaries. The compressive stress is released by atomic diffusion and rearrangement, resulting in the migration of Sn atomic layers perpendicular to the stress direction, expanding along the grain boundaries towards the roots of the tin whiskers, and contributing to the growth of tin whiskers.

3. Spontaneous process: As long as there are free Sn and Cu atoms, tin whisker growth will continue and is a spontaneous process.

Figure 1: SEM image of Sn whisker growth on SnCu-coated leads.(a): Tube-shaped whiskers on the surface of SnCu coating.(b): Some sharp and bent whiskers.Figure 2 Flute-like whiskers and whiskers that are fully angled at a sharp angle

The main factors affecting the growth of tin whiskers

1. grain orientation and size: columnar crystals and single-crystal columnar crystals are more likely to lead to the growth of tin whiskers, while fine grains are also more likely to produce tin whiskers.

2. Thickness of coating: 2~3μm thick coating has the highest possibility of tin whiskering under high stress.

3. Material under the coating: Ni as the backing material has a small tendency to grow tin whiskers, while Cu as the backing material has a large tendency to grow tin whiskers.

4. Temperature and humidity: the growth of tin whiskers depends on the temperature and humidity, and the possibility of tin whisker growth is smaller under low temperature and low humidity conditions.

5. Material purity: pure tin surface is most likely to make tin whisker growth.

Measures to inhibit the growth of tin whiskers

1. Alloying: adding alloying elements such as Bi or Ag in the plating bath can effectively prevent the growth of tin whiskers, but need to pay attention to alloy processing and cost issues.

2. Diffusion hindering layer: add a diffusion hindering layer between Sn and Cu, such as plating a thin layer of Ni, can slow down the reaction between Sn and Cu, so as to inhibit the growth of tin whiskers.

3. Surface pretreatment: in the Cu substrate or lead frame surface pre-plating NiPdAu, etc., can also effectively inhibit the growth of tin whiskers.

4. Optimisation of process parameters: Control soldering temperature as low as possible and humidity as low as possible to reduce the possibility of tin whisker growth.

Fig. 3 SEM photographs of short whiskers on pure Sn coating

The effect of surface oxides on tin whisker growth 1. Necessary conditions: Only metals that produce a surface oxide film will show tin whisker growth. 2. Thickness of the oxide film: too thick a surface oxide film will prevent the growth of tin whiskers, while too thin is conducive to the growth of tin whiskers. 3. Oxide film discontinuity: the discontinuity of the oxide film provides a channel for tin whisker growth. 4. Tin whisker morphology: the existence of the surface oxide film prevents the tin whisker to the side of the growth of tin whiskers, to maintain a uniform cross-section of tin whiskers. In short, the growth of tin whiskers is a complex process involving a number of factors. By understanding the growth mechanism of tin whiskers and taking corresponding inhibition measures, the impact of tin whiskers on the reliability of electronic devices can be effectively reduced.