Mechanism of tin mustache formation and methods of prevention

What exactly is the reason why whiskers are formed on the tin layers applied to the copper substrate? Also read what can prevent their formation.

Electroplated tin and predominantly tin alloys are currently used as alternatives to tin and lead alloys in most electronic components. It is known, however, that these alternatives are prone to the formation of tin whiskers, the so-called 'Wiskers' that can cause short circuits in electronic systems.

In the case of tin finishing a copper (or copper alloy based) substrate, the main cause of tin whiskers is compressive stress. The stresses are mainly caused by the irregular growth of the intermetallic compound (IMC) layer even under standard environmental conditions.

It is known that tin whiskers are easily formed on galvanized tin deposits formed on a copper layer and, on the other hand, are not observed on tin-lead electroplated deposits. It should be noted here that there are significant differences in the metallic structure of the tin deposit and the tin-lead alloy, and it has a direct impact on the formation of tin whiskers.

The crystal structure modified tin scale (similar to lead tin scale) is able to prevent whisker formation by dissipating and displacing the stresses that cause whisker formation.

Figure 1. Diagram of making tin whiskers.

Source: The Elimination of Whiskers from Electroplated Tin, © Uyemura

As shown in Figure 1, the stresses along the inner boundaries of the large, columnar structure of the tin deposit are responsible for tin whisker formation. Stress can be internal or external. The main source of internal stresses is the uneven increase in the thickness of the IMC layer over time under standard environmental conditions (30 ° C, 60% RH for 4,000 hours). Another factor that causes internal stresses to occur is exposure to elevated temperatures and high humidity (55 ° C, 85% RH, 4,000 hours), which facilitate oxidation and / or corrosion. Internal stresses can also be induced by thermal cycling (-55 ° C to 85 ° C, 1,500 cycles) due to the mismatched coefficient of thermal expansion of the various materials.

It is also known that external stress also initiates the growth of the mustache, an example of which is the stress caused by the press-fit joints.

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