Identifying Whisker Causes and Overcoming Inspection Problems

Problems and history of whiskers

Short circuits leads to failures of electrical products and devices.
In 1946, there were many failures in products on the market caused by short-circuits in radios that used variable capacitors plated with cadmium. This put the spotlight on cadmium whiskers.
In the 1950s, a small amount of lead was added to an alloy to slow down whisker growth and this countermeasure spread widely. Since 2000, with the trend to ban the use of lead in products, tin plating has come into use. The use of tin, however, led to whiskers causing failures in products ranging from wristwatches, to nuclear reactors, and even NASA's satellites and space shuttles, leading to a refocusing on identifying and eliminating whiskers.

Causes of whiskers

Whiskers are believed to occur mainly due to influences from the following factors:

• Diffusion in intermetallic compounds
• Galvanic corrosion*
• External stress
• Stress generated by a difference in the coefficient of thermal expansion

With many other possible factors, the underlying mechanism of whiskering is still unclear.

Galvanic corrosion, also known as bimetallic corrosion, occurs when two or more different metals are brought into electrical contact and there is a potential difference between them.

Environments where whiskers occur

Tin whiskers and zinc whiskers occur and grow easily because their atoms actively diffuse at around room temperature. The following are typical environmental conditions in which whiskers may occur:

• Room temperature
• Temperature cycling
• Oxidation and corrosion
• External pressure
• Electromigration*

Whiskers occur due to electromigration in semiconductors having high current density or special packaging such as flip chip packaging.

Electromigration is the transportation of metal atoms caused by the electric current that passes through an integrated circuit. For example, in an aluminium wire, whiskers and hillocks can occur and grow at the anode as a result of aluminium atoms moving in the direction of the flow of electrons.

Examples of tin whisker evaluation tests

Examples of the types and conditions of tests currently conducted to evaluate tin whiskers are shown below.

Room temperature storage test

Observation of the growth of tin whiskers that occur due to the influence of diffusion in intermetallic compounds
Environment: 30 ±2°C/60 ±3%RH, time: 4000 hours

Constant temperature and humidity test

Observation of the growth of tin whiskers that occur due to galvanic corrosion
Environment: 55 ±3°C/85 ±3%RH, time: 2000 hours

Temperature cycle test

Observation of the growth of tin whiskers that occur due to a difference in the coefficient of thermal expansion
Environment: low temperature −55 ±5°C or −40 ±5°C/high temperature 85 ±2°C or 125 ±2°C, number of cycles: 2000

External stress test

Observation of the growth of tin whiskers that occur due to the influence of external stress
Type: connector engagement test (using actual products), load test (using a zirconia ball with a diameter of 0.1 mm and a load of 300 gf for 500 hours)

Key countermeasures - Magnified observation and evaluation

It is possible to understand product failure risks in advance and take proactive countermeasures for such risks if the conditions of tin whisker occurrence and growth can be analysed and evaluated using magnified observation in each test. Magnified observation and analysis enables risks to be understood and countermeasures to be taken before the product is released to the market. For example, selecting different materials or changing the circuit design can impact the occurrence of whiskers.
For conventional magnified observation of tin whiskers, general optical microscopes or scanning electron microscopes (SEMs) have been used. Instead of light, SEMs use a beam of electrons that have a short wavelength to perform nano-order observation. In recent years, however, breakthroughs in optical system and image processing technologies have led to the development of digital microscopes that enable observation with clear images and simple operations. Such digital microscopes can be used to observe and evaluate tin whiskers effectively with clear images.

Solutions to problems in tin whisker observation using SEMs

With the VHX Series 4K Digital Microscope

The newly developed optical system and 4K CMOS image sensor eliminate the need for vacuum chambers, making it possible to observe targets in non-vacuum environments while tracking the field of view at magnifications of up to 6000x. Observation is possible with clear, high-resolution 4K images and without any time-consuming preparation.

With the free-angle observation system and high-accuracy XYZ motorised stage, field-of-view alignment, rotation, and oblique axis motion are optimally performed for tilted observation, thereby allowing users to observe freely and easily from any angle.

Furthermore, depth-composition automatically captures fully-focused images while maintaining high magnification and resolution. A single click is able to move the stage to the desired location, automatically focus, and capture an image, decreasing the time required for observation.

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Solutions to problems in tin whisker observation using microscopes

With the VHX Series 4K Digital Microscope

The high-resolution HR lens and motorised revolver enable magnified observation with clear 4K images through seamless zoom between 20x and 6000x without lens replacement.

Real-time composition brings an entire target into focus, even if it has surface irregularities or height changes. The fully motorised design enables all operations to be performed with a mouse, dramatically improving work efficiency.

Using the free-angle observation system and high-accuracy XYZ motorised stage enables tilted observation, which was not possible with conventional microscopes. Grown tin whiskers forming nodule-like projections can be effectively observed at the desired location and from the desired angle.

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