Identifying Whisker Causes and Overcoming Inspection Problems

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.

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.

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 aluminum wire, whiskers and hillocks can occur and grow at the anode as a result of aluminum atoms moving in the direction of the flow of electrons.

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)

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 analyzed 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.

Conventional problems with SEMs
Before observing tin whiskers, the preparation, including sample positioning in the chamber and evacuating the chamber to establish a vacuum or low-vacuum status, takes time and effort.
Observation is only available from directly above the sample. To observe three-dimensional whiskers, a series of preparation procedures needs to be carried out every time the angle is changed.

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.

Conventional problems with optical microscopes
If tin whiskers occur in three dimensions or a sample that has tin whiskers is three-dimensional, conventional optical microscopes only focus on part of the tin whiskers.
Furthermore, tilted observation is not available, causing time-consuming adjustments that require an experienced user.

Real-time composition brings an entire target into focus, even if it has surface irregularities or height changes. The fully motorized design enables all operations to be performed with a mouse, dramatically improving work efficiency.
Using the free-angle observation system and high-accuracy XYZ motorized 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.