Not completely understood, electrically conductive tin whiskers are crystalline structures between 1-2 millimeters (mm) that can grow from surfaces where tin is used as a final finish; surfaces finished with electroplated tin are particularly susceptible to whisker growth. Although their occurrence was originally documented during the 1940s, no real solution has yet been devised to prevent their development, which may reach 10 mm in some cases. This is unfortunate because tin whiskers have the capacity for generating arcing and short circuits between electrical elements of printed circuit boards (PCBs) and related electronic equipment.
Tin Whiskers: Their Origin and Impact
Physically, tin whiskers result from the spontaneous growth of tiny, filiform hairs or tendrils upon tin surfaces. These structures can create electrical paths, often within the presence of compressive stress during component operation. Because they usually develop in a functional environment that supports short circuits or arcing, tin whiskers don't need to be airborne to damage electronics. Among other problems, the four main risks with tin whiskers are:
- Stable short circuits in low voltage, high impedance circuits.
- Transient short circuits may develop where tin whiskers span tightly-spaced circuit elements maintained at different electrical potentials.
- Metal vapor arcs result when a whisker-short occurs in a high-current/voltage environment. They are perhaps the most destructive of electronic system failures attributed to tin whiskers.
- Contamination from debris resulting from tin whisker presence can interfere with component performance.
- Behaving like miniature antennas in fast digital circuits or at frequencies above 6 GHz, generating a negative impact on circuit impedance and stimulating reflections.
- Causing failures in relays, a source of deep concern for relay-functions as important as those for nuclear power facilities.
- In outer space (or any vacuum), tin whiskers can short circuit high-power components, ionizing and potentially conducting hundreds of amperes of current, exponentially increasing the short circuit’s damage.
- Tin whiskers have caused malfunction and recall of medical pacemakers.
- Whiskers located in computer disk drives can break, resulting in bearing failures or head crashes.
Conformal Coatings Mitigate the Effects of Tin Whiskers
Selecting a tin whiskers’ mitigation strategy is important; because the source of their growth is unknown, they cannot be entirely eliminated. Although ceramic coatings have proven successful, conformal films made from polymeric compounds such as vapor-deposited parylene, or wet application acrylic and urethane, deflect whiskers away from the coating surface. For instance, studies conducted by NASA seeking tin whisker control for space craft have shown urethane conformal coatings successfully mitigate tin whisker growth. In addition, some acrylic wet coatings, such as HumiSeal 1B31, also mitigate tin whisker’s problems. For various reasons, other conformal coatings -- epoxy, and silicone – are less effective minimizing the development of tin whiskers and their impact on PCB performance.
Perhaps the most effective conformal coating for alleviation of tin whisker related issues is parylene. Deposited in gaseous form, through a chemical vapor deposition (CVD) process, parylene seeps deep into substrate surfaces, penetrating spaces as minute as 0.01mm. In doing so, it forms a pinhole-free protective film that is ultra-thin but exceptionally durable. Chemically inert and of high tensile strength, parylene retains its stability throughout a wide range of temperatures. Because it can be applied at room temperature, parylene application is stress-free. These properties combine to support superior mitigation of tin whiskers.
. However they are applied, conformal coatings create a physical barrier over electronic components that stops tin whisker damage. Conformal coatings:
- Form a protective film that safeguards assembly circuitry and components, physically separating them from each other.
- Substantially diminish tin whiskers bridging between the separated components.
- Lower whiskers’ capacity to generate arcing and shorts.
Tin whiskers can generate arcing and short circuits leading to systemic failures in PCBs and similar electrical assemblies, significantly damaging and otherwise altering their performance expectations. Vital devices, equipment and facilities such as pacemakers, power plants, and even satellites have had their function diminished by the presence of tin whiskers. Determining methods for preventing or slowing tin whisker growth is difficult because:
- outside of some evidence they are the product of mechanically- and thermally-induced stresses,
- the exact mechanism behind their development is not fully understood.
Where they develop, mitigation of tin whiskers is essential to limiting their impact on assembly performance. Conformal compound coatings such as parylene, and to a lesser extent acrylic and urethane, can stop tin whiskers from;
- penetrating an applied protective barrier,
- bridging electrical components and
- creating arcing or a short.
While it is impossible at the moment to completely prevent the occurrence of tin whiskers, their mitigation with conformal coatings will dramatically limit whisker growth and equipment damage. Vapor-deposited parylene and wet coatings such as acrylic and urethane, provide generally good tin whisker defense. Other traditional wet conformal coating materials such as epoxy and silicone are mostly ineffective as protection against the development and effect of tin whiskers.