Tin Whiskers pose a serious reliability issue to electronics assemblies with RoHS Lead-free design, whose components are having pure Tin or Matte Tin. Several instances have been reported where tin whiskers have caused system failures in both earth and space-based applications. There have also been whisker-induced failures in medical devices, weapon systems, power plants, and consumer products. Tin Whiskers Risk Mitigation Plans are the precautions to help avoiding/controlling Tin Whiskers.

Tin Whiskers Risk Mitigation Plans for Avoiding/Controlling Tin Whiskers

The uncertainties associated with tin whisker growth make it extremely difficult to predict when tin whiskers may appear. The following list provides some suggestions for reducing the risk of tin whisker induced failures.

  • Avoid the use of PURE TIN plated components if possible. Utilization of procurement specifications that have clear restrictions against the use of pure tin plating is highly recommended. Most (but not all) of the commonly used military specifications currently have prohibitions against pure tin plating. Studies have shown that alloying tin with a second metal reduces the propensity for whisker growth. Alloys of tin and lead are generally considered to be acceptable where the alloy contains a minimum of 3% lead by weight. Although some experimenters have reported whisker growth from tin-lead alloys, such whiskers have also been reported to be dramatically smaller than those from pure tin plated surfaces and are believed to sufficiently small so as not to pose a significant risk for the geometries of today’s microelectronics.
  • Post Procurement. It can be dangerous to rely on the part manufacturer’s certification that pure tin plating was not used in the production of the product supplied. There are several instances where the procurement specification required “No Pure Tin”, but the product supplied was later determined to be pure tin. In some of these instances, tin whisker growths were also discovered. Users are advised to analyze the plating composition of the products received as an independent verification. When simple avoidance of pure tin plating is not a viable option (such as in cases where its use is discovered late in system integration/test), then the following approaches may also be considered to reduce risk.
  • Solder Dip the plated surfaces sufficiently using a tin-lead solder to completely reflow and alloy the tin plating. Obviously, special precautions are required to prevent thermal shock induced damage, to prevent loss of hermeticity and to avoid thermal degradation. This approach may have limited success since it may be difficult to ensure that the entire surface is properly reflowed.
  • Replate the whisker prone areas. Some manufacturers may be willing to strip the pure tin plate from finished products and re-plate using a suitable alternate plating material such as tin/lead or Nickel. Caution is advised if considering use of an external plated finish (e.g., Sn-Pb or Cu) on top of an existing pure tin deposit. There is some evidence that whiskers may still form from the pure tin layer and protrude through the thin external deposit.
  • Conformal Coat or foam encapsulation over the whisker prone surface can significantly reduce the risk of electrical short circuits caused by whiskers. The choice of coating material, thickness and possible degradation with time/environmental exposure can impact the effectiveness of the coating.
  • Evaluate Application Specific Risks. A variety of application specific considerations may be used to assess the risk of whisker induced failures and assist in making “use as-is” or “repair/replace” decisions. These factors include circuit geometries that are sufficiently large to preclude the risk of a tin whisker short, mission criticality, mission duration, collateral risk of rework, schedule and cost.