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  Home | NASA Parts Selection List (NPSL) | Prohibited Materials Section | Pure Tin Plating Prohibition

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Pure Tin Plating Prohibition

Policy:

Pure tin plating is prohibited as a final finish on EEE parts and associated hardware

Reference:

Rationale:

Pure tin finishes are susceptible to the spontaneous growth of electrically conductive single crystal structures known as tin whiskers. Over time these whiskers may grow to be several millimeters (mm) long. Tin whiskers are capable of causing electrical failures ranging from parametric deviations to sustained plasma arcing that can result in catastrophic short circuits.

Tin Whiskers

Several instances have been reported where tin whiskers have caused system failures in both earth and space-based applications including at least 3 separate events where a tin whisker induced short circuit resulted in complete failure of a commercial satellite.

The general risks fall into four categories:

  1. Stable short circuits in low voltage, high impedance circuits. In such circuits there may be insufficient current available to fuse the whisker open and a stable short circuit results. Depending on the diameter and length of the whisker, it can take more than 50 milliamps (mA) to fuse one open. More typical is ~10mA
  2. Transient short circuits. At atmospheric pressure, if the available current exceeds the fusing current of the whisker, the circuit may only experience a transient glitch as the whisker fuses open.
  3. Metal Vapor ArcingIf a tin whisker initiates a short in an application environment possessing high levels of current and voltage, then a VERY DESTRUCTIVE phenomenon known as a Metal Vapor Arc can occur.  The ambient pressure, temperature and the presence of arc suppressing materials also affect metal vapor arc formation. In a metal vapor arc, the solid metal whisker is vaporized into a plasma of HIGHLY CONDUCTIVE metal ions (more conductive than the solid whisker itself). This plasma can form an ARC capable of carrying HUNDREDS OF AMPERES. Such arcs can be sustained for long duration (several seconds) until interrupted by circuit protection devices (e.g., fuses, circuit breakers) or until other arc extinguishing processes occur. This kind of arcing is happening in the metal vapor.  When an arc quenching agent (e.g., air) is present, more power must be installed into the event to replace power lost to the non-interesting processes happening in the quenching agent.  Therefore, as air pressure is reduced, less power is required to initiate and sustain a whisker-induced metal vapor arc.  For example, past experiments** have demonstrated that at atmospheric pressures of about 150 torr, a tin whisker could initiate a sustained metal vapor arc where the supply voltage was approximately 13 Volts (or greater) and supply current was 15 Amps (or greater).  Tin (or other materials) from the adjacent surfaces can help to sustain the arc until the available material is consumed or the supply current is interrupted. Metal vapor arcs in vacuum are reported to have occurred on at least three commercial satellites resulting in blown fuses that rendered the spacecraft non-operational.
    **  J.H. Richardson, and B.R. Lasley, "Tin Whisker Initiated Vacuum Metal Arcing in Spacecraft Electronics," 1992 Government Microcircuit Applications Conference, Vol. XVIII, pp. 119 - 122, November 10 - 12, 1992.Basic Info on Tin Whiskers
  4. Debris/Contamination.Whiskers or parts of whiskers may break loose and bridge isolated conductors or interfere with optical surfaces

Recommendations:

At this time, the only sure way of avoiding tin whiskers is not to use parts plated with pure tin. Despite procurement specification requirements that prohibit pure tin, errors sometimes occur resulting in pure tin plated components being delivered. Therefore, users are advised to independently test and analyze the plating composition of the products received as a verification that pure tin plating is not in use. Simple visual inspection is generally not sufficient because pure tin plating may appear similar to other plated finishes such as tin/lead or nickel.

Utilization of procurement specifications that have clear restrictions against the use of pure tin plating is still 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 can reduce the propensity for whisker growth. Generally speaking, alloys of tin and lead (Pb) are acceptable where the alloy contains a minimum of 3% Pb by weight. The effects on tin whisker formation of alloying materials other than Pb are not well understood.

In the event pure tin plated parts cannot be avoided, there are some additional processing techniques that may be used to reduce but not eliminate the risks associated with tin whiskers. The effectiveness of these approaches is variable and most require further evaluation to determine their suitability for long duration missions. Examples of these approaches include:

  • Solder dipping pure tin plated terminations and leads using a leaded solder. The effectiveness of this approach at covering all pure tin plated surfaces can be variable.
  • Application of conformal coat material to pure tin plated surfaces. Conformal coat appears to reduce the growth rate of tin whiskers, but whiskers are still capable of growing through some conformal coat materials such as polyurethanes.
  • For some device types manufacturers may be willing to replate surfaces using finishes such as tin/lead or nickel which are substantially less prone to whisker formation.

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Responsible NASA Official: Michael Sampson, NEPAG Manager
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Last Modified: September 20, 2001
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