Tin Whisker Anecdote: 20 Years to Failure

NASA
Anecdote #1: 20 Years to Failure

Anecdote #1: Field Failure Caused by Tin Whisker Short ~20 Years after System Fielded

Circa 2002 the tin whiskers shown below caused a field failure of an electronic system that is used in the "electric power utility" industry. The system was originally put into field service circa 1983.

This "actual" failure is considered to be significant because it took ~ 20 YEARS before the whiskers grew in such a manner as to create a short. Previous published literature has given the impression that the tin whisker growth process may occur within a maximum of 8 to 10 years (typically much shorter). Unfortunately, the information provided about this event is not sufficient to know if

a) the whisker growth remained dormant for a very long time, or
b) the growth process was continuous and slow for the entire period.

This type of failure is of particular importance to applications that require very long term reliability and for which field servicing may be impractical or impossible (space missions included). The lack of accepted "accelerated" test methods for judging whisker propensity is a further complication.

See below for more FACTS about this failure.

Optical Photos of Matte Tin Plated Dual In-Line Package (DIP) IC Leads with Tin Whiskers that Caused a Permanent Short Between Adjacent Pins

		Photos show Tin Whiskers growing from Matte Tin Plated IC Leads Manufactured in 1980.

Additional Photos of a "spare assembly" manufactured during essentially the same time period

Facts:

IC Description:

	Standard Dual In-Line Package (DIP) IC
	Manufactured in 1980
	Leads are copper based with "pure matte tin plated finish" (confirmed via SEM/EDS).
	IC manufacturer contacted in 2002 confirms the finish used in production during the 1980s was pure "MATTE" tin. They claim they never used "bright" finishes.

IC Installation:

	IC was hand soldered (tin/lead solder) into a through hole PC board in 1980. Solder joint is on Back Side of board
	Large portions of every lead (away from the solder joint) remained unreflowed and uncoated by the tin/lead solder.

Circuit Application:

	After assembly, this electronic assembly began field service circa 1983. This system is used in the "electric power utility" industry.
	Typically the circuit is exposed to 40°C with "normal" humidity
	Circuit is normally " unpowered" except for a few hours per month when the circuit is powered as part of a system test
	10 Volts maximum available to the IC (90 Volts maximum available to other components of the circuit)

FIELD FAILURE:

	Failure occurred circa 2002 (> 20 YEARS after system was first assembled) due to a permanent "low current" short caused by a tin whisker that bridged two adjacent pins on the IC.
	Estimated shorting distance between pins is 30 - 40 mils (0.75 - 1.0 mm).
	Failure analysis showed numerous tin whiskers on all leads of the IC.
	Most (but not all) of the whiskers are near the bend in the IC lead where it exits the plastic package.
	Some whiskers also appear along the stamped edges of the leads.
	Still other whiskers are found growing from the flat surface of the lead away from bends or edges.
	No whiskers were observed immediately adjacent to the solder joints themselves although a few are visible near the lead egress from the through hole.
	At the time of this report additional spare electronic assemblies (never put into field service) built during the same time frame as the assembly that failed were being retrieved for more detailed inspection. Initial findings indicate the spare assemblies also have tin whiskers growing from this same IC type, but the ICs are of a slightly different lot date code.

Responsible NASA Officials: Michael Sampson/NASA GSFC Code 306 Dr. Henning Leidecker/NASA GSFC Code 562	Additional Researchers: Jong Kadesch/Orbital Sciences Corp. Jay Brusse/Perot Systems	Last Updated: June 9, 2008
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