[tinwhiskers] Re: [LF] FW: [SMART] Very useful RoHS review conference report
- From: "Bob Landman" <rlandman@xxxxxxxxxxxxxxxxx>
- To: "tin whiskers forum" <tinwhiskers@xxxxxxxxxxxxx>, "'\(Leadfree Electronics Assembly Forum\)'" <Leadfree@xxxxxxx>
- Date: Mon, 25 Jan 2010 14:38:54 -0500
Dave, Ed, and others:
The link I got from the NASA website and it is indeed broken. I found an
article I'd downloaded from NASA that referenced this link which does work
http://www.fda.gov/ICECI/Inspections/InspectionGuides/InspectionTechnicalGuides/ucm072921.htm
Here are some details from the report: I am continuing to search for the
mentioned CLASS I recall.
-Bob Landman
Tin Wiskers Problems, Causes, and Solutions
DEPT. OF HEALTH, EDUCATION, AND
WELFARE PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION
*ORA/ORO/DEIO/IB*Date: 3/14/86 Number: 42
Related Program Areas:
Medical Devices
_______________________________________________________________
ITG SUBJECT: TIN WHISKERS - PROBLEMS, CAUSES, AND SOLUTIONS
Recently, a little-known phenomenon called tin whiskering caused the recall of
several models of a pacemaker. This incident revealed tin whiskers to be a
general threat to all users and manufacturers of medical devices that
incorporate electronic circuitry. To prevent future problems, field personnel
will need to educate themselves and manufacturers. This guide is intended to
help in this endeavor by describing the problems, causes, and solutions
associated with tin whiskers.
TIN WHISKERS
later in the report:
ment, and Criminal Investigations>
Inspections>
Inspection Guides
Section Contents Menu
Inspections
Inspection Guides
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Tin Wiskers Problems, Causes, and Solutions
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DEPT. OF HEALTH, EDUCATION, AND
WELFARE PUBLIC HEALTH SERVICE
FOOD AND DRUG ADMINISTRATION
*ORA/ORO/DEIO/IB*Date: 3/14/86 Number: 42
Related Program Areas:
Medical Devices
_______________________________________________________________
ITG SUBJECT: TIN WHISKERS - PROBLEMS, CAUSES, AND SOLUTIONS
Recently, a little-known phenomenon called tin whiskering caused the recall of
several models of a pacemaker. This incident revealed tin whiskers to be a
general threat to all users and manufacturers of medical devices that
incorporate electronic circuitry. To prevent future problems, field personnel
will need to educate themselves and manufacturers. This guide is intended to
help in this endeavor by describing the problems, causes, and solutions
associated with tin whiskers.
TIN WHISKERS
Tin whiskers are metal filaments which grow from tin. They are extremely thin,
1-2uM typically, and grow as straight, kinked, or spiraled single crystals of
tin. They can reach a length of 9mm (3/8") and carry 10mA of current before
burning up. The electrical resistance of a tin whisker 3mm (1/8") long is about
50 ohms. Because of their current carrying ability and low electrical
resistance, whiskers are a threat to electronic circuits.
The ability of tin whiskers to cause electronic circuit problems was
established in 1951. Many sudden failures and intermittent problems were
associated with tin whiskers because of their ability to short closely spaced
electronic circuits. Whiskers were found to grow across circuit connections
and, because of their thin, brittle nature, would break free and lodge across
circuits. Investigation into preventive measures was started, but solutions
developed slowly due to the complex nature of tin whisker growth.
The exact cause of tin whisker growth is still not fully understood. It is
known that a whisker grows from its base and that the tin around the base does
not thin as the whisker grows. It seems that the energy for growth comes from
microstrains present in the tin or from externally applied pressure. Tin atoms
appear to diffuse along screw dislocations within the tin and are pushed
outwards by stresses. Growth rate varies tremendously, and it may be unsteady.
Whiskers can fully develop in minutes or take decades to form. Spurts of growth
may occur.
The growth of tin whiskers is not directly related to the surrounding medium.
Whiskers will grow in sealed components, under high vacuum, and in low or high
humidity. Temperature has some effect on the rate of growth, and the thickness
of tin deposits affects whisker density. An obvious factor affecting whisker
growth is pressure. High-compression pressure from bolts or screws will always
produce whiskers in tin deposits.
It was 1974, two decades after the problem was recognized, that scientifically
valid methods were established for controlling whisker growth. Two methods are
currently used. The most common is to avoid using tin. Other metals or alloys
of tin are used instead with solder (tin/lead) being the most popular. The
other method is known as "reflow." After the tin is in place, the tin coated
part is heated to a temperature above tin's melting point. This heating
releases any stress that exists within the tin deposit.
The FDA became interested in tin whiskers as the result of pacemaker failures.
A group of pacemakers from a single manufacturer were found to have a high rate
of failure due to tin whiskers growing from the tin-plated case of the
pacemaker crystal component. An electrical bridge between the crystal and its
case disabled the crystal component, resulting in the total loss of pacemaker
output. The FDA issued a Class I recall for the affected devices and initiated
a follow-up investigation.
Examination of the manufacturing process revealed that the manufacturer's
specification for the crystal component should have prevented tin-whisker
growth. The crystal component specification called for gold, nickel, or solder
plating. Any one of these case coatings would have prevented the tin whisker
problem. The manufacturer, however, failed to test the crystal components for
proper material composition. It relied on its vendor to deliver proper
components. Unfortunately, a bad batch of crystal components resulted in 80
percent of the affected devices having tin-plated crystal components.
From: ddhillma@xxxxxxxxxxxxxxxxxxx [mailto:ddhillma@xxxxxxxxxxxxxxxxxxx]
Sent: Monday, January 25, 2010 1:15 PM
To: Bob Landman
Subject: Re: [LF] [tinwhiskers] FW: [SMART] Very useful RoHS review conference
report
Hi Bob - the link below did not work:
) FDA forced Medtronic to recall their implanted cardiac defibrilators (from
patients bodies) when whiskers shorted the devices.
http://www.fda.gov/ora/inspect_ref/itg/itg42.html
Do you know what the valid link is?
Dave
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