The FDA issued a Class I recall for the affected devices and initiated a follow-up investigation.Įxamination of the manufacturing process revealed that the manufacturer's specification for the crystal component should have prevented tin-whisker growth. An electrical bridge between the crystal and its case disabled the crystal component, resulting in the total loss of pacemaker output. 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. The FDA became interested in tin whiskers as the result of pacemaker failures. This heating releases any stress that exists within the tin deposit. 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. Other metals or alloys of tin are used instead with solder (tin/lead) being the most popular. It was 1974, two decades after the problem was recognized, that scientifically valid methods were established for controlling whisker growth. High-compression pressure from bolts or screws will always produce whiskers in tin deposits. An obvious factor affecting whisker growth is pressure. Temperature has some effect on the rate of growth, and the thickness of tin deposits affects whisker density. Whiskers will grow in sealed components, under high vacuum, and in low or high humidity. The growth of tin whiskers is not directly related to the surrounding medium. Whiskers can fully develop in minutes or take decades to form. Growth rate varies tremendously, and it may be unsteady. Tin atoms appear to diffuse along screw dislocations within the tin and are pushed outwards by stresses. It seems that the energy for growth comes from microstrains present in the tin or from externally applied pressure. It is known that a whisker grows from its base and that the tin around the base does not thin as the whisker grows. The exact cause of tin whisker growth is still not fully understood. Investigation into preventive measures was started, but solutions developed slowly due to the complex nature of tin whisker growth. Whiskers were found to grow across circuit connections and, because of their thin, brittle nature, would break free and lodge across circuits. Many sudden failures and intermittent problems were associated with tin whiskers because of their ability to short closely spaced electronic circuits. The ability of tin whiskers to cause electronic circuit problems was established in 1951. Because of their current carrying ability and low electrical resistance, whiskers are a threat to electronic circuits. The electrical resistance of a tin whisker 3mm (1/8") long is about 50 ohms. They can reach a length of 9mm (3/8") and carry 10mA of current before burning up. They are extremely thin, 1-2uM typically, and grow as straight, kinked, or spiraled single crystals of tin. Tin whiskers are metal filaments which grow from tin. This guide is intended to help in this endeavor by describing the problems, causes, and solutions associated with tin whiskers. To prevent future problems, field personnel will need to educate themselves and manufacturers. This incident revealed tin whiskers to be a general threat to all users and manufacturers of medical devices that incorporate electronic circuitry. Recently, a little-known phenomenon called tin whiskering caused the recall of several models of a pacemaker. ITG SUBJECT: TIN WHISKERS - PROBLEMS, CAUSES, AND SOLUTIONS
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