Abstract: For nearly four decades, the published upper temperature
limit for tin contact surfaces has been 60oC. Two potential failure
mechanisms have been identified. The first applies to thick deposits
and is based on the tin softening at temperature, causing local collapse
at the interface with a consequential loss in normal force. The second
failure mechanism arises from the consumption of free tin due to intermetallic
formation. As the intermetallic layer grows, the apparent hardness
of the tin deposit increases. This higher hardness combined with less
free tin to extrude through the surface oxide diminishes metal-to-metal
contact. For both cases, contact resistance will increase. During
the discussion on this topic at last year's symposium, it was pointed
out that automotive tin connectors are routinely exposed to temperatures
in the range of 100 to 125oC. Further, a comment was made that the
separable interface had become permanent after exposure to elevated
temperature for one week. Connector resistance and withdrawal force
were measured after exposure and compared to baseline values. Arhennius
plots were used to determine to what degree results demonstrated thermal
dependence.
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