Tin Whisker Risk Assessment of a Tin Surface Finished Connector

Estimated reading time: 2 minutes

Rockwell Collins initially investigated and qualified Samtec’s SEARAY solder charge connector (hereafter referred to as "the connector") technology with a focus on solder joint integrity. However, the connector in question, which has a matte tin finish on the solder tail, was found to have a potential risk of tin whiskers due to a small region of the connector that was not protected by either solder poisoning or the connector’s mechanical configuration.

An investigation was conducted in accordance with modified JESD201 tin whisker susceptibility protocols to assess the possible tin whisker risk concern. The investigation results, coupled with plating and reflow soldering data inputs, demonstrated that the connector’s tin whisker risk was extremely low and deemed acceptable.

Background

The implementation of the RoHS European Union Directive in 2005 led to the use of pure tin as an accepted surface finish for PCBs and component terminations. A drawback of pure tin surface finishes is the potential to form tin whiskers. Tin whiskers are a metallurgical phenomenon that is associated with tin rich/pure tin materials and has been a topic of intense industry interest.

The acceptance and usage of pure tin by the electronics industry component fabricators is understandable as the pure tin surface finishes are inexpensive, are simple plating systems to operate and have reasonable solderability characteristics. However, the commercial electronics segment, which uses the majority of electronic components, often has product life cycles that are measured in months.

In contrast, high performance/harsh environment electronics typically have product life cycles that are measured in decades and therefore are much more susceptible to the potential long term threat of tin whiskers. Industry studies have shown that pure tin surfaces can be “poisoned” with tin/lead solder provided the tin plating is consumed/converted during the soldering process. With many short components such as resistors or capacitors, the soldering process consistently and repeatedly eliminates any tin whisker risk. However, the component lead geometry, solder paste deposit and component pad dimensions all factor into whether a component can be successfully poisoned by the soldering process.

Connectors are typically not an issue, in terms of solder poisoning, as connector suppliers utilize the connector housing design/configuration to eliminate potential shorting from a tin whisker or ensure that the areas with pure tin surface finishes will be poisoned as part of the soldering process.

Previously, Rockwell Collins completed a connector technology qualification that included the Samtec SEARAY solder charge connector. The connector had very robust thermal cycle solder joint integrity and utilized a matte tin (i.e., pure tin) surface finish on the non-contact pin surfaces. During metallographic cross-sectioning phase of the connector’s qualification effort, it was observed that a region of matte tin was not poisoned during the soldering process nor was the tin whisker shorting potential in this region mitigated by the connector design/configuration. An investigation was conducted to determine if this region of the connector posed a tin whisker risk threat.

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Editor's Note: This article originally appeared in the February issue of SMT Magazine.