This is the first of a two-part series that looks at tin whisker problems facing design engineers in industries such as defense, aerospace and medical equipment. The exempt OEMs don’t need to use RoHS-compliant parts, but they may soon find that non-compliant parts are unavailable. So, some engineers are considering using compliant parts that may be susceptible to whisker growth.
Even as commercial electronics OEMs switch comfortably to tin alloy solder and finishes, there remains concern about tin whiskers in the exempt industries of defense, aerospace and medical products. Since they are exempt, companies in these industries don’t have to use RoHS-compliant parts. They can use leaded parts that are not generally susceptible to the growth of tin whiskers – which can break off and cause a component to fail.
However, these industries have long depended on commercial-off-the-shelf (COTS) components because they’re cheap and they work well. Plus, many parts suppliers have decided to discontinue their non-compliant lines or charge more for them. These leaves exempt OEMs with the prospect of paying more for non-compliant parts, changing their designs to avoid using compliant parts, or, using COTS parts that are now made with tin alloy solders and finishes that don’t contain lead.
This third alternative will likely be the preferred course since it’s cheap and hopefully the tin alloy materials will not easily grow whiskers. Problem is, the defense and aerospace industries create systems that are expected to last decades. Commercial products have life spans of just a few years. Chances are, whiskers won’t grow in your DVD player during its cozy four-year life span, but a component in military equipment that flies at 30,000 feet and is expected to last 25 years may be a completely different story.
One of the dilemmas facing design engineers in defense an aerospace is that we simply don’t know what causes tin whiskers, so our ability to make sure they don’t occur remains iffy. A Lockheed Martin engineer who spoke on the condition of anonymity notes that the lack of good whisker information makes whisker mitigation a difficult prospect. “Nobody has determined the physics of tin whisker growth,” said the engineer. “Stress must be relieved in the tin, if not, time is pushed out in the form of a whisker.”
He notes that by including lead in the tin solder and finishes, you can mitigate whisker growth. “Lead has traditionally performed the function of the stress relief and lowered the melting point,” said the engineer. “Other metals in the tin may perform the stress relief – such as antimony and silver.” He also noted whisker growth depends on the setting of the component. “On a ceramic substrate or board, the stresses are so low, whiskers seldom grow, and those that do are not long,” said the engineer. “The degree of risk depends on the spacing between components and exposed PCB board.”
He also notes certain coatings can depress whisker growth. “Parylene, except for parylene C, is good as an overcoat to inhibit whisker dangers,” said the engineer. “The whiskers seldom grow and push their way through the parylene; if they do, the won’t push through very far.” He also notes that those whiskers that push through the parylene will not be long enough to break off and short our the component.
The second article in this series will look at the lack of adequate studies to understand whiskers. The next article will also look at tin whisker danger in microelectronic device packages.
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