Nickel-coating Solves Electrical Problem
THE MAGAZINE DEVOTED TO NICKEL AND ITS APPLICATIONS
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TIN WHISKERS such as these can form spontaneously on surfaces coated with tin.
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ONE SOLUTION is to coat electrical components with nickel instead of tin.
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INTEGRATED CIRCUITS such as this are essential components in wireless communications devices and computer
hard drives.
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Nickel offers a solution to trimming 'tin whiskers'
By Dean Jobb
Nickel Magazine, March 2005 -- Nickel is being used to combat a problem the electronics industry thought it had solved long ago: hair-like strands of metal that can form on soldered surfaces, causing electrical components to fail or short out.
These "tin whiskers," as they are known, have plagued everything from pacemakers to missile guidance systems and caused millions of dollars in damage. They have even disabled the central processors of communications satellites, knocking out television, radio and pager services back on Earth and turning the spacecraft into expensive space junk.
The whiskers can form spontaneously on surfaces coated with tin (as well as cadmium and zinc) under various environmental conditions. They can appear within days or take up to a decade to form.
The traditional solution has been to add lead to the solder, so that typically it is 40% lead and 60% tin. But with the European Union banning lead from electronic components in 2006 and China and other jurisdictions poised to follow suit, the industry is searching for alternatives.
Agere Systems of Allentown, Pennsylvania, U.S.A., a supplier of integrated circuits for manufacturers of wireless communications devices and computer hard drives, is devising a process to coat the copper leads of electronic components with a thin layer of pure nickel before the solder is applied.
While metallurgists and scientists are still debating what causes tin whiskers, John Osenbach, a consulting member of the company’s technical staff, says they appear to be the product of internal stresses that are created as tin bonds with copper.
Agere’s solution is to apply a layer of pure nickel, seven-tenths of a micron thick, before copper connections are soldered onto circuit boards. "The nickel is essentially a barrier between the copper and tin," Osenbach says. "It substantially improves the whisker resistance."
An off-the-shelf nickel sulphamate solution is used to coat the copper leads. A thicker layer would provide a better barrier between the tin and copper – "denser is always better," Osenbach says – but the leads often have to be bent after manufacture, and thicker layers of nickel have been found to crack under the strain.
There are potential alternatives to using nickel. Anoplate Corporation of Syracuse, New York, U.S.A., has developed a solder for the electronics industry made from tin and bismuth, which has proven effective against tin whiskers. Another possible solution is annealing, that is, heating the solder and copper at a temperature below the melting point of the tin to improve the bond between the metals. Researchers are still trying to determine the best combination of temperature and heating times.
The International Electronics Manufacturing Initiative has undertaken its own research on nickel undercoatings, and has corroborated Agere’s findings. Agere added a nickel sulphamate bath to its production line about eight months ago, providing an option to customers looking for lead-free components.
Agere recommends nickel undercoating for all lead-free components that will have an operating life in excess of ten years. "It’s a thin layer," Osenbach says, "but it’s an important layer."
Dean Jobb is a Wolfville, Nova Scotia-based freelance writer.
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