Can Nickel Barriers Eliminate Tin Whiskers?
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Like it or not, lead-free requirements are coming and will be here to stay, along with one of their biggest problems: tin whiskers. Agere Systems (Allentown, Pa.) recently announced promising results from using a nickel barrier layer to hinder the growth of tin whiskers when tin is used as a pad or pin finish over copper.
Tin is ubiquitous in soldered electronic interconnecti
ons and pin finishes, because it wets well in soldering processes and makes good metallurgical contact with typical pad and pin metals. It also has a well documented tendency to form whiskers (Fig. 1 ) that can cause shorts and puncture conformal coatings. It is commonly understood that whisker formation in tin is a stress-relief mechanism. Using lead as a companion metal for tin has been effective in terms of relieving stress, but now other ways must be developed for relieving stress in tin solders.
Five years ago, another subsidiary of Lucent Technologies, Electroplating Chemicals and Services (Murray Hill, N.J.), stepped forward as a proponent of matte tin,1 which has since become a lead-free finish of choice. Matte tin is simply tin that is plated using a solution without brighteners.
Organic brightener molecules can get trapped inside the tin and cause multiple problems. They act as nucleation points for grain growth. This reduces tin grain sizes and increases the number of tin grains, which increases the chances for intergrain stresses. They can also outgas during solder attach operations.
The use of matte tin was an improvement in dealing with tin whiskers, but according to the results Agere Systems released, it is not a solution by itself. Test vehicles with matte tin plated on copper were subjected to accelerated life tests at 60°C and 93% relative humidity for 15 weeks. Those that had not experienced a 260°C reflow before the test did not grow whiskers, but those that