The well worn but nevertheless important story of microscopic whisker growth on tin was given a new twist recently when engineers at Agere Systems have found a mix of semiconductor packaging ingredients that prevent the creation of these whiskers in lead free packaging designs. The company's formula eliminates lead from the process and avoids the potentially devastating flaw of whiskers that can create short circuits or break and cause other defects
Lead-free semiconductor packaging is required by the European Union in just over a year, but will be implemented globally in nearly every semiconductor package produced — impacting the trillions of microchips produced worldwide every year. Every company manufacturing or using chips in electronic products is working on finding the right combination of materials to allow its products to be sold throughout Europe and in other countries adopting similar legislation.
The key to Agere's discovery is taking into account the customer's needs when evaluating the quality of the product before shipping it.
Most chip packages being shipped today use a layer of tin and lead over copper. As companies move toward lead-free packaging, many of these packages will be using a combination of just tin over copper and will be processed by electronics equipment manufacturers at temperatures significantly hotter than packages with lead in them.
Agere's work has demonstrated that tin over copper packaging will pass today's industry standard tests developed for products containing lead. However, when using the products like the customer uses them, Agere has observed that commercially available tin over copper packaging form "whiskers" that can create electrical shorts or break off and cause other system failures.
Layer of nickel
Three tests have been proposed by the JEDEC Solid State Technology Association with guidance from the National Electronics Manufacturing Initiative (NEMI) to more effectively screen for the susceptibility to tin whiskers. Of the three tests, two display no discernable difference between matte-tin on copper and nickel undercoated matte-tin on copper. Agere has demonstrated that a layer of nickel between the copper and the layer of tin provides improvements in the third test using the customer environment.
"We are unveiling these findings in hopes that the electronics industry will adopt our approach to avoid the problems Agere observed in currently accepted copper and tin packages," said Melissa Grupen-Shemansky, Ph.D., Agere's director of packaging and interconnect technology. "We evaluated multiple options being used by other semiconductor companies in a scientifically valid study over a prolonged timeframe and found that Agere's tin-nickel-copper combination resolved the tin whisker problem seen after high-temperature, high-humidity storage."
Early results from an independent study conducted by the NEMI tin whisker test group corroborate the Agere findings on whiskers found on commercially available tin over copper packages. NEMI will publish their findings in 2005.
Many companies moving to lead-free packages use a combination of just tin and copper in their packaging. However, the tin-copper combination shows whisker formation in lifetime reliability studies when subjected to the assembly conditions seen by electronics equipment providers. In Agere's evaluation of samples from multiple packaging vendors, these solutions would not be acceptable to the electronics industry in applications where long-term reliability must be assured. The company's research has proven that a tin, nickel and copper combination is a reliable alternative.
"The impact of tin whisker growth in lead-free semiconductor packaging would be disturbing to the electronics industry because businesses and consumers risk increased equipment failure as a result of electrical shorts caused by these tin whiskers," said John Pittman, vice president of assembly and test operations for Agere. "The nickel solution can help facilitate the widespread adoption of lead-free packaging. The good news is that nickel is common in packaging processes today, so a "whisker-free" tin-nickel-copper solution can be easily implemented by the industry.
However, there will still be those in the industry that point out that whiskering can be encouraged by some of the routine operations found in electronic manufacturing and assembly processes that bend, compress and stress component compunds.
Time will of course tell on this and that in itself is a problem with whiskering as sometimes it can take several years for them to develop.