Shaving the whiskers
for lead-free production
Agere Systems says that its engineers have found the right mix of packaging ingredients to enable the semiconductor industry to successfully implement lead-free packaging. Dr Mike Cooke reports.
Lead-based solders have been used to join metals at least since its use to join copper in Mesopotamia in about 3000BC. "Hard" gold or silver based solders are even older. Mesopotamians were using solder techniques in about 4000BC. Since electronics depends on electron conduction and metals are usually the best conductors, it was natural that soldering would become a vital mainstay of the industry.
Since high temperatures often damage electronic components, it is unsurprising that the relatively low melting temperature of lead-based solders would attract. The snag is well known - lead attacks the human central nervous system, kidney and blood cells. Lead at or above 80µgrams/decilitre of blood can cause convulsions, coma, and even death. Now that the removal of lead from automotive fuel is practically complete, government agencies have moved their focus to other lead-using sectors.
Japan and the European Union are leading the push. The EU's Restriction of Use of Certain Hazardous Substances (RoHS) directive bans the use of lead, mercury, cadmium and other hazardous materials in electronics manufacturing. Under the directives, manufacturers have until July 1, 2006, to ensure that the banned substances are not present in new equipment sold into the market. For Japan, the Ministry of International Trade and Industry (MITI) has decreed that commercial hardware will be 75% lead-free by January 1, 2006.
Problematic solution
Removing lead from the most common solders would leave tin (Sn), a metal used in alloy to lower the melting point of the solder from lead's 327¼C to 183¼C in the "eutectic" (minimum melting point) 62Sn/38Pb composition used by the ancient Romans. On its own, tin melts at 232¼C. As for many metal alloys, the sum of the whole is also stronger than its parts.
Couldn't we live with tin's 232¼C? Possibly, the industry is already planning to process circuit boards with lead-free solders at significantly higher "reflow" temperatures of the order of 260¼C compared with the traditional 220¼C. But there is another problem - whiskers. These are spontaneous growths from tin coatings (Figures 1 & 2) created via a stress-driven mechanism that can yield whiskers of sufficient length to disrupt electrical connections causing microchip failure. Much effort has gone in to finding lead-free solder replacements based on tin alloys that don't whisker. These may involve three or four metals, as well as tin.
Three tests have been proposed by the JEDEC Solid State Technology Association with guidance from the US National Electronics Manufacturing Initiative (NEMI) to screen for the susceptibility to tin whiskering. Many processes can pass some but not all of these. In particular, thermal processing and storage can often whisker previously whisker-free samples.
Shaving the whiskers
Agere's research has found that a layer of nickel between the tin solder plate and a copper leadframe eliminates whisker growth, even after reflow and storage. The researchers compared the Agere nickel barrier technique with four other solutions for tin on copper. Three of the four alternatives used methane sulphonic acid (MSA) as the tin plating chemistry, while one used a "mixed acid". Agere's nickel barrier used MSA.
Without reflow, none of the five platings developed whiskers. However, after 260¼C reflow and isothermal storage (60¼C/93% relative humidity) for 15 weeks, Agere's nickel barrier solution was the only one not to form whiskers. The maximum whisker lengths for the remaining techniques ranged between 50µm and 64µm (Figure 3).
"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," says Melissa Grupen-Shemansky, PhD, Agere's director of packaging and interconnect technology.
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.
Agere is shipping lead-free semiconductor products to customers using the new packaging process developed by the company with a complete catalogue due in 2005.
