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NIST Seeks Cure for Metallic Whiskers

You may remember several years ago metallic "whiskers" were suspected of causing problems with multiple in-orbit satellites, including the loss of Galaxy 4. These whiskers are believed to form as a result of a metal's releasing stress generated by electroplating. Researchers at the National Institute of Standards and Technology (NIST) working with the International Electronics Manufacturing Initiative (INEMI) are trying to identify the origins of the whisker producing stresses. They reported that the surfaces of tin-copper deposits developed extremely long whiskers, while pure tin deposits produced only hillocks, a more benign growth.

The problem has become more acute as environmental groups have campaigned against lead-containing solders and protective layers on electronic components. The European Union will ban use of lead in all electrical and electronic equipment sold in EU nations next July.

Pure electroplated tin and lead-free tin alloys tend to spontaneously grow metallic whiskers, which can short out connections, causing equipment to fail. NIST researchers found that the whiskers and hillocks form when boundaries between individual grains have a column-shaped structure. Hillocks form when the boundaries move laterally. If the copper impurities hold the columnar boundaries immobile, whiskers form. Tin-lead deposits don't have this problem because the boundaries are randomly structured.

NIST researchers are investigating ways to eliminate the stresses and create structures without column grains. One way to accomplish this may be to use an interrupted alternating current electroplating process to disrupt the formation of columnar boundaries, instead of the traditional continuous current method. This could yield a structure similar to that of a tin-lead deposit, but without the environmental dangers of lead.

To view pictures of hillocks and whiskers, see the NIST Techbeat article NIST Seeking Cure for Electronics-Killing Whiskers.