ELECTRODEPOSITED Pb-FREE SOLDER AND WHISKER PREVENTION.

A technology important to electronics manufacturing is the electrodeposition of Sn-based protective coatings to guarantee solderability. It has been known for many years that the use of pure Sn as a protective coating on copper and copper alloys can result in the growth of hair-like Sn crystals known as "whiskers." These whiskers can be 1 [micro]m to 2 [micro]m in diameter and several millimeters long, and are capable of carrying 10 mA to 20 mA of current. It has also long been known that the addition of Pb to the coating effectively suppresses whisker growth, but, with the advent of Pb-free electronics finishing, the risk of tin whiskers is again a significant concern. With today's finer pitch devices, the whiskers can cause electrical shorts and failure.

Whiskers are generally believed to grow to relieve residual stress in electrodeposited Sn. However, the origin of this stress is not at all clear. In the more than 50 years since the first documented observation of tin whiskers, a fundamental mechanism of tin whisker formation has never been fully elucidated. Without a good understanding of the mechanism of whisker growth, the electronics industry has yet to devise a good test for determining the propensity for coatings to grow whiskers. In the current program, NIST researchers are focusing their efforts on developing an understanding of whisker formation in pure Sn and Sn-Cu alloy electrodeposits. This study is being carried out within the context of a study of the effect of plating methods on grain size, residual stress and alloy composition, and is expected to indicate plating approaches for prevention of whisker formation.

The NIST study has revealed seven types (shapes) of tin whiskers that form on matte and bright pure Sn and Sn-Cu alloy coatings. The type is dependent on substrate material, such as rolled annealed Cu, electrodeposited Cu, evaporated Cu, rolled phosphor bronze, rolled brass, and mild steel. However, the study found that Sn whisker growth could he prevented on a substrate/Sn deposit combination known to grow whiskers by depositing a thin (0.1 [micro]m to 2.0 [micro]m) Ni coating on the substrate before deposition of the Sn or Sn-Cu coating. Currently the investigation of whisker formation is focused on intermetallic formation by copper-tin interdiffusion, substrate effects on tin nucleation, and inclusion of organic or inorganic impurities in the tin deposits. Special emphasis is being focused on the measurement of residual stress using x-ray diffraction analysis.