Preventing whiskers in electrodeposited tin for semiconductor lead frame applications

A new pure tin process is described which produces a deposit with a crystal orientation combination that minimises whisker growth, particularly when used with a specific pre-treatment process. A recommended etch depth of minimum 2.5 μm has been shown to reduce whisker growth, particularly with thin (3 μm) tin coatings. Crystal orientation is determined by X-ray diffraction and the angles between adjacent crystal planes are calculated from the pattern produced. A minimum proportion of small angles is required for lowest whisker risk. Angles smaller than 22° have been shown to be critical. The predominant orientations of the deposits from new tin process are 101, 211, 112, and 312. This combination of orientations has 8 percent of angles in the critical range. A typical pure tin process with a high tendency for whiskering may have a texture of 431, 321, and 211 orientations, and this combination produces 65 percent of angles smaller than 22°.