Metallurgical Failure Modes of Wire Bonds

Various metallurgical failure modes of gold and aluminum wire bonds are described. Examples are taken from both low and high power devices. Whenever possible, known methods of avoiding these failure modes are given. Wire bond failure modes can be divided into two categories. The first is comprised of those failure modes that are caused by poorly controlled or poorly designed manufacturing processes that result in lower product yield or higher per unit bonding cost, as well as those processes that predispose the device to early field failure. The second category is comprised of the failure modes of adequately made bonds that are caused to fail by environmental stresses during the operating life of the device. The most frequent causes of failures are discussed in detail. These include non-optimized bonding schedules, cracks in the heels of ultrasonic bonds, intermetallic formation, poor metallization, and inadequate glassivation removal. Assuming that the package is hermetic, or for plastic devices that humidity and other corrosion producers are not present, then the primary wire bond failure modes in the second category result from environmental temperature exposure and the number of power or thermal cycles experienced by the device during operation. Both of these can induce intermetallic formation. The latter can cause metallurgical flexure-fatigue at the bond heel or in the wire.