Wafer bumping: Low-alpha lead considerations

摘要

As a result of decades of manufacturing flip chip packages, there exists a large database of reliability information about lead solder bumps. IBM was the first to warn us about the elusive issues of alpha particle emission from the solder. The "spontaneous" soft errors that occurred in devices led to several months of investigation. The problem was traced to an air jet used by a nitric acid vendor to clean bottles. The air jet contained a small amount of alpha-emitting material that contaminated a very small number of bottles. Soft errors result when an alpha particle penetrates a device, creating a trail of electron-hole pairs. While this process seldom causes permanent damage, it changes the state of the device. Alpha particles are most often produced by the radioactive decay of various atomic species in the solder. A simplified approach would identify lead (Pb) as the source of the problem, but the problem is multi-faceted. Accordingly, an effective low-alpha strategy should include more than a single approach.