Effects of probe damage on wire bond integrity

The electrical performance and reliability of ail manufactured integrated circuit devices is evaluated through wafer level test. During wafer testing, probe needles are brought in mechanical contact with aluminum bond pads and electrical contact is made as the probes "scrub" through the oxide and contaminants on the pad surface. This scrubbing action causes the probe needle to disturb or damage the pad metal. This scrub damage can adversely impact the wire bond quality at assembly and add extra costs by lowering bond and assembly yields. As the amount of pad damage increases, the strength and integrity of the bond is reduced. If the bond is too weak, the ball could potentially lift off the pad during the bond process, at assembly, or at some point during the service life of the device. This effect is exacerbated for smaller bond pads since a larger percentage of the total pad area is damaged during probe. This paper describes a controlled experimental study carried out at Texas Instruments that involved wire bonding to unprobed and probed aluminum bond pads. The probed wafers were generated such that pads had one, two, three, or four touchdowns offset from one another with measured damage ranging from 10-45% of total pad area. Analyses on bonded units concluded that the critical damage threshold is about 20%. Probe damage beyond this limit resulted in an increased incidence of lifted ball bonds and general degradation of the bond process. A package reliability test carried out on dies with >20% pad damage yielded no failures indicating perhaps that excessive pad damage is more of a quality issue than a reliability concern, at least for the one package and die size used in this study.