Investigation of bond pad etching chemistries for passivation crack

Wire bonding is still a very common method for connecting the pads on a chip to the package. During the ultrasonic wire bonding process, several failures such as ball neck failure, missing ball, bond metal peeling or crack etc., may be generated. Of those failures, bond pad peeling or crack is a phenomenon detected after bonding process and is identified as a critical reliability problem and is known as a complex defect to investigate. Bond pad cracks pose a high reliability risk and potential failure during environmental stress testing. Damage to the bond pad may be the result of sub optimized probe or wirebond process parameters, as well as poor pad design. In addition, bond pad cracks may be unintentionally induced by the cratering test chemical etch solution. There is a case where an assembly folk reported had a bond pad crack, but none of the parts have failure during electrical test or even after reliability stress. In such case, we believe the crack found at assembly was an artifact induced by etching chemical, resulting the over-rejecting the parts. This paper specifically discusses a comparative analysis of various bond pad etching methods and their impact on bond pad cracking. There are few interesting findings will also be shared during the discussions. Failure analysis results are also briefly discussed.