Elimination of integrated circuit bond pad crater test over rejection

Abstract

Wirebond interconnect reliability in integrated circuit, IC, chip is one of the key characteristic for the IC's performance during its function. One of the common and known interconnect reliability failures is cratering or the Wirebonding or Probing process related damage in bond pad surface and underlying material. This study determined that cratering is not only Wirebond or Probe process induced. Cratering can also be induced by the chemical test preparation, the etching process. Samples for pad cratering test are pulled out after Wirebond. To determine if there is damage in the pad, the wire-pad intermetallic is etched. The complete etching process shows the ball bond lifting and then revealing the pad surface. If not optimized, the etching process in the extreme side or over etch will shift the stress in the pad as the wire lifts. This mean a portion of the pad peels off with the wire resulting to pad damage. This phenomenon was validated in a screening design of experiment, DOE covering temperature before etch start, amount of etching chemicals, volume of samples, and etch time as key input variables. Temperature before etch start was the significant factor at 95% confidence level and was optimized. To avoid over etch; the process should start at 40 degrees Celsius. The cratering test procedure was revised and since then bond pad crater test over rejection was eliminated.