Cu wire and Pd-Cu wire package reliability and molding compounds

Abstract

Cu wire is drastically replacing Au wire due to surge of Au price. However, Cu wire package has poorer humidity reliability than Au wire package. Although Pd coated Cu wire package could show better humidity reliability than Cu wire, it is still worse than Au. Enough information regarding failure mechanism was not available. For failure analysis, x-section has been widely used to identify the Cu/Al IMC after failure. However, the x-section is the results of corrosion reaction and doesn't show the IMC status before corrosion. Therefore, the failure mechanism could not be estimated precisely. We used chemical model simulation to predict what kinds of IMC could be created after wire bonding, then which IMC could be corroded more easily during HAST. The Desorption energy was used to estimate reactivity between specified Cu/Al IMC and chlorine ion. The simulation suggested that the formation of Cu rich and Cu poor Cu/Al IMC and the Cu rich IMC was estimated to be corroded by chlorine ion. These chemical model simulations are the effective way to have fundamental understanding of the mechanism of Cu/Al IMC corrosion. Furthermore, chemical model simulation for Pd coated Cu wire was done to explore the effect of Pd existence and distribution of Pd in Cu/Al IMC. Dispersed Pd contributed to create new IMC of Cu/Al/Pd instead of easily corroded Cu rich Cu/Al IMC. Cu and Al diffusion and also Cl ion diffusion were inhibited by Pd at surface. Even Cl ion catching effect by Pd is also discussed.To improve humidity reliability performance with Cu wire, we developed new ion trapper using chemical model simulation technique. Developed molding compounds with the ion trapper showed significant improvement at bias HAST with Cu wire, which was even better than conventional Cu wire compatible molding compounds. IntroductionOne of the concerns to use Cu wire has been failure during humidity reliability test such as PCT, unbias HAST and bias HAST. [1,2] Failure mode of humidity reliability test was open with observed corroded layer and crack. We also confirmed that open failure at positive pad at bias HAST.[3] Conventional Br contained molding compound showed worse HAST results. However, even green molding compound showed HAST failure. We explored the factors of humidity reliability failure for Cu wire packages. As a result, extracted chlorine ion from molding compounds turned to be major factor while pH of extracted water turned to be minor factor through bias HAST. We also examined impact of wire bonding strength, bias and reliability condition.

Effect of isothermal aging on the growth behavior of Cu/Al intermetallic compounds

“…These results suggest that IMC compositions are: Cu 9 Al 4 for IMC I; Cu 3 Al 2 for IMC II; and CuAl for IMC III. Cu 3 Al 2 was also found by Abe et al [10] as noted earlier. The similar IMC formation in this study and Abe's study may be attributed to the similarity in the composition of Al layer which included Cu.…”

“…However, some other studies report that only Cu 9 Al 4 will be added to the CuAl 2 [9]. Abe et al claimed that they identified Cu 3 Al 2 and CuAl while the initial layers were Cu and AlSiCu [10]. Therefore, the understanding of the sequence of IMC phase transformations at the Cu/Al bonds remains incomplete.…”

Importance of Cu/Al Intermetallic coverage in copper wire bonding with sensitive pad structure

“…EDS analysis at Point 2 showed absence of Clion as corrosion only occurs at the Cu/Al IMC layer. Due to the uneven distribution of Cu/Al IMC and insufficient coverage percentage, this eventually results in many vacancies that will attract reaction with Clion [1], thus causing Cell 1 to fail biased HAST test. To understand the effects of the wafer lot variation that may cause a difference in Cu/Al IMC formation between Cell 1 and Cell 2, further SEM analysis was carried out on the bond pads.…”

The corrosion performance of Cu alloy wire bond on Al pad in molding compounds of various chlorine contents under biased-HAST

“…[1,2] One of the major corrosives in industrial products is the chlorine content of epoxy molding compound (EMC). [3,4] Many literature have been reported that the Cu wire bonding suffered chlorine ion attack on Cu-Al IMC after thermal annealing, pressure cooker test (PCT), highly accelerated stress test (HAST) or biased-HAST. [5,6,7] In Cu-Al binary system, five intermetallic phases could form during 300°C.…”