Corrosion of palladium coated copper wire bonds under high temperature storage reliability test

• Main Author: Cha, Chan Lam
• Format: Thesis
• Language: English; English
• Published: 2021
• Subjects: T Technology (General); TS Manufactures
• Online Access: http://eprints.utem.edu.my/id/eprint/25978/; https://plh.utem.edu.my/cgi-bin/koha/opac-detail.pl?biblionumber=121056

Palladium coated copper (PCC) wire is emerging as an alternative to bare copper (Cu) wire in the semiconductor industry due to better manufacturing yield and reliability stress performance. However, the acceptance of PCC wire for automotive device is still low due to the stringent reliability requirement by Automotive Electronic Council (AEC) Q100 standard and limited proven record in automotive application. Previous works showed PCC wire fails the High Temperature Storage (HTS) stress test at temperature of 175°C. Corrosion was observed on ball bond and stitch bond, causing severe degradations in the ball shear and stitch pull values. This research aims to determine the performance of PCC wire, to characterize the critical influencing factors driving the PCC wire corrosion at HTS stress test in accordance to AEC Q100 Grade 0 requirement, and to model the PCC wire failure mechanism. Based on failure analysis (SEM) results, there are many voids observed on expose Cu surface on PCC wire after stress, EDX analysis result detected sulfur element on the void region, thus, it is confirmed the Copper Sulfide (CuS) formation had occurred and Cu void formation is leading the early failure of PCC wire under HTS stress test. This research work concluded there are three critical factors that need to be controlled to meet the AEC Q100 Grade 0 quality requirement, first is the mold compound sulfur parts per milliom (ppm) level should control <10 ppm to mitigate the CuS formation. The second factor was the gold silver (AuAg) plating thickness on lead frame, the CuS formation rate was slower if the AuAg plating thickness is thinner. 8 nanometer (nm) AuAg lead frame thickness passed the AEC Q100 Grade 0 requirements with minimum stitch pull value 3.65 gram force (gf), however, the low process capability index (Cpk) might be a concern for automotive application. Thus, controlling the lead frame plating thickness could not be recommended as one of the solutions to PCC wire application. The third factor was the PCC wire corrosion was more severe under 175°C at 1000 hours (hrs) if compare to 150°C at 2000 hrs, even both condition were defined as equivalent in AEC Q100 standard; This result indicates the activation energy for CuS formation was not equal to the typical value 0.7 eV used by the standard. Based on the experimental data analysis, a model on galvanic corrosion accelerated by the presence of metal catalyst was proposed. The sulfur element comes from the adhesion promoter in the mold compound. The palladium coated on the PCC wire acted as a catalyst to enables the corrosion reaction to happen at lower activation energy. This research study had also concluded the material selection especially the mold compound type is very crucial to prevent PCC wire corrosion issue.