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

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The importance of copper/aluminum (Cu/Al) Intermetallic coverage (IMC) for package reliability with Copper (Cu) wire was studied in great detail. A sensitive device with 0.8µm thin aluminum (Al) metallization was selected as the most stringent test vehicle. For such sensitive devices, it was found that aggressive bonding parameters such as high ultrasonic (USG) bonding method have to be avoided to prevent pad cratering issues. The challenges were found to be balancing between no cratering reject, yet achieving targeted Cu/Al IMC coverage, ball shear strength per unit area and wire pull test responses. Through comprehensive bonding parameters optimization, the Cu/Al intermetallic coverage (IMC) percentage and its distribution beneath the bonded ball were found to be very important factors to achieve biased Highly Accelerated Stress Test (biased-HAST) reliability robustness. In addition, the interaction effects of epoxy molding encapsulation (EME) selection with different pH and halogen content towards the bond reliability was also studied as biased-HAST failures are related to halogen corrosion from the EME. Samples were assembled with combinations of good IMC (>60%) and poor IMC (<60%), coupled with two different EME which are Control EME used for gold wire and Copper EME which are specifically formulated with the same base resin material as Control EME but with different pH and chloride (Cl-) content. The samples were subjected to MSL3/260°C pre-conditioning, followed by biased-HAST (130°C/85%RH) stressing. Electrical testing was done after reliability stress, and followed by package decapsulation and wire pull testing. It was found that by ensuring IMC coverage to be >60%, the sensitive pad devices were still able to meet all reliability stressing requirements regardless of the pH and Cl- range of the EME used. Conversely, if the IMC coverage is relatively poor (<60%), the biased HAST reliability depended on the choice of EME, with only the high pH and low Cl- ppm level able to pass requirements. This shows that the optimization of the Cu wire bond process to obtain good IMC coverage is the most critical factor in Cu wire bonding reliability performance.