Simplified Fluxless Bonding via Ultraviolet Activation: Enabling High-Reliability Fine-Pitch Interconnections

초록

This study examines a fluxless bonding approach assisted by ultraviolet (UV) irradiation to enhance solder joint formation and interfacial integrity. Flip-chip test vehicles incorporating a daisy-chain structure were fabricated, and thermal compression (TC) bonding was performed with in-situ UV irradiation applied during the heating stage. Cross-sectional electron microscopy revealed that UV-assisted fluxless bonding produced continuous interfacial intermetallic compound (IMC) layers comparable to those formed in conventional flux-assisted joints. Energy-dispersive spectroscopy confirmed effective metallurgical bonding without void formation. X-ray photoelectron spectroscopy further clarified the surface-specific effects of UV irradiation on Cu and Sn. UV exposure suppressed CuO formation on Cu, maintaining a metallic or Cu2O-dominant surface state that promotes interfacial reactions. In contrast, Sn showed a time-dependent response: limited UV exposure restrained SnO2 growth and preserved a reactive SnO-rich surface, whereas excessive irradiation accelerated full oxidation to SnO2 leading to degraded wetting behavior and reduced joint reliability. These results demonstrate that properly controlled UV irradiation enables effective fluxless surface activation, resulting in clean interfaces and reliable interconnect formation.

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