Controlling Extrinsic Chloride Ions Effect on Copper Wirebond Reliability

Abstract Copper (Cu) wirebond (WB) reliability in encapsulated microelectronic devices is influenced by intrinsic and extrinsic chloride ions. Previous work showed that intrinsic extractable chloride (Cl) ions that are part of the epoxy mold compound (EMC) can cause Cu-Al intermetallic (IMC) or Al pad corrosion under moisture-temperature reliability conditions. Another mode of Cl corrosion occurs due to the entrapment of Cl foreign material (FM) particles that can release Cl ions after molding the device. Intrinsic EMC and extrinsic Cl ions in encapsulated EMC must be defined, analyzed, and controlled to limit the potential for corrosion. Extrinsic Cl ions from FM (Foreign Material) particles can originate from human sources, the environment, assembly processes, and assembly tools. This paper focuses on extrinsic Cl ions from Cl-FM. Cl-FM embedded in EMC can be determined by dissolving the EMC in a solvent followed by filtration. The isolated FM was characterized by various techniques including: Scanning Electron Microscopy - energy-dispersive X-ray (SEM – EDX) and Fourier-transform infrared spectroscopy (FTIR) to determine the structure of the Cl-FM compounds. The FTIR Analysis determines the presence of C-Cl bond by identifying stretching bond frequency in the region of 800 – 600 cm−1 that determines if the Cl-FM particle is organic. The combination of analytical techniques was used to identify the source of the Cl-FM. The types of Cl FM particles discovered were both organic and inorganic in nature, and of different shapes and sizes. The smaller size particles < 30μm posed a challenge for some particle type identification accuracies. Airborne particles are one of the main sources of Cl FM particles that can be entrapped in the EMC. Airborne particles are created by environmental dust, human wear, abrasion and handing, and machine maintenance. The primary Cl FM particles were found to be carbon-chlorine polymers such as polyvinyl chloride (PVC), Polyvinylidene chloride (PVDC), and inorganic chlorides including metal chlorides. Many of the EMC materials found to have entrapped natural and synthetic fibrous materials with embedded Cl compounds. The 175°C mold process and 260°C reflow thermal effect degrading the Cl FM particle in the EMC matrix could generate Cl depending on the additives and modifiers. Investigations were performed to develop hypotheses of how Cl FM particles are generated in the EMC and how the FM particles could be controlled.