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Controlling Extrinsic Chloride Ions Effect on Copper Wirebond Reliability
Keywords: Copper Wirebond, Extrinsic, Mechanism
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 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 particles can originate from human sources, the environment, the assembly process, 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 Microscope - 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 type 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 source mechanism Cl FM particles becoming that can be entrapped in the EMC. The presence of airborne particles in the device prior to molding is a function of the type of air flow (turbulent, laminar), and air velocity. 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 175C mold process and 260C 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.
Sheila F. W. Chopin, Technical Staff Memberm MMTS & Materials Mgr.
NXP Semiconductor
Austin, Texas

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