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Growth of Tin Whiskers in Relation to IMC Development

Growth of Tin Whiskers in Relation to IMC Development. Thomas Bitner. Scanning Electron Microscope (SEM). Tungsten source Highest melting point and lowest vapor pressure of all metals, relatively cheap cost Wehnelt Negatively charged, forces electrons downward Positively charged plate

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Growth of Tin Whiskers in Relation to IMC Development

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  1. Growth of Tin Whiskers in Relation to IMC Development Thomas Bitner

  2. Scanning Electron Microscope (SEM) • Tungsten source • Highest melting point and lowest vapor pressure of all metals, relatively cheap cost • Wehnelt • Negatively charged, forces electrons downward • Positively charged plate • Magnetic focusing “lenses”

  3. Secondary electrons • Electrons ejected from around nucleus by inelastic collisions • Everhart-Thornley detector • Counts secondary electrons • Backscattered electrons • Coulomb force (electrical charge) • Speed electron is backscattered determined by the element • Used to show contrast in areas of differing chemical composition

  4. Energy Dispersive X-ray Spectroscopy (EDX) • Electrons from higher energy fill in gap in lower shell • Excess energy emitted, often as Auger electron or X-ray • X-rays characteristic of their element • Can be detected by an energy dispersive spectrometer

  5. Focused Ion Beam (FIB) • Gallium source (Ga+ ions) • Many differences from electrons • 130,000x mass of an electron • 370x momentum for same energy • Milling/cutting • Scanning (Secondary electrons and ions) • Deposition of other elements • (Pt, W, SiO2, C)

  6. Internal Stresses • Oxidation • Atomic settling • External forces • Coefficient of thermal expansion, or CTE, mismatch • Atomic peening • Intermetallic layer, or IMC

  7. Intermetallic Layer (IMC) • Cu6Sn5, Cu3Sn • Cu6Sn5 under ambient conditions • Cu3Sn temp >60o C • Solid state diffusion • Kirkendall effect • One metal diffuses faster than the other • Vacancies in lattice structure • IMC under compressive stress

  8. Whisker formation • How do they form? • Um… • Why do they form? • Multiple theories • Most agree on stress relief

  9. Data • Only whiskers at higher and lower pressures • Incubation • 31 days for 2mTorr • 37 days for 16, 18 mTorr • 51 days for 12 mTorr • Whisker density not uniform • Sn layer uneven • No IMC directly after sputtering • No IMC in 20 mTorr sample as of July 15, but possible IMC in 2 mTorr sample as of July 15 • IMC supsected to have an effect on, but not essential for Sn whisker growth

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