Tham khảo tài liệu 'the materials science of coatings and substrates part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | Diffusion 101 HEATING TIME HOURS Figure 10c Reliability vs. heating time for mil pure gold plate on copper with no underplate and with various thicknesses of nickel underplate. Heated at 200 C. 12 5 10 20 50 100 200 500 1000 HEATING TIME .HOURS Figure lOd Reliability vs. heating time for mil pure gold plate on copper with no underplate and with various thicknesses of nickel underplate. Heated at 300 C. 102 Electrodeposition 3- Concentration gradients-Since the driving force for diffusion is toward uniform composition concenttation gradients are important. The greater the difference in concentration the greater the magnitude of the diffusion reaction. 4- Lattice Structure-Some types of lattice structure are more conducive to diffusion than others. For example with the more complicated structures such as hexagonal close packed diffusion does not occur at the same rate in all directions as it does in cubic lattice systems. 5- Grain size-This is a less important factor since diffusion occurs much faster along grain boundaries than through the grains as discussed earlier. However it is important to remember that the smaller the grain size the more the grain boundaries. 6- Impurities and other alloying elements- Alloying elements and or impurities can noticeably influence diffusion. An example of this is shown in Table 1 which lists the maximum times that thick 5pm gold and gold alloy deposits can be heated at various temperatures before they become unreliable from a contact resistance viewpoint. It is clearly evident that alloy gold deposits degrade more quickly than pure deposits especially at high temperature 29 7- Cold worfc-Diffusion occurs more rapidly when a metal has been cold worked since dislocation densities are increased and grain size is reduced. Of importance from the viewpoint of electrodeposition is the fact than many electrodeposits often appear quite comparable to cold worked metals. For example electroplated copper has exhibited behavior .
